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.
Reaction with hydrogen halides
Alcohols react with hydrogen halides according to the following equations.
The alcohol may be primary, secondary or tertiary and the hydrogen halide may be HCl, HBr or HI.Order of reactivity of alcohols towards the reaction is:-secondary->-primary">
tertiary > secondary > primary
The order of reactivity of hydrogen halides is HI > HBr > HCl.
Distinction between primary, secondary and tertiary alcohols
This test is based upon relative reactivities of various alcohols towards HCl in pressure of ZnCl2. In this test, the alcohol is treated with Lucas reagent, which is equimolarmixture of HCl and ZnCl2. Alcohols are soluble in Lucas reagent and form a clear solution. On reaction alkyl chlorides are formed which being insoluble result in cloudiness in the solution.If cloudiness appears immediately, tertiary alcohol is indicated. If cloudiness appears within five minutes, secondary alcohol is indicated.
If cloudiness appears only upon heating, primary alcohol is indicated.
Reaction with phosphorus trihalides
Alcohols react with phosphorus trihalides (PCl3, PBr3 or PI3) to form alkyl halides.PBr3 and PI3 are generally prepared in situ by the reaction between phosphorus and bromine or iodine.
Alkyl chlorides are prepared by reacting an alcohol with thionylchloride or phosphorus trichloride or phosphorus pentachloride.
The reaction with thionyl chloride is preferred as the by products (SO2 and Cl2) formed are gases and are easily removed from the reaction mixture.