I85PREP
⌘ K
GTLE Subject Specialism Chemistry study materials
GTLE Subject Specialism Chemistry study resources

GTLE Subject Specialism Chemistry: Organic Reactions and Mechanisms

GTLE Subject Specialism Chemistry · Work through your exam, one topic at a time.

All guide sections

Organic Reactions and Mechanisms

Mechanism questions ask you to predict the major product and justify it with a mechanism. The exam tests substitution versus elimination competition, electrophilic addition, radical substitution, aromatic substitution patterns and carbonyl chemistry, plus the stereochemistry each pathway imposes.

Rules to apply

  • SN2 is one step with backside attack and inversion, fastest for methyl and primary halides with strong nucleophiles in polar aprotic solvents. SN1 proceeds through a carbocation with racemisation, fastest for tertiary halides in polar protic solvents.
  • E2 needs a strong base and an anti-periplanar beta-hydrogen; Zaitsev's rule favours the more substituted alkene, while bulky bases such as t-BuOK give the Hofmann product. E1 accompanies SN1 on heating.
  • Electrophilic addition to alkenes follows Markovnikov's rule; hydride or methyl shifts can intervene when a carbocation can become more stable. Peroxides reverse the regiochemistry for HBr only.
  • Aromatic substitution: activators such as -OH, -NH2, -R and -OR are ortho/para directing; deactivators such as -NO2, -SO3H, -CN and -CHO are meta directing; halogens are deactivating but ortho/para.
  • Acyl substitution reactivity: acid chloride > anhydride > ester > amide. Grignard addition to carbonyls extends the carbon chain.

Worked example

2-bromo-2-methylbutane is heated with ethanolic KOH. The substrate is tertiary and the base is not especially strong, so ionisation to a tertiary carbocation dominates. Loss of a beta-hydrogen gives 2-methyl-2-butene (trisubstituted) and 2-methyl-1-butene (disubstituted). Zaitsev's rule makes 2-methyl-2-butene the major product, with some SN1 ether as a by-product. Using t-BuOK in t-BuOH instead would make the Hofmann alkene dominant.

Traps

  • Choosing SN2 for a tertiary halide, or SN1 for a methyl halide.
  • Forgetting that E2 requires an anti-periplanar beta-hydrogen.
  • Drawing Markovnikov products with hydrogen on the more substituted carbon.
  • Ignoring rearrangement when a secondary carbocation sits next to a quaternary centre.
  • Predicting meta substitution from a halogen, which is ortho/para despite being deactivating.