Question

Describe a sequence of reactions by which hept-1-yne can be straightforwardly prepared from hept-1-ene.

1) Br₂ ; 2) NaNH₂
1) Br₂; 2) NaNH₂, heat.
1) Br₂; 2) NaOH
1) HBr; 2) NaNH₂, heat

Answer 1

The conversion of hept-1-ene to hept-1-yne involves the initial addition of bromine to form a dibromoalkane, followed by an elimination reaction using sodium amide under heat to produce the alkyne.

Step-by-step explanation:

The preparation of hept-1-yne from hept-1-ene involves a sequence of halogenation and elimination reactions. The first step is the addition of bromine (Br₂) to the double bond of hept-1-ene, resulting in a dibromoalkane. The next step is the treatment with sodium amide (NaNH₂), which acts as a strong base to deprotonate one of the vicinal hydrogen atoms next to a bromine, leading to a bromoalkyne through an elimination reaction. This sequence of reactions is represented as follows:

• 1) hept-1-ene + Br₂ → 1,2-dibromoheptane
• 2) 1,2-dibromoheptane + NaNH₂, heat → hept-1-yne + NaBr + NH₃

Elimination reactions, such as the one involving NaNH₂, may compete with substitution reactions. It's important to note that the conditions using NaNH₂ must be carefully controlled to avoid harsh conditions that may lead to decomposition. The reaction sequence mentioned does not involve NaOH or direct addition of HBr, as these steps would not yield the desired alkyne product.