racemic mixture
/ ray-SEE-mik (racemate: RASS-uh-mate) /
Imagine a bin filled with an exactly equal number of left and right gloves, all jumbled together. The bin as a whole has no overall handedness — for every left glove there is a matching right one. A racemic mixture is the molecular version of that bin: a 50:50 blend of a chiral compound and its mirror image.
A racemic mixture, or racemate, contains equal amounts of the two enantiomers of a chiral substance. Because the two enantiomers rotate plane-polarized light by equal amounts in opposite directions, their effects cancel exactly and the mixture shows no net optical rotation — it is optically inactive even though every single molecule in it is chiral. A racemate is often written with the (plus/minus) symbol or the prefix rac- before the name. Crucially it is a mixture of two distinct substances, not a new compound.
Racemic mixtures appear constantly because most ordinary reactions that create a new stereocenter, starting from achiral reagents in an achiral environment, produce both enantiomers with exactly equal probability. The nucleophile (or whatever) is just as likely to approach from either face. This is why a synthetic drug made without special chiral control comes out racemic, and it is why so much effort goes into either making one enantiomer selectively or separating the two afterward — the topic of resolution.
Adding HBr across a simple symmetric alkene to make a new stereocenter gives a racemic mixture: the proton and bromide add from either face with equal odds, producing exactly 50% (R) and 50% (S). A polarimeter reads zero rotation for the product.
Equal enantiomers from an achiral reaction; the optical rotations cancel to zero.
A racemate and a meso compound are both optically inactive, but for different reasons and they are not the same thing. A racemate is a mixture of two chiral molecules cancelling each other; a meso compound is one achiral molecule cancelling itself internally. You can in principle separate a racemate into its two halves, but a meso compound cannot be 'separated' — it is a single substance.