Question

Arrange the following compounds according to the decreasing order of heat of combustion.

• A. a: ii > iii > i > iv
  b: ii > iv > i > iii
• B. a: ii > iv > iii > i
  b: iii > i > iv > ii
• C. a: ii > i > iii > iv
  b: iii > iv > i > ii
• D. a: iv > iii > i > ii
  b: ii > i > iv > iii

Answer 1

The correct option is C a: ii > i > iii > iv

b: iii > iv > i > ii

(a) The greater the number of C atoms, more is the heat of combustion, but when the number of C atoms is same, the order is straight chain > less branching > more branching.

So, the order is as follows :

hexane > Pentane > 2-Methyl butane (isobutane) > 2, 2-dimethyl propane (neopentane).

More is the branching, lesser is the heat of combusion, and greater is the stability of comopound.

(b) Structure (iii) is straight chain thus releases more heat on combustion than others as is thermodynamically less stable. (Higher the energy in a compound, less stable is that compound compared to the products and to its isomers.) Hence, more heat of combustion will be released. Structure (i) is an eclipsed structure, whereas (ii) is Gauche structure, hence (i) is less stable than (ii) has higher potential energy than (ii)and release more energy.

The order of heat of combustion would be $iii>iv>i>ii$