(S)- $CH X_{3} - CH (OTs) - CH X_{2} - OCH X_{3}$ (OTs = tosylate) reacts with sodium cyanide in DMSO by a clean SN2 pathway. The product $CH X_{3} - CH (CN) - CH X_{2} - OCH X_{3}$ , 3-methoxy-2-methylpropanenitrile, is

A(S), formed with inversion of geometry; the descriptor letter is unchanged
Ba racemic mixture
C(R), formed with retention of geometry
D(R), formed with inversion of geometry

Answer: A

In the substrate the priorities are

OTs > CH X_{2} OCH X_{3} > CH X_{3} > H

. In the product

CH X_{2} OCH X_{3}

(O,H,H) outranks

CN

(N,N,N), so the order becomes

CH X_{2} OCH X_{3} > CN > CH X_{3} > H

: the incoming group now sits in the second rather than the first position. Inversion of geometry combined with this priority change leaves the letter unchanged, giving (S).

Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on SN1 vs SN2: substrate, nucleophile, solvent, leaving group

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(S)-CHX3​−CH(OTs)−CHX2​−OCHX3​ (OTs = tosylate) reacts with sodium cyanide in DMSO by a clean SN2 pathway. The product CHX3​−CH(CN)−CHX2​−OCHX3​, 3-methoxy-2-methylpropanenitrile, is

Answer: A

More questions on SN1 vs SN2: substrate, nucleophile, solvent, leaving group

(S)- $CH X_{3} - CH (OTs) - CH X_{2} - OCH X_{3}$ (OTs = tosylate) reacts with sodium cyanide in DMSO by a clean SN2 pathway. The product $CH X_{3} - CH (CN) - CH X_{2} - OCH X_{3}$ , 3-methoxy-2-methylpropanenitrile, is