For CH3CH2CH2Br reacting with NaCN in DMF, which mechanism occurs and what is the major product?

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Multiple Choice

For CH3CH2CH2Br reacting with NaCN in DMF, which mechanism occurs and what is the major product?

SN2 substitution on a primary alkyl halide is favored when the nucleophile is strong and the solvent is polar aprotic. Sodium cyanide provides CN−, which attacks the carbon bearing the bromine from the backside and displaces Br− in one concerted step. Because the carbon attached to the leaving group is primary, there is no stable carbocation formed, so SN1 is not operative and rearrangement can’t occur. CN− is not a strong enough base to promote significant elimination on this substrate, so elimination to propene is unlikely. The result is substitution of Br by CN, giving CH3CH2CN (propionitrile, also called propanenitrile). If that carbon were a stereocenter, SN2 would cause inversion, but in this primary substrate there isn’t a stereochemical center to invert.

SN2 substitution on a primary alkyl halide is favored when the nucleophile is strong and the solvent is polar aprotic. Sodium cyanide provides CN−, which attacks the carbon bearing the bromine from the backside and displaces Br− in one concerted step. Because the carbon attached to the leaving group is primary, there is no stable carbocation formed, so SN1 is not operative and rearrangement can’t occur. CN− is not a strong enough base to promote significant elimination on this substrate, so elimination to propene is unlikely. The result is substitution of Br by CN, giving CH3CH2CN (propionitrile, also called propanenitrile). If that carbon were a stereocenter, SN2 would cause inversion, but in this primary substrate there isn’t a stereochemical center to invert.

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