Apply base effects in E2/SN2 competition

Compare bases on the same haloalkane with leaving group, concentrations, solvent and temperature stated.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Apply base effects in E2/SN2 competition

Hold the substrate constant

Compare bases on the same haloalkane with leaving group, concentrations, solvent and temperature stated.

Carbon attack depends on nucleophilicity and steric access, whereas β-H removal depends on basicity, access to H and the E2 geometric requirement.

Strength, nucleophilicity and bulk

A strong base readily accepts a proton; an effective nucleophile rapidly donates an electron pair to carbon under the stated conditions.

Steric bulk around the donor atom obstructs backside carbon attack more than approach to an exposed β-H, often shifting a strong bulky reagent toward E2.

Compare cyanide, ethoxide and tert-butoxide

CN⁻ is a relatively unhindered carbon nucleophile and with 1-bromobutane strongly supports SN2 substitution to pentanenitrile.

Ethoxide can give both paths; tert-butoxide is a strong bulky base whose carbon attack is hindered, so β-H abstraction and E2 are enhanced, especially for a secondary substrate at higher temperature.

A reagent can be both a base and a nucleophile, but the two attacks have different targets. SN2 needs access to Cα; E2 needs access to Hβ, so crowding around the donor atom can suppress carbon attack more strongly than proton removal.

Fix 2-bromobutane

At 298 K with ethoxide, 2-bromobutane can give ethoxy substitution and butene elimination products.

Replacing ethoxide by equal-concentration tert-butoxide increases steric obstruction to α-carbon attack while retaining strong proton acceptance.

Try this

Predict the direction of the product-fraction change.

Check your answer

A strong answer should include a higher butene fraction and lower substitution fraction, qualified to the matched conditions.

Use a cyanide control

Supplied reaction: 1-bromobutane with CN⁻; the observed organic product comes from substitution at the primary carbon.

Decide which atom of cyanide donates its lone pair and which bond breaks. Do not call CN⁻ incapable of basic behaviour universally; use the fixed primary substrate and observed substitution evidence.

Try this

Draw the SN2 arrows and name the organic product.

Check your answer

One concerted step: an arrow from the lone pair on cyanide's carbon to C1, and an arrow from the C–Br bond onto Br, which leaves as Br⁻. The product is CH₃CH₂CH₂CH₂CN, pentanenitrile.

Add temperature evidence

Supplied data: for one secondary bromide/base pair, raising the temperature changes the measured alkene:substitution ratio from 42:58 to 67:33.

Say what the change shows about elimination under these conditions, then what further evidence would be needed to name the elimination mechanism.

Try this

Interpret the ratio change and state its limit.

Check your answer

Alkene rises from 42% to 67% of the product, so heating favours elimination for this pair. Temperature alone does not identify E2: that needs evidence such as a rate that depends on [base] or a concerted-step signature.

Do not merge basicity with nucleophilicity

A bulky reagent may be strongly basic yet poor at backside attack because its donor atom is shielded.

A small reagent may be effective at carbon attack; substrate crowding and solvent still belong to any fully controlled comparison.

Try this

Better reasoning: ‘The strongest base must give the fastest SN2 reaction.’

Check your answer

Reject it; SN2 needs accessible nucleophilic attack, while strong bulky bases can favour E2 proton removal.

Reconcile reagent and product data

Record base charge, strength, donor-site bulk, substrate access, β-H geometry, temperature and separate substitution/alkene yields.

Later, try the comparison; return to the H3 course page for whole-course synthesis.

Hold the substrate constant, compare reagent size and basicity, quote the product fractions and show whether the electron pair moves to Cα or Hβ. Include all coupled arrows for the chosen pathway.

Try this

State the base-effect evidence needed for an E2/SN2 judgement.

Check your answer

Your answer should include a fixed substrate, controlled conditions, steric/basicity reasoning, electron-flow target and product ratio.