48 Stereochemistry Chair conformation.The lowest energy conformation of cyclohexa- ne.All the vicinal C-H bonds are staggered;this conformation has very little angle strain or torsion strain Chiral.A chiral(or handed)molecule is one that is not superimpos- able on its mirror image.The adjective is ideally restricted to single mol- ecules.An object such as a helix can also be described as chiral. Chirality.The property of non-identity of an object with its mirro image.An object,e.g.a molecule in a given configuration or conforma- tion,is said to be chiral when it is not identical with its mirror image. cis.Groups or atoms are cis when they lie on the same side of an iden tifiable reference plane in a molecule. Concerted.In a concerted reaction the bonding changes occur in a single step and simultaneously. Configuration.The three-dimensional arrangement (or sequence)in space of atoms,or functional groups that oisomer In order to change a configuration,bond breakin g and bond re-forming in a different sequence must occur.Enantiomers have opposite configu- rations.Configurations are denoted by R/S;D/L;E/Z.Configuration should be contrasted with conformation,in which changes are brought about only by bond rotation. Conformation.A description of the arrangement or disposition in space of the atoms,or functional groups,in a molecule.Conformations can change by varying a dihedral angle,i.e.by rotating about a single bond,but not by breaking a bond.Amides(Chapter 4)are a special case. Conformation (boat).A conformation of cyclohexane that,by virtuc of two eclipsed C-C bonds and a 1,4 non-bonded H-H interaction,is of higher energy than the chair conformation.So called because it resem- bles a boat when viewed sideways on. Conformation (chair).The lowest energy conformation of cyclo- hexane n which all CCho ds are staggered and which has ery little angle or torsional strain.This conformation resembles a chair when viewed sideways on. Conformation (eclipsed).When,for example,in ethane the dihedral of (synperiplanar)and 120(anticlinal)are eclipsed;the C-H bonds are eclipsed here also.Some rigid molecules,e.g.bicyclo[2.2.1]heptane, have the C-H bonds at C(2),C(3).C(5)and C(6)locked in an eclipsed conformation.Chair conformation. The lowest energy conformation of cyclohexa￾ne. All the vicinal C–H bonds are staggered; this conformation has very little angle strain or torsion strain. Chiral. A chiral (or handed) molecule is one that is not superimpos￾able on its mirror image. The adjective is ideally restricted to single mol￾ecules. An object such as a helix can also be described as chiral. Chirality. The property of non-identity of an object with its mirror image. An object, e.g. a molecule in a given configuration or conforma￾tion, is said to be chiral when it is not identical with its mirror image. Cis. Groups or atoms are cis when they lie on the same side of an iden￾tifiable reference plane in a molecule. Concerted. In a concerted reaction the bonding changes occur in a single step and simultaneously. Configuration. The three-dimensional arrangement (or sequence) in space of atoms, or functional groups, that characterizes a stereoisomer. In order to change a configuration, bond breaking and bond re-forming in a different sequence must occur. Enantiomers have opposite configu￾rations. Configurations are denoted by R/S; D/L; E/Z. Configuration should be contrasted with conformation, in which changes are brought about only by bond rotation. Conformation. A description of the arrangement or disposition in space of the atoms, or functional groups, in a molecule. Conformations can change by varying a dihedral angle, i.e. by rotating about a single bond, but not by breaking a bond. Amides (Chapter 4) are a special case. Conformation (boat). A conformation of cyclohexane that, by virtue of two eclipsed C–C bonds and a 1,4 non-bonded H...H interaction, is of higher energy than the chair conformation. So called because it resem￾bles a boat when viewed sideways on. Conformation (chair). The lowest energy conformation of cyclo￾hexane, in which all C–C bonds are staggered, and which has very little angle or torsional strain. This conformation resembles a chair when viewed sideways on. Conformation (eclipsed). When, for example, in ethane the dihedral angle φ between a pair of hydrogens on adjacent carbons is 0°; this is the least stable conformation. In butane, both C–C–C–C dihedral angles of 0° (synperiplanar) and 120° (anticlinal) are eclipsed; the C–H bonds are eclipsed here also. Some rigid molecules, e.g. bicyclo[2.2.1]heptane, have the C–H bonds at C(2), C(3), C(5) and C(6) locked in an eclipsed conformation. 48 Stereochemistry