54 Stereochemistry Nucleophiles have either (a)at least one unshared pair of electrons and aneutral,e.g.Me N.or negatively charged.e.g.I.Nucleophe arbon with expulsio of a leaving group (S)ora car- bocation(S1).(b)A double bond which attacks,for example,bromine in the addition of Br,to a double bond,or an aromatic ring which attacks,for example,a nitronium ion in aromatic nitration.In these reac- tions (a)saturated carbon and the carbocation and (b)bromine and nitro n um ion,respectively,themselves act as complementary elec trophiles. OBs.See Brosylate(4-bromobenzenesulfonate,4-BrC H SO,),above. Optically active compound.A compound,necessarily chiral,and non-racemic,that rotates the plane of polarized light in a particular direc tion. Optically pure.Mainly superseded by 'enantiomerically pure' Orthogonal.Term used to describe p-orbitals and t bonds in mutual- ly perpendicular planes,a nitriles and allenes.Such orbitals and bon do not interact.The two HCH planes at each terminal car- bon in allene are also orthogonal. OTs.See Tosylate(toluene-4-sulfonate.4-MeC H,SO,).below. Overhauser effect.See Nuclear Overhauser effect Pericyclic reactions.These reactions occur by cyclic concerted elec tron shifts and are sub-divided into cycloadditions,electrocyclic and cheletropic reactions,and sigmatropic rearrangements.The first two types are considered in the text. Plane of symmetry.The presence of a plane of symmetry in n a mol- ecule means that it can be superimposed on its mirror image.Molecules with a plane of symmetry,in any conformation,cannot be chiral. Plane polarized light.Light that emerges from a polarizer,which acts as a filter to block the light waves other than those oscillating in a single plan An integral part of a polarimeter,and very important to detection of chirality. Polarimeter.An instrument for determination of the number of degrees by which a particular sample of a chiral compound rotates the light.This enables the specific rotation (see below)to Prochiral.Adjective used to describe (a)a molecule with two enan- tiotopic atoms,e.g.the methylene hydrogens of ethanol,or (b)a mole- cule such as PhCOMe,with two enantiotopic faces.The term does not usually apply to molecules with diastereotopic atoms and faces,as thesNucleophiles have either (a) at least one unshared pair of electrons and may be neutral, e.g. Me3 N, or negatively charged, e.g. I– . Nucleophiles attack saturated carbon with expulsion of a leaving group (SN2) or a car￾bocation (SN1). (b) A double bond which attacks, for example, bromine in the addition of Br2 to a double bond, or an aromatic ring which attacks, for example, a nitronium ion in aromatic nitration. In these reac￾tions (a) saturated carbon and the carbocation and (b) bromine and nitronium ion, respectively, themselves act as complementary elec￾trophiles. OBs. See Brosylate (4-bromobenzenesulfonate, 4-BrC6 H4 SO3 ), above. Optically active compound. A compound, necessarily chiral, and non-racemic, that rotates the plane of polarized light in a particular direc￾tion. Optically pure. Mainly superseded by ‘enantiomerically pure’. Orthogonal. Term used to describe p-orbitals and π bonds in mutual￾ly perpendicular planes, as in alkynes, nitriles and allenes. Such orbitals and bonds do not interact. The two HCH planes at each terminal car￾bon in allene are also orthogonal. OTs. See Tosylate (toluene-4-sulfonate, 4-MeC6 H4 SO3 ), below. Overhauser effect. See Nuclear Overhauser effect. Pericyclic reactions. These reactions occur by cyclic concerted elec￾tron shifts and are sub-divided into cycloadditions, electrocyclic and cheletropic reactions, and sigmatropic rearrangements. The first two types are considered in the text. Plane of symmetry. The presence of a plane of symmetry in a mol￾ecule means that it can be superimposed on its mirror image. Molecules with a plane of symmetry, in any conformation, cannot be chiral. Plane polarized light. Light that emerges from a polarizer, which acts as a filter to block the light waves other than those oscillating in a single plane. An integral part of a polarimeter, and very important to detection of chirality. Polarimeter. An instrument for determination of the number of degrees by which a particular sample of a chiral compound rotates the plane of polarized light. This enables the specific rotation (see below) to be calculated. Prochiral. Adjective used to describe (a) a molecule with two enan￾tiotopic atoms, e.g. the methylene hydrogens of ethanol, or (b) a mole￾cule such as PhCOMe, with two enantiotopic faces. The term does not usually apply to molecules with diastereotopic atoms and faces, as these 54 Stereochemistry