Question

Show how you would convert

(a) oct-3-yne to cis-oct-3-ene.

(b) pent-2-yne to trans-pent-2-ene.

(c) cis-cyclodecene to trans-cyclodecene.

(d) but-1-yne to cis-hex-3-ene.

Short answer

(a)oct-3-yne to cis-oct-3-ene

(b) pent-2-yne to trans-pent-2-ene

(c) cis-cyclodecene to trans-cyclodecene

(d) but-1-yne to cis-hex-3-ene

Step-by-step solution

Partial catalytic hydrogenation to Alkene

In catalytic hydrogenation, alkynes are converted to alkanes.

Here,to convert alkyne to cis-alkene, some reagents are used, stopping the alkene reaction. This process is known as partial catalytic hydrogenation of an alkene.

The Lindalar’s catalyst is used to make cis-alkene, and metal ammonia is used to get trans-alkene.

Subpart (a) Conversion of oct-3-yne to cis-oct-3-ene.

Using Lindlar’s catalyst, one can convert oct-3-yne to cis-oct-3-ene.

Here, the product is a cis alkene as Lindlar’s catalyst produces cis-alkene.

Subpart (b) Conversion of pent-2-yne to trans-pent-2-ene.

Using metal in ammonia Na/NH₃ , one can convert pent-2-yne to trans-pent-2-ene.

Here, the product is a trans alkene.

Subpart (c) Conversion of cis-cyclodecene to trans-cyclodecene.

Cis-cyclodecyne can be prepared by halogenation followed by the addition of sodium amide NaNH₂ .

Cyclodecyne can be converted to cyclodecene with the addition of Na/NH₃ metal in ammonia.

Here, the product is a trans alkene.

Subpart (d) Conversion of but-1-yne to cis-hex-3-ene.

But-1-yne can be converted to hex-3-yne by adding sodium amide and ethyl bromide.

Later, hex-3-yne is converted tocis-hex-3-eneby adding hydrogen molecule and Lindlar’s catalyst.

Here, the product is a cis alkene.