Organic Chemistry
Structures, mechanisms, synthesis and analytical identification.
Before you begin
Structures, mechanisms, synthesis and analytical identification.
Learning goals
- Representations and Nomenclature (A Level Organic)
- Isomerism and Stereochemistry (A Level Organic)
- Alkanes and Free-radical Substitution
- Alkenes and Electrophilic Addition
- Halogenoalkanes: SN1/SN2 and Elimination
- Alcohols: Oxidation and Dehydration
- Organic Mechanisms: Curly Arrows, Electrophiles, Nucleophiles
- Arenes and Electrophilic Substitution
- Carbonyls: Nucleophilic Addition and Tests
- Carboxylic Acids and Derivatives
- Amines, Amides, Amino Acids
- Polymers: Addition and Condensation
Syllabus statements covered
- interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: — hydrocarbons (alkanes, alkenes and arenes)
- interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: — halogen derivatives (halogenoalkanes and halogenoarenes)
- interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: — hydroxyl compounds (alcohols and phenols)
- interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: — carbonyl compounds (aldehydes and ketones)
- interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: — carboxylic acids and derivatives (acyl chlorides and esters)
- interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: — nitrogen compounds (amines, amides, amino acids and nitriles)
- describe sp3 hybridisation, as in ethane molecule, sp2 hybridisation, as in ethene and benzene molecules, and sp hybridisation, as in ethyne molecule
- explain the shapes of, and bond angles in, the ethane, ethene, benzene, and ethyne molecules in relation to σ and π carbon-carbon bonds
- predict the shapes of, and bond angles in, molecules analogous to those specified in (c)
- describe constitutional (structural) isomerism
- describe cis-trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of π bonds [use of E, Z nomenclature is not required]
- explain what is meant by a chiral centre
- deduce whether a given molecule is chiral based on the presence or absence of chiral centres and/or a plane of symmetry
- recognise that an optically active sample rotates plane-polarised light and contains chiral molecules
- recognise that enantiomers have identical physical properties except in the direction in which they rotate plane-polarised light [usage of the term diastereomers is not required]
- recognise that enantiomers have identical chemical properties except in their interactions with another chiral molecule
- recognise that different stereoisomers exhibit different biological properties, for example in drug action
- deduce the possible isomers for an organic molecule of known molecular formula
- identify chiral centres and/or cis-trans isomerism in a molecule of given structural formula
- interpret and use the following terminology associated with organic reactions: — functional group
- interpret and use the following terminology associated with organic reactions: — degree of substitution: primary, secondary, tertiary, quaternary
- interpret and use the following terminology associated with organic reactions: — homolytic and heterolytic fission
- interpret and use the following terminology associated with organic reactions: — carbocation
- interpret and use the following terminology associated with organic reactions: — free radical
- interpret and use the following terminology associated with organic reactions: — electrophile (Lewis acid), nucleophile (Lewis base)
- interpret and use the following terminology associated with organic reactions: — addition, substitution, elimination, condensation, hydrolysis
- interpret and use the following terminology associated with organic reactions: — oxidation and reduction [in equations for organic redox reactions, the symbols [O] and [H] are acceptable]
- interpret and use the following terminology associated with organic reactivities: — delocalisation
- interpret and use the following terminology associated with organic reactivities: — electronic effect (electron-donating and electron-withdrawing effect)
- interpret and use the following terminology associated with organic reactivities: — steric effect (steric hindrance)
- apply (a) and (b) to the understanding of mechanisms in terms of organic structure and bonding
- recognise that the mechanisms of polar reactions involve the flow of electrons from electron-rich to electron-poor sites
- explain the general unreactivity of alkanes, including towards polar reagents
- describe the mechanism of free-radical substitution with particular reference to the initiation, propagation and termination reactions, exemplified by reaction of ethane with chlorine
- describe the chemistry of alkanes as exemplified by the following reactions of ethane: — combustion
- describe the chemistry of alkanes as exemplified by the following reactions of ethane: — free-radical substitution by chlorine and by bromine in the presence of ultraviolet light at room temperature (see also 11.3(c), 11.3(k))
- recognise the environmental consequences of: — carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the internal combustion engine and of their catalytic removal
- recognise the environmental consequences of: — gases that contribute to the enhanced greenhouse effect
- explain the general reactivity of alkenes towards electrophilic reagents/electrophiles
- describe the mechanism of electrophilic addition in alkenes, using bromine (Br2 (in CCl4)) with ethene as an example
- describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: — electrophilic addition of steam (H2O(g) with H3PO4 catalyst), hydrogen halides (HX(g)) and halogens (X2(aq) or X2 (in CCl4)) (see also 11.3(d), 11.3(l))
- describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: — reduction via catalytic hydrogenation, using H2(g) and Ni catalyst (catalytic addition of hydrogen; see also 8(j))
- describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: — oxidation by cold, alkaline solution of manganate(VII) ions to form the diol
- describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: — oxidation by hot, acidified solution of manganate(VII) ions leading to the rupture of the carbon-to-carbon double bond in order to determine the position of alkene linkages in larger molecules
- apply Markovnikov’s rule to the addition of hydrogen halides to unsymmetrical alkenes, and explain the composition of products in terms of the stability of carbocation intermediates
- explain, in terms of delocalisation of π electrons, the difference between benzene and alkene: — reactivity towards electrophiles
- explain, in terms of delocalisation of π electrons, the difference between benzene and alkene: — preference of benzene to undergo substitution rather than addition reaction
- describe the mechanism of electrophilic substitution in arenes, using the mono-bromination of benzene as an example
- describe the effect of the delocalisation of electrons in arenes in such reactions
- describe the chemistry of the benzene ring as exemplified by the following reactions of benzene and methylbenzene (see also Section 4, 11.3(e) and 11.3(m)(i)): — electrophilic substitution reactions with chlorine using AlCl3 as catalyst and with bromine using AlBr3 as catalyst (recognise the use of Lewis acid as catalysts)
- describe the chemistry of the benzene ring as exemplified by the following reactions of benzene and methylbenzene (see also Section 4, 11.3(e) and 11.3(m)(i)): — nitration with a mixture of concentrated nitric acid and concentrated sulfuric acid, with the reaction mixture maintained at 30°C for methylbenzene and 50°C for benzene (recognise concentrated sulfuric acid as a Brønsted-Lowry acid catalyst)
- describe the chemistry of the benzene ring as exemplified by the following reactions of benzene and methylbenzene (see also Section 4, 11.3(e) and 11.3(m)(i)): — Friedel-Crafts alkylation with halogenoalkanes using AlCl3 or AlBr3 as catalyst (recognise the use of Lewis acid as catalysts)
- describe the chemistry of the alkyl side-chain of benzene ring as exemplified by the following reactions of methylbenzene: — free-radical substitution by chlorine and by bromine in the presence of ultraviolet light at room temperature
- describe the chemistry of the alkyl side-chain of benzene ring as exemplified by the following reactions of methylbenzene: — complete oxidation to give benzoic acid using hot alkaline KMnO4 and then dilute acid, or using hot acidified KMnO4
- predict whether halogenation will occur in the side-chain or aromatic nucleus in arenes depending on reaction conditions
- apply the knowledge of positions of substitution in the electrophilic substitution reactions of mono-substituted arenes
- interpret the different reactivities of halogenoalkanes, with particular reference to hydrolysis, and to the relative strengths of the carbon-halogen bonds
- explain the unreactivity of chlorobenzene compared to halogenoalkanes towards nucleophilic substitution, in terms of the delocalisation of the lone pair of electrons on the halogen and steric hindrance
- describe and explain the mechanisms of nucleophilic substitutions in halogenoalkanes: — SN1, in terms of stability of the carbocation intermediates
- describe and explain the mechanisms of nucleophilic substitutions in halogenoalkanes: — SN2, in terms of steric hindrance in the halogenoalkanes
- recall the chemistry of halogenoalkanes as exemplified by: — the following nucleophilic substitution reactions of bromoethane: hydrolysis using NaOH(aq) and heat formation of nitriles using KCN in ethanol and heat formation of primary amines by reaction with ammonia in ethanol heated under pressure
- recall the chemistry of halogenoalkanes as exemplified by: — the elimination of hydrogen bromide from 2-bromopropane using NaOH in ethanol and heat
- explain the stereochemical outcome in nucleophilic substitution involving optically active substrates (see also 11.3(n)): — inversion of configuration in SN2 mechanism
- explain the stereochemical outcome in nucleophilic substitution involving optically active substrates (see also 11.3(n)): — racemisation in SN1 mechanism
- suggest characteristic reactions to differentiate between: — different halogenoalkanes (see also 11.3(f))
- suggest characteristic reactions to differentiate between: — halogenoalkanes and halogenoarenes (see also 11.3(g)) e.g. hydrolysis, followed by testing of the halide ions
- explain the uses of fluoroalkanes and fluorohalogenoalkanes in terms of their relative chemical inertness
- recognise the effect of chlorofluoroalkanes (CFCs) on the ozone layer, and that their proposed replacements, hydrofluoroalkanes (HFCs) and hydrochlorofluoroalkanes (HCFCs), have significant environmental impact too [the mechanistic details of how CFCs and HCFCs deplete the ozone layer are not required]
- recall the chemistry of alcohols, exemplified by ethanol: — combustion
- recall the chemistry of alcohols, exemplified by ethanol: — nucleophilic substitution to give halogenoalkanes using hydrogen halides, HX or PCl5
- recall the chemistry of alcohols, exemplified by ethanol: — reaction with sodium
- recall the chemistry of alcohols, exemplified by ethanol: — oxidation to: carbonyl compounds using acidified K2Cr2O7, heat with distillation carboxylic acids using primary alcohols, acidified KMnO4 and heat (under reflux) carboxylic acids using primary alcohols, acidified K2Cr2O7 and heat (under reflux)
- recall the chemistry of alcohols, exemplified by ethanol: — dehydration to alkenes using concentrated H3PO4 catalyst and heat
- suggest characteristic distinguishing reactions for the different classes of alcohols (primary, secondary and tertiary alcohols), e.g. mild oxidation
- deduce the presence of a CH3CH(OH)– group in an alcohol from its reaction with warm alkaline aqueous iodine to form tri-iodomethane
- recall the chemistry of phenol, as exemplified by the following reactions: — with bases
- recall the chemistry of phenol, as exemplified by the following reactions: — with sodium
- recall the chemistry of phenol, as exemplified by the following reactions: — nitration of the benzene ring (using dilute HNO3) to form a mixture of 2-nitrophenol and 4-nitrophenol, and bromination of the benzene ring (using aqueous Br2) to form 2,4,6-tribromophenol
- explain the relative acidities of water, phenol and ethanol in aqueous medium (interpret as Brønsted-Lowry acids)
- explain the reactivity of carbonyl compounds towards nucleophilic reagents, such as hydrogen cyanide
- describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones
- describe the formation of aldehydes and ketones from, and their reduction to, primary and secondary alcohols respectively (using LiAlH4, or using H2(g), Ni catalyst)
- describe the reactions of hydrogen cyanide (using KCN catalyst) with aldehydes and ketones (see also 11.3(o))
- describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect the presence of carbonyl compounds
- deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of simple tests (i.e. warming in Fehling’s and Tollens’ reagents; ease of oxidation)
- deduce the presence of a CH3CO– group in a carbonyl compound from its reaction with warm alkaline aqueous iodine to give tri-iodomethane
- describe the formation of carboxylic acids from: primary alcohols using acidified KMnO4 or acidified K2Cr2O7, and heat (under reflux) aldehydes using acidified KMnO4 or acidified K2Cr2O7, and heat (under reflux) nitriles by heating with dilute acid (or dilute alkali followed by acidification)
- describe the reactions of carboxylic acids in the formation of: — salts via reaction with metals, alkalis or carbonates
- describe the reactions of carboxylic acids in the formation of: — esters via condensation with alcohols (using concentrated H2SO4 catalyst and heat), using ethyl ethanoate as an example
- describe the reactions of carboxylic acids in the formation of: — acyl chlorides via reaction with PCl5 using ethanoyl chloride as an example
- describe the reactions of carboxylic acids in the formation of: — primary alcohols via reduction with LiAlH4, using ethanol as an example
- explain the acidity of carboxylic acids and of chlorine-substituted ethanoic acids in terms of their structures
- describe the hydrolysis of acyl chlorides with water
- describe the condensation reactions of acyl chlorides with alcohols, phenols and primary amines
- explain the relative ease of hydrolysis of acyl chlorides, alkyl chlorides and aryl chlorides
- describe the formation of esters from the condensation reaction of acyl chlorides, using phenyl benzoate as an example
- describe the acid and base hydrolysis of esters using aqueous acid (or aqueous alkali) and heat
- describe the formation of amines as exemplified by: ethylamine through reduction of amide using LiAlH4 and nitrile using LiAlH4 or H2(g), Ni catalyst (see also Section 11.5) phenylamine through the reduction of nitrobenzene using Sn and concentrated HCl with heating, followed by NaOH(aq)
- describe the reaction of amines in the formation of salts
- describe and explain the basicity of primary, secondary and tertiary amines in the gaseous phase (interpret as Lewis bases)
- explain the relative basicities of ammonia, ethylamine and phenylamine in aqueous medium, in terms of their structures
- describe the reaction of phenylamine with aqueous bromine
- describe the formation of amides from the condensation reaction between RNH2 and R'COCl
- explain why amide is neutral in terms of delocalisation of the lone pair of electrons on nitrogen
- describe the chemistry of amides, exemplified by the following reactions: — hydrolysis using aqueous acid (or aqueous alkali) and heat
- describe the chemistry of amides, exemplified by the following reactions: — reduction to amines with LiAlH4
- describe the acid/base properties of amino acids
- recognise polymers as macromolecules built up from monomers, with average relative molecular mass of at least 1000 or at least 100 repeat units
- classify and explain the difference between addition polymers (as exemplified by poly(alkenes)) and condensation polymers (as exemplified by polyesters and polyamides)
- describe proteins as an example of condensation polymers made up of α-amino acids as monomers, forming peptide (amide) bonds
- describe the hydrolysis of proteins using aqueous acid (or aqueous alkali) and heat
- recognise that poly(alkenes) are chemically inert and can therefore be difficult to biodegrade (see also 11.3(c))
- recognise that polyesters and polyamides are generally biodegradable by hydrolysis (see also 11.8(h) and 11.9(h)(i))
- recognise that materials are a finite resource and the importance of recycling plastics, considering the economic, environmental and social factors
Lessons
Work through them in order.
Structure and isomerism
Hydrocarbons, halogenoalkanes and alcohols
- Alkanes and Free-radical SubstitutionExplain free-radical substitution through initiation, propagation and termination.
- Alkenes and Electrophilic AdditionExplain electrophilic addition to alkenes and predict its products.
- Halogenoalkanes: SN1/SN2 and EliminationCompare SN1, SN2 and elimination pathways for halogenoalkanes.
- Alcohols: Oxidation and DehydrationPredict the oxidation and dehydration products of alcohols.
Mechanisms and arenes
Carbonyl, carboxylic, nitrogen and polymer chemistry
- Carbonyls: Nucleophilic Addition and TestsExplain nucleophilic addition to carbonyls and interpret their tests.
- Carboxylic Acids and DerivativesPredict the reactions of carboxylic acids and their derivatives.
- Amines, Amides, Amino AcidsPredict the properties and reactions of amines, amides and amino acids.
- Polymers: Addition and CondensationLink monomers, linkages and repeat units in addition and condensation polymers.
Practise and check
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Topic reference
This topic covers organic structures, reaction mechanisms and synthesis. The lessons start with representations and isomerism, then work through each functional group, its reactions and its mechanisms.
Be comfortable with:
- Chemical Bonding: electronegativity, bond polarity, sigma and pi bonds, orbital overlap and intermolecular forces.
- Acids and Bases: electron-pair donor and acceptor language, and acid–base equilibria.
- Energetics: bond-energy and stability comparisons.
- Reaction Kinetics: activation energy, rate equations and mechanistic evidence.
Before synthesis work, be able to count carbon atoms in skeletal formulae, conserve charge and atoms, and identify the functional group that actually reacts.
Quick Reference
| Evidence or condition | Reaction family | First reasoning move |
|---|---|---|
| UV light with a halogen | free-radical substitution | use homolytic fission and single-headed arrows |
| electron-rich C = C with Br₂ or HX | electrophilic addition | polarise the reagent and form the more stable intermediate where relevant |
| benzene with Cl₂/AlCl₃ or Br₂/AlBr₃ | electrophilic substitution | generate a stronger electrophile and restore aromaticity |
| warm aqueous nucleophile with a halogenoalkane | nucleophilic substitution | compare substrate, nucleophile and solvent for SN1/SN2 |
| hot ethanolic base with a halogenoalkane | elimination | remove H⁺ and the leaving group to form C = C |
| aldehyde or ketone with HCN | nucleophilic addition | attack the δ + carbonyl carbon |
| acyl chloride with water, alcohol or ammonia | nucleophilic acyl substitution | addition is followed by elimination |
Use this sequence in every unfamiliar problem: functional group → reagent role → essential condition → reaction family → product → evidence or mechanism.
Common Exam Traps
- Drawing a curly arrow from an electrophile or positive charge instead of from an electron pair.
- Treating every bromine reaction alike: alkene addition, arene ring substitution and methylbenzene side-chain substitution require different conditions and mechanisms.
- Giving a reagent but omitting the solvent, temperature, catalyst or distillation/reflux choice that determines the product.
- Calling 2,4-DNPH an aldehyde test; it detects aldehydes and ketones.
- Forcing one mechanism label from the substrate alone when solvent, nucleophile and kinetic evidence also matter.
- Showing the right product with impossible charges, missing atoms or an unregenerated catalyst.
- Memorising a route in only one direction and then failing a synthesis question that begins from the product.
- Importing H2 mechanism requirements into H1 8873.
After drawing any mechanism, audit four things: electron source, electron destination, charge conservation and regeneration of any catalyst.