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Carbonyl compounds add on one mole of alcohol to give hemi−acetals and hemi−ketals while addition of two moles of alcohols give acetals and ketals.
Generally, hemi−acetals and hemi−ketals are not isolated while isolable products are acetals and ketals.
Conversion of hemi−acetal to acetal is a specific acid catalysis reaction. Hemi−acetal is first protonated, which then loses H2O molecule to give carbocation. Formation of this carbocation is the rate limiting step. Carbocation is then attacked by neucleophile to give final product, acetal.
This reaction is not so favourable with ketones under these conditions (with simple alcohols) but reaction can be favoured if 1, 2 or 1, 3−diols are used. With diols, cyclic ketals
are formed. The reaction with simple alcohols was not favourable because entropy decreases
(ΔS = −ve) as 3 molecules give 2 molecules on reaction while with diols, entropy change is zero, so reaction becomes favourable.
Acetals or ketals are stable in neutral or basic conditions but in acidic medium, they undergo acid catalysed cleavage similar to that of ethers to regenerate carbonyl compounds. Thus, this reaction is used to protect carbonyl groups. For example, we want to convert 2−carbethoxy cyclopentanone to 2−hydroxy methyl cyclopentanone. This can be achieved by protecting keto group and reducing −CO2Et group to −CH2OH by LiAlH4.
CH3−CH=O + 2C2H5OH CH3CH(OC2H5)2 + H2O