Question

16 multiple choice 2 points which of the following alkenes has the smallest molar heat of hydrogenation (ie, releases the least heat upon hydrogenation)? trans - 2 - butene 2 - methyl - 2 - butene 2,3 - dimethyl - 2 - butene cis - 2 - butene previous

Explanation:

Step1: Understand molar heat of hydrogenation

The molar heat of hydrogenation is inversely related to the stability of the alkene. More stable alkenes have smaller molar heats of hydrogenation.

Step2: Analyze alkene stability

Alkene stability increases with the degree of substitution (more alkyl groups attached to the double - bond carbons).

• trans - 2 - butene has two alkyl groups attached to the double - bond carbons.
• 2 - methyl - 2 - butene has three alkyl groups attached to the double - bond carbons.
• 2,3 - dimethyl - 2 - butene has four alkyl groups attached to the double - bond carbons.
• cis - 2 - butene has two alkyl groups attached to the double - bond carbons, but it is less stable than the trans isomer due to steric hindrance.

Step3: Compare stabilities

Since 2,3 - dimethyl - 2 - butene is the most highly substituted (most stable) among the given alkenes, it has the smallest molar heat of hydrogenation.

Answer:

C. 2,3-dimethyl-2-butene