Explain giving reasons:
(i) Boiling point of an alcohol is higher than the corresponding alkane.

(ii) Lower alcohols are soluble in water, while the higher alcohols are not.

(iii) Boiling point of mono hydric alcohols increases with increase in carbon atoms.

Since a large amount of energy is needed to break the extensive hydrogen bonds, which hold a large number of alcohol molelcules together, so the boiling points of alcohols are significantly higher than the corresponding alkanes, in which no such hydrogen bonding can take place.

(ii) The solubility of an alcohol in water depends upon its capacity to form hydrogen bonds with water molecules. However, the non-polar hydrocarbon part of the alcohol molecule is unable to form hydrogen bonds with other. Consequently, the lower alcohols are soluble in water, due to the formation of hydrogen bonds between highly polarised hydroxyl group (-OH), present in alcohols as well as water. However, in higher alcohols the hydrocarbon part becomes larger in size and, therefore, the solubility effect due to hydroxyl groups (-OH), of alcohol is out weighed by the counter large ‘repulsive effect’ of non-polar hydrocarbon part of the alcohol molelcule. Hence, higher alcohols become in soluble in water. The large inductive effect (+I effect) of the hydrocarbon part (or alkyl group) in higher alcohols also reduces the polar character of hydroxyl group (–OH) in higher alcohols, which in turn reduces their capacity to form hydrogen bonding with water.

(iii) With the increase in molar mass of alcohol, the intermolecular Van der Waals forces of attraction increases, thereby the boiling point also increases. Thus b.p. of C₄H₅OH > C₃H₇OH > C₂H₅OH > CH₃OH