Question

Rank the sets of compounds from lowest to highest boiling point. justify your answer

- propanol and isopropanol, - butanol, isobutanol, and t-butyl alcohol,- ethanol, ethylene glycol and glycerine

Answer 1

Increasing order of boiling points:

isopropanol < propanol ;

t-butyl alcohol < isobutanol < butanol ;

ethanol<ethylene glycol < glycerine;

This increase inthe bp's is due to the Van der Waals dispersion forces, which areproportional to surface area. So as you increase the length of the chain, you also increase the surface area, which means that you increase the ability of individual molecules to attract each other. More the inter molecular interactions, higher will be the heat required to separate the molecules apart.

Well, coming to the structural isomers like propanol and isopropanol, butanol and it's isomers, there is a difference in their boiling points with respect to the number of carbons (carbon chain) . This is due to the fact that branching of the chain makes the molecule more compact and thereby decreases the surface area. Therefore, the intermolecular attractive forces which depend upon the surface area, also become small in magnitude on account of branching. Consequently,the boiling points of the branched chain compounds are lesser than the straight chain isomers.

With increaseing the number of hydroxy groups from ethanol to glycol and to glycerine, the hydrogen bonding interactions increase increasing the inter molecular interactions. Therefore more heat would require to break the inter molecular attractions and hence the boiling points are high with increasing hydroxy groups.