Indian Forest Service (Mains), Chemistry optional · 2021 · Paper II

Q1(d). Kinetic enolate aldol sequence

Question

Complete 3-methylbutan-2-one —(LDA, −78 °C)→ A —(CH₃CHO)→ B —(H₂SO₄)→ C (major), and give the mechanism.

A is the methyl-side lithium enolate; B is (CH₃)₂CHCOCH₂CH(OH)CH₃ after protonation; C is predominantly the E conjugated enone (CH₃)₂CHCOCH=CHCH₃.

Bulky LDA at low temperature removes a proton from the less hindered methyl side, giving the kinetic enolate (CH₃)₂CHC(O⁻)=CH₂ with Li⁺.
Enolate carbon attacks acetaldehyde, making the new C–C bond; protonation of the alkoxide furnishes the β-hydroxy ketone B.
Acid protonates OH; loss of water and removal of the α-H create the conjugated C=C bond.
The less crowded E enone is generally favoured; a room-temperature equilibrating enolate preparation could change the regioselectivity.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Coversdirected aldol condensationkinetic enolateLDAaldol dehydrationα,β-unsaturated ketone