combustion can be used to measure their relative stabilities.
1) 2-Methylpropene cannot be compared directly with other butene isomers.
H 3 CC CH 2
CH 3
+ H 2 CH 3 CHCH 3
CH 3
2-Methylpropene Isobutane
Pt
2) Isobutane and butane do not have the same enthalpy so a direct comparison of
heats of hydrogenation is not possible.
- 2-Methylpropene is the most stable of the four C 4 H 8 isomers:
CH 3 CH 2 CH CH 2 + 6 O 2 4 O 2 + 4 H 2 O ∆Ho = −2719 kJ mol-1
CC + 6 O 2 4 O 2 + 4 H 2 O ∆Ho = −2712 kJ mol-1
H
H 3 C CH 3
H
CC + 6 O 2 4 O 2 + 4 H 2 O ∆Ho = −2707 kJ mol-1
CH 3
H 3 C H
H
H 3 CC CH 2 + 6 O^2 4 O 2 + 4 H 2 O ∆Ho^ = −2703 kJ mol-1
CH 3
- The stability of the butene isomers:
H 3 CC CH 2
CH 3
>>CC >
CH 3
H 3 C H H 3 C CH 3
H
CC
H H
CH 3 CH 2 CH CH 2
7.3C OVERALL RELATIVE STABILITIES OF ALKENES
- The greater the number of attached alkyl groups (i.e., the more substituted
the carbon atoms of the double bond), the greater is the alkene’s stability.
