Chemical properties of monohydric alcohols

In alcohols and phenols, -OH group is the functional group. Thus the chemical properties of alcohols generally involve the reactions of -OH group. They can undergo substitution as well as elimination reaction.

Reactions of alcohols are classified into three types.

1) Reactions involving cleavage of -OH bond.

Reaction with metals

Alcohols and phenols react with metals such as sodium, potassium and aluminium to yield corresponding alkoxides and hydrogen.

In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides.

The above reactions show that alcohols and phenols are acidic in nature. In fact, alcohols and phenols are Bronsted acids i.e., they can donate a proton to a strong base (B:).

On treating in alkoxide with water, the starting alcohol is obtained.

This reaction shows that water is a better proton donar than alcohol i.e., alcohols are very weak acids even feeble than water. They do not turn blue litmus to red but treated with active metals liberate hydrogen.

The acidic character of alcohols is due to the presence of polar O-H group. Due to greater electro negativity of oxygen atom, the shared pair between O and H is drawn towards the oxygen atom helping in release of H⁺ ion.

An electron - releasing alkyl group (-CH₃, -C₂H₅) increases electron density over the oxygen atom tending to decrease the polarity of O-H bond. This decreases the acid strength. In addition, the alkoxide ion formed is also destabilised due to concentration of negative charge on oxygen atom by electron releasing inductive effect of alkyl group. Thus the release of H⁺ ion is difficult.

The acid strength of alcohols decreases in the following order:

The tertiary alcohols are least acidic while primary alcohols (with only one alkyl group) are most acidic.

Consider the reaction

This reaction shows that alkoxide ion is a better proton acceptor then hydroxide ion, which show that alkoxides are stronger bases (sodium ethoxide is a stronger base than sodium hydroxide)

The basic strength of the alkoxides follow the order

Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs over oxygen, which makes alcohols proton acceptors.