Several distinct mechanisms are possible for nucleophilic substitution reactions at saturated C atoms, depending on the substrate, nucleophile, leaving group and the reaction conditions. By far the most common are the SN1 and SN2 mechanisms.
Simple alkyl groups, methyl and primary alkyl groups always react by the SN2 mechanism and never by SN1.
Why methyl and primary alkyl groups react by the SN2 mechanism? What are the steps in an SN2 mechanism?
Partly because the corresponding cations are unstable and partly because it is easy for the nucleophile to attack the C atom since it is surrounded by H atoms – and not by bulky alkyl groups.
Experimental results show the following in SN2 reactions:
In this mechanism (Fig. 1) there is backside attack1. The nucleophile attacks the carbon atom on the opposite side from the leaving group – 180 away from the leaving group – and the carbon atom turns inside out as the reaction proceeds
The reaction is a one-step process with no intermediate
The C-Nu bond is formed (where Nu nucleophile) as the C-X bond is broken (where X the leaving group) to generate transition state 1.
If the C atom under attack is a stereogenic center the result will be inversion of configuration
The kinetics of the rection is 2nd order
There is absence of rearrangement (absence of intermediate carbocations)
The rate of reaction decreases in the order: -CH3 > 1o > 2o> 3o
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| Fig. 1: Mechanism for the SN2 reaction |
The energy necessary to break the C-X bond is supplied by the simultaneous formation of the C-Nu bond. The transition state 1 is shown in Fig.1
The group X must leave as the group Nu comes in because the carbon atom cannot have more than eight electrons in its outer shell. At the transition state, the central C atom has an sp2 hybridization with an approximately perpendicular p orbital. One lobe of this p orbital overlaps with the nucleophile and the other with the leaving group. This is why a backside attack always occurs in SN2 reactions because in a hypothetical front-side attack both the nucleophile Nu and the leaving group X would have to overlap with the same lobe of the p orbital.
During the transition state the three non-reacting substituents and the central carbon are on the same plane.
There is a large amount of evidence for the SN2 mechanism:
- Kinetic evidence
- Stereochemical evidence
Kinetic evidence
Since both the nucleophile and the substrate are involved in the rate-determining step , the reaction should be first order in each component, second order overall.
The rate expression for SN2 reactions has been found to be:
Rate = k * [nucleophile] * [substrate] = k * [Nu] * [RX]
This rate law has been found to apply. The 2 in SN2 reactions stands for bimolecular.
References
1. L. Sun et al., J.A.C.S., 123, 5753 (2001)
2. R. Bruckner, “Advanced Organic Chemistry – Reaction Mechanisms”, 2ndEdition, Elsevier, 2002
3. M.B. Smith & J. March “March’s Advanced Organic Chemistry”, 6thEdition, Wiley-Interscience, 2007
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