52 Stereochemistry dichloromethane (CH,CL)the chlorines are geminal.In NMR spec- Geometric isomers.Geometric isomers have different configurations at a double bond,e.g.(E)-and (Z)-1,2-dichloroethene;in this context, cis/trans isomerism is also used. Helicity.The sense of twist of,say,a screw or helix.Defined as right handed if,in proceeding along the axis of the screw or helix,the sense of twist is clockwise. Heterolytic.Adjective used to describe the cleavage of a bond X-Y in which Y gains both bonding electrons to give Y-,leaving X". Homochiral.Molecules are homochiral if they possess the same sense of chirality,e.g.the naturally occurring a-amino acids which all have L configuration.Also used by some to denote enantiomeric purity. Homolytic bond cleavage.Occurs when a single bond between two atoms in A-B is broken so that both A and B retain one of the bond ing electrons. Homotopic.Adjective that can be used to describe a molecule that contains (a)sp hybridized carbons.In,for example,CH,CL,separate en by deuterium give identical molecules are Also us mole cules that contain(b sp2 hybridized carbons.In,for example,R'R2C=O,the carbonyl group is homotopic when R R2.Addition of a different R3 group (via a Grignard reagent)gives identical tertiary alcohols whether addition to the carbonyl group occurs at the top or bottom face. Hybridization.A mathematical mixing of atomic orbitals(two or more in number)that gives the same number of hybrid orbitals.The hybrid orbitals retain some of the properties of the component atomic orbitals (see Table 1).Hybrid is from the Latin for 'mongrel'. Table 1 Characteristics of hybrid orbitals in methane,ethene and ethyne D E 4×sD3 0 109°28 3×sp3 120° 2×SD 2 A number of s orbitals per molecule involved in forming hybrid orbitals.B number of 2p per molecule involved in forming hybrid orbtals.C=number and type of hybrid orbitals to fo bond anges at C.ie.HCH in methane.HCC in ethene in ethyne.respectively after hybridization.Note that for each row the sum of the entries in columns C and D is 4.dichloromethane (CH2 Cl2 ) the chlorines are geminal. In NMR spec￾troscopy a geminal coupling constant, 2 J, occurs between nuclei sepa￾rated by two bonds. Geometric isomers. Geometric isomers have different configurations at a double bond, e.g. (E)- and (Z)-1,2-dichloroethene; in this context, cis/trans isomerism is also used. Helicity. The sense of twist of, say, a screw or helix. Defined as right￾handed if, in proceeding along the axis of the screw or helix, the sense of twist is clockwise. Heterolytic. Adjective used to describe the cleavage of a bond X–Y in which Y gains both bonding electrons to give Y– , leaving X+. Homochiral. Molecules are homochiral if they possess the same sense of chirality, e.g. the naturally occurring α-amino acids which all have L configuration. Also used by some to denote enantiomeric purity. Homolytic bond cleavage. Occurs when a single bond between two atoms in A–B is broken so that both A and B retain one of the bond￾ing electrons. Homotopic. Adjective that can be used to describe a molecule that contains (a) sp3 hybridized carbons. In, for example, CH2 Cl2 , separate replacement of each hydrogen by deuterium gives identical molecules. The hydrogens are homotopic. Also used for molecules that contain (b) sp2 hybridized carbons. In, for example, R1 R2 C=O, the carbonyl group is homotopic when R1 = R2 . Addition of a different R3 group (via a Grignard reagent) gives identical tertiary alcohols whether addition to the carbonyl group occurs at the top or bottom face. Hybridization. A mathematical mixing of atomic orbitals (two or more in number) that gives the same number of hybrid orbitals. The hybrid orbitals retain some of the properties of the component atomic orbitals (see Table 1). Hybrid is from the Latin for ‘mongrel’. 52 Stereochemistry Table 1 Characteristics of hybrid orbitals in methane, ethene and ethynea AB C D E 1 3 4 × sp3 0 109° 28′ 1 2 3 × sp3 1 120° 1 1 2 × sp 2 180° a A = number of s orbitals per molecule involved in forming hybrid orbitals. B = number of 2p orbitals per molecule involved in forming hybrid orbitals. C = number and type of hybrid orbitals per molecule formed after hybridization. D = number of p orbitals per molecule remaining after hybridization that can be used to form multiple bonds, as in, say, alkenes and alkynes. E = bond angles at C, i.e. ∠ HCH in methane, ∠ HCC in ethene and ∠ HCC in ethyne, respectively, after hybridization. Note that for each row the sum of the entries in columns C and D is 4