The SN2 reaction involves three things interacting:
  • A nucleophile (a Lewis base that is using an electron pair to form a new bond);
  • A leaving group (an atom or group that will pull an existing bonding electron pair away from the reaction site)
  • The substrate carbon, on which the leaving group is being replaced with the nucleophile.
The structures below are predictions based on an MO calculation.

Reactants
 Transition State
 Products
MeBr.pdb SN2_TS.pdb MeCN.pdb
CH3Br + -CN

Total E (aq) = -35.0 kcal/mol (-146.5 kJ/mol)
C-Br Distance
Energy (aq) = -24.4 kcal/mol (-101.8 kJ/mol)
Relative to reactants: +10.6 kcal/mol
C-Br Distance
C-C Distance 		CH3CN + -Br
Total E (aq) = -77.5 kcal/mol (-324.0 kJ/mol)
Relative to reactants: -42.5 kcal/mol
C-C Distance
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 Spacefilling model
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Ball & Stick

Note several things:
  1. There is an activation barrier of about 10.6 kcal/mol (45 kJ/mol). 
  2. The reaction is exothermic by about 42.5 kcal/mol (178 kJ/mol).
  3. The transition state structure is symmetric; the carbon is being inverted.
  4. The bond lenghts in the transition state are all longer than in either reactant or product--evidence of partial bonding.
  5. The nucleophile (cyanide) approaches from the opposite side of the molecule from where the bond to the leaving group (bromide) is being broken.  (There is an MO explanation for this:  the lone pair on the nucleophile interacts with the antibonding MO for the C-Br bond.) Go back to the top and click "Stereochemuistry" to see how the stereochemistry of a secondary center is affected.
The details will vary, but all SN2 reactions follow this general motif.

Because this is a single step reaction that brings nucleophile and substrate together, the rate is second order overall--first order in nucleophile, first order in substrate:

Rate = k[MeBr][-CN]

The SN2 terminology means Substitution, Nucleophilic, 2nd order.

Several things control the rate constant (and therefore the rate):
-Strength of the nucleophile.
-Leaving group ability.
-Steric hindrance of the reacting carbon
-Solvent stabilization of ionic reactants, products or transition states.