Question

Which one of the following has the smallest heat of hydrogenation per mole ?

• A. 1-butene
• B. trans-2-butene
• C. cis-2-butene
• D. 1,3-butadiene

Answer 1

Answer: (B)

Among alkenes-heat of hydrogenation depends on :
(a) The number of double bonds-greater the number of double bonds, greater the amount of heat evolved in hydrogenation.Hence, $1,3$-butadiene has highest heat of hydrogenation among these.
(b) Relative stability of alkenes-greater the stability, smaller the heat evolved in hydrogenation. Trans-$2$-butene is most stable among three given butenes, has least heat of hydrogenation.