The Williamson ether reaction follows an SN2 bimolecular nucleophilic substitution mechanism. In an SN2 reaction mechanism there is a backside attack of an electrophile by a nucleophile and it occurs in a concerted mechanism (happens all at once). In order for the SN2 reaction to take place there must be a good leaving group which is strongly electronegative, commonly a halide.
In the Williamson ether reaction there is an alkoxide ion (RO-) which acts as the nucleophile, attacking the electrophilic carbon with the leaving group, which in most cases is an alkyl tosylate or an alkyl halide. The leaving site must be a primary carbon, because secondary and tertiary leaving sites generally prefer to proceed as an elimination reaction. Also, this reaction does not favor the formation of bulky ethers like di-tertbutyl ether, due to steric hindrance and predominant formation of alkenes instead.
Conditions:
Since alkoxide ions are highly reactive, they are usually prepared immediately prior to the reaction, or are generated in situ. In laboratory chemistry, in situ generation is most often accomplished by the use of a carbonate base or potassium hydroxide, while in industrial syntheses phase transfer catalysis is very common. A wide range of solvents can be used, but protic solvents and apolar solvents tend to slow the reaction rate strongly, as a result of lowering the availability of the free nucleophile. For this reason, acetonitrile and N,N-dimethylformamide are particularly commonly used.
A typical Williamson reaction is conducted at 50 to 100° C and is complete in 1 to 8 h. Often the complete disappearance of the starting material is difficult to achieve, and side reactions are common. Yields of 50-95% are generally achieved in laboratory syntheses, while near-quantitative conversion can be achieved in industrial procedures.
Catalysis is not usually necessary in laboratory syntheses. However, if an unreactive alkylating agent is used (e.g. an alkyl chloride) then the rate of reaction can be greatly improved by the addition of a catalytic quantity of a soluble iodide salt (which undergoes halide exchange with the chloride to yield a much more reactive iodide, a variant of the Finkelstein reaction). In extreme cases, silver compounds such as silver oxide may be added.
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