In which of the following clean SN2 reactions of an enantiopure substrate does the product carry the same CIP letter ( $R$ or $S$ ) as the substrate, even though the configuration at the carbon has been inverted? (Work out the priorities in substrate and product.)

A $(R)$ -2-bromo-1-methoxypropane, $CH X_{3} OCH X_{2} - CH (Br) - CH X_{3}$ , with $NaCN$ in DMSO, giving 3-methoxy-2-methylpropanenitrile
B $(S)$ -2-chlorobutane with $NaI$ in acetone, giving 2-iodobutane
C $(R)$ -2-bromo-1-fluoropropane, $FCH X_{2} - CH (Br) - CH X_{3}$ , with $NaCN$ in DMSO, giving 3-fluoro-2-methylpropanenitrile
D $(R)$ -2-bromooctane with $NaN X_{3}$ in DMF, giving 2-azidooctane

Answer: A, C

In the two cyanide reactions the substrate order is

Br > CH X_{2} X > CH X_{3} > H

(X = OCH3 or F), but in the product the group

CH X_{2} X

with (X,H,H) outranks the cyano carbon (N,N,N) because O and F outrank N. The order of the top two groups is swapped, which exactly cancels the effect of the inversion, so the letter stays

R

(RDKit CIP check). For 2-bromooctane with azide, and for 2-chlorobutane with iodide, the incoming group takes over the top priority, so the order is unchanged and the letter flips:

R \to S

and

S \to R

Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on SN2 mechanism, rate, stereochemistry, steric effects

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In which of the following clean SN2 reactions of an enantiopure substrate does the product carry the same CIP letter (R or S) as the substrate, even though the configuration at the carbon has been inverted? (Work out the priorities in substrate and product.)

Answer: A, C

More questions on SN2 mechanism, rate, stereochemistry, steric effects

In which of the following clean SN2 reactions of an enantiopure substrate does the product carry the same CIP letter ( $R$ or $S$ ) as the substrate, even though the configuration at the carbon has been inverted? (Work out the priorities in substrate and product.)