Inversion of configuration in Alkyl halides takes place:

The correct answer is Option (2) → In \(S_N2\) reaction

In an S_N2 reaction (substitution, nucleophilic, bimolecular) the nucleophile attacks the carbon from the side opposite to the leaving group. The new C–Nu bond forms while the C–X bond breaks, in one step through a transition state in which the carbon is bonded to five groups. As the leaving group goes, the other three groups on carbon flip over like an umbrella turned inside out, so the product has the inverted configuration. The rate depends on both reactants (second order):

    \( \displaystyle \ce{HO- + CH3Cl -> CH3OH + Cl-}\qquad \text{Rate} = k[\ce{CH3Cl}][\ce{OH-}] \)

For an optically active halide such as 2-bromooctane, S_N2 gives the alcohol of the opposite configuration.

Why the other options are incorrect

• Option (1) In \(S_N1\) reaction — Incorrect. S_N1 goes through a planar carbocation that the nucleophile can attack from either face, so an optically active halide gives racemisation (both retention and inversion), not clean inversion.

• Option (3) In electrophilic substitution — Incorrect. This is the typical reaction of arenes (nitration, halogenation of benzene), in which an electrophile replaces H on a ring; alkyl halides react with nucleophiles, and no configuration is inverted.

• Option (4) In elimination reaction — Incorrect. β-elimination (dehydrohalogenation) removes HX to form an alkene; the carbon does not keep a substituent whose configuration could be inverted.

NCERT (2026-27 edition):
Substitution Nucleophilic Bimolecular (S_N2) – “The reaction between CH₃Cl and hydroxide ion to yield methanol and chloride ion follows a second order kinetics, i.e., the rate depends upon the concentration of both the reactants. … As the attacking nucleophile approaches closer to the carbon, C-H bonds still keep on moving in the same direction till the attacking nucleophile attaches to carbon and leaving group leaves the carbon. As a result configuration is inverted, the configuration … of carbon atom under attack inverts in much the same way as an umbrella is turned inside out when caught in a strong wind. This process is called as inversion of configuration.”