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SN2 (substitution nucleophilic bimolecular) reactions proceed by a concerted one-step mechanism in which a nucleophile attacks the backside of the carbon bearing the leaving group, simultaneously displacing it through a trigonal bipyramidal transition state. This backside attack causes Walden inversion of configuration at the stereocenter. SN2 reactivity decreases with increasing steric hindrance: methyl > primary > secondary >> tertiary. Rate depends on the concentrations of both the substrate and the nucleophile, reflecting the bimolecular transition state.
Draw the transition state explicitly (trigonal bipyramidal, partial bonds to both nucleophile and leaving group) for each example. Practice the four-factor analysis — substrate class, nucleophile strength, solvent, leaving group — and predict whether SN2 will occur. Confirm stereoochemical outcomes.
SN2 reactions are the cleanest, most predictable substitution reactions in organic chemistry. The mechanism is a single concerted step: a nucleophile approaches the carbon bearing the leaving group from the backside — directly opposite the leaving group — and attacks at the same moment the leaving group departs. There is no intermediate; the entire process occurs through a single transition state where the carbon is transiently pentacoordinated (five bonds) in a trigonal bipyramidal geometry with partial bonds to both the nucleophile and the leaving group. Because the two reactants must collide in this precise geometry, the rate depends on both concentrations: rate = k[substrate][nucleophile]. That is the "bimolecular" in SN2.
The stereochemical consequence of backside attack is Walden inversion, sometimes described as an umbrella flipping inside out. Imagine the carbon at the center of the umbrella, with three substituents as the ribs. When the nucleophile attacks from one face and the leaving group departs from the other, the three remaining substituents invert through the trigonal bipyramidal transition state and end up on the opposite side from where they started. This is always 100% stereospecific in SN2 — every molecule reacts with complete inversion. Whether this changes the R or S label depends on how the CIP priorities of the substituents compare before and after, which is a labeling consequence, not a mechanistic one.
Steric hindrance is the dominant factor controlling SN2 reactivity. The nucleophile must attack the backside of the electrophilic carbon, which requires an unobstructed approach. Methyl substrates have no alkyl groups at all, so backside access is wide open. Primary substrates have one alkyl group — some steric hindrance but still fast. Secondary substrates have two alkyl groups flanking the reaction center — notably slower. Tertiary substrates have three alkyl groups fully crowding the backside — SN2 is essentially impossible. This is not a matter of thermodynamics but of transition-state accessibility: the incoming nucleophile and the three substituents must all fit in the transition state simultaneously.
The stereochemistry you studied in enantiomers and chirality is essential here. SN2 is a powerful stereochemical tool: if you start with an enantiopure substrate, you get an enantiopure product with inverted configuration. Synthetic chemists use this predictability to construct molecules with defined stereocenters. The reaction is also sensitive to solvent: polar aprotic solvents (DMSO, acetone, DMF) are ideal because they dissolve ionic nucleophiles without surrounding and deactivating them with solvent hydrogen-bonding. Polar protic solvents (water, methanol) solvate nucleophiles and reduce their reactivity, slowing SN2.
Finally, keep in mind that the SN2 mechanism is one tool among several for substitution reactions. Its competitor, SN1, operates through a two-step mechanism involving a carbocation intermediate and is favored by tertiary substrates, stable carbocations, and polar protic solvents. The contrast between them — concerted vs. stepwise, inversion vs. racemization, steric sensitivity vs. carbocation stability — will become a recurring theme as you move through organic mechanisms. Recognizing which pathway dominates under a given set of conditions (substrate class, nucleophile, solvent) is a core skill in predicting reaction outcomes.