Animated Solution for Chemistry - Haloalkanes & Haloarenes: A solution of $(-)$-chloro-1-phenylethane in toluene racemises slowly in the presence of a small amount of ${\text{SbCl}}_5$, due to the formation of

Imagine you are looking at a flask containing a solution of $(-)$-chloro-1-phenylethane dissolved in toluene. This starting material is optically active, meaning it consists of a single enantiomer that can rotate plane-polarized light.

Now, we introduce a small amount of antimony pentachloride, ${\text{SbCl}}_5$. Toluene is a non-polar solvent, so it doesn't actively participate as a nucleophile. The real magic happens between the alkyl halide and the ${\text{SbCl}}_5$.

Antimony pentachloride is a powerful Lewis acid. It has a strong affinity for electron pairs, specifically those on halogen atoms. When it encounters our alkyl halide, it acts as a halogen carrier. It coordinates with the chlorine atom and effectively pulls it away, taking the bonding electrons with it to form the complex anion ${\text{SbCl}}_6^{-}$.

Once the chloride leaving group departs, the central carbon atom is left with only three bonds and a positive charge. This is our carbocation intermediate.

Because it only has three electron domains, the central carbon rehybridizes to $s{p}^2$. Geometrically, this means the carbocation is perfectly flat, or trigonal planar. The empty p-orbital sits perpendicular to this plane, extending equally above and below it.

Racemisation

Now, the ${\text{SbCl}}_6^{-}$ complex is unstable and will eventually return the chloride ion (${\text{Cl}}^{-}$) to the carbocation. Because the carbocation is planar, the incoming nucleophile sees no difference between the top face and the bottom face.

It can attack from the front or from the back with exactly equal probability ($50\%$ chance for each).

If it attacks from the front, it recreates the original stereocenter. If it attacks from the back, it creates the opposite enantiomer. This equal mixture of d-form and l-form enantiomers is optically inactive and is known as a racemic mixture. The slow loss of optical activity over time is called racemisation, and it is the ultimate proof that the reaction proceeded via a planar carbocation intermediate.