inversion
Summary
Inversion (Walden inversion) is the flip in three-dimensional arrangement at a stereocenter during an SN2 reaction, because the nucleophile attacks from the side opposite the leaving group. The most testable point: a chiral substrate gives a product with inverted geometry, which is often but not always R → S.
Detail
Meaning: In an SN2 reaction the nucleophile attacks the carbon 180° from the leaving group (backside attack). Bond formation and bond breaking happen in one concerted step. The other three substituents pass through a planar transition state and flip like an umbrella in the wind. The product therefore has the opposite spatial arrangement at that carbon. In biology, a chromosomal inversion is a different idea: a chromosome segment reversed end to end.
Equation: rate = k[Nu][RX]. Rate is in mol L⁻¹ s⁻¹. k is the second-order rate constant in L mol⁻¹ s⁻¹. [Nu] is the nucleophile concentration and [RX] is the substrate concentration, both in mol L⁻¹.
On the MCAT: A passage gives an enantiomerically pure alkyl halide, such as (R)-2-bromobutane, and a strong nucleophile, often in a polar aprotic solvent. You name the product's configuration, usually (S) when the priorities keep their order. Methyl and primary substrates favor SN2, and steric hindrance at tertiary carbons blocks backside attack. Expect to link inversion to second-order kinetics.
Common confusion: Inversion describes the geometry, not the label. The R/S label depends on CIP priorities, and the incoming nucleophile may rank differently from the leaving group, so R can stay R despite inversion. Also, SN1 does not give clean inversion. The planar carbocation can be attacked from either face, so the product is racemic or close to it.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.