amazon.co.ukA* revision for organic mechanisms: free-radical substitution, electrophilic addition, SN1/SN2 nucleophilic substitution, elimination and nucleophilic addition, described step by step with curly-arrow conventions for AQA, Edexcel, OCR, WJEC and CCEA.
IUPAC naming: find the longest carbon chain containing the highest-priority functional group, number from the end giving the lowest locants, name substituents as prefixes in alphabetical order and the functional group as a suffix. Priority order (high to low): carboxylic acid > ester > acyl chloride > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkane. Examples: CH₃CH₂CH(OH)CH₃ is butan-2-ol; CH₃CH₂COOCH₂CH₃ is ethyl propanoate (acid part as base name, alkyl part as prefix).
Functional groups and their reactions: alkanes (free-radical substitution); alkenes (electrophilic addition); halogenoalkanes (nucleophilic substitution, elimination); alcohols (substitution, elimination, oxidation, esterification); aldehydes/ketones (nucleophilic addition, oxidation of aldehydes); carboxylic acids (esterification, reduction); esters (hydrolysis); amines (bases, nucleophiles); nitriles (reduction, hydrolysis). Skeletal formulae show bonds as lines with vertices as carbons — be fluent converting between displayed, structural and skeletal forms.
Isomerism: structural isomers have the same molecular formula but different atom ordering (chain, position, functional-group isomerism). E/Z (geometric) isomerism requires a C=C bond with restricted rotation and two different groups on each carbon: Z when the highest-priority groups (CIP rules — higher atomic number first) are on the same side, E when opposite. But-2-ene has cis (Z) and trans (E) forms. Optical isomerism: a chiral carbon (four different groups) gives non-superimposable mirror images (enantiomers) rotating plane-polarised light equally in opposite directions; racemic mixtures are 50:50 and optically inactive. SN1 products are racemic (planar carbocation attacked from both sides) — a classic A* link between mechanisms and stereochemistry.
Free-radical substitution (alkanes + halogens, UV light): three stages. Initiation: UV supplies energy for homolytic fission of the Cl–Cl bond — each atom takes one electron, forming two radicals: Cl₂ → 2Cl•. Propagation: Cl• + CH₄ → CH₃• + HCl (radical abstracts H, new radical forms), then CH₃• + Cl₂ → CH₃Cl + Cl• (radical regenerated — chain reaction). Termination: any two radicals combine: Cl• + Cl• → Cl₂; CH₃• + Cl• → CH₃Cl; CH₃• + CH₃• → C₂H₆. Curly-arrow convention: radicals use single-headed 'fish-hook' arrows showing one-electron movement; a dot represents the unpaired electron. Limitations to quote: further substitution (poly-substitution giving CH₂Cl₂ etc.) and mixture of products at termination — limiting CH₄ excess favours monosubstitution. Ozone depletion mechanism (CFCl₃ → Cl• + CFCl₂•; Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂) uses the same radical chain logic.
Electrophilic addition (alkenes): the C=C π bond is electron-rich and attracts electrophiles. Mechanism for propene + HBr, step by step: (1) the π electron pair forms a bond to the H of H–Br; simultaneously (or immediately after) the H–Br bond breaks heterolytically — both electrons go to Br. Curly arrows: from the π bond to the H, and from the H–Br bond to Br. (2) A carbocation forms on one alkene carbon; Markovnikov's rule: H adds to the carbon already carrying more hydrogens, so the positive charge sits on the more substituted carbon (secondary/tertiary carbocations are stabilised by electron-donating alkyl groups via the inductive effect). (3) The Br⁻ lone pair donates to the positive carbon, forming the C–Br bond: product 2-bromopropane (major). Minor product 1-bromopropane arises via the primary carbocation.
With Br₂ no polar solvent needed — Br₂ is polarised as it approaches the electron-rich π bond. Exam tip: always draw the intermediate carbocation with the positive charge, and justify the major product by carbocation stability. Hydration of alkenes (H₃PO₄ catalyst, steam) follows the same electrophilic-addition mechanism to make alcohols industrially.
Nucleophiles are electron-pair donors (OH⁻, CN⁻, NH₃, H₂O) attacking electron-deficient δ+ carbon bonded to a halogen (C–X is polar: X more electronegative). SN2 — substitution nucleophilic bimolecular — is concerted, one step: (1) the nucleophile's lone pair attacks the δ+ carbon from the side opposite (180° to) the leaving group; (2) simultaneously the C–X bond breaks heterolytically, both electrons to X. Curly arrows: from nucleophile lone pair to carbon, and from C–X bond to X. A transition state shows partial bonds C···Nu and C···X with a dashed wedge — the 'umbrella turning inside out'. Rate = k[halogenoalkane][nucleophile]; favoured by primary halogenoalkanes (little steric hindrance, no stable carbocation) and polar aprotic solvents. Inversion of configuration occurs at a chiral centre.
SN1 — substitution nucleophilic unimolecular — is stepwise: (1) the C–X bond breaks first, heterolytically, forming a planar carbocation intermediate — rate-determining step; (2) the nucleophile attacks the planar carbocation from either side equally, giving a racemic mixture if the carbon was chiral. Rate = k[halogenoalkane] only. Favoured by tertiary halogenoalkanes (carbocation stabilised by three alkyl groups' inductive effect; steric hindrance blocks backside attack). Standard exam comparison: 2-bromo-2-methylpropane reacts by SN1, 1-bromobutane by SN2 — justify by carbocation stability AND steric hindrance for full marks.
Conditions: hydrolysis with warm aqueous NaOH (or water, slower) gives alcohols; with KCN in ethanol gives nitriles (extends chain by one C); with excess ethanolic NH₃ gives amines (excess NH₃ minimises further substitution). Bond enthalpy trend: C–I weakest, hydrolyses fastest — rate measured by AgNO₃/ethanol (AgI precipitate fastest, AgCl slowest), a required OCR/AQA rate comparison. Nucleophilicity order in polar protic solvents: I⁻ > Br⁻ > Cl⁻ (weaker bond, more polarisable).
Elimination (dehydrohalogenation): halogenoalkane + hot, concentrated ethanolic KOH (reflux) → alkene + KBr + H₂O. Step-by-step mechanism (E2): (1) the hydroxide lone pair abstracts a β-hydrogen (on the carbon adjacent to the C–X carbon); (2) the C–H bond electron pair forms a C=C π bond between the α and β carbons; (3) simultaneously the C–X bond breaks heterolytically to X⁻. Curly arrows: OH⁻ lone pair to the β-H, C–H bond to the C–C gap (forming π), C–X bond to X. Aqueous KOH gives substitution (alcohol); ethanolic KOH gives elimination — condition determines mechanism, standard comparison question. With asymmetric halogenoalkanes, the major alkene has more alkyl groups (more stable — Zaitsev-type preference).
Oxidation of alcohols with acidified K₂Cr₂O₇ (orange → green, Cr³⁺): primary alcohols → aldehydes (distil immediately, to prevent further oxidation) → carboxylic acids (reflux with excess oxidant); secondary → ketones; tertiary resist oxidation (no C–H on the carbinol carbon, so no hydrogen to remove). Oxidation mechanism concept: loss of two H (one from O–H, one from C–H) forming the new C=O. Distinguishing tests: aldehydes reduce Tollens' reagent ([Ag(NH₃)₂]⁺) giving a silver mirror and Fehling's gives brick-red Cu₂O; ketones give no reaction. 2,4-DNP (Brady's reagent) gives an orange precipitate with any C=O, confirming carbonyl presence but not distinguishing aldehyde from ketone. Distinguishing the alcohol classes: oxidise then test, or use iodoform (CH₃CHOH– and CH₃CO– give yellow CHI₃ precipitate with I₂/NaOH).
Required practical (AQA RP5 / OCR PAG5): distillation of a primary alcohol oxidation product. Apparatus: round-bottom flask, still head with thermometer, condenser, receiver. Thermometer bulb level with the side-arm so it reads the vapour temperature; collect the fraction boiling at the aldehyde's boiling point. Sealing: anti-bumping granules prevent violent boiling. Then test the distillate with Tollens'/Fehling's to show incomplete oxidation produced aldehyde, or reflux further to convert to the acid. Preparation of purified organic solids (recrystallisation) is covered under Titrations and Analytical Techniques.
Nucleophilic addition to carbonyls: the C=O is polarised (O more electronegative → C δ+), so nucleophiles attack the carbon. Mechanism for carbonyl + HCN (NaCN/H₂SO₄ catalyst), step by step: (1) the CN⁻ lone pair attacks the δ+ carbonyl carbon; the π bond pair moves onto the oxygen — curly arrow from CN⁻ lone pair to C, from C=O π bond to O — forming an alkoxide intermediate with O⁻; (2) the alkoxide oxygen abstracts a proton from HCN: arrow from O⁻ lone pair to H, from H–CN bond to CN⁻ (regenerating CN⁻ — catalyst). Product: hydroxynitrile (extends chain by one carbon, creates a chiral centre when the two groups on C differ — racemic mixture because planar C=O is attacked from both sides). Ketones react more slowly than aldehydes: two alkyl groups donate electron density, reducing the δ+ character, and sterically hinder attack.
Reduction: NaBH₄ provides H⁻ (hydride) — same nucleophilic-addition mechanism: aldehydes → primary alcohols, ketones → secondary alcohols. Carboxylic acids need LiAlH₄ (stronger). Esterification: carboxylic acid + alcohol, conc. H₂SO₄ catalyst, reflux → ester + water; mechanism involves protonating the C=O, alcohol addition, then loss of water. Ester hydrolysis: hot aqueous acid (reversible, gives acid + alcohol) or hot aqueous alkali — saponification, irreversible, gives carboxylate salt + alcohol (basis of soap-making from fats).
Aromatic chemistry: benzene's delocalised π-system (6 electrons over 6 carbons) makes it stable — evidence against Kekulé: hydrogenation enthalpy −208 kJ/mol vs expected −360 kJ/mol for cyclohexa-1,3,5-triene; equal C–C bond lengths (0.139 nm, between single and double); reluctance to undergo addition. Nitration mechanism, step by step: (1) electrophile generation: HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + 2HSO₄⁻ + H₃O⁺ (conc. acids, 50–60 °C); (2) attack: the π-electron pair of one C=C in benzene bonds to NO₂⁺, forming the arenium (Wheland) intermediate — a carbocation with the positive charge delocalised over five carbons (horseshoe); (3) deprotonation: HSO₄⁻ removes the H⁺ from the sp³ carbon, the C–H bond electrons restore the delocalised ring — nitrobenzene + H₂SO₄ (catalyst regenerated). Higher temperatures promote dinitration; −NO₂ deactivates the ring (electron-withdrawing), so further substitution needs harsher conditions.
Friedel-Crafts acylation: RCOCl + AlCl₃ → RCO⁺ (acylium ion) + AlCl₄⁻; the acylium ion attacks the ring as in nitration; AlCl₃ is regenerated on deprotonation (catalyst). Products: aryl ketones (e.g. benzene + CH₃COCl → phenylethanone). Acylation is used in synthesis because it prevents multiple acylation (the ketone deactivates the ring) and fixes chain length. Directing effects: electron-donating groups (−OH, −NH₂, alkyl) activate the ring and direct 2,4-substitution; electron-withdrawing (−NO₂, −COR) deactivate and direct 3-substitution. Phenol is more reactive than benzene (O lone pair delocalised into the ring, increasing electron density) and reacts with bromine water without a catalyst (white precipitate of 2,4,6-tribromophenol).