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allenes the two元bonds ethe two terminal hydro Angstrom (A).Unit of length named after the Swedish physicist,now being superseded by nanometer(nm).I A =10-10m so that a bond length of 1.54 A is given by 0.154 nm.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 which
46 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 hydrogens 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 Unit of length named after the Swedish physicist, now being superseded by nanometer (nm). 1 Å = 10–10 m so that a bond length of 1.54 Å is given by 0.154 nm.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, asymmetric 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. diagram 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°
Glossary 47 Atactic.This adjective describes a polymer whose stereogenic centres along the polymer chain are randomly oriented. Atronisomers Stereoisomers that arise from restricted rotation around a single bond in which the barrier to rotation is sufficiently high are atropisomers,and the stereoisomers referred to in this case are enan- tiomers. Axial bonds.In the chair conformation of cyclohexanes,axial bonds are described as being:(a)parallel to the C axis,or(b)perpendicular to a general plane that contains the majority of carbon atom Axial chirality.Chirality that has its origins in the non-planar dispo- sition of groups with respect to an axis,and exemplified by 1,3- dichloroallene (Chapter 5,structure 4),certain alkylidenecyclohexanes (Chapter 5,structure 6)and 4-substituted cyclohexanone oximes Bridgehead carbon.In an unsubstituted hydrocarbon that consists of two or more fused or bridged rings,with two or more carbons in com- mon.the bridgehead carbons are tertiary carbons common to two or more rings.Bridgehead carbons can be substituted by,for example.Cl. OH,etc.In bicy .2.1]hept ane the bridgehead ca bons are C(1)and C(4);in bicyclo[4..0decane they are C(1)and C(6) Brosylate (4-bromobenzenesulfonate,4-BrC H SO,).Slightly better leaving groupgroup than tosylate in substitution and elimination reactions.Usually abbreviated to OBs. Cahn-Ingold-Prelog(CIP)R/S convention.The most widely used method for assignment of configuration to stereogenic centres (and chi- ral axes).Substituents at stereogenic centres are given ranking numbers 1,2,3 and 4,associated with decreasing atomic number;configuration is then assigned as in Chapter 2,Section 2.2.2.The configuration of chi ral axes is given in Chapter 5,Section 5.3 Canonical forms(also called resonance structures).These are differ- ent Lewis structures that are alternative ways of representing the actual structure of a molecule that contains delocalized bonds One draws sev eral possible structures that all have the same number of u npaired elec. trons,and in which the relative positions of the nuclei are the same.Each canonical structure contributes in proportion to its stability,so that the structure is a weighted average of all canonical structures.These delo- calized canonical structures have no individual existence.As an example, aromatic compounds and amides are weighted averages of two or m canonical fo or Lewis structure n the case of an amide,these are. for example,RN-C(=O)R2 and R,N+=C(O-)R2
Atactic. This adjective describes a polymer whose stereogenic centres along the polymer chain are randomly oriented. Atropisomers. Stereoisomers that arise from restricted rotation around a single bond in which the barrier to rotation is sufficiently high for the stereoisomers to be isolated. Certain ortho disubstituted biphenyls are atropisomers, and the stereoisomers referred to in this case are enantiomers. Axial bonds. In the chair conformation of cyclohexanes, axial bonds are described as being: (a) parallel to the C3 axis, or (b) perpendicular to a general plane that contains the majority of carbon atoms. Axial chirality. Chirality that has its origins in the non-planar disposition of groups with respect to an axis, and exemplified by 1,3- dichloroallene (Chapter 5, structure 4), certain alkylidenecyclohexanes (Chapter 5, structure 6) and 4-substituted cyclohexanone oximes. Bridgehead carbon. In an unsubstituted hydrocarbon that consists of two or more fused or bridged rings, with two or more carbons in common, the bridgehead carbons are tertiary carbons common to two or more rings. Bridgehead carbons can be substituted by, for example, Cl, OH, etc. In bicyclo[2.2.1]heptane the bridgehead carbons are C(1) and C(4); in bicyclo[4.4.0]decane they are C(1) and C(6). Brosylate (4-bromobenzenesulfonate, 4-BrC6H4SO3). Slightly better leaving group group than tosylate in substitution and elimination reactions. Usually abbreviated to OBs. Cahn–Ingold–Prelog (CIP) R/S convention. The most widely used method for assignment of configuration to stereogenic centres (and chiral axes). Substituents at stereogenic centres are given ranking numbers 1, 2, 3 and 4, associated with decreasing atomic number; configuration is then assigned as in Chapter 2, Section 2.2.2. The configuration of chiral axes is given in Chapter 5, Section 5.3. Canonical forms (also called resonance structures). These are different Lewis structures that are alternative ways of representing the actual structure of a molecule that contains delocalized bonds. One draws several possible structures that all have the same number of unpaired electrons, and in which the relative positions of the nuclei are the same. Each canonical structure contributes in proportion to its stability, so that the structure is a weighted average of all canonical structures. These delocalized canonical structures have no individual existence. As an example, aromatic compounds and amides are weighted averages of two or more canonical forms (or Lewis structures); in the case of an amide, these are, for example, R1 2 N–C(=O)R2 and R1 2 N+=C(O– )R2 . Glossary 47
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 cyclohexane. 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 superimposable on its mirror image. The adjective is ideally restricted to single molecules. 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 conformation, 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 identifiable 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 configurations. 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 resembles a boat when viewed sideways on. Conformation (chair). The lowest energy conformation of cyclohexane, 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
Glossary 49 Conformation (skewed).As above,but with the value of between O°and6o°：there are an infinite number of skewed conformations. Conformation (staggered).As above but with =60;in the case of ethane the most stable conformation butane has a gauche staggered conformation in which the C-C-C-C dihedral angle is 60and an antiperiplanar staggered conformation in which the corresponding dihe- dral angle is 180 Conrotatory.This adjective indicates that both p-orbitals at the ter. minal carbons (and the substituents at these carbons)of a conjugated acyclic hydrocarbon rotate in the same sense,both clockwise or both anti-clockwise,during ring closure.Also applied to the reverse step, namely ring opening. Cycloaddition reaction.In the context of pericyclic reactions,this refers to two ules,the same or different with one or me bonds. that combine to form a cyclic compound with creation of two new o bonds in a concerted,bimolecular reaction.Of course,cycload- dition reactions may proceed stepwise,but these are not pericyclic reac- tions. DL convention.Employed,frequently in conjunction with Fischer pro jections,to assign configuration to sugars and amino acids,with D-glyc- eraldehyde (see p.24)as reference compound. Dehydrogenases.Enzymes that operate in conjunction with a cofac- usually NADH.Despite the nar dehyd conditions Dextrorotatory (d).Description given to a chiral compound that rotates the plane of polarized light (usually of wavelength 589.6 nm,the sodium D line)in a clockwise sense as the observer looks into the prop agating beam Diastereoisomers.Stereoisomers that are not enantiomers.This is a wide definition,which includes geometric isomers such as,for example, (Z)-and (E)-1,2-dichloroethene.Note that diastereoisomers may be (a) both chiral,(b)one may be chiral and the other not,e.g.tarta ic acids (Chapter 3)and (c)both may be achiral,e.g.cis-and trans-1,3-disubsti- tuted cyclobutanes and cis-1,4-and trans-1,4-disubstituted cyclohexanes (Chapter 6). Diastereotopic.This adjective applies to molecules that contain (a) CODH.COXYZY that comtains a stereogemie sentre.usu on, e.g. in a molecule type ally,but not necessarily,adjacent to C(1).Replacement,separately,of
Conformation (skewed). As above, but with the value of φ between 0° and 60°; there are an infinite number of skewed conformations. Conformation (staggered). As above but with φ = 60°; in the case of ethane, the most stable conformation. Butane has a gauche staggered conformation in which the C–C–C–C dihedral angle is 60° and an antiperiplanar staggered conformation in which the corresponding dihedral angle is 180°. Conrotatory. This adjective indicates that both p-orbitals at the terminal carbons (and the substituents at these carbons) of a conjugated acyclic hydrocarbon rotate in the same sense, both clockwise or both anti-clockwise, during ring closure. Also applied to the reverse step, namely ring opening. Cycloaddition reaction. In the context of pericyclic reactions, this term refers to two molecules, the same or different, with one or more π bonds, that combine to form a cyclic compound with creation of two new σ bonds in a concerted, bimolecular reaction. Of course, cycloaddition reactions may proceed stepwise, but these are not pericyclic reactions. DL convention. Employed, frequently in conjunction with Fischer projections, to assign configuration to sugars and amino acids, with D-glyceraldehyde (see p. 24) as reference compound. Dehydrogenases. Enzymes that operate in conjunction with a cofactor, usually NADH. Despite the name, dehydrogenases catalyse both dehydrogenation (oxidation) and reduction reactions, under appropriate conditions. Dextrorotatory (d). Description given to a chiral compound that rotates the plane of polarized light (usually of wavelength 589.6 nm, the sodium D line) in a clockwise sense as the observer looks into the propagating beam. Diastereoisomers. Stereoisomers that are not enantiomers. This is a wide definition, which includes geometric isomers such as, for example, (Z)- and (E)-1,2-dichloroethene. Note that diastereoisomers may be (a) both chiral, (b) one may be chiral and the other not, e.g. tartaric acids (Chapter 3) and (c) both may be achiral, e.g. cis- and trans-1,3-disubstituted cyclobutanes and cis-1,4- and trans-1,4-disubstituted cyclohexanes (Chapter 6). Diastereotopic. This adjective applies to molecules that contain (a) sp3 hybridized carbon, e.g. C(1), in a molecule of type RC(1)H2 –C(2)XYZ (X ≠ Y ≠ Z) that contains a stereogenic centre, usually, but not necessarily, adjacent to C(1). Replacement, separately, of Glossary 49
50 Stereochemistry each of the C(1)hydrogens by D gives two diastereoisomers.and the eogenic cen tre,e.g.MeCOCH(Me)Ph.Attack at the two faces of the carbonyl group by a Grignard reagent RMgBr,where R Me,gives a pair of diastereoisomeric alcohols,usually in unequal amounts.The faces of the carbonyl group are said to be diastereotopic. Disrotatory(see Conrotatory).The opposite of conrotatory:in disro tatory ring closures of conjugated acyclic hydrocarbons,the p-orbitals at the terminal carbons,together with the substituents at these carbons, rotate in opposite senses,one clockwise and the other anti-clockwise. Electrocyclic reactions.A sub-class of pericylic reactions.Concerted intramolecula ring-forming reactions,e.g.of conjugated dienes or trienes in which a new o bond is formed by way of a cyclic transition state.The cyclic product contains one fewer double bond.Term also to the reverse racton.The simplest examplstr buta-1,3-diene and cyclobutene Electrophile(from Greek 'electron loving).Term defined by Ingold (J.Chem.Soc.,1933,1191),together with nucleophile (see below).A reagent that acquires a share in the electrons of another(foreign)mole- cule during a reaction.Exemplified by Br,in electrophilic bromination of an alk ne,and nitronium on(NO,)in electrophilic nitration of ben zene.In these reactions the alkene and benzene,respectively,are com- plementary nucleophiles. Elution.In column chromatography,which is used to separate com- pounds()a substrate is oured onto a column acking (statio y phase)which may be the case of cellulose,or compounds 37 and 38 in Chapter 3.The solvent passes downward through the column and emerges through a tap at the bottom.In so doing there are two opposing factors at work:(a)adsorp- tion of the substrate(s)on the stationary phase,(b)de-adsorption, washing-of of the substrate by the sove ent.This pro cess of tion/de-adsorption is repeated continually as the substrate moves down the column.The role of the solvent is elution.and a substrate is eluted more rapidly the less it is adsorbed on the stationary phase.By means of differential diastereomeric interactions with a chiral stationary phase. it is possible to separate enantio Enantiomeric excess;enantiomeric ratio.Ways of expressing the enantiomeric make-up of a sample.A sample that consists of,say,90% (+)enantiomer and 10%(-)enantiomer has an enantiomeric excess (ee) of 80%and an enantiomeric ratio of 9:1
each of the C(1) hydrogens by D gives two diastereoisomers, and the hydrogens are said to be diastereotopic. (b) An sp2 hybridized carbon, such as a carbonyl group in a molecule that contains a stereogenic centre, e.g. MeCOCH(Me)Ph. Attack at the two faces of the carbonyl group by a Grignard reagent RMgBr, where R ≠ Me, gives a pair of diastereoisomeric alcohols, usually in unequal amounts. The faces of the carbonyl group are said to be diastereotopic. Disrotatory (see Conrotatory). The opposite of conrotatory; in disrotatory ring closures of conjugated acyclic hydrocarbons, the p-orbitals at the terminal carbons, together with the substituents at these carbons, rotate in opposite senses, one clockwise and the other anti-clockwise. Electrocyclic reactions. A sub-class of pericylic reactions. Concerted intramolecular ring-forming reactions, e.g. of conjugated dienes or trienes in which a new σ bond is formed by way of a cyclic transition state. The cyclic product contains one fewer double bond. Term also applies to the reverse reaction. The simplest example is interconversion of buta-1,3-diene and cyclobutene. Electrophile (from Greek ‘electron loving’). Term defined by Ingold (J. Chem. Soc., 1933, 1191), together with nucleophile (see below). A reagent that acquires a share in the electrons of another (foreign) molecule during a reaction. Exemplified by Br2 in electrophilic bromination of an alkene, and nitronium ion (NO2 +) in electrophilic nitration of benzene. In these reactions the alkene and benzene, respectively, are complementary nucleophiles. Elution. In column chromatography, which is used to separate compounds (including enantiomers), a solution of a substrate is poured onto a column of a packing (or stationary phase) which may be chiral, as in the case of cellulose, or compounds 37 and 38 in Chapter 3. The solvent passes downward through the column and emerges through a tap at the bottom. In so doing there are two opposing factors at work: (a) adsorption of the substrate(s) on the stationary phase, (b) de-adsorption, or ‘washing-off’ of the substrate by the solvent. This process of adsorption/de-adsorption is repeated continually as the substrate moves down the column. The role of the solvent is elution, and a substrate is eluted more rapidly the less it is adsorbed on the stationary phase. By means of differential diastereomeric interactions with a chiral stationary phase, it is possible to separate enantiomers. Enantiomeric excess; enantiomeric ratio. Ways of expressing the enantiomeric make-up of a sample. A sample that consists of, say, 90% (+) enantiomer and 10% (–) enantiomer has an enantiomeric excess (ee) of 80% and an enantiomeric ratio of 9:1. 50 Stereochemistry