# Why is sn2 concerted?

May 30, 2016

Because the intermediate is short-lived.

A concerted reaction, like an ideal ${\text{S}}_{N} 2$, is simply one that happens in essentially one step.

SN2 MECHANISM

The typical ${\text{S}}_{N} 2$ mechanism is a backside-attack of the electrophilic carbon, inverting the stereochemistry at that carbon.

• For this mechanism example, the $\text{N"-="C} {:}^{-}$ simply approaches carbon-2 from behind, and the three groups on carbon-2 "flip" backwards.

• Then, $\text{N"-="C} {:}^{-}$ attaches by donating a pair of electrons to carbon-2's antibonding orbital, and ${\text{Br}}^{-}$ leaves after retracting its bonding pair of electrons.

(The planar transition state illustrates a halfway-point between the initial and final stereochemistry.)

STRENGTH OF NUCLEOPHILE VS STRENGTH OF C-LG BOND

For an ideal ${\text{S}}_{N} 2$ reaction, the $\text{C"-"LG}$ bond is strong enough that it doesn't break on its own before the nucleophile attacks, and it's up to the nucleophile to "brute-force" the reaction.

If the nucleophile isn't that strong:

A nucleophile that isn't strong enough leaves an opportunity for the $\text{C"-"LG}$ bond to break before the nucleophile has a chance to backside-attack.

This forms a carbocation intermediate through the active participation of only the substrate initially, as in an ${\text{S}}_{N} 1$ reaction mechanism. (The nucleophile attacks later.)

If the nucleophile is fairly strong:

A strong-enough nucleophile allows for the mechanism to involve the participation of both molecules (hence ${\text{S}}_{N} \textcolor{b l u e}{2}$).

In this situation, essentially no carbocation intermediate forms (if it does, it is short-lived).

Hence, the ideal ${\text{S}}_{N} 2$ mechanism is concerted.