The SN2 Reaction

This movie depicts an SN2 reaction between the hydroxide anion (HO-) and methyl chloride . In this reaction, a new bond is formed between the nucleophile, HO-, and the carbon atom, while the carbon-chlorine bond is broken. The departing chloride anion is referred to as the leaving group. The species being attacked by the nucleophile, namely methyl chloride, is referred to as the electrophile.
The term SN2stands for Substitution reaction, Nucleophilic, 2nd order (also called bimolecular). According to the SN2 mechanism, there is a single transition state because bond-breaking and bond-making occur simultaneously. Notice that for this to occur, the nucleophile must approach from the backside of the carbon-leaving group bond (so-called backside attack ). Look for the backside attack in the movie.

Notice that there is not intermediate in an SN2 reaction, just a transition state. A transition state has no real lifetime, it is the highest energy point on the reaction coordinate as starting materials transition into products.

Bimolecular reaction A bimolecular reaction, such as the SN2 reaction, is one in which two reactants take part in the transition state of the slow or rate-determining step of a reaction. For this reason, the concentrations of both the nucleophile and the alkyl halide are proportional to the observed SN2 reaction rate.

Nucleophilicity Because the nucleophile is involved in the rate-determining step of SN2 reactions, stronger nucleophiles react faster. Stronger nucleophiles are said to have increased nucleophilicity. In the gas phase, there is a correlation between increased relative nucleophilicity and increased base strength, although there are many exceptions to this trend in solution (see below). In general, within a period of the periodic table, nucleophilicity increases from right to left. Furthermore, for different reagents with the same nucleophilic atom, an anion is a better nucleophile than a neutral species.

Solvent effects In solution the comparison of nucleophilicity vs. basicity is complicated by solvation effects. Polar aprotic solvents (e.g. acetonitrile, dimethylsulfoxide, dimethylformamide, etc.) are good at solvating cations, but not anions. As a result, the nucleophiles are not highly solvated, and the relative order of nucleophilicities is similar to that observed in the gas phase. Polar protic solvents (e.g. alcohols, water) are much better at solvating anions, so nucleophiles are highly solvated in these solvents. The relative order of nucleophilicities can be changed dramatically by this solvation. Solvation of nucleophiles by polar protic solvents also inhibits the nucleophile’s ability to take part in an SN2 reaction, so SN2 reactions are much slower in polar protic solvents compared with polar aprotic solvents.

Inversion of stereochemistry If the halide leaving group is attached to a stereocenter, the configuration of the stereocenter is inverted during an SN2 reaction. This is because the nucleophile enters from the opposite side of the molecule as the departing group (“backside attack”), thus the molecule inverts analogous to an umbrella inverting in the wind. You can see this molecular inversion in the above movie, but in this case, the inversion does not influence stereochemistry since neither the reactants nor products are chiral.

Inhibition by steric hindrance SN2 reactions are particularly sensitive to steric factors, since they are greatly retarded by steric hindrance (crowding) at the site of reaction. In general, the order of reactivity of alkyl halides in SN2 reactions is: methyl > 1° > 2°. The 3° alkyl halides are so crowded that they do not generally react by an SN2 mechanism.