Question

Question: Each of the following syntheses requires more than one step. How

would you carry them out?

(a)

b)

Short answer

a)

b)

Step-by-step solution

Conversion of alkynes to aldehydes

Alkynes get converted to alkenes in the presence of hydrogen and Lindlar’s catalyst. The alkenes are further converted to alcohols by treating with ozone, zinc and ${\mathbf{H}}_{\mathbf{3}}{\mathbf{O}}^{\mathbf{+}}$ .

Reagents needed for the synthesis of the given product from the reactant in reaction (a)

(a)The given reaction shows the conversion of 1-pentyne to butanal. The reactant, 1-pentyne can be treated with${\mathrm{H}}_2$ and Lindlar’s catalyst to produce 1-pentene. This further reacts with${\mathrm{O}}_3$ , zinc and acetic acid to produce butanal. The reaction can be given as:

Conversion of alkynes to alkenes 

Alkynes are treated with ${\mathbf{NaNH}}_{\mathbf{2}}\mathbf{,}{\mathbf{NH}}_{\mathbf{3}}$ in the presence of alkyl bromide to give a higher alkyne. This further reacts with Li in ammonia to form the alkene.

Reagents needed for the synthesis of the given product from the reactant in reaction (b)

(b)The reaction shows the conversion of 4-methyl-1-pentyne to trans-6-methyl-3-heptene.In the first step, the lower alkyne, 4-methyl-1-pentyne is converted to a higher alkyne, 6-methyl-hept-3-yne by reacting with${\mathrm{NaNH}}_2$ in liquid ammonia and${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}$ . The resultant alkyne further reacts with Li in ammonia to produce trans-6-methyl-3-heptene. The reaction can be given as: