Advices For Studying Organic Chemistry

4.10 THE RELATIVE STABILITIES OF CYCLOALKANES:

RING STRAIN

1. Ring strain: the instability of cycloalkanes due to their cyclic structures.⇒
   angle strain and torsional strain.

4.10A HEATS OF COMBUSTION

1. Heat of combustion: the enthalpy change for the complete oxidation of a
   compound ⇒ for a hydrocarbon means converting it to CO 2 and water.

1. For methane, the heat of combustion is –803 kJ mol–1 (–191.9 kcal mol–1):

CH 4 + 2 O 2 CO 2 + 2 H 2 O ∆H° = –803 kJ mol–1

2) Heat of combustion can be used to measure relative stability of isomers.

CH 3 CH 2 CH 2 CH 3 + 6 21 O 2 4 CO 2 + 5 H 2 O ∆H° = –2877 kJ mol–1
(C 4 H 10 , butane) –687.6 kcal mol–1

CH 3 CHCH 3

CH 3

+ 621 O 2 4 CO 2 + 5 H 2 O ∆H° = –2868 kJ mol–1

(C 4 H 10 , isobutane) –685.5 kcal mol–1

i) Since butane liberates more heat (9 kJ mol–1 = 2.15 kcal mol–1) on

combustion than isobutene, it must contain relative more potential energy.

ii) Isobutane must be more stable.