PROBLEM 12.5 - Organic Chemistry

Question

Question: Explain why heats of hydrogenation cannot be used to determine the relative stability of 2-methylpent-2-ene and 3-methylpent-1-ene.

Step-by-Step Solution

Answer

Answer

Heats of hydrogenation cannot be used to determine the relative stability of 2-methylpent-2-ene and 3-methylpent-1-ene because the hydrogenation product of both reactants is not identical.

Heat of hydrogenation

Catalytic hydrogenation of alkenes is an exothermic reaction. The hat evolved during this reaction is called the heat of hydrogenation

 

Relation between heat of hydrogenation and stability

The stability of a carbon-carbon double bond is determined using heat of hydrogenation.

The smaller the value of heat of hydrogenation greater is the stability of the carbon-carbon double bond.

Relative stability of 2-methylpent-2-ene and 3-methylpent-1-ene

The hydrogenation product of 2-methylpent-2-ene and 3-methylpent-1-ene can be represented as:

hydrogenation reaction of 2-methylpent-2-ene and 3-methylpent-1-ene

The hydrogenation products formed for 2-methylpent-2-ene and 3-methylpent-1-ene are different. 

Since the products formed are different, their energies cannot be compared.

Therefore, the heats of hydrogenation cannot be used to determine the relative stability of 2-methylpent-2-ene and 3-methylpent-1-ene.