Question

$\underset{\left(I\right)}{\text{1,2-Dihydroxybenzene}}\underset{\left(II\right)}{\text{1,3-Dihydroxybenzene}}$
$\underset{\left(III\right)}{\text{1,4-Dihydroxybenzene}}\underset{\left(IV\right)}{\text{Hydroxybenzene}}$

The increasing order of boiling points of the above alcohols is:

• A. $III<II<I<IV$
• B. $III<II<IV<I$
• C. $IV<I<III<II$
• D. $IV<I<II<III$

Answer 1

The correct option is D $IV<I<II<III$

We know that more the surface area, greater will be the intermolecular forces. So among 1,2-Dihydroxybenzene, 1,3-Dihydroxybenzene and 1,4-Dihydroxybenzene; 1,4-Dihydroxybenzene has the highest intermolecular forces or in other words extent of hydrogen bonding will be highest in 1,4-Dihydroxybenzene. Again in 1,2-Dihydroxybenzene intramolecular hydrogen bonding will also be present. So extent of intermolecular forces is least in 1,2-Dihydroxybenzene and is highest in 1,4-Dihydroxybenzene among the three dihydroxybenzenes given here.
Again, molecular mass of hydroxybenzene will be less than that of the dihydroxybenzenes. Since with increase in molecular mass, intermolecular force also increases thus boiling point of hydroxy benzene will be least. Moreover since only one hydroxy group is present in hydroxybenzene, the extent of hydrogen bonding will also be less in this case. So the correct order of boiling point would be
$IV<I<II<III$