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Glossary 505 Asymmetric hydrogenation A highly selective addition of hydrogen to alkenes used in the synthesis of amino acids. Atomic orbital In quantum mechanics, a region associated with the electrons surrounding an atom. Aufbau principle The assignment of electrons to atomic orbitals. Axial bond A bond parallel to the principal molecular axis of, usually, a cycloalkane ring. Azacycloalkane A saturated nitrogen heterocycle, such as azacyclohexane (piperidine), C₅H₁₁N. Azide ion a nucleophile useful in introducing an amino group. Azo group the functional group of several brilliant dyes, the azobenzenes. Backside displacement A concerted mechanism in which the nucleophile approaches from the side op- posite the leaving group displacement, for example). Baeyer-Villiger oxidation Addition of peroxycarboxylic acid to a ketone followed by rearrangement to an ester. Base (1) The conjugate base of an acid; or (2) one of the four heterocyclic subunits of a nucleic acid. Benzene The aromatic compound C₆H₆, a remarkably stable conjugated cyclic system. Benzoic acid An aromatic carboxylic acid, Benzyl also called the phenylmethyl group-with a corresponding resonance-stabilized ben- zylic radical. Benzylic resonance Resonance stabilization of the phenylmethyl (benzyl) radical, C₆H₅CH₂ (also cation and anion). Benzyne a highly reactive intermediate, with a bent triple bond. Bimolecular reaction A process. usually second-order, in which two molecules participate in the transi- tion state. Boat conformation An unstable cyclohexane structure, with carbons 1 and 4 tilted out of the plane in the same direction. The 1,1-dimethylethoxycarbonyl group, a protecting group in peptide synthesis. Bond dissociation energy The energy required to break a chemical bond through homolytic cleav- age. or the energy released in forming that bond. Bond length The separation between nuclei for maximum chemical bonding, in that a change would re- lease no further energy. Bond-angle strain Relative instability of a molecule due to distortion of (for example) the tetrahedral carbon bond angle away from 109.5°. Bond-line notation Molecular representation using straight lines for the main carbon chain and omitting all hydrogens. Branched alkane An alkane in which one or more methylene hydrogens have been replaced by an alkyl group. Bridgehead carbon A carbon atom shared by two rings. Buckminsterfullerene "Buckyball." or the most prevalent closed-shell allotrope of carbon. Carbamic acid RNH-COOH. an amide derivative of carbonic acid. Carbamic ester RNH-COOR', an amide derivative of carbonic acid with common name urethane. Carbanion Negatively charged carbon. Carbene a highly reactive species such as methylene (H₂C:). Carbenoid A reactive substance that. like a carbene. converts alkenes to cyclopropanes stereospecifically. Carbocation Positively charged carbon. Carbocyclic compound A ring of only carbon atoms. Carbohydrate A saccharide. which may be a sugar, starch. or cellulose. α-Carbon The carbon adjacent to a carbonyl group. NMR spectrometry NMR spectrometry used to reveal molecular structure by observation of nuclei of the isotope carbon-13. Carbonyl group the functional group characteristic of aldehydes. ketones. and. together with an OH group. carboxylic acids.