R-group

Meaning: R is a placeholder for any carbon-containing group, usually alkyl, attached to a functional group. R–OH stands for any alcohol and RCOOH for any carboxylic acid. R′ and R″ mark other groups on the same molecule, which may be the same as R or different, as in a ketone R–CO–R′. Aryl groups are often written Ar. R is not inert. It affects the functional group by induction (alkyl groups donate electron density) and by sterics (bulky groups block approach).

On the MCAT: A passage shows a general reaction and asks what happens when R changes. Typical trends: a bulkier R slows SN2 reactions and nucleophilic addition to carbonyls. More alkyl substitution stabilizes carbocations. Electron-withdrawing groups on R, such as halogens, make RCOOH more acidic, while electron-donating alkyl groups make it slightly less acidic. In biochemistry, the R group of an amino acid is its side chain, which makes the amino acid nonpolar, polar, acidic or basic.

Common confusion: R is not an element, a specific group or a radical. Different labels such as R and R′ do not require different groups. R is only notation, so read the actual structure in the question to judge its steric and electronic effect.