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In the reaction depicted below shouldn't $\ce{Br}$ be replaced by $\ce{SCH3-}$ ion as $\ce{Br-}$ is a good leaving group and the $\ce{C-F}$ bond is quite strong due to back bonding of 2pπ-2pπ orbitals?

2-bromo-1-fluoro-4-nitrobenzene with sodium methanethiolate

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    $\begingroup$ What is the rate limiting step in this reaction? What is the structure of the intermediate? This should help you think about what happens. $\endgroup$ – bon Jul 24 '17 at 7:18
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    $\begingroup$ I'd suggest looking up nucleophilic aromatic substitution in a textbook; the issues you are asking about should be described in any decent text. In particular, any decent text should be able to explain why (1) the 2- and 4-positions relative to the electron-withdrawing group are activated towards substitution and (2) the order of reactivity is F > Cl > Br > I. $\endgroup$ – orthocresol Jul 24 '17 at 8:31

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