Question

Which of the following statement regarding heat of hydrogenation of alkenes is true?

• A. Cis-butene has higher heat of hydrogenation then trans-2-butene
• B. trans-2-pentent has higher heat of hydrogenation then Cis-2-butene
• C. trimethylethlene and tetramethylethlene have same heat of hydrogenation.
• D. Propene has higher heat of hydrogenation then ethylene

Answer 1

The correct option is A Cis-butene has higher heat of hydrogenation then trans-2-butene

solution:

Heat of hydrogenation is directly related to stability alkene i.e. more is the stability of alkene smallest is the heat of hydrogenation per mole. Among the given molecules Symmetrical alkenes are highly stable. Furthermore, Trans-2-butene is more stable than cis-2-butene because in trans-2-butene the bulky methyl groups are far apart whereas in cis-2-butene the bulky groups are crowded together hence there are stearic repulsions. Hence,trans-2-butene has the smallest heat of hydrogenation per mole.

hence the correct option:A