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Give the preparation of alcohols from alkenes. |
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Answer» Solution :`to` The alcohols are prepared from alkenes by two ways : (1) Acid catalysed hydration : In presence of acid, the hydration of alkenes gives alcohols as per Markovnikov.s rule. The acid act as a catalyst in the reaction.  `to` Mechanism of the reaction : Step-1 : Protonation of alkene to form carbocation by electrophilic attack of`H_(3)O^(+)` `H_(2)+ H^(+) to H_(3)O^(+)`  Step 2: Nucleophilic attack of water on carbocation.  Step 3: Deprotonation to form an alcohol.  `to` The electrophilic attack by H2O+ is a slow step in the reaction and thus it is a rate determining step. As the electrophile is added in a slow step of the reaction, it is known as electrophilic addition reaction. `to` (ii) Hydroboration-oxidation of alkenes : Diborane `(BH_(3))_(2)` REACTS with alkenes to give trialkyl boranes as addition product. The trialkyl borane is oxidized to trialkyl borate by hydrogen peroxide in presence of aqueous alkaline solution.  `to` The addition of boranes to double bond takes place in such a manner that the boron atom gets attached `sp^(2)` carbon carrying GREATER NUMBER of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov.s rule. In this method the alcohol is obtained in excellent yield. |
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