SN2 reaction

The S_N2 reaction is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed synchronously, i.e., in one step. S_N2 is a kind of nucleophilic substitution reaction mechanism. Since two reacting species are involved in the slow (rate determining) step, this leads to the term substitution nucleophilic (bi-molecular) or S_N2, the other major kind is S_N1.^[1] Many other more specialized mechanisms describe substitution reactions.

The reaction type is so common that it has other names, e.g. "bimolecular nucleophilic substitution", or, among inorganic chemists, "associative substitution" or "interchange mechanism".

Reaction Mechanism

The reaction most often occurs at an aliphatic sp³ carbon center with an electronegative, stable leaving group attached to it (often denoted X), which is frequently a halide atom. The breaking of the C–X bond and the formation of the new bond (often denoted C–Y or C–Nu) occur simultaneously through atransition state in which the carbon under nucleophilic attack is pentacoordinate, and approximately sp² hybridised. The nucleophile attacks the carbon at 180° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the C–X σ* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed.

If the substrate under nucleophilic attack is chiral, this can lead, although not necessarily, to an inversion of stereochemistry called a Walden inversion(the nucleophile attacks the electrophilic carbon center, inverting the tetrahedron, much like an umbrella turning inside out in the wind).

In an example of the S_N2 reaction, the attack of Br⁻ (the nucleophile) on an ethyl chloride (the electrophile) results in ethyl bromide, with chloride ejected as the leaving group.

S_N2 reaction of chloroethane with bromide ion:

S_N2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore this mechanism usually occurs at an unhindered primary carbon centre. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an S_N1 rather than an S_N2 mechanism, (an S_N1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed).

Factor affecting the rate of reaction

i) Substrate

The substrate plays the most important part in determining the rate of the reaction. This is because the nucleophile attacks from the back of the substrate, thus breaking the carbon-leaving group bond and forming the carbon-nucleophile bond. Therefore, to maximise the rate of the S_N2 reaction, the back of the substrate must be as unhindered as possible. Overall, this means that methyl and primary substrates react the fastest, followed by secondary substrates. Tertiary substrates do not participate in S_N2 reactions, because of steric hindrance. Moreover, compounds like '1-chloro 1-ethene' too do not undergo nucleophillic substitution easily because the carbon to chlorine bond is said to be of partial double bond character, thus is harder to break. Another factor leading to an S_N2 reaction due to substrate involves the stability and ease by which the carbocation is formed after removing the leaving group. This means the more stable the carbocation is after removing the leaving group, the more likely it is an S_N1 reaction will occur instead of an S_N2. Among the stabilization methods to be considered are: resonance stabilization, hyper-conjugative stabilization, inductive effect stabilization, or the formation of an aromatic ring molecule (as in the case of 7-chloro cyclohept-1, 3, 5-triene, as it will form a tropolium carbocation which is aromatic).

ii) Nucleophile

Like the substrate, steric hindrance affects the nucleophile's strength. The methoxide anion, for example, is both a strong base and nucleophile because it is a methyl nucleophile, and is thus very much unhindered. tert-Butoxide, on the other hand, is a strong base, but a poor nucleophile, because of its three methyl groups hindering its approach to the carbon. Nucleophile strength is also affected by charge and electronegativity: nucleophilicity increases with increasing negative charge and decreasing electronegativity. For example, OH^- is a better nucleophile than water, and I^- is a better nucleophile than Br^- (in polar protic solvents). In a polar aprotic solvent, nucleophilicity increases up a column of the periodic table as there is no hydrogen bonding between the solvent and nucleophile; in this case nucleophilicity mirrors basicity. I^- would therefore be a weaker nucleophile than Br^- because it is a weaker base. Verdict - A strong/anionic nucleophile always favours S_N2 manner of nucleophillic substitution.

iii) Solvent

The solvent affects the rate of reaction because solvents may or may not surround a nucleophile, thus hindering or not hindering its approach to the carbon atom. Polar aprotic solvents, like tetrahydrofuran, are better solvents for this reaction than polar protic solvents because polar protic solvents will hydrogen bond to the nucleophile, hindering it from attacking the carbon with the leaving group. A polar aprotic solvent with low dielectric constant or a hindered dipole end will favour S_N2 manner of nucleophillic substitution reaction. Examples: DMSO, DMF, acetone etc. In polar aprotic solvent, nucleophilicity parallels basicity.

iv) Leaving group

The leaving group affects the rate of reaction because the more stable it is, the more likely that it will take the two electrons of its carbon-leaving group bond with it when the nucleophile attacks the carbon. Therefore, the weaker the leaving group is as a conjugate base, and thus the stronger its corresponding acid, the better the leaving group. Examples of good leaving groups are therefore the halides (except fluoride) and tosylate, whereas HO^- and H₂N^- are not.

v) Reaction kinetics

The rate of an S_N2 reaction is second order, as the rate-determining step depends on the nucleophile concentration, [Nu⁻] as well as the concentration of substrate, [RX].

r = k[RX][Nu⁻]

This is a key difference between the S_N1 and S_N2 mechanisms. In the S_N1 reaction the nucleophile attacks after the rate-limiting step is over, whereas in S_N2 the nucleophile forces off the leaving group in the limiting step. In other words, the rate of S_N1 reactions depend only on the concentration of the substrate while the S_N2 reaction rate depends on the concentration of both the substrate and nucleophile. In cases where both mechanisms are possible (for example at a secondary carbon centre), the mechanism depends on solvent, temperature, concentration of the nucleophile or on the leaving group.

S_N2 reactions are generally favored in primary alkyl halides or secondary alkyl halides with an aprotic solvent. They occur at a negligible rate in tertiary alkyl halides due to steric hindrance.

It is important to understand that S_N2 and S_N1 are two extremes of a sliding scale of reactions, it is possible to find many reactions which exhibit both S_N2 and S_N1 character in their mechanisms. For instance, it is possible to get a contact ion pairs formed from an alkyl halide in which the ions are not fully separated. When these undergo substitution the stereochemistry will be inverted (as in S_N2) for many of the reacting molecules but a few may show retention of configuration. Sn2 reactions are more common than Sn1 reactions.