Question

Show how Wittig reactions might be used to synthesize the following compounds. In each case, start with an alkyl halide and a ketone or an aldehyde.

(a)

(b)

(c)

(d)

Short answer

(a)

(b)

(c)

(d)

Step-by-step solution

Wittig reaction

Adding a phosphorus stabilized carbanion to a ketone or aldehyde was given by Wittig. In Wittig reaction, the carbonyl group of a ketone or an aldehyde is converted into a new C=C bond where no bond was present before.

Synthesis of the compounds

(a)

Alkyl halide (isopropyl bromide) reacts with triphenyl phosphine to form the phosphonium salt (isopropyl triphenyl phosphonium salt) which again reacts with butylylithium (BuLi)to give the ylide.

Isopropyl bromide phosphonium salt ylide

Benzaldehyde (an aldehyde) reacts with the ylide to give the desired product.

benzaldehyde ylide

(b)

Alkyl halide (methyl iodide) reacts with triphenyl phosphine to form the phosphonium salt (methyl triphenyl phosphonium salt) which again reacts with butylylithium (BuLi) to give the ylide.

methyl iodide phosphonium salt ylide

Acetophenone (a ketone) reacts with the ylide to give the desired product.

acetophenone ylide

(c)

Alkyl halide reacts with triphenyl phosphine (PPh₃) to form the phosphonium salt which again reacts with butylylithium (BuLi) to give the ylide.

alkyl halide phosphonium salt ylide

A ketone reacts with the ylide to give the desired product.

ketone ylide

(d)

Alkyl halide (ethyl bromide) reacts with triphenyl phosphine to form the phosphonium salt (ethyl triphenyl phosphonium salt) which again reacts with butylylithium to give the ylide.

ethyl bromide phosphonium salt ylide

Cyclohexanone (a ketone) reactswith the ylide to give the desired product.

cyclohexanone ylide