Question 1

What is the difference between E1, E2 and E1cb?

Accepted Answer

All three mechanisms form an alkene by breaking a C-H bond and a C-leaving-group bond, but the timing differs. E1 loses the leaving group first to form a carbocation, E2 breaks both bonds in one concerted step, and E1cb removes hydrogen first to form a carbanion or enolate before the leaving group departs.

Question 2

Why does 3-hydroxybutanal dehydrate by E1cb?

Accepted Answer

The hydroxide leaving group is poor, so direct ionisation to a carbocation is not favourable. The aldehyde group makes the adjacent hydrogen sufficiently acidic for hydroxide to remove it, producing a resonance-stabilised enolate; hydroxide then leaves in the slower elimination step to form but-2-enal.

Question 3

Can rate law alone distinguish E2 from E1cb?

Accepted Answer

No. Both E2 and E1cb can show second-order rate laws because base participates in the rate expression. Evidence for a reversible carbanion-forming step, especially hydrogen/deuterium exchange in recovered starting material, supports E1cb rather than E2.

Question 4

Why does the same alkyl halide give E1 with a weak base and E2 with a strong base?

Accepted Answer

A tertiary halide can ionise in a polar protic weak-base medium to give a stable carbocation, so E1 becomes viable. A strong base instead removes a beta-hydrogen while the leaving group departs in the same event, favouring E2; both routes can yield the same alkene.

Question 5

Does E1 have a rearrangement possibility?

Accepted Answer

Yes. E1 proceeds through a carbocation intermediate, and carbocations can rearrange through hydride or alkyl shifts when a more stable carbocation can form. E2 has no carbocation intermediate, and E1cb proceeds through a carbanion or enolate, so rearrangement is not the expected feature of those mechanisms.

Question 6

What is the product of E1cb dehydration in aldol condensation?

Accepted Answer

Dehydration of 3-hydroxybutanal produces but-2-enal, an alpha,beta-unsaturated aldehyde conjugated with the carbonyl group. This elimination is the dehydration stage that converts the aldol addition product into the aldol condensation product.

E1 vs E2 vs E1cb: Bond-Breaking Timing in 3D

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