Question 31 of 39intermediate💡 UnderstandShort Answer2 marks

Illustrate with examples the limitations of Williamson synthesis for the preparation of certain types of ethers.

Correct Answer

The Williamson synthesis involves SN2 attack of an alkoxide ion on an alkyl halide. A major limitation is that secondary and tertiary alkyl halides cannot be used effectively because SN2 reactions are hindered by steric factors.

When secondary or tertiary alkyl halides are used, elimination reactions compete with substitution, leading to the formation of alkenes instead of the desired ethers. For example, to prepare tert-butyl methyl ether, one cannot use tert-butyl bromide with sodium methoxide as elimination would occur. Instead, the correct approach is to use methyl bromide with sodium tert-butoxide, where the bulky group is on the alkoxide side and the primary alkyl halide undergoes SN2 attack.

Exercise: EXERCISES | Q: 7.25 | (Page Page 32)
For More Understanding

Explanation

The textbook context clearly states that Williamson synthesis involves SN2 attack of an alkoxide ion on a primary alkyl halide. Since SN2 reactions are sterically hindered, secondary and tertiary alkyl halides cannot be used effectively. The context mentions that 'Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method' - but this requires using the secondary/tertiary group as the alkoxide component (not as the alkyl halide). The limitation is specifically about using secondary/tertiary alkyl halides, which would lead to elimination products (alkenes) rather than ethers.

Solution Steps

  1. Step 1: Identify that Williamson synthesis proceeds via SN2 mechanism on alkyl halide

  2. Step 2: Recognize that SN2 is hindered for secondary and tertiary alkyl halides

  3. Step 3: Explain that elimination competes with substitution for hindered substrates

  4. Step 4: Provide example showing correct choice of reactants for bulky ethers

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