Glossary Achiral.A molecule that is superimposable on its mirror image(p.19) is achiral. Allenes.Propa-1.2-diene(CH,=C-CH)and derivatives.In allenes the two元bonds ethe two terminal hydro Angstrom (A).Unit of length named after the Swedish physicist,now being superseded by nanometer(nm).I A =10-10m so that a bond length of 1.54 A is given by 0.154 nm. Anisochronous.In NMR spectrosco opy this m ic used to indicate that different chemical shifts. Anomers.Anomers are cyclic diastereoisomers that differ only at the hemiacetal carbon.The anomeric carbon in a sugar is the only carbon that is bonded to two oxygen atoms. 二二二 of a ne in a molecule. fo r example,asym metric aldol reactions.The main chain is drawn in the plane of the paper and substituents on opposite sides of the plane are termed anti,e.g.dia- gram 19 in Chapter 3. Anti addition.Anti addition of X-Y occurs when X and Y are added to opposite faces of a double bond. Anti clinal.When the C-C-C-C dihedral angle is between 90 and 150°，i.e.l20±30°，the conformation is said to be anti clinal,. Anti periplanar(sometimes called Anti).Term given to a molecular fragment,e.g.X-C(1)-C(2)-Y,or C(1)-C(2)-Y (in this second case,a air of electron s the X-C bond),in which46 Glossary Aims Achiral. A molecule that is superimposable on its mirror image (p. 19) is achiral. Allenes. Propa-1,2-diene (CH2 =C=CH2 ) and derivatives. In allenes the two π bonds are orthogonal (see below), as are the two terminal hydro￾gens at one end, with respect to those at the other. Ångstrom (Å). Unit of length named after the Swedish physicist, now being superseded by nanometer (nm). 1 Å = 10–10 m so that a bond length of 1.54 Å is given by 0.154 nm. Anisochronous. In NMR spectroscopy this term is used to indicate that two or more nuclei have different chemical shifts. Anomers. Anomers are cyclic diastereoisomers that differ only at the hemiacetal carbon. The anomeric carbon in a sugar is the only carbon that is bonded to two oxygen atoms. Anti (see also Anti addition; Anti periplanar). Substituents are anti if they are on opposite sides of a defined reference plane in a molecule. Anti is used to assign stereochemistry to products of, for example, asym￾metric aldol reactions. The main chain is drawn in the plane of the paper and substituents on opposite sides of the plane are termed anti, e.g. dia￾gram 19 in Chapter 3. Anti addition. Anti addition of X–Y occurs when X and Y are added to opposite faces of a double bond. Anti clinal. When the C–C–C–C dihedral angle is between 90° and 150°, i.e. 120 ± 30°, the conformation is said to be anti clinal. Anti periplanar (sometimes called Anti). Term given to a molecular fragment, e.g. X–C(1)–C(2)–Y, or C(1)–C(2)–Y (in this second case, a lone pair of electrons in a p-orbital replaces the X–C bond), in which the dihedral angle is 180°, or more generally 180 ± 30°