Question

Question: How would you carry out the following reactions?

Short answer

Alkynes can be transformed into aldehydes by hydroboration oxidation reaction. They undergo hydroboration-oxidation reaction to produce an enol. This gets tautomerized to form an aldehyde. Alkynes can be converted to alkenes by reacting it with hydrogen in the presence of Lindlar’s catalyst.

Step-by-step solution

Conversion of alkynes to various compounds

Alkynes can be transformed into aldehydes by hydroboration oxidation reaction. They undergo hydroboration-oxidation reaction to produce an enol. This gets tautomerized to form an aldehyde. Alkynes can be converted to alkenes by reacting it with hydrogen in the presence of Lindlar’s catalyst.

 Reagent used for carrying out reaction (a),(b),(c),(d),(e) and (f)

(a)The reaction shows the conversion of 1-butyne to butanone. Terminal alkynes undergo hydration with dilute H₂SO₄in the presence of H_gSO₄ and gets converted to an enol. The enols get tautomerized to give a ketone. This conversion can be achieved with the help of the reagent H₂O , H₂SO₄ and H_gSO₄ . The reaction can be given as:

(b)The reaction shows the conversion of 1-butyne to butanal. Terminal alkynes when hydrated using hydroboration-oxidation reaction yield an enol which tautomerizes to form an aldehyde. The reagent used for this conversion is BH₃ , THF, H₂O₂ and^-OH . The reaction can be given as:

(c)The reaction shows the conversion of ethynylbenzene to propynylbenzene. Terminal alkynes are converted to acetylides by treating with NaNH₂ in liquid ammonia. The acetylides can be further converted to higher alkynes by treating with alkyl halides. So the reagent used for this conversion is NaNH₂ , NH₃and CH₃Br . The reaction can be given as:

(d)The reaction shows the conversion of propynylbenzene to cis-1-propenylbenzene. Alkynes can be converted to the corresponding alkenes by treating with hydrogen in the presence of Lindlar’s catalyst. Hence, the reagent for this conversion is H₂ and Lindlar’s catalyst. The reaction can be given as:

(e)The reaction shows the conversion of 1-butyne to propanoic acid. Alkynes get cleaved when treated with powerful oxidizing agents like KMnO₄. Internal alkynes produce carboxylic acid as the product. Terminal alkynes give carbon dioxide along with carboxylic acid. The reaction can be given as:

(f)The reaction shows the conversion of 1-hexene to 1-hexyne. The treatment of alkenes with bromine yields a dibromo alkane which undergoes dehydrohalogenation with alcoholic KOH. This eliminates two HBr molecules to produce alkyne as the product.

The reaction can be given as: