Lactone—A cyclic ester; named according to the Greek letter of the carbon closing the ring and for
the straight-chain form of the compound.
Leaving group—The molecular fragment that retains electrons after heterolysis; must be stable in
solution.
Lewis acid—An electron acceptor in the formation of a covalent bond.
Lewis base—An electron donor in the formation of a covalent bond.
Lowest unoccupied molecular orbital (LUMO)—The lowest-energy molecular orbital that does not
contain electrons; in UV spectroscopy, electrons are excited from the HOMO to the LUMO.
Magnetic quantum number (ml)—Describes the orbital in which an electron is found; possible
values range from −l to +l.
Meso compound—A molecule that has chiral centers but is not optically active because it has an
internal plane of symmetry.
Mesylate—A compound containing the functional group –SO 3 CH 3 , derived from methanesulfonic
acid.
Michael addition—A reaction in which a carbanion attacks an α,β-unsaturated carbonyl.
Mobile phase—A liquid (or a gas in gas chromatography) that is run through the stationary phase in
chromatography.
Molecular orbital—The resulting electron structure when two atomic orbitals combine.
Multiplet—Peaks that have more than four shifts in NMR spectroscopy.
Newman projection—A method of visualizing a compound in which the line of sight is down a
carbon–carbon bond axis.
Node—In orbital structure, an area where the probability of finding an electron is zero.