Rgroup

R group, or residual group, is a placeholder in organic chemistry for a substituent or side chain attached to a core molecule. The exact identity varies by compound and is not fixed; it may be an alkyl, aryl, a heteroatom-containing group, or hydrogen. In schematic diagrams, multiple substituents are often denoted as R, R', and R'' to indicate that their identities can differ and that they influence the molecule's properties.

R groups are used to express generality in mechanisms and naming. They allow chemists to discuss a class of compounds without listing every possible substituent. When a specific example is needed, the R group is replaced with the actual substituent in question. The term also appears in discussions of structure–property relationships, where changing the R group can alter reactivity, polarity, bulk, and stability.

In biology, "R group" commonly refers to the side chain of amino acids. This R group distinguishes amino acids and determines their chemical behavior. For example, glycine has R = hydrogen, while alanine has a methyl group as its R.

In materials and pharmaceutical chemistry, varying R groups is a core strategy for tuning solubility, permeability, potency, and selectivity. While the term describes a general concept, precise studies specify the exact R group because its identity largely governs a molecule’s properties and performance.