can also be effected using thionyl chloride OH + SOCl Pyridine Cl HCl + SO Methanol Thionyl chloride chloromethane This reaction also follows the S N i mechanism in the presence of pyridine. XII U11-Hydroxy XII U11-Hydroxy - - - - . Elimination reactions of alcohols When alcohols are heated with a suitable dehydrating agents like sulphuric acid, the H and OH present in the adjacent carbons of alcohols are lost, and it results in the formation of a carbon – carbon double bond. Phosphoric acid, anhydrous ZnCl , alumina etc., can also be used as dehydrating agents.
Example CH H SO CH = CH + H O K Mechanism Primary alcohols undergo dehydration by E mechanism O .. .. SO H CH = CH + Ethene H O ethanol – OSO H - H + Tertiary alcohols undergo dehydration by E mechanism. It involves the formation of a carbocation.
Protonation of alcohol Step : O .. .. OSO H – OSO H Step : Dissociation of oxonium ion to form a carbocation. Slow H C + H O Step : Deprotonation of carbocation to form an alkene H C fast H C - methylprop - - ene – H + Order of reactivity: The relative reactivities of alcohols in the dehydration reaction follows the order primary < secondary < tertiary XII U11-Hydroxy XII U11-Hydroxy - - - - Evaluate yourself Identify the products in the following reactions.
Write their IUPAC names and mention the mechanism involved in the reactions. i) cyclopentanol H SO ∆ ? ii) butan– –ol NaBr H SO ? iii) neopentyl alcohol PCl ?
Saytzeff’s rule During intramolecular dehydration, if there is a possibility to form a carbon – carbon double bond at different locations, the preferred location is the one that gives the more (highly) substituted alkene i.e., the stable alkene. For example, the