Question

Choose the correct order for heat of combustion of the following compounds:

• A. $\left(i\right)>\left(ii\right)>\left(iii\right)$
• B. $\left(i\right)>\left(iii\right)>\left(ii\right)$
• C. $\left(ii\right)>\left(iii\right)>\left(i\right)$
• D. $\left(iii\right)>\left(ii\right)>\left(i\right)$

Answer 1

The correct option is C $\left(ii\right)>\left(iii\right)>\left(i\right)$

More the stability, lesser is the heat of combustion.
Checking the stability of these compounds :
(i) and (iii) are more stable than (ii) because of the presence of conjugated double bonds in them.
So, (ii) is the least stable and has maximum heat of combustion.
between (i) and (ii), the stability of (i) is more due to presence of benzene ring. It has the maximum conjugation (delocalisation of pi electrons) and is aromatic.
So, the heat of combustion will be minimum for (i)
The correct order for stability is
$\left(i\right)>\left(iii\right)>\left(ii\right)$
and the correct order for heat of combustion is :
$\left(ii\right)>\left(iii\right)>\left(i\right)$