Relative Stability of Alkenes:
Various alkenes differ in stability. The extent of stability of alkenes depends upon following factors:
Position of double bond:
1-Butene is less stable than 2-butene which is evident from their heats of combustions. 1-butene being least stable liberates more heat on combustion.
CH2 = CH – CH2 – CH3 + 6O2 → 4CO2 + 4 H2O
1-Butene ∆H = -2719 kJ.mol-1
CH3 – CH = CH – CH3 + 6O2 → 4CO2 + 4 H2O
2-Butene ∆H = -2712 kJ.mol-1
The measurement and comparison of heat of combustion of different alkenes , give us a clue about the relative stability of these alkenes.
Number and nature of substituent:
More is the number of substituents on doubly bonded carbons more is the stability of alkenes. e.g. stability order in following is:
CH3 CH3 CH3
│ │ │
CH3 – C = C – CH3 > CH3 – C = CH – CH3 > CH3 –
CH = CH – CH3
If electronegative group is attached to double bonded carbon it decreases the stability, while alkyl groups increases the stability.
CH3 – C = CH – CH3 > CH3 – CH = CH – CH3 > CH3
– C = CH – CH3
Orientation of substituents on double bonded carbon:
Trans form is more stable than cis form.