Question

27 multiple choice 2 points which sequence ranks the following compounds in order of increasing heat of hydrogenation, δhhyd? 1 cis - 2 - butene 2 1 - butene 3 cyclohexene 1 < 2 < 3 1 < 3 < 2 2 < 1 < 3 3 < 1 < 2 3 < 2 < 1 previous

Explanation:

Step1: Understand heat of hydrogenation

The heat of hydrogenation ($\Delta H_{hyd}$) is inversely related to the stability of the alkene. More stable alkenes have lower heat of hydrogenation.

Step2: Analyze stability of alkenes

• Cyclohexene: It has some ring strain, but the double - bond is part of a six - membered ring.
• cis - 2 - butene: It is a disubstituted alkene. In cis - 2 - butene, the two methyl groups are on the same side of the double bond. There is some steric hindrance, but it is more stable than a monosubstituted alkene.
• 1 - butene: It is a monosubstituted alkene.

The order of stability of alkenes (from most stable to least stable) is: cyclohexene (due to ring - strain relief and double - bond in a relatively stable ring system) > cis - 2 - butene (disubstituted) > 1 - butene (monosubstituted).

Since stability is inversely related to heat of hydrogenation, the order of heat of hydrogenation (from lowest to highest) is: cyclohexene (3) < cis - 2 - butene (1) < 1 - butene (2).

Answer:

3 < 1 < 2