Heats of Hydrogenation
•
In general, a conjugated system is more thermodynamically‐stable
than an isolated system.
Alkene or Diene
•
•
Product
ΔHohydrog (kJ/mol)
CH3CH2CH=CH
CH CH2 CH3CH2CH2CH3
–126
126
CH2=CHCH=CH2 CH3CH2CH2CH3
–236
If 1,3‐butadiene's double bonds behaved independently then it would be
expected to yield 252 kJ/mol on hydrogenation rather than the 236 observed.
That is, 1,3‐butadiene is about 16 kJ/mol more stable than expected.
1