SN2 reactions
- The rate of the reaction depends on the nature and concentration of BOTH the nucleophile and the substrate
For CH3Cl + Br–
CH3Br + Cl– rate = k[CH3Cl][Br–]
This suggests a single step mechanism involving collision of the nucleophile with the carbon bearing the leaving group.
The diagram of the transition state shows that the carbon undergoing substitution has partial bonds to both nucleophile and leaving group.
At points on the curve just to the left of the green dot, the C-Br bond is being formed.
Place your mouse on the curve just to the left of the green dot..
Then move it along the curve to the right and left changes going through the transitiion state. - If substitution occurs at a stereogenic centre, the configuration of that centre in the product of substitution is opposite to that of the reactant. The reaction is said to proceed with inversion. Usually, but not always, the Cahn-Ingold-Prelog designation changes also (R to S or S to R).
Flick your mouse from the point on the curve to the left of the peak to the point on the curve to the right and then back again.
Note how before the peak the hydrogens point toward Br, and after the peak they point toward Cl.
The change is similar to an umbrella turning inside out.
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CH3CH2Br (1°) > (CH3)2CHBr (2°) > (CH3)3CBr (3°) Nucleophile attacks at carbon bearing the green dot. - The rate of reaction is slower if the carbon bearing the leaving group is more highly substituted.
The presence of groups larger than H at the carbon bearing the leaving group increases crowding in the transition state and therefore raises its energy. The rate of reaction is lower due to the activation energy being higher.