dehydration of , – dimethyl – - butanol gives a mixture of alkenes. The secondary carbocation formed in this reaction undergoes rearrangement to form a more stable tertiary carbocation. OSO H OSO H CH CH - H O - H CH CH CH Methyl shift H C H C CH = CH , - dimethylbut - - ene ( %) H C CH (CH ) , - dimethylbut - - ene ( %) , - dimethylbut - -ene ( %) - H + - H + Elimination) Elimination) ( Carbocation ) ( Carbocation ) (two alkyl Substitution) (four alkyl Substitution) (one alkyl Substitution) (a) (b) XII U11-Hydroxy XII U11-Hydroxy - - - - Evaluate yourself : What is the major product obtained when , – dimethyl pentan - – ol is heated in the presence of H SO . Oxidation of alcohols The important reactions of alcohols are their oxidation to give carbonyl compounds.
The commonly used oxidising agent is acidified sodiumdichromate. Oxidation of primary alcohols give an aldehyde which on further oxidation gives the carboxylic acids. To stop the oxidation reaction at the aldehyde / ketone stage, pyridinium chlorochromate (PCC) is used as an oxidising agent. Example Na Cr O (O) CHO Na Cr O (O) COOH ethanol ethanal ethanoicacid acidified acidified CH Na Cr O (O) Na Cr O (O) Propan - - ol Propanone ethanoicacid acidified acidified CHO PCC Propan - - ol Propanal Tertiary alcohols do not undergo oxidation reaction under normal conditions, but at elevated temperatures, under strong oxidising agent cleavage of C –C bond takes place to give a mixture of carboxylic acid.
Swern oxidation In this method, dimethyl sulfoxide (DMSO) is used as the oxidising agent, which converts alcohols to ketones / aldehydes. In this method an alcohol is treated with DMSO and oxalyl chloride followed by the addition of triethylamine. CH CH + H C S + Cl Cl Propan - - ol DMSO Oxalylchloride