Why is sn2 concerted?
Because the intermediate is short-lived.
A concerted reaction, like an ideal
For this mechanism example, the
#"N"-="C":^(-)#simply approaches carbon-2 from behind, and the three groups on carbon-2 "flip" backwards.
#"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
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
In this situation, essentially no carbocation intermediate forms (if it does, it is short-lived).
Hence, the ideal