56 Stereochemistry stereogenic centres)to which four different groups are bonded.In the case of sulfoxides,one of the groups isa lone pair of eletrons. Stereoisomers.Isomers that differ in three-dimensional spatial arrangements(configurations)but which have the same connectivity(this term indicates which atom is bonded to which)and includes (see above) enantiomers,diastereoisomers and geometric isomers. Stereoselective.In stereoselective reactions from a suitable starting material,two (or more)stereoisomers are formed in unequal amounts For example,in a stereoselective reaction,camphor reacts with LiAlH to give mainly isoborneol together with borneol.When one stereoisomer is formed exclusively the reaction is termed completely stereoselective. Stereospecific.Types of reactions that can be shown by reactants that exist as stereoisomers.If each stereoisomer of a reactant gives a dif ferent stereoisomeric product,the reaction is stereospecific.Examples include the S.2 reaction,anti addition of bromine to alkenes,and the Diels-Alder reaction(a syn addition in which the relative configuration of substituents on the dienophile is retained) Suprafacial.A suprafacial reaction takes place at the ends of the same face of a n electron system,usually in an alkene.Used to describe,for example,photochemical 2+2]cycloadditions of alkenes,and also the OsO,mediated formation of 1,2-diols from alkenes Syn(see also Syn addition;Syn periplanar).Substituents are syn if they are on the same side of a defined reference plane in a molecule.Syn is also employed to assign stereochemistry to products,e.g.of aldol reac- tions.With the main chain drawn in the plane of the paper,substituents othe same side of the plane are termed yn.e strct re 18 in Chapte Syn addition.Syn addition of X-Y occurs when X and Y are added to the same face of a double bond. 90 Syn periplanar.The conformation of,say,butane when the C-C-C-C dihedral angle is 0.The term is justified up to values of 30. Syndiotactic.Adjective sed to describe a polymer,.g.polypr in which with respect to the polymer chain (drawn ir the plane of the paper)the methyl groups are represented as alternately in front of and behind the plane.The monomer units exist as repeating pairs. Tautomerism.Isomerism brought about by rapid and reversible inter- conversion of,say,a ketone with an a-hydrogen and an enol,as instereogenic centres) to which four different groups are bonded. In the case of sulfoxides, one of the groups is a lone pair of electrons. Stereoisomers. Isomers that differ in three-dimensional spatial arrangements (configurations) but which have the same connectivity (this term indicates which atom is bonded to which) and includes (see above) enantiomers, diastereoisomers and geometric isomers. Stereoselective. In stereoselective reactions from a suitable starting material, two (or more) stereoisomers are formed in unequal amounts. For example, in a stereoselective reaction, camphor reacts with LiAlH4 to give mainly isoborneol together with borneol. When one stereoisomer is formed exclusively the reaction is termed completely stereoselective. Stereospecific. Types of reactions that can be shown by reactants that exist as stereoisomers. If each stereoisomer of a reactant gives a dif￾ferent stereoisomeric product, the reaction is stereospecific. Examples include the SN2 reaction, anti addition of bromine to alkenes, and the Diels–Alder reaction ( a syn addition in which the relative configuration of substituents on the dienophile is retained). Suprafacial. A suprafacial reaction takes place at the ends of the same face of a π electron system, usually in an alkene. Used to describe, for example, photochemical [2 + 2] cycloadditions of alkenes, and also the OsO4 mediated formation of 1,2-diols from alkenes. Syn (see also Syn addition; Syn periplanar). Substituents are syn if they are on the same side of a defined reference plane in a molecule. Syn is also employed to assign stereochemistry to products, e.g. of aldol reac￾tions. With the main chain drawn in the plane of the paper, substituents on the same side of the plane are termed syn, e.g structure 18 in Chapter 3. Syn addition. Syn addition of X–Y occurs when X and Y are added to the same face of a double bond. Syn clinal. The conformation of, say, butane when the C–C–C–C dihe￾dral angle has values between 30° and 90°. Syn periplanar. The conformation of, say, butane when the C–C–C–C dihedral angle is 0°. The term is justified up to values of 30°. Syndiotactic. Adjective used to describe a polymer, e.g. polypropene, in which with respect to the polymer chain (drawn in the plane of the paper) the methyl groups are represented as alternately in front of and behind the plane. The monomer units exist as repeating pairs. Tautomerism. Isomerism brought about by rapid and reversible inter￾conversion of, say, a ketone with an α-hydrogen and an enol, as in 56 Stereochemistry