Show how the following ethers might be synthesized using (1) alkoxymercuration– demercuration and (2) the Williamson synthesis. (When one of these methods cannot be used for the given ether, point out why it will not work.)
a. 2-methoxybutane
b. ethyl cyclohexyl ether
c. 1-methoxy-2-methylcyclopentane

Verified step by step guidance

1. Step 1: Understand the two methods for ether synthesis. (1) Alkoxymercuration–demercuration involves the addition of an alcohol to an alkene in the presence of mercuric acetate and subsequent reduction with sodium borohydride. (2) The Williamson synthesis involves the reaction of an alkoxide ion with a primary alkyl halide or tosylate via an SN2 mechanism. Note that the Williamson synthesis is not suitable for tertiary alkyl halides due to steric hindrance and elimination side reactions.
2. Step 2: For (a) 2-methoxybutane: (1) Alkoxymercuration–demercuration: Identify the alkene precursor. The alkene would be 2-butene, and the alcohol used would be methanol. The reaction proceeds via Markovnikov addition, where the methoxy group attaches to the more substituted carbon. (2) Williamson synthesis: Use sodium methoxide (CH3O⁻) as the nucleophile and 2-bromobutane as the alkyl halide. This works because 2-bromobutane is a secondary alkyl halide, which is still reactive in SN2 reactions.
3. Step 3: For (b) ethyl cyclohexyl ether: (1) Alkoxymercuration–demercuration: Identify the alkene precursor. The alkene would be cyclohexene, and the alcohol used would be ethanol. The reaction proceeds via Markovnikov addition, where the ethoxy group attaches to the more substituted carbon. (2) Williamson synthesis: Use sodium ethoxide (C2H5O⁻) as the nucleophile and cyclohexyl bromide as the alkyl halide. This works because cyclohexyl bromide is a primary alkyl halide, which is ideal for SN2 reactions.
4. Step 4: For (c) 1-methoxy-2-methylcyclopentane: (1) Alkoxymercuration–demercuration: Identify the alkene precursor. The alkene would be 1-methylcyclopentene, and the alcohol used would be methanol. The reaction proceeds via Markovnikov addition, where the methoxy group attaches to the more substituted carbon. (2) Williamson synthesis: This method is not suitable because the desired ether has a methoxy group on a tertiary carbon, and tertiary alkyl halides are prone to elimination rather than substitution in SN2 reactions.
5. Step 5: Summarize the results. Both alkoxymercuration–demercuration and Williamson synthesis can be used for (a) and (b). However, for (c), only alkoxymercuration–demercuration is suitable because the Williamson synthesis is not effective for tertiary alkyl halides due to steric hindrance and elimination side reactions.

Key Concepts

Alkoxymercuration-Demercuration

Alkoxymercuration-demercuration is a two-step reaction used to synthesize ethers from alkenes. In the first step, an alkene reacts with mercuric acetate in the presence of an alcohol, leading to the formation of an alkoxymercurial intermediate. The second step involves the reduction of this intermediate, typically using sodium borohydride, to yield the ether. This method is particularly useful for synthesizing ethers from unsymmetrical alkenes.

Williamson Ether Synthesis

The Williamson ether synthesis is a method for producing ethers through the nucleophilic substitution of an alkoxide ion with a primary alkyl halide. This reaction is favored when using primary halides to avoid steric hindrance, which can lead to elimination reactions instead of substitution. The alkoxide is generated by deprotonating an alcohol with a strong base, making it a versatile and widely used method for ether synthesis.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In the context of ether synthesis, steric hindrance can affect the choice of reagents and the feasibility of certain reactions, such as the Williamson synthesis. For example, using secondary or tertiary alkyl halides can lead to elimination rather than substitution, making it crucial to consider steric factors when planning ether synthesis.