Side chain

In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called "main chain" or backbone. The placeholder R is often used as a generic placeholder for side chains, the R historically being derived from radical or rest or random.

In polymer science, the side chain or pendant chain is oligomeric or polymeric offshoot extends from the backbone chain of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its crystallinity and density. An oligomeric branch may be termed a short-chain branch and a polymeric branch may be termed a long-chain branch. Side groups are different from side chains; they are neither oligomeric nor polymeric.

In proteins (composed of amino acids) the side chains are attached to the alpha-carbon atoms of the amide backbone.