Methyl tert-butyl ether is best prepared by…

Methyl tert-butyl ether is best prepared by the Williamson synthesis from one of two routes, and is then cleaved with 1 equivalent of cold HI. Which…

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Methyl tert-butyl ether is best prepared by the Williamson synthesis from one of two routes, and is then cleaved with

1

equivalent of cold

\mathrm{HI}

. Which combination correctly gives the preferred route and the main cleavage products?

A. $\mathrm{CH_3ONa}+(\mathrm{CH_3})_3\mathrm{CBr}$ ; cleavage gives $\mathrm{CH_3I}$ and $(\mathrm{CH_3})_3\mathrm{COH}$
B. $(\mathrm{CH_3})_3\mathrm{CONa}+\mathrm{CH_3I}$ ; cleavage gives $\mathrm{CH_3I}$ and $(\mathrm{CH_3})_3\mathrm{COH}$
C. $\mathrm{CH_3ONa}+(\mathrm{CH_3})_3\mathrm{CBr}$ ; cleavage gives $(\mathrm{CH_3})_3\mathrm{CI}$ and $\mathrm{CH_3OH}$
D. $(\mathrm{CH_3})_3\mathrm{CONa}+\mathrm{CH_3I}$ ; cleavage gives $(\mathrm{CH_3})_3\mathrm{CI}$ and $\mathrm{CH_3OH}$

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Answer: (D) $(\mathrm{CH_3})_3\mathrm{CONa}+\mathrm{CH_3I}$ ; cleavage gives $(\mathrm{CH_3})_3\mathrm{CI}$ and $\mathrm{CH_3OH}$

Williamson synthesis is

\mathrm{S_N2}

, so the halide must be primary: sodium tert-butoxide with methyl iodide gives the ether, whereas methoxide with tert-butyl bromide would give mostly isobutene by E2. For cleavage, the ether is protonated and the C-O bond that can form the more stable carbocation breaks (

\mathrm{S_N1}

): the tert-butyl cation forms and is captured by

\mathrm{I^-}

, giving

(\mathrm{CH_3})_3\mathrm{CI}

and methanol. Methyl iodide is formed only when no tertiary or benzylic group is present (as for primary-alkyl ethers).

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