Why is sn2 concerted?

Because the intermediate is short-lived.

A concerted reaction, like an ideal #"S"_N2#, is simply one that happens in essentially one step.

SN2 MECHANISM

The typical #"S"_N2# mechanism is a backside-attack of the electrophilic carbon, inverting the stereochemistry at that carbon.

• For this mechanism example, the #"N"-="C":^(-)# simply approaches carbon-2 from behind, and the three groups on carbon-2 "flip" backwards.

• Then, #"N"-="C":^(-)# attaches by donating a pair of electrons to carbon-2's antibonding orbital, and #"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 #"S"_N2# reaction, the #"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 #"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 #"S"_N1# 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 #"S"_Ncolor(blue)(2)#).

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

Hence, the ideal #"S"_N2# mechanism is concerted.