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Question
Which one of the following has the smallest heat of hydrogenation per mole?
Which one of the following has the smallest heat of hydrogenation per mole?
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Solution
The correct option is B trans-2-butene
Heat of hydrogenation is related to stability of molecules; higher the stability, lower is the heat of hydrogenation.
Butadiene, CH2=CHCH=CH2 has two double bonds so its heat of hydrogenation will be more than the other three.
The order of stability of alkenes,
R2C=CR2>R2C=CHR>RCH=CHR
Tetra substituted alkenes are most stable due to hyperconjugative structures.
Unsubstituted alkenes are least stable due to less hyperconjugation.
Thus, stability order of the given monoalkenes is
The trans-2-butene is more stable than the cis-2-butene because in the cis-isomer the two bulky groups are crowded together so it has more vander waal's strain than the trans-isomer.
Hence, trans isomer has more stability and its heat of hydrogenation is less.
Heat of hydrogenation is related to stability of molecules; higher the stability, lower is the heat of hydrogenation.
Butadiene, CH2=CHCH=CH2 has two double bonds so its heat of hydrogenation will be more than the other three.
The order of stability of alkenes,
R2C=CR2>R2C=CHR>RCH=CHR
Tetra substituted alkenes are most stable due to hyperconjugative structures.
Unsubstituted alkenes are least stable due to less hyperconjugation.
Thus, stability order of the given monoalkenes is
The trans-2-butene is more stable than the cis-2-butene because in the cis-isomer the two bulky groups are crowded together so it has more vander waal's strain than the trans-isomer.
Hence, trans isomer has more stability and its heat of hydrogenation is less.
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