PROBLEM 12.9 - Organic Chemistry

Question

Question: Which alkyne has the smaller Heat of hydrogenation $\mathrm{HC}\equiv {\mathrm{CCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3 \mathrm{or} {\mathrm{CH}}_3\mathrm{C}\equiv {\mathrm{CCH}}_2{\mathrm{CH}}_3$ ? Explain your choice.

Step-by-Step Solution

Answer

Answer 

The heat of hydrogenation is smaller for ${\mathrm{CH}}_3\mathrm{C}\equiv {\mathrm{CCH}}_2{\mathrm{CH}}_3$ .

Heat of hydrogenation

In an organic molecule, unsaturated bonds are higher in energy compared to saturated bonds.

During hydrogenation reaction, triple bond or double bonds are reduced, and new single bonds are formed, due to which energy is released in the form of heat. This is known as the heat of hydrogenation.

Comparing two organic molecules for heat of hydrogenation

The measure of heat of hydrogenation can be used to determine stability only if the hydrogenation products are identical.

A more stable molecule will have less heat of hydration value.

Double bond and heat of hydrogenation

The product obtained after hydrogenation for both the alkynes is the same. 

Hydrogenation reaction

The heat of hydration is lesser for ${\mathrm{CH}}_3\mathrm{C}\equiv {\mathrm{CCH}}_2{\mathrm{CH}}_3$  because it has an internal double bond. 

Internal double bonds are more stable due to more substitutions as compared to terminal double bonds.