OCR A Chemistry A – Compare Every Mechanism
Cheat Sheet
1. Electrophilic Addition vs Nucleophilic Addition
Electrophilic addition: alkenes, electron-rich C=C attracts electrophile.
Nucleophilic addition: aldehydes/ketones, δ+ carbon attracts nucleophile.
Mark-scheme phrases: 'π bond is a region of high electron density'; 'carbon atom in carbonyl group
is δ+'; 'nucleophile donates a lone pair'.
2. Nucleophilic Substitution vs Elimination
Substitution forms alcohols; elimination forms alkenes.
Mark-scheme phrases: 'hydroxide ion acts as a nucleophile'; 'hydroxide ion acts as a base';
'elimination forms a carbon-carbon double bond'.
3. SN1 vs SN2
SN1: two-step, carbocation intermediate, tertiary favoured.
SN2: one-step, no intermediate, primary favoured.
Key phrases: 'carbocation intermediate is formed'; 'tertiary carbocation is more stable'; 'steric
hindrance reduces nucleophilic attack'.
4. Electrophilic Addition vs Electrophilic Substitution
Alkenes undergo addition; benzene undergoes substitution to retain aromaticity.
Key phrases: 'benzene is stabilised by delocalisation'; 'addition would destroy aromaticity';
'delocalised π system is retained'.
5. Benzene vs Phenol
Phenol is more reactive due to oxygen lone pair increasing ring electron density.
Key phrases: 'lone pair overlaps with the π system'; 'electron density in the ring increases'; 'ring is
activated towards electrophilic attack'.
6. Benzene vs Methylbenzene
Methyl group donates electron density and activates the ring.
Key phrases: 'methyl group is electron-releasing'; 'electron density in the ring is increased'.
7. Nucleophilic Addition vs Addition–Elimination
Aldehydes/ketones: addition only. Acyl chlorides: addition then elimination.
Key phrases: 'chloride is a good leaving group'; 'tetrahedral intermediate is formed'.
8. Free Radical Substitution vs Electrophilic Substitution
Radicals in alkanes vs electrophiles in benzene.
Key phrases: 'homolytic fission produces radicals'; 'a radical contains an unpaired electron'.
9. Haloalkane Hydrolysis
Reactivity: iodoalkane > bromoalkane > chloroalkane.
Key phrases: 'C–I bond has the lowest bond enthalpy'; 'less energy is required to break the bond'.
OCR Favourite 6-Markers
Why benzene undergoes substitution not addition; why phenol reacts faster than benzene; why
tertiary haloalkanes react faster in SN1.
Cheat Sheet
1. Electrophilic Addition vs Nucleophilic Addition
Electrophilic addition: alkenes, electron-rich C=C attracts electrophile.
Nucleophilic addition: aldehydes/ketones, δ+ carbon attracts nucleophile.
Mark-scheme phrases: 'π bond is a region of high electron density'; 'carbon atom in carbonyl group
is δ+'; 'nucleophile donates a lone pair'.
2. Nucleophilic Substitution vs Elimination
Substitution forms alcohols; elimination forms alkenes.
Mark-scheme phrases: 'hydroxide ion acts as a nucleophile'; 'hydroxide ion acts as a base';
'elimination forms a carbon-carbon double bond'.
3. SN1 vs SN2
SN1: two-step, carbocation intermediate, tertiary favoured.
SN2: one-step, no intermediate, primary favoured.
Key phrases: 'carbocation intermediate is formed'; 'tertiary carbocation is more stable'; 'steric
hindrance reduces nucleophilic attack'.
4. Electrophilic Addition vs Electrophilic Substitution
Alkenes undergo addition; benzene undergoes substitution to retain aromaticity.
Key phrases: 'benzene is stabilised by delocalisation'; 'addition would destroy aromaticity';
'delocalised π system is retained'.
5. Benzene vs Phenol
Phenol is more reactive due to oxygen lone pair increasing ring electron density.
Key phrases: 'lone pair overlaps with the π system'; 'electron density in the ring increases'; 'ring is
activated towards electrophilic attack'.
6. Benzene vs Methylbenzene
Methyl group donates electron density and activates the ring.
Key phrases: 'methyl group is electron-releasing'; 'electron density in the ring is increased'.
7. Nucleophilic Addition vs Addition–Elimination
Aldehydes/ketones: addition only. Acyl chlorides: addition then elimination.
Key phrases: 'chloride is a good leaving group'; 'tetrahedral intermediate is formed'.
8. Free Radical Substitution vs Electrophilic Substitution
Radicals in alkanes vs electrophiles in benzene.
Key phrases: 'homolytic fission produces radicals'; 'a radical contains an unpaired electron'.
9. Haloalkane Hydrolysis
Reactivity: iodoalkane > bromoalkane > chloroalkane.
Key phrases: 'C–I bond has the lowest bond enthalpy'; 'less energy is required to break the bond'.
OCR Favourite 6-Markers
Why benzene undergoes substitution not addition; why phenol reacts faster than benzene; why
tertiary haloalkanes react faster in SN1.
