Apply substrate effects in E2/SN2 competition

Compare substrates with the same strong nucleophile/base, leaving group, concentration, solvent and temperature to isolate substitution at carbon versus β-H removal.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Apply substrate effects in E2/SN2 competition

Hold the reagent constant

Compare substrates with the same strong nucleophile/base, leaving group, concentration, solvent and temperature to isolate substitution at carbon versus β-H removal.

SN2 requires backside access to the α-carbon; E2 requires at least one β-H and a reactive periplanar arrangement.

Competing concerted pathways

SN2 forms a nucleophile–carbon bond as the leaving group departs; α-carbon crowding strongly raises its barrier.

E2 removes β-H as C=C forms and the leaving group departs; substitution can shift competition toward E2, but a substrate without β-H cannot eliminate.

Follow methyl to tertiary

Methyl halides have no β-carbon and undergo SN2 rather than E2; unhindered primary halides normally retain strong SN2 access.

Secondary halides show substantial competition, while tertiary halides block backside SN2 and can favour E2 with strong base if an accessible β-H exists.

Ask two yes-or-no questions for every substrate: can the nucleophile reach the back of Cα, and is there an appropriately aligned β-H? These checks explain methyl, primary, secondary, tertiary and neopentyl cases more reliably than labels alone.

Compare bromoethane and tert-butyl bromide

With ethoxide under matched warm conditions, bromoethane offers an accessible primary carbon and can substitute.

tert-Butyl bromide blocks SN2 approach but has nine β-H, so E2 formation of 2-methylpropene gains strongly.

Try this

Predict which substrate has greater SN2 and which greater E2 tendency.

Check your answer

A strong answer should include bromoethane for SN2 and tert-butyl bromide for E2, with access and β-H evidence.

Place a secondary substrate

Supplied substrates: bromoethane and 2-bromobutane, each with ethoxide.

For each, judge how hindered backside approach to the α-carbon is and where β-H atoms are available.

Try this

Explain why 2-bromobutane shows stronger competition than bromoethane.

Check your answer

Bromoethane's primary α-carbon is open to backside attack, so substitution dominates. 2-Bromobutane's secondary α-carbon is more hindered and it has β-H on both sides, so elimination competes strongly. Measured product fractions are needed before calling either path exclusive.

Use a no-beta-hydrogen control

Supplied substrate: neopentyl bromide, (CH₃)₃CCH₂Br, with ethoxide.

For E2, look for H on the β-carbon; for SN2, judge backside access to the α-carbon.

Try this

Evaluate E2 and SN2 for neopentyl bromide with ethoxide.

Check your answer

E2 is impossible: the only β-carbon is quaternary and bears no H. SN2 is possible but very slow, because the adjacent tert-butyl group blocks backside approach even though the reacting carbon is primary.

Do not say substitution helps both equally

Increasing α-substitution progressively blocks the precise backside trajectory required by SN2.

E2 can tolerate a substituted α-carbon, but still depends on β-H geometry and the chosen base.

Try this

Better reasoning: ‘A tertiary halide reacts fastest by SN2 because it has more alkyl groups.’

Check your answer

Reject it; tertiary crowding blocks backside attack and strong-base reaction with β-H instead supports E2.

Check the substrate clues

List α-substitution, backside access, every β-carbon, β-H availability and any periplanar constraint before comparing product pathways.

Next hold the substrate fixed and vary base strength, nucleophilicity and steric bulk.

Keep reagent and conditions fixed, compare Cα access and β-H availability, then connect the observed substitution and alkene fractions to the two concerted pathways.

Try this

State the substrate checklist for E2/SN2 competition.

Check your answer

Your answer should include reaction-centre substitution, backside sterics, β-H existence, geometry and matched-condition product evidence.