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Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason. |
Answer» Solution :ACID - catalysed dehydration of `1^(@)` alcohols to ethers occurs by `S_(N)2` reaction involving nucleophilic attack by the alcohol molecule on the protonated alcohol molecule.  However `2^(@)` and `3^(@)` alcohols, give alkenesrather than ether under these conditions. Due to steric hindrance, nucleophilic attack by the alcohol molecule on the protonated alcohol molecule does not occur. Instead protonated `2^(@) and 3^(@)` carbocations. These carbocations prefer to lose a proton to from alkenes rather than undergoing nucleophilic attack by alcohol molecule to FORM ethers, as shown below taking the example of propan -2- ol, a `2^(@)` alcohol.  SIMILARLY, `3^(@)` alcohols give alkenes rather than ethers, again due to steric hindrance.  `underset("(Not formed)")underset("prooxy) propane")underset("2-Methyl-2-(2-methyl")(CH_(3)-overset(CH_(3))overset("|")underset(CH_(3))underset("|")"C "-O-overset(CH_(3))overset("|")underset(CH_(3))underset("|")"C "-CH_(3))overset((CH_(3))(3)COH)underset(-H^(+))rarrunderset("carbocation")underset("tert-Butyl")(CH(3)-underset(CH_(3))underset("|")overset(CH_(3))overset("|")(C^(+))underset(-H^(+))rarr)underset("2-Methylprop-1-ene")(CH_(3)-overset(CH_(3))overset("|")"C "=CH_(2))` |
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