racemization

Start with a single, pure left-handed molecule and run an SN1 reaction. The product comes out as an even mixture of left- and right-handed forms — the "handedness" has been scrambled. This loss of a single, defined handedness, replaced by a 50:50 mix of both mirror-image forms, is racemization. The resulting equal mixture is called a racemic mixture, and unlike either pure enantiomer it does not rotate plane-polarized light.

Why does it happen in SN1? The mechanism goes through a flat, trigonal carbocation. That planar cation has lost all memory of which side the leaving group was on — its three groups lie in a plane with empty lobes above and below. The incoming nucleophile is then free to bond from the top face or the bottom face with roughly equal probability. Bonding from one face regenerates the original handedness; from the other face it gives the mirror image. Equal chances of each yields a (near) 50:50 racemic product.

Racemization is the stereochemical signature that distinguishes SN1 from SN2. SN2 inverts cleanly (one product, opposite handedness); SN1 racemizes (both products, equal amounts). In practice SN1 racemization is often not perfectly 50:50 — the leaving group can briefly linger near the face it just left (an ion pair), slightly shielding it and giving a little extra inversion — but the hallmark of SN1 is substantial loss of optical purity. This matters hugely in drug synthesis, where the two enantiomers can have very different biological effects, so an unwanted racemization can ruin a product.