An introduction to A Level organic chemistry

• Benzene has six equal bonds, not three double bonds. A racemic mixture rotates plane-polarised light by exactly zero.

• 29.1.1 I can understand — understand that the compounds in the table on page 47 contain a functional group which dictates their physical and chemical properties
• 29.1.2 I can interpret — interpret and use the general, structural, displayed and skeletal formulas of the classes of compound stated in the table on page 47
• 29.1.3 I can understand — understand and use systematic nomenclature of simple aliphatic organic molecules (including cyclic compounds containing a single ring of up to six carbon atoms) with functional groups detailed in the table on page 47, up to six carbon atoms (six plus six for esters and amides, straight chains only for esters and nitriles)
• 29.1.4 I can understand — understand and use systematic nomenclature of simple aromatic molecules with one benzene ring and one or more simple substituents, for example 3-nitrobenzoic acid or 2,4,6-tribromophenol
• 29.2.1 I can understand — understand and use the following terminology associated with types of organic mechanisms: (a) electrophilic substitution (b) addition-elimination
• 29.3.1 I can describe — describe and explain the shape of benzene and other aromatic molecules, including sp² hybridisation, in terms of σ bonds and a delocalised π system
• 29.4.1 I can understand — understand that enantiomers have identical physical and chemical properties apart from their ability to rotate plane polarised light and their potential biological activity
• 29.4.2 I can understand — understand and use the terms optically active and racemic mixture
• 29.4.3 I can describe — describe the effect on plane polarised light of the two optical isomers of a single substance
• 29.4.4 I can explain — explain the relevance of chirality to the synthetic preparation of drug molecules including: (a) the potential different biological activity of the two enantiomers (b) the need to separate a racemic mixture into two pure enantiomers (c) the use of chiral catalysts to produce a single pure optical isomer (Candidates should appreciate that compounds can contain more than one chiral centre, but knowledge of meso compounds and nomenclature such as diastereoisomers is not required.)

Chapters 30 to 36 teach the reactions of every class introduced here. They will use these names, these formulae and these two mechanism terms without defining them again.

• “Benzene has three C=C double bonds.” Correct Benzene has six equal C–C bonds, each 0.139 nm long, with a bond order between one and two. The six p electrons are delocalised over the whole ring. The circle in the hexagon stands for that π system. The alternating Kekulé picture is a model that the evidence rejects.
• “A racemic mixture rotates plane-polarised light a little.” Correct A racemic mixture rotates it by exactly zero. It is an equimolar mixture of the two enantiomers, and their equal and opposite rotations cancel. A small rotation means an unequal mixture, not a racemate.
• “Enantiomers have slightly different melting points, so they can be told apart that way.” Correct Their physical and chemical properties are identical — melting and boiling point, density, solubility and spectra — apart from two things: the direction in which they rotate plane-polarised light, and their potential biological activity.
• “Electrophilic substitution is what alkenes do.” Correct Alkenes undergo electrophilic addition, and their π bond is used up. Arenes undergo electrophilic substitution: an H on the ring is replaced and the delocalised ring survives.
• “6-methylcyclohexanol” and “5-nitrobenzoic acid” Correct The principal group fixes C1. You then count round the ring in whichever direction gives the lower locants. The correct names are 2-methylcyclohexanol and 3-nitrobenzoic acid.
• “2-chloromethylbenzene is C₆H₅CH₂Cl.” Correct In the syllabus’s name 2-chloromethylbenzene, the Cl is on the ring, next to the methyl group. C₆H₅CH₂Cl has its chlorine on the side-chain and is a different compound, (chloromethyl)benzene.
• “C₆H₅CH₂OH is a phenol.” Repair In a phenol the OH is bonded directly to a ring carbon. Here it is on the CH₂ side-chain, so the compound is an alcohol, phenylmethanol.
• “CH₃CONHCH₃ is a secondary amine.” Repair It has a C=O next to the nitrogen, so it is an amide. It is a secondary amide, N-methylethanamide. A secondary amine, R–NH–R′, has no C=O.
• A displayed formula of chlorobenzene drawn with alternating C=C bonds and six drawn C–H bonds. Repair The syllabus does not expect a displayed formula where a benzene ring is part of the molecule. Draw the hexagon with a circle and write Cl at one vertex. Alternating double bonds also misrepresent benzene’s six equal bonds.
• “6-methylcyclohexanol.” Repair Number the other way round the ring from the OH carbon: 2-methylcyclohexanol.
• “1-cyclohexanol” or “cyclohexan-1-ol.” Repair With only one group on the ring, no locant is written: cyclohexanol.
• “1-nitro-3-benzoic acid” or “5-nitrobenzoic acid” for O₂NC₆H₄COOH. Repair COOH names the parent and its carbon is C1. The locants are the lowest set: 3-nitrobenzoic acid.
• “2-chloromethylbenzene” for C₆H₅CH₂Cl. Repair C₆H₅CH₂Cl has its chlorine on the side-chain and is (chloromethyl)benzene. 2-chloromethylbenzene has Cl on the ring, next to the methyl group (30.1.3).
• “1,5-dinitrobenzene.” Repair Counting the other way gives 1,3, which is lower: 1,3-dinitrobenzene.
• “Electrophilic substitution is the mechanism for alkenes.” Repair Alkenes undergo electrophilic addition. Arenes undergo electrophilic substitution and keep the ring.
• “Addition–elimination is just nucleophilic addition.” Repair It starts the same way. Then the tetrahedral intermediate eliminates Cl⁻ and the C=O re-forms. A ketone has no leaving group, so its addition stops after the first step.
• “Benzene has three double bonds.” Repair It has six equal C–C bonds, with a bond order between one and two, and the six p electrons are delocalised. Draw the circle, not alternating double bonds.
• “The benzene bond energy is the average of C–C and C=C.” Repair The Data section gives 520 kJ mol⁻¹, which is above the average of 480. The difference is the delocalisation.
• “Benzene is sp³ hybridised because it is a ring.” Repair Every carbon in benzene is sp², and the molecule is planar. Cyclohexane is the sp³ ring, and it is puckered.
• “The π system of benzene is in the plane of the ring.” Repair The π system is above and below the plane. The σ framework is in the plane.
• “Enantiomers have different melting points.” Repair Their physical properties, melting point included, are identical apart from one: the direction in which they rotate plane-polarised light. The only other difference is their potential biological activity.
• “A racemic mixture rotates plane-polarised light a little.” Repair It does not rotate it at all. Equal amounts of the two enantiomers give equal and opposite rotations that cancel exactly.
• “A racemate forms because the reaction is reversible.” Repair It forms because a planar intermediate or planar carbonyl carbon is attacked from either side with equal probability.
• “The enantiomers are easy to separate, using their different solubilities.” Repair Their solubilities are identical. That is exactly why separation is difficult and costly, and why chiral catalysts are used instead.
• “The (+) enantiomer is the one with the OH on the wedge.” Repair The sign of rotation is measured with a polarimeter. It cannot be read off a wedge-and-hash drawing.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. When you reach chapter 30, re-answer “why does benzene substitute rather than add?” using the delocalisation energy. At chapter 32, re-explain the phenol oxygen’s p-orbital overlap. At chapter 33, re-state what addition–elimination means before learning its curly arrows. At chapter 34, re-classify amines and amides. At chapter 36, re-explain why a synthesis through a planar intermediate gives a racemate. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 29 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• Topic 29 is the language every Paper 4 organic question is written in. Expect to be asked to identify a class from a structure; to draw or interpret a structural or skeletal formula; to give the systematic name of a ring compound; to state which of the six mechanism types a reaction follows; to describe and explain the bonding in benzene in terms of sp² hybridisation, σ bonds and a delocalised π system; and to define optically active and racemic mixture. You may also have to explain why a drug is made as one enantiomer. There is no multiple-choice paper on A Level content.
• The one calculation is a bond-energy comparison. The Data section gives C–C 350, C=C 610 and C⋯C (benzene) 520 kJ mol⁻¹, and you compare six benzene bonds with three single plus three double bonds. The values are supplied in the examination, so learn the method rather than the numbers. A rotation angle, an optical-activity result or a set of polarimeter readings may be given as data to interpret.
• No laboratory procedure belongs to this topic. Its practical use is as a data-handling context. You may be asked to decide from polarimeter readings whether a product is one enantiomer or a racemate, to name the variables that must be held constant, to read an angle to half a scale division, to calculate a percentage error, and to suggest an improvement.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.