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Explain the intermolecular dehydration of alcohols. State the limitations of this method. |
Answer» Solution :In the presence of protic acids such as `H_(2)SO_(4)` or `H_(2)PO_(4)`, the alcohol dehydrates to form either alkene or ether depending upon the reaction conditions. For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K and at 413 K, ethoxyethane is obtained.  The formation of ether is a NUCLEOPHILIC bimolecular substitution reaction (`S_(N)2`) involving the attack of alcohol molecule on a protonated alcohol. The reaction takes place as follows:  Order of dehydration of alcohols to form ethers : `1^(@) gt2^(@) gt3^(@)` `to`Limitations of the method : `to` (i) The method is suitable for the preparation of ethers having primary alkyl groups only. The alkyl group should be unhindered, and the temperature of the reaction must be low otherwise the alkene will be formed in MAJOR proportion. If the alcohol is secondary or tertiary, the elimination FAVOURS over substitution and as a RESULT, alkene will be obtained as a major product. `to` (ii) The method is not suitable for the preparation of unsymmetrical ethers (mixed ethers). This is because of combination of two different alcohols that would result in the formation of mixture of three ethers which are not easy to separate. For example, the ethyl methyl ether cannot be prepared by this method. |
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