# SN2 - Second-order Nucleophilic Substitution

## Substrate Effects by Side Chain Branchings in $SN2$ Reactions

The reaction rate of an $SN2$ reaction is reduced with an increase in the reaction center's degree of substitution. This effect arises from the steric shielding of the reaction center by its substituents. However, the reaction rate may not only be influenced by the substituents that are directly bound to the reaction center. Substituents of carbons that are adjacent to the reaction center may influence the reaction rate, as well. This effect is illustrated by the following examples of halogen exchanges with different alkyl bromides.

Fig.1
Halogen exchange.
Tab.1
Relative reaction rates of halogen exchanges
Alkyl halideStructureRelative reaction rate
Ethyl bromide $CH3CH2Br$ 1
Propyl bromide $CH3CH2CH2Br$ 0.8
Isobutyl bromide $(CH3)2CHCH2Br$ 0.036
Neopentyl bromide $(CH3)3CCH2Br$ 0.00002

The relative reaction rates in the examples can be explained as follows. The effect that a chain extension of ethyl bromide by one methylen group, which yields propyl bromide, may have on the reaction is very small (1→0.8). Additional linear chain extensions have no further effect on the reaction rate (examples are not depicted), as many conformations in which the reaction center does not undergo any further shielding are always possible, due to the conformational flexibility of the linear alkyl chain and the increase in distance between the chain members and the reaction center. However, a branching in the position adjacent to the reaction center considerably influences the reaction rate of $SN2$ reactions. The rate of the $SN2$ reaction in association with neopentyl bromide amounts to only a fifty thousandth part of the reaction rate that is found with ethyl bromide. The steric shielding of the reaction center in $SN2$ reactions by vicinal substituents is illustrated by the three-dimensional, interactive molecular models (Chime plugin) below, in particular, by the spacefill presentation.

Fig.2
Ethyl bromide
Fig.3
Propyl bromide
Fig.4
Isobutyl bromide
Fig.5
Neopentyl bromide

Summary:

1. The linear extension of the alkyl group of n-alkyl halides has only a very small (ethyl to propyl) or virtually no (propyl to higher alkyl) significant impact on the reaction rate of an $SN2$ reaction.
2. If an alkyl substituent is branched adjacent to the reaction center, the reaction rate of an $SN2$ reaction is considerably reduced compared to an unbranched alkyl substituent, as the nucleophile's approach to the reaction center, which is required for an $SN2$ reaction, is then sterically hindered.

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