$cis$ -1-Bromo-3-methylcyclohexane, which is $(1 R, 3 S)$ -1-bromo-3-methylcyclohexane (numbering from the carbon bearing bromine), is treated with $NaI$ in acetone and the reaction occurs by a single SN2 step at C1. The product is

A $(1 S, 3 S)$ -1-iodo-3-methylcyclohexane (trans)
B $(1 R, 3 S)$ -1-iodo-3-methylcyclohexane (cis)
C $(1 R, 3 R)$ -1-iodo-3-methylcyclohexane (trans)
Da 1:1 mixture of $(1 R, 3 S)$ and $(1 S, 3 R)$ isomers

Answer: A

SN2 inverts only C1, so the iodo and methyl groups go from cis to trans. At C1 the priorities are

Br

(or

I

), then C2 (which leads to the more substituted C3), then C6, then H; the order is the same in substrate and product, so the descriptor at C1 changes from

R

S

. C3 is unchanged (

S

). The product is

(1 S, 3 S)

-1-iodo-3-methylcyclohexane, the trans isomer (CIP labels checked by RDKit).

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

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cis-1-Bromo-3-methylcyclohexane, which is (1R,3S)-1-bromo-3-methylcyclohexane (numbering from the carbon bearing bromine), is treated with NaI in acetone and the reaction occurs by a single SN2 step at C1. The product is

Answer: A

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

$cis$ -1-Bromo-3-methylcyclohexane, which is $(1 R, 3 S)$ -1-bromo-3-methylcyclohexane (numbering from the carbon bearing bromine), is treated with $NaI$ in acetone and the reaction occurs by a single SN2 step at C1. The product is