Why boiling point of alcohol is higher than ether and alkane ?

Why boiling point of alcohol is higher than ether?

Oxygen is more electronegative than hydrogen. For this reason, O – H bond  of alcohol is polar one. 

As a result , alcohol molecules can form inter molecular hydrogen bond, through which alcohol molecules become associated and they are staying as gigantic molecule.

Hence, a large amount heat energy is required to break this large number of hydrogen bond .Consequently, the boiling point of alcohol molecule is very much higher.

On the other hand , ether molecules are slightly polar. So, among the ether molecule ,a weak dipole-dipole force of attraction act.

Therefore, to separate these ether molecule a less amount of heat energy is required .

Consequently, the boiling   point of ether is less than alcohol .

For example , the molecular weight of ethanol (46) is same as the molecular weight  (46) of dimethyl ether.

 

Yet, the boiling point of ethanol is 78.7ᵒC  whereas the boiling point of dimethyl ether is – 24.9ᵒC.

Why solubility of alcohol in water decreases with increasing molecular mass ?

Alcohol is soluble in water due to formation of inter molecular hydrogen bond with water molecule . 

But, with increasing molecular mass the solubility of alcohol in water gradually decreases and ultimately alcohol with higher mass are insoluble in water.

This is due to the fact that alcohol with lower molecular mass can form inter molecular hydrogen bond with water molecule easily.

Because, in case of lower alcohol the priority of polar hydrophilic part ( - OH )  more prominent than non polar hydrophobic part  ( -R ,alkyl group ).

On the other hand, in case of higher alcohol that is alcohol with higher molecular mass, the non polar hydrophobic part of alcohol ( - R , alkyl group ) is more prominent than polar hydrophilic part ( -OH ).

Hence, these alcohol have low tendency to form hydrogen bond with water molecule . 

Consequently, they are insoluble in water .

Why the boiling point of alcohol is higher than alkane?

Oxygen is more electronegative than hydrogen. For this reason, O – H bond  of alcohol is polar one. 

As a result , alcohol molecules can form inter molecular hydrogen bond , through which alcohol molecules become associated and they are staying as gigantic molecule.

Hence, a large amount heat energy is required to break this large number of hydrogen bond .Consequently, the boiling point of alcohol molecule is very much higher.

On the other hand, alkane are completely non polar. So, a weak Vander waals force of attraction acts among the alkane molecules, that is alkane molecules are staying as individual molecule.

Hence, to separate these alkane molecule a very much less amount of heat energy is required.Consequently, the boiling point of alkane is very much less than alcohol.

For example , the molecular weight of ethanol (46) is near about same as the molecular weight of propane ( 44 ).

But, the boiling point of ethanol is 78.7ᵒC  where as the boiling point of propane is - 42ᵒC which is very much less.

Summary 

• Why boiling point of alcohol is higher than ether ?
• Why solubility of alcohol in water decreases with increasing molecular mass ?
• Why the boiling point of alcohol is higher than alkane ?

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