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Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:
- Content: a clear outline of the content covered by the delivery guide;
- Thinking Conceptually: expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject;
- Thinking Contextually: a range of suggested teaching activities using a variety of themes so that different activities can be selected that best suit particular classes, learning styles or teaching approaches.
Learners should be able to demonstrate and apply their knowledge and understanding of:
DF(j) the term cracking; the use of catalysts in cracking processes; techniques and procedures for cracking a hydrocarbon vapour over a heated catalyst
DF(l) the terms aliphatic, aromatic, arene, saturated, unsaturated, functional group and homologous series
DF(m) the nomenclature, general formulae and structural formulae for alkanes, cycloalkanes, alkenes and alcohols (names up to ten carbon atoms)
DF(n) balanced equations for the combustion and incomplete combustion (oxidation) of alkanes, cycloalkanes, alkenes and alcohols
DF(o) the addition reactions of alkenes with the following, showing the greater reactivity of the C=C bond compared with C–C:
(i) bromine to give a dibromo compound, including techniques and procedures for testing compounds for unsaturation using bromine water
(ii) hydrogen bromide to give a bromo compound
(iii) hydrogen in the presence of a catalyst to give an alkane (Ni with heat and pressure or Pt at room temperature and pressure)
(iv) water in the presence of a catalyst to give an alcohol (concentrated H2SO4, then add water; or steam/H3PO4/heat and pressure)
DF(p) addition polymerisation and the relationship between the structural formula of the addition polymer formed from given monomer(s), and vice versa
DF(q) the terms addition, electrophile, carbocation; the mechanism of electrophilic addition to alkenes using ‘curly arrows’; how the products obtained when other anions are present can be used to confirm the model of the mechanism
DF(r) structural formulae (full, shortened and skeletal)
DF(s) structural isomerism and structural isomers
DF(t) stereoisomerism in terms of lack of free rotation about C=C bonds when the groups on each carbon differ; description and naming as:
(i) E/Z for compounds that have an H on each carbon of C=C
(ii) cis-trans for compounds in which one of the groups on each carbon of C=C is the same
DF(u) the benefits and risks associated with using fossil fuels and alternative fuels (biofuels and hydrogen); making decisions about ensuring a sustainable energy supply
OZ(j) the recognition of and formulae for examples of members of the following homologous series:
(i) haloalkanes, including systematic nomenclature
OZ(k) the characteristic properties of haloalkanes, comparing fluoro-, chloro-, bromo- and iodo-compounds, considering the following aspects:
(ii) nucleophilic substitution with water and hydroxide ions to form alcohols, and with ammonia to form amines
OZ(l) the terms substitution and nucleophile
OZ(m) the use of the SN2 mechanism as a model to explain nucleophilic substitution reactions of haloalkanes using ‘curly arrows’ and partial charges
OZ(n) the possible dependence of the relative reactivities of the haloalkanes on either bond enthalpy or bond polarity and how experimental evidence determines that the bond enthalpy is more important
OZ(o) homolytic and heterolytic bond fission
OZ(p) the formation, nature and reactivity of radicals and:
(i) explanation of the mechanism of a radical chain reaction involving initiation, propagation and termination
(ii) the radical mechanism for the reaction of alkanes with halogens
(iii) use of ‘half curly arrows’ in radical mechanisms
OZ(q) the chemical basis of the depletion of ozone in the stratosphere due to haloalkanes; the ease of photodissociation of the haloalkanes (fluoroalkanes to iodoalkanes) in terms of bond enthalpy
WM(a) the formulae of the following homologous series: carboxylic acids, phenols, acid anhydrides, esters, aldehydes, ketones, ethers
WM(b) primary, secondary and tertiary alcohols in terms of the differences in structures
WM(c) the following properties of phenols:
(i) acidic nature, and their reaction with alkalis but not carbonates (whereas carboxylic acids react with alkalis and carbonates)
(ii) test with neutral iron(III) chloride solution, to give a purple colouration
(iii) reaction with acid anhydrides (but not carboxylic acids) to form esters
WM(d) the following reactions of alcohols and two-step syntheses involving these reactions and other organic reactions in the specification:
(i) with carboxylic acids, in the presence of concentrated sulfuric acid or concentrated hydrochloric acid (or with acid anhydrides) to form esters
(ii) oxidation to carbonyl compounds (aldehydes and ketones) and carboxylic acids with acidified dichromate(VI) solution, including the importance of the condition (reflux or distillation) under which it is done
(iii) dehydration to form alkenes using heated Al2O3 or refluxing with concentrated H2SO4
(iv) substitution reactions to make haloalkanes
WM(e) techniques and procedures for making a solid organic product and for purifying it using filtration under reduced pressure and recrystallisation (including choice of solvent and how impurities are removed); techniques and procedures for melting point determination and thin layer chromatography
WM(f) techniques and procedures for preparing and purifying a liquid organic product including the use of a separating funnel and of Quickfit or reduced scale apparatus for distillation and heating under reflux
WM(g) the principles of green chemistry in industrial processes
WM(h) the term elimination reaction
PL(a)(i) amino acid chemistry:
- the general structure of amino acids
- proteins as condensation polymers formed from amino acid monomers
- the formation and hydrolysis of the peptide link between amino acid residues
PL(b) the primary, secondary and tertiary structure of proteins; the role of intermolecular bonds in determining the secondary and tertiary structures, and hence the properties of proteins
PL(c) DNA and RNA as condensation polymers formed from nucleotides, which are monomers having three components (phosphate, sugar and base):
(i) the phosphate units join by condensation with deoxyribose or ribose to form the phosphate–sugar backbone in DNA and RNA
(ii) the four bases present in DNA and RNA join by condensation with the deoxyribose or ribose in the phosphate–sugar backbone
(iii) two strands of DNA from a double-helix structure through base pairing
PL(d) the significance of hydrogen bonding in the pairing of bases in DNA and relation to the replication of genetic information; how DNA encodes for RNA which codes for an amino acid sequence in a protein
PL(e) molecular recognition (the structure and action of a given pharmacologically active material) in terms of:
(i) the pharmacophore and groups that modify it
(ii) its interaction with receptor sites
(iii) the ways that species interact in three dimensions (size, shape, bond formation, orientation)
PL(k) the formulae and systematic nomenclature of members of the following homologous series: carboxylic acids, phenols, acyl chlorides, acid anhydrides, esters, aldehydes, ketones, diols, dicarboxylic acids, primary aimnes, diamines; naming nylon structures
PL(l) the formulae for the following functional groups: primary amide, secondary amide
PL(m) the hydrolysis of esters and amides by both aqueous acids and alkalis, including salt formation where appropriate
PL(n) the reactions of acyl chlorides with amines and alcohols
PL(o) the differences between addition and condensation polymerisation
PL(p) the relationship between the structural formula of a condensation polymer and the structural formulae of its monomer(s) and vice versa
PL(q) optical isomerism:
(i) diagrams to represent optical stereoisomers of molecules
(ii) the use of the term chiral as applied to a molecule and identifying carbon atoms that are chiral centres in molecules
(iii) enantiomers as non-superimposable mirror image molecules
CD(c) fats and oils consist mainly of mixed esters of propane-1,2,3-triol with varying degrees of unsaturation
CD(d) the formulae of arenes and their derivatives (aromatic compounds)
(i) the delocalisation of electrons in these compounds
(ii) how delocalisation accounts for their characteristic properties
CD(e) the two common representations of the benzene molecule and their relation to:
(i) the shape of the molecule
(ii) bonding in the molecule (including a treatment of enthalpy change of hydrogenation)
CD(f) naming the individual functional groups mentioned elsewhere in the specification within a polyfunctional molecule and making predictions about the properties of the polyfunctional molecule; testing for these functional groups in a compound, using reactions mentioned in the specification
CD(g) the following electrophilic substitution reactions of arenes and the names of the benzene derivatives formed:
(i) halogenation of the ring
(ii) nitration, including the mechanism
(iv) Friedel–Crafts alkylation and acylation
CD(h) the formation of diazonium compounds and the coupling reactions that these undergo to form azo dyes
CD(i) the following reaction involving carbonyl compounds (aldehydes and ketones):
(i) oxidation of aldehydes to carboxylic acids using acidified dichromate, under reflux
(ii) reaction with Fehling’s solution and Tollens’ reagent
(iii) reaction with cyanide ions to form the cyanohydrin
CD(j) use of organic reactions and reaction conditions mentioned here and elsewhere in the specification to suggest and explain synthetic routes for preparing organic compounds
CD(k) the mechanism of the nucleophilic addition reaction between a carbonyl compound and CN−, using ‘curly arrows’ and partial charges
CD(l) organic mechanisms:
(i) use of the following terms to classify organic reactions: addition, condensation, elimination, substitution, oxidation, reduction, hydrolysis
(ii) use of ‘curly arrows’ and partial charges, where appropriate, to describe unfamiliar mechanisms, given appropriate information
Approaches to teaching the content
In the Chemistry B (Salters) specification, organic chemistry is integrated with other chemistry topics in modules throughout the course, rather than being covered in a few large segments. This means that teaching can draw on key ideas where these are needed to help make sense of the behaviour of organic compounds, such as structure, bonding, energetics and redox. In some cases, classes of organic compounds are introduced in one module but their reactions are only covered in detail in a later module where this is more appropriate.
The Salters approach to teaching organic chemistry places a particular emphasis on molecular structures, including 3-D representations, and on biologically important molecules such as proteins, DNA and RNA. It also encourages learners to understand the way in which organic molecules react in terms of the movement of electrons in reaction mechanisms.
Classes of organic compounds
In the module Developing fuels (DF), the basic language of organic chemistry is established to be built on in later sections. This includes the use of the terms aliphatic, aromatic, arene, saturated and unsaturated. It is worth pointing out to learners at this stage the vast number of different organic compounds that exist and the use that we make of them. This will emphasise the importance of this branch of chemistry.
Because new classes of organic compounds are met in several modules throughout the course, it is helpful to remind learners of what they have done before when introducing a new functional group. Developing a flow chart or spider diagram pinned to the wall of the lab over time is a useful way of achieving this.
The treatment of organic chemistry in DF begins with hydrocarbons and uses alkanes, cycloalkanes and alkenes to establish basic ideas about nomenclature and structure. These ideas are also applied to alcohols at this point.
Haloalkanes are covered in detail in The ozone story (OZ). This is a good opportunity to remind learners of the hydrocarbon chemistry they met in DF. Amines are also met in this module and are the first example of nitrogen-containing organic molecules.
In What’s in a medicine (WM) learners are introduced to further homologous series in which oxygen atoms are important, including carboxylic acids, phenols, acid anhydrides, esters, aldehydes, ketones and ethers.
In Polymers and life (PL) treatment of simple types of compounds met in earlier modules is extended to include diols, dicarboxylic acids and diamines. There is an opportunity here for learners to check that their prior learning is secure. Acyl chlorides, amides and nylons are also introduced in this module, as well as biologically important molecules such as amino acids, proteins, DNA and RNA. Studying condensation polymers such as proteins and nylons provides a good opportunity to contrast them with addition polymers met in DF in the context of alkenes.
Arenes and their derivatives are covered in Colour by design (CD). Fats and oils are also met in this module.
Nomenclature, formulae and structure of organic compounds
Basic ideas about the names, formulae and structure of organic compounds are introduced in DF in the context of alkanes, alkenes and alcohols.
A good place to start is with the names and full formulae of alkanes with up to ten carbon atoms. This introduces learners to the common prefixes that they will use throughout the course and to the idea of homologous series in which each member has the same general formula.
The differences between full, structural and skeletal formulae will be new to most learners and it is worth spending time making explicit exactly what is required for each type of structure. It is particularly helpful to stress that hydrogen atoms are not shown in skeletal formulae. Encouraging learners to use a molecular drawing package such as ‘ChemSketch’ is a very worthwhile strategy. They can quickly learn to use basic tools in the package and produce structures that are often much clearer than the hand drawn alternatives. The skills learnt at this point will be useful throughout the rest of the course.
Asking learners to draw full and/or skeletal formulae of alkanes with four or more carbon atom reveals that more than one possible structure is possible (see Isomerism below). Using molecular models such as MolyMod® or the ‘Orbit’ system is an excellent way of helping them appreciate the difference between structures and the three-dimensional nature of the structures. Different structures require unique names to avoid ambiguity. Rules for naming alkanes can be given to learners at this point. An effective alternative approach is to provide the names of given structures and ask learners to devise the nomenclature rules themselves.
Treatment of the names, formulae and structure of alkanes leads on naturally to do the same with alkenes. It is important that learners appreciate that the one letter difference in the name describes very different compounds.
The introduction of alcohols in DF brings with it the need to name and draw formulae for compounds with a functional group as well as a hydrocarbon chain. It is particularly effective to give learners examples of matching structures and names and ask them to devise general naming rules. This helps them to appreciate why the rules are necessary to describe each compound uniquely. Another approach is to prepare a sheet leading learners through the rules for naming alcohols and use this as a self-study activity, followed by an opportunity for learners to discuss their work in small groups.
Learners are expected to recognise the formulae of further types of organic compounds at different points during the course. Haloalkanes and amines appear in OZ, carboxylic acid, phenols, acid anhydrides, esters, aldehydes, ketones and ethers in WM.
Naming individual compounds is a slightly higher demand skill than recognising members of particular homologous series. Apart from hydrocarbons and alcohols, this is only a requirement for haloalkanes at AS Level (in OZ). For A Level only, naming is required for carboxylic acid, phenols, acyl chlorides, acid anhydrides, esters, aldehydes, ketones and amines (PL) and for some derivatives of benzene (CD). Learners also need to be able to name different forms of nylon in PL. Organic chemistry ‘dominoes’ can be produced for an activity in which names and formulae and/or structures need to be matched together.
Learners develop their ability to draw two-dimensional and three-dimensional structures of organic molecules throughout the course. This is particularly important when comparing primary, secondary and tertiary alcohols in WM, and primary and secondary amides in PL.
Learners are required to understand the relationship between the structures of polymers and their monomers for both addition polymers in DF and condensation polymers in PL. Asking groups of learners to make models of monomers and to join with other groups to produce polymer models is an effective way of helping them appreciate this relationship. In addition, learners can represent monomers themselves, combining with other learners to form a section of polymer chain in a role play.
The structure of biologically important molecules is a key component of PL. Learners begin with the structure of amino acids, move on to the primary, secondary and tertiary structures of proteins, and then to a detailed account of the structure of RNA and DNA. Molecular modules again prove very useful in helping learners to appreciate how amino acids can combine to form dipeptides and proteins. It is really helpful if learners can make models of short sections of DNA themselves. Many sets of instructions for representing DNA using cheap materials are available on the internet. An approach popular with learners uses different types of confectionery to represent the key components of the molecule. Biology departments in schools and colleges may have their own models that can be borrowed.
The structure of benzene is covered in detail in CD. The structures of dyes and fats and oils are also covered in this module. By this stage of the course, learners should be very adept at using a molecular drawing package, which is a particularly valuable skill for drawing more complex molecules.
Structural isomerism should arise naturally in DF when the structures and names of alkanes are explored. Asking learners to use molecular models to make different structures for molecules with the same molecular formula will enable them to appreciate why some structures are different and some are the same. Working in small groups provides an opportunity for learners to discuss their structures and to articulate why they are different. Applying similar strategies to the structure of alcohols can extend the ideas about structural isomerism include the position of the OH group in the molecule.
The idea of stereoisomerism, first met in DF, will be new to many learners and so will need to be introduced with care. Molecular models are almost essential as a tool to help them appreciate the differences between E/Z and cis-trans isomers and the importance of the lack of free rotation around a carbon–carbon double bond. The reasons for labelling some isomers as E/Z and some as cis-trans is quite subtle and needs to be made explicit to learners. Some useful screencasts illustrating these points are produced by the Khan Academy.
Optical isomers are met in PL in the context of amino acids. Words like ‘chiral’ and ‘enantiomer’ will be new to learners and can present a barrier to learning. Carefully worded questions can be used to draw out their meaning from learners. Once again, the use of molecular models is very important. A small mirror used with a molecular model can help learners appreciate the meaning of the term ‘non-superimposable mirror image’. Learners also need to practice drawing three-dimensional shapes using wedges and dotted lines, which were first introduced in DF. The difference between optical isomers can be illustrated by the different smells of the isomers of limonene or spearmint and caraway seeds.
Characteristic reactions of organic compounds
Learners can explore many of the characteristic reactions of different classes of organic compounds through class experiments and teacher-led demonstrations. It is important, however, that these practical activities are supported by questions that require learners to think about the reactions involved. Teachers need, therefore, to be very clear about the aims and expected outcomes of practical activities. The ‘Getting Practical’ project provides excellent guidance on this issue.
There is a danger that the characteristic reactions can become simply a long list to be learned by rote. This can be avoided by explaining the reactions clearly in terms of the electrostatic interactions between molecules, and illustrating them in some cases by reaction mechanisms (see Reaction type and reaction mechanisms below).
The lack of reactivity of alkanes (DF) can be explained by their non-polar nature and contrasted with the much greater reactivity of alkenes due to the presence of the electron rich carbon–carbon double bond. This can be illustrated with a typical electrophilic addition: addition of drops of cyclohexane and cyclohexene to bromine dissolved in hexane. Including limonene, a compound that can be extracted from orange peel, in this experiment adds an interesting context to this test for unsaturation.
The reactions of haloalkanes in OZ should be explained in terms of the electronegativity of the halogen atom and the consequent polarity of the carbon–halogen bond, and the attraction of nucleophiles, such as hydroxide ion, water and ammonia, to the electron deficient carbon. Experiments in which different halobutanes dissolved in ethanol react with silver nitrate solution can be used to investigate the relative reactivities of haloalkanes.
Although alcohols are introduced in DF their reactions are not fully explored until WM. Phenols are also introduced in this module. Comparing the reactions of ethanol and phenol in simple test-tube experiments helps learners to see the difference that a benzene ring compared to a hydrocarbon chain makes to the behaviour of the OH group. In a more investigative approach, learners can be asked to carry out a number of test-tube experiments on the compounds known to be ethanol, phenol and salicylic acid to identify which compound is which. Organic syntheses can be used to illustrate the uses of these reactions (see Practical techniques below).
Acid–base chemistry plays an important part in the reactions of carboxylic acids, amines and amino acids in PL. Learners can be asked to explain and construct equations for reactions of ethanoic acid, butylamine and glycine carried out in test tubes. Some learners will need particular help in writing the formulae of ions formed in the reactions between amino acids and hydrochloric acid and sodium hydroxide.
Many learners find the hydrolysis of esters and amides by aqueous acids and alkalis in PL difficult to follow. Working in groups, learners can discuss together the expected products when specific compounds are heated with hydrochloric acid and sodium hydroxide. Alternatively, groups of learners can be given a card-matching activity in which they link together reactants and hydrolysis products.
Electrophilic substitution reactions of arenes are introduced in CD. A safe and reliable learner experiment to illustrate this type of reaction is the nitration of methyl benzoate. Learners can make azo dyes by reacting phenylamine and benzocaine with phenol and naphthalene-2-ol. Supplying the reagents pre-frozen in test tubes reduces the time taken by this experiment and allows learners to review the reactions in the same session.
Reactions of carbonyl compounds are also studied in this module. Simple test-tube experiments with Fehling’s solution, Tollens’ reagent and acidified potassium dichromate solution can be used to show the difference in behaviour of aldehydes and ketones.
Towards the end of the course, a number of strategies can be used to help learners check their recall and understanding of the reactions of organic compounds. One approach is to ask them to carry out specified tests on unknown compounds to identify them. This can be made a little more demanding by giving the identity of the unknowns and asking learners to devise their own tests. Another approach is to give learners a spider diagram in which the reactions between different classes of organic compounds are shown. In some cases it may be more appropriate for learners to create their own diagrams. Learners can then be asked to use their diagrams to devise multi-step synthetic routes to convert given starting materials to a number of different compounds, including reagents and conditions for each step.
Reaction type and reaction mechanisms
Organic chemistry can appear quite daunting to some learners if they perceive it to be made up of a large number of unrelated reactions. Helping them to realise that most reactions fall into one of a small number of categories can go a long way to changing this perception. Reaction mechanisms can also have a great impact on learners’ views of organic chemistry as they realise that they can explain why reactions occur rather than just describing them. An effective way for learners to get to grips with reaction mechanisms is for them to construct models or animations of them.
Types of organic reaction and different reaction mechanisms are developed throughout the course, and require learners to learn and use a new kind of vocabulary. In DF, for example, they come across the terms ‘electrophile’, ‘electrophilic’ and ‘carbocation’ for the first time. It is well worth spending time making sure that the meaning of these words is clear to all learners. The meaning and use of curly arrows is particularly important. Learners are often rather vague and indiscriminate in their use of curly arrows because they are not really clear about what they represent.
The terms ‘substitution’ and ‘nucleophile’ are introduced in OZ in the context of the reactions of haloalkanes. Note that only the SN2 mechanism is required.
The radical chain reaction mechanism is also used in OZ to explain the reactions of alkanes with halogens. Some learners find this mechanism particularly difficult to follow. Carefully worded questions can be used to check that learners appreciate the difference between homolytic and heterolytic fission, and that they understand the meaning of the term ‘radical’ before describing this mechanism. The use of curly arrows must be developed at this point to include ‘half curly arrows’, which show the movement of unpaired electrons. Asking learners to label stages in the reaction as initiation, propagation or termination steps helps them focus on the differences between these stages. A very effective teacher demonstration that can be used to introduce radical reactions is to expose a mixture of cyclohexane and bromine in a test tube to sunlight. This can then be compared to the outcome with a test tube containing a similar mixture that has been wrapped in aluminium cooking foil to keep out the light.
In WM, the terms ‘oxidation’, ‘reduction’, ‘dehydration’ and ‘elimination’ are used to describe some of the reactions of alcohols but no mechanisms are required for these types of reactions.
The idea of condensation reactions is an important concept in PL. It can be introduced using the example of amino acids as building blocks of proteins, and deepened to explain the formation of the ‘backbone’ structure of DNA and the way in which bases are bonded to this backbone. Molecular models again play an important role in helping learners appreciate that a small molecule is ‘lost’ when two reactants combine in a condensation reaction. Once firmly established, the same principles can be applied by learners to explain the formation of polymers, such as polyesters and nylons. The ‘nylon rope trick’, in which strands of nylon are made from its monomers, is a quick and effective way of demonstrating a condensation reaction. This is a good place to remind learners that the formation of an ester that they met in WM is also an example of a condensation reaction.
The term ‘hydrolysis’ is introduced in PL to describe the reaction of esters and amides, including proteins and nylons, with dilute acid or alkali. It is helpful to cover the reactions of esters and amides at the same time so that learners can appreciate the similarities between the reactions. The hydrolysis of a nylon by prolonged heating under reflux with sodium hydroxide solution, followed by the isolation of the diacid from which it was made, is a challenging but rewarding experiment for learners to carry out.
Learners further develop their understanding and use of reaction mechanisms in CD to explain the nitration of benzene as an example of electrophilic substitution. This mechanism is a bit more complex than others that learners have met before and they may need help to appreciate why an electrophile is produced by mixing concentrated nitric and sulfuric acids. Asking learners why benzene does not react with nitric acid alone is a good way to stimulate their thinking about this issue.
The mechanism of nucleophilic addition reactions between aldehydes and ketones and cyanide ions is also covered in CD. By now learners should be familiar with the conventions for drawing out reaction mechanisms and can be asked to suggest a mechanism that takes account of the polarity of the carbon–oxygen double bond, and the fact that the cyanide ion is a powerful nucleophile.
Towards the end of the course, learners should be asked to use curly arrows and partial charges to describe unfamiliar mechanisms, to check that they have understood the principles involved and have not simply tried to memorise the mechanisms that they have met. This is also a good time to summarise the different types of reactions that have been met during the course, perhaps by using an activity in which learners place given reactions into a table containing columns for each type of reaction.
In the study of organic chemistry in this course, learners will meet a number of new practical techniques. This provides an opportunity for them to widen and develop their practical skills. Many of these techniques are required for the Practical Endorsement, and will also help prepare learners for questions on practical work in written assessments.
In DF, learners should carry out an experiment to crack a hydrocarbon vapour over a heated catalyst, collecting and testing the gas produced. Vaseline or liquid paraffin are suitable hydrocarbons to use. Learners should be warned about the dangers of cold water ‘sucking back’ into the hot apparatus. The use of a ‘Bunsen valve’ is sometimes recommended but this can cause more problems than it solves. When the experiment is completed the apparatus should be dismantled in a fume cupboard to avoid getting fumes into the laboratory. Cracking of hydrocarbons can also be carried out using a reduced scale technique.
The processes of heating under reflux and distillation are needed in WM when ethanol is oxidised to ethanoic acid. This experiment provides an opportunity to introduce learners to ‘Quickfit’ apparatus. They will need careful guidance about how to set up and use this equipment. Short video clips are available such as those from:
They explain the principles and use of this apparatus and are a good way of preparing learners in advance of the laboratory session. The videos should, however, be followed by teacher demonstration of the techniques within the laboratory before learners try them out themselves.
Learners meet the techniques and procedures for making and purifying a liquid organic compound in WM. The synthesis of a haloalkane such as 2-chloro-2-methylpropane from 2-methylpropan-2-ol is a good example. This procedure requires learners to use a separating funnel, possibly for the first time. They will need to be shown how to use this piece of apparatus safely and asked to think about which liquid layer to retain. Using a separating funnel is quite a tricky technique and learners should be given sufficient time to carry this operation out. It is helpful to ask learners what impurities might be present in the liquid product and to discuss ways of removing them. The practical steps of purification will then have much greater meaning. The final stage of the purification process is distillation. This is another opportunity for learners to set up and use Quickfit apparatus.
Learners also meet the techniques and procedures for making and purifying a solid organic compound in WM. This may provide further opportunities for practising refluxing. The purification also includes filtration under reduced pressure, using either a Hirsch or a Büchner funnel. Learners may not have met this technique before, so a short video clip and teacher demonstration can help learners appreciate both how to use the technique and why it is effective. These preparations can be quite time consuming and might be split into two sessions. This type of activity extends the experience of learners, who may only be used to experiments that can be completed in one session.
A further session is usually needed to purify and analyse the solid product. Here learners meet the techniques of recrystallisation, thin layer chromatography and melting point determination. Short videos illustrating these techniques are available to help prepare learners in advance of the session in the laboratory. These would need to be followed by a teacher demonstration of some key manipulative techniques, such as drawing out a tube to make it suitable for spotting on a thin layer chromatography plate, sealing a melting point tube and observing when a solid just melts. Some teachers may decide to make their own video clips to demonstrate these new techniques to learners, so that the video focuses on what they feel to be the most important features of the technique. Tablet computers and mobile phones provide a very versatile method of achieving this. Carefully worded questions can be used to promote a discussion about these practical techniques, such as how recrystallisation removes both soluble and insoluble impurities, where to place spots of solution on a thin layer chromatography plate and how the presence of impurities affects the melting point of a solid.
Extended practical procedures, such as those described above, involve hazards that learners are not usually exposed to when they carry out more tightly controlled test-tube experiments. It is good practice to ask learners to devise their own risk assessments for the experiments they are about to carry out, so that they are aware of the dangers involved and the control measures that are necessary. This could involve both online and offline research. Publications by CLEAPSS are reliable sources of information.
Common misconceptions or difficulties students may have
The idea that there is a ‘special’ branch of chemistry called organic chemistry can be a problem for some learners; it may suggest that it is somehow different from other areas of chemistry, with its own special set of rules governing it. It is important to draw out general principles so that learners realise that organic chemistry is still governed by the same things that apply to all other branches of chemistry. Some learners tend to compartmentalise their learning and it is helpful for teachers to make explicit links between different areas of chemistry to reinforce the idea that the different branches are part of a coherent whole. It is helpful, however, to point out that it is the unique ability of carbon atoms to form chains, rings and multiple bonds that is responsible for the vast number of organic compounds.
The similarity of some names of different classes of compounds, such as alkane and alkene, amine and amide, carboxylic acid, amino acid and acid anhydride, propanone and propanal and chloroethane and ethanoyl chloride, can be off-putting to many learners. Activities such as card matching between names and formulae, can help learners appreciate the need for rules for naming compounds so that they are not ambiguous. The use of the term ‘carbonyl compound’ can be particularly problematic since it covers both aldehydes and ketones. The term ‘nylon’ can also be a cause for concern, since some learners think that the name represents a single compound rather than being a generic name for polyamides.
Problems with nomenclature can occur right at the start of the course when naming alkanes. Learners are faced with remembering the prefixes for hydrocarbons with up to ten carbon atoms, and then are immediately asked to modify these words as they switch from alkane to alkyl group. Getting learners to work in groups to make models of and name branched alkanes provides an opportunity for them to use the words in conversation, and to achieve a sound understanding of rules for naming compounds at an early stage. Later on in the course, the introduction of the term ’phenyl’ can be described as the arene equivalent of an alkyl group.
The terms ‘electrophile’ and ‘nucleophile’ will be new to most learners. It is important that learners link these words with the attraction to electron-rich or electron-deficient centres, and with the process of forming dative bonds. The use of appropriate role plays can be an effective way of reinforcing these ideas.
There is a gradual progression in the way learners represent molecules from Key Stage 3 to Key Stage 4 and then Post-16. They may begin by using ‘Lego®’ bricks and ‘jigsaw’ models, move on to using 2-D formulae and then on to 3-D representations. Problems can arise if learners keep using an earlier representation when a more sophisticated one is necessary. It is worth checking what learners understand by the representations that we use. In particular, it is important that they associate a line drawn between symbols for elements in structural formulae with a pair of electrons, and a double line with two pairs of electrons. Later in the course, they need to develop their ideas about representing electrons to accommodate the delocalised electrons systems in arenes and dye molecules.
Some learners find it difficult to visualise 3-D structures from 2-D drawings, so getting them to make routine use of molecular models is especially helpful. Twisting sausage-shaped balloons together is also a really effective and fun way of reinforcing the 3D shapes of simple molecules. It is often better to make models without adding hydrogen atoms so that learners focus on the carbon chain and the functional group involved. Models are particularly useful in helping learners appreciate how the component parts are related in a complex molecule such as DNA and RNA.
The structures and names of esters are often seen by learners as complicated and confusing. Esters are often thought of as derivatives of an acid in which the hydrogen of an acid is replaced by an alkyl of phenyl group from an alcohol or phenol. The structure to match this description will be shown with the acid group on the left and the alkyl group on the right. Unfortunately, the naming system puts names the other way round with the alkyl group on the left and the name derived from the acid on the right! Learners should be asked to make models of esters and name them, and should be given structures to name in which the alkyl group is shown on the left. Learners need to be able to apply their ideas about the structure of esters to the more complex examples of fats and oils.
Predicting the structure of addition polymers based on the structure of their monomers may also cause problems. These problems are often seen with the structure of poly(propene) where learners may be tempted to add the methyl group to the main polymer chain instead of forming a side chain. A good way to avoid this problem is to use a role play where learners act out the role of monomers, using their arms to form bonds with other monomers and with their legs acting as methyl groups.
Learners often think that alkanes have more isomers than they actually have because they draw structures with alkyl groups added at a right angle at the end of the molecule. A good analogy to use is to point out that a tree may have branches from the side of the trunk but not from the top.
Using molecular models is a good way of helping learners appreciate the lack of rotation around a carbon–carbon double bond in an alkene. It is well worth asking learners to make a model of a cyclic structure at this point so that they see that there is a lack of free rotation around this structure as well.
Some learners find it difficult to identify chiral centres in molecules and so predict that the compound will have optical isomers. It is better to start with a simple example, such as butan-2-ol, before moving to examples of amino acids and then to complex structures of medicines or dyes. It is important for learners to appreciate that these isomers arise because of the way four different groups are arranged tetrahedrally around carbon atoms. Asking why the amino acid glycine does not have optical isomers is a good way of making this point.
Many learners find reaction mechanisms difficult to interpret and even more difficult to construct. One of the reasons for this is that they try to memorise the mechanisms rather than understanding what they represent. A key point to get over is that organic reaction mechanisms show the movement of electrons and the consequences of their movement.
Learners are used to writing chemical equations showing only the reactants and products. This is like looking at a journey from the point of view of the point of departure and destination only. A reaction mechanism shows what happens during the journey. Reaction mechanisms are a unifying force in organic chemistry. Instead of memorising hundreds of unrelated reactions, learners can fit reactions into a small number of categories and make sense of them by applying some general principles.
A good starting point is to get learners to show partial charges on polar bonds, full charges on ions and relevant lone pairs of electrons when drawing out mechanisms. When learners consider how mechanisms proceed, it is through an understanding of how the electrons in bonds are distributed. The use of d+ and d- are standard ways for denoting this, but it is also helpful sometimes to illustrate the distribution pictorially as an electron cloud that is bigger at the d- end, so that learners appreciate that this is what is meant by an increase in negative charge. It is worth asking learners about the polarity of bonds such as C–OH, C–NH2 , C–Cl, H–OH and H–NH2.
The next step is for learners to be clear that a curly arrow represents the movement of a pair of electrons, whether this is a lone pair of electrons or a pair of electrons in a covalent bond. Following on from this is the importance of showing precisely where an arrow starts and ends. This is the major weakness for many learners. Bound up with showing curly arrows is the idea that every movement of an electron pair has a consequence. It may result in the formation of a new bond or the formation of an ion or both. Drawing a parallel with the formation of a dative bond can be helpful. Asking ‘what happens next’ type of questions after drawing a curly arrow is a useful way of focusing learners’ attention on the consequences of electron movements. Numbering electron pairs in bonds, and lone pairs in reactant molecules, and showing these numbers as a reaction mechanism proceeds is another way of keeping track of them.
The electrophilic addition reaction mechanism is often illustrated by the reaction between ethene and bromine, which requires learners to include an initial step in which the bromine molecule is polarised when it approaches the carbon–carbon double bond. A better approach might be to start with the mechanism of the ethane–hydrogen bromide reaction in which the hydrogen–bromine bond in already polar. Moving from this to the ethene–bromine reaction makes the issue of bond polarisation easier to grasp.
Radical reactions have their own type of problems. One issue is that some learners try to write out the steps in the mechanism without showing the unpaired electrons. Unfortunately, most molecular models use plastic connectors to represent a pair of electrons and so do not lend themselves to representing the unpaired electrons needed to represent radical reactions. A system is needed in which the individual electrons can be shown, such as using balls of modelling clay and cocktail sticks.
Learners can also find electrophilic substitution mechanisms difficult to understand and describe. To explain them clearly, they need to have a sound understanding of the structure of benzene and the meaning of the circle in the representation of the benzene ring. It is a good idea to contrast the electrophilic substitution reaction of benzene by bromine with the electrophilic addition reaction of bromine with ethene to draw out from learners why the outcomes are so different.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
General principles introduced during the course are used to explain the behaviour of organic compounds in different modules. Learners often find it difficult to make connections between different topics in chemistry so making explicit links to where ideas have been met before is very helpful. When using a concept in organic chemistry, that has been introduced in a different context it is a good opportunity for learners to recap their knowledge and understanding of the basic principles involved.
Ideas about s- and p-bonds, for example, which are introduced in DF, are used to explain the structure of ethene in DF and benzene in CD, and the mechanism for electrophilic addition reactions in DF and electrophilic substitution in CD. A good understanding of p-bonds is also essential for learners to fully understand how stereoisomerism arises in alkenes, in terms of lack of rotation about the C–C double bond.
The idea that some bonds are polar is a key requirement for learners to understand the mechanism of nucleophilic substitution reactions in OZ. The difference between homolytic and heterolytic bond fission is also described in OZ, which leads nicely into the mechanism of radical reactions. Understanding of dative bonds, introduced in EL, is needed to appreciate what is meant by a nucleophile in nucleophilic substitution, and addition reactions mechanisms in OZ and CD. It is helpful to draw parallels between the behaviour of nucleophiles in the context of organic reactions, and ligands in the context of inorganic complex formation.
Learners need to have a good understanding of hydrogen bonding, first met in OZ, to explain the structure of DNA and RNA. This includes the formation of the sugar–phosphate ‘backbone’, bonding of bases to the backbone and, most importantly, the way in which bases bond together to from a double helix.
Both bond enthalpy, introduced in DF, and bond polarity, met in OZ, are needed to help decide what factor determines the relative reactivity of haloalkanes in OZ. Interpretation of the reaction of the haloalkanes with silver nitrate solution, which is used to decide this issue, assumes that learners know about the formation of insoluble silver halides covered in ES.
The principles of heterogeneous catalysis are introduced in DF. These ideas are used in DF to help learners understand cracking of alkanes to form alkenes, and the reverse reaction in which alkenes are reduced to alkanes. Catalysis is again needed to describe the dehydration of alcohols to form alkenes in WM, and the Friedel–Crafts alkylation and acylation of arenes in CD.
Shapes of simple covalent molecules are first covered in EL. These ideas are developed further in DF to help learners understand why E/Z and cis-trans isomers can exist. The use of solid and dashed wedges to represent 3-D structures is also introduced in DF, and this form of representation is used again in PL to represent the differences between the structures of optical isomers.
This is an effective approach to learning how to construct reaction mechanisms. Learners:
- place molecular models of reagents on a large sheet of white paper and take a digital photograph
- draw on the paper appropriate partial or full charges, lone pairs of electrons and curly arrows, using felt-tipped pens and then take another photograph
- rearrange the molecules to produce the next stage of the reaction mechanism, and take a further photograph
- continue this until products are formed.
Learners can upload the photographs into a slide presentation, which will show the sequence of changes in the reaction mechanism. Alternatively, learners can use a tablet computer or mobile phone application that can display still images in a continuous sequence as a short video clip. Suitable applications include ‘i-motion HD’ and ‘Clayframes’. If learners work in small groups, they can discuss what models to make and what annotations they should add to the paper. Learners from different groups can comment on the accuracy and effectiveness of the animations produced.
Hydrocarbons, including alkanes and alkenes, are introduced in DF through the study of motor vehicle fuels. This module also covers combustion reactions, and requires learners to gain an understanding of the benefits and risks associated with both fossil fuels and alternative fuels.
Haloalkanes are important in the study of the atmosphere in OZ because of their potential impact on the ozone layer. Learners study the chemical basis for the depletion of ozone in the stratosphere, which provides a relevant context for the study of radical mechanisms.
A study of medicines, such as aspirin, in WM leads to much functional group chemistry, including the behaviour of alcohols, carboxylic acids and esters, and methods of analysis, such as thin layer chromatography and melting point determination. Learners also study the principles of green chemistry in the context of the industrial synthesis of medicines.
PL starts with the use of enzymes in food production and the uses of condensation polymers, such as nylons and polyesters. The module moves on to the development of models of the DNA structure and ends with a description of the Human Genome Project. Organic chemistry strongly interlinks with ideas about structure and bonding in this module, through contexts such as the structure of enzymes and DNA, and molecular recognition.
CD begins with an explanation of the colour of carrots and Kekule’s dream about the structure of benzene before moving on to the design of synthetic dyes, such as picric acid, chrysoidine and mauveine. The module ends with a look at food dyes, food testing and case studies to illustrate the synthesis of organic molecules.
This is a comprehensive resource that provides details about the extraction and purification of paracetamol tablets, and the preparation and analysis of paracetamol. The preparation includes the nitration of phenol, the reduction of a nitro group to an amine and the formation of an amide.
The chemistry involved means this resource is more suitable for use in the second year of the course, which provides a good link back to the chemistry of medicines covered earlier in WM.
The resource is an infographic that identifies the major organic compounds contributing to the flavour, aroma or colour of common herbs and spices. Posters like this can be used for ‘functional group spotting’, and potentially for thinking about synthesis routes (though not all compounds depicted here will be suitable for that purpose).
There are many other useful infographics that can be used as sources of information and as colourful posters covering topics in which organic compounds are important, such as hay fever medications, cosmetics, antibiotics, insect repellents and the chemistry behind football shirts and sunscreens.
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