Glossary

achiral: the opposite of chiral; also called nonchiral. An achiral molecule can be superimposed on its mirror image.
acid-base reaction: a neutralization reaction in which the products are a salt and water.
activated complex: molecules at an unstable intermediate stage in a reaction.
activation energy: the energy that must be supplied to chemicals to initiate a reaction; the difference in potential energy between the ground state and the transition state of molecules. Molecules of reactants must have this amount of energy to proceed to the product state.
addition: a reaction that produces a new compound by combining all of the elements of the original reactants.
adduct: the product of an addition reaction.
alcohol: an organic chemical that contains an - OH group.
aldehyde: an organic chemical that contains a - CHO group. Water addition to terminal alkynes forms aldehydes.
alicyclic compound: an aliphatic cyclic hydrocarbon, which means that an alicyclic compound contains a ring but not an aromatic benzene ring (a six-carbon ring with three double bonds).
aliphatic compound: a straight- or branched-chain hydrocarbon; an alkane, alkene, or alkyne.
alkane: a hydrocarbon that contains only single covalent bonds. The alkane general formula is CnH2n+2.
alkene: a hydrocarbon that contains a carbon-carbon double bond. The alkene general formula is CnH2n.
alkoxide ion: an anion formed by removing a proton from an alcohol; the RO-ion.
alkoxy free radical: a free radical formed by the homolytic cleavage of an alcohol - OH bond; the RO• free radical.
alkyl group: an alkane molecule from which a hydrogen atom has been removed. Alkyl groups are abbreviated as "R" in structural formulas.
alkyl halide: a hydrocarbon that contains a halogen substituent, such as fluorine, chlorine, bromine, or iodine.
alkyl-substituted cycloalkane: a cyclic hydrocarbon to which one or more alkyl groups are bonded. (Compare with "cycloalkyl alkane.")
alkyne: a hydrocarbon that contains a triple bond. The alkyne general formula is CnH2n-2.
allyl group: the H2C = CHCH2 - group.
allylic carbocation: the H2C = CHCH2+ ion.
analogue: in organic chemistry, chemicals that are similar to each other, but not identical. For example, the hydrocarbons are all similar to each other, but an alkane is different from the alkenes and alkynes because of the types of bonds they contain. Therefore, an alkane and an alkene are analogues.
angle of rotation: (α) in a polarimeter, the angle right or left in which plane-polarized light is turned after passing through an optically active compound in solution.
angle strain: the strain created by the deformation of bond angles from their normal values.
angular momentum quantum number (l): the second number in Schrödinger's electron wave equation, which tells the shape of the orbital.
anion: a negatively charged ion.
anti addition: the addition of atoms to opposite sides of a molecule. (Compare with "syn addition.")
antibonding molecular orbital: a molecular orbital that contains more energy than the atomic orbitals from which it was formed; in other words, an electron is less stable in an antibonding orbital than it is in its original atomic orbital.
anti-Markovnikov addition: a reaction in which the hydrogen atom of a hydrogen bromide bonds to the carbon of a double bond that is bonded to fewer hydrogen atoms. The addition takes place via a free-radical intermediate rather than a carbocation. (Compare with "Markovnikov rule.")
Arrhenius theory: a theory (limited to aqueous systems) that defines an acid as a compound that liberates hydrogen ions and a base as a compound that liberates hydroxide ions. A neutralization is the reaction of a hydrogen ion with a hydroxide ion to form water.
asymmetric center: see stereogenic center.
atom: the smallest amount of an element; a nucleus surrounded by electrons.
atomic mass (A): the sum of the weights of the protons and neutrons in an atom. (A proton and neutron each have a mass of 1 atomic mass unit.)
atomic number (Z): the number of protons or electrons in an atom.
atomic 1s orbital: the spherical orbital nearest the nucleus of an atom.
atomic orbital: a region in space around the nucleus of an atom where the probability of finding an electron is high.
atomic p orbital: an hourglass-shaped orbital, oriented on x, y, and z axes in three-dimensional space.
atomic s orbital: a spherical orbital.
aufbau buildup: the order in which electrons fill atomic orbitals according to energy factors.