R group—In general, an alkyl chain; in amino acid chemistry, the variable side chain on the α-
carbon.
Racemic mixture—A mixture where both (+) and (–) enantiomers are present in equal
concentrations.
Reduction—Gain of electrons causing a decrease in oxidation state; decreasing bonds to oxygen or
other heteroatoms in a molecule.
Relative configuration—The spatial arrangement of groups in a chiral molecule compared to
another chiral molecule.
Resonance—Delocalization of π electrons which increases stability of a molecule.
Resonance structure—A possible arrangement of π electrons in a molecule; the actual electronic
structure of a molecule is the weighted average of resonance structures, based on their stability.
Retardation factor (Rf)—A ratio used in thin-layer chromatography to identify a compound;
calculated as how far the compound traveled relative to how far the solvent front traveled.
Retro-aldol reaction—The reverse of an aldol condensation reaction, in which a carbon–carbon
bond is cleaved with heat and base, yielding two aldehydes, two ketones, or one of each.
Reverse-phase chromatography—The opposite of traditional thin-layer chromatography, in which
the stationary phase is nonpolar and the mobile phase is polar.
Ring strain—Energy created in a cyclic molecule by angle strain, torsional strain, and nonbonded
strain; determines if a ring is stable enough to stay intact.
s-orbital—An atomic orbital that is spherical and symmetrical, centered on the nucleus.
Saponification—The process by which fats are hydrolyzed under basic conditions to produce soap.
Separatory funnel—A piece of laboratory equipment used in extraction; immiscible solvents are
separated by gravity, causing the denser layer to sink to the bottom, where it can be removed by
turning the stopcock at the bottom.