Groups

Groups Fig. . : Structures of methanol, phenol and methoxymethane Give IUPAC names of the following compounds: The bond angle in alcohols is slightly less than the tetrahedral angle ( °- ¢ ). It is due to the repulsion between the unshared electron pairs of oxygen. In phenols, the –OH group is attached to sp hybridised carbon of an aromatic ring. The carbon– oxygen bond length ( pm) in phenol is slightly less than that in methanol. This is due to (i) partial double bond character on account of the conjugation of unshared electron pair of oxygen with the aromatic ring (Section . . ) and (ii) sp hybridised state of carbon to which oxygen is attached. In ethers, the four electron pairs, i.e., the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (–R) groups. The C – O bond length ( pm) is almost the same as in alcohols. . . Preparation of Alcohols Alcohols are prepared by the following methods: . From alkenes (i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule. Mechanism The mechanism of the reaction involves the following three steps: Step : Protonation of alkene to form carbocation by electrophilic attack of H O + . H O + H + ® H O + Step : Nucleophilic attack of water on carbocation. Step : Deprotonation to form an alcohol.