why the carbon atoms containing 2 or more hydroxy group is unstable,but not in case of halogen.

Dear student,

An Alcohol functional group is a hydroxyl group bonded to an sp³ hybridized carbon.  It can be regarded as a derivative of water, with an alkyl group replacing one of the hydrogens.  If an aryl group is present rather than an alkyl, the compound is generally called a phenol rather than an alcohol.  Also, if the hydroxyl group is bonded to one of the sp² hybridized carbons of an alkenyl group, the compound is referred to as an enol.  The oxygen in an alcohol has a bond angle of around 109° (c.f. 104.5° in water), and two nonbonded electron pairs.