Question

multiple choice 2 points
which of the following alkenes has the largest molar heat of hydrogenation (le, releases the most heat upon hydrogenation)?
2,3 - dimethyl - 2 - butene
trans - 2 - butene
2 - methyl - 2 - butene
1 - hexene
cis - 2 - butene
previous

Explanation:

Step 1: Stability of alkenes

The stability of alkenes increases with the degree of substitution (more alkyl groups attached to the double - bond carbons) and with the trans configuration (less steric hindrance compared to cis).
1 - hexene is a monosubstituted alkene.
2,3 - dimethyl - 2 - butene is a tetrasubstituted alkene.
trans - 2 - butene and cis - 2 - butene are disubstituted alkenes (trans is more stable than cis due to less steric hindrance).
2 - methyl - 2 - butene is a trisubstituted alkene.

Step 2: Relationship between stability and heat of hydrogenation

The more stable an alkene is, the less heat it releases upon hydrogenation (smaller molar heat of hydrogenation).
Since 1 - hexene is the least stable (monosubstituted) among the given alkenes, it has the largest molar heat of hydrogenation.

Answer:

1 - hexene