Chapter 20.3: Stereoisomerism Stereoisomerism involves isomers which have different arrangements of atoms in space but do not differ in connectivity or bond multiplicity (i.e. whether single, double, or triple) between the isomers themselves.

Structural isomers: 2 compounds have different structural formulae (atoms not arranged in same order) Stereoisomers: 2compounds have the same structural formula (spatial arrangements of atoms is different)

Configurational: what we will focus on here Conformational: hard to tell difference due to interconversion via free rotation around bonds

Stereoisomers: Configurational Isomers Permanent difference in geometry (cannot be interconverted but can be separated) Also called geometric isomers Cis-trans These isomers can be observed in 2 types of molecules: Double bonded molecules Cyclic molecules

Stereoisomers: Configurational Isomers cis-trans The substituents do not have to be on adjacent carbons The cis-trans configuration is relative to the plane of the ring

Stereoisomers: Configurational Isomers Permanent difference in geometry (cannot be interconverted but can be separated) Also called geometric isomers E/Z isomers Used primarily for double bonds where the cis-trans terminology does not work: having different substituents on the carbons Follow the rules of priority: Higher atomic number has higher priority If atoms are same, follow rule to next atom bonded (so longer hydrocarbons have higher priority) C3H7 > C2H5 > CH3 > H and Br > Cl > F If priority groups are on same side of double bond = Z isomer If priority groups are on opposite sides of double bond = E isomer

Used primarily for double bonds where the cis-trans terminology does not work: having different substituents on the carbons Follow the rules of priority: Higher atomic number has higher priority If atoms are same, follow rule to next atom bonded (so longer hydrocarbons have higher priority) C3H7 > C2H5 > CH3 > H and Br > Cl > F

Used primarily for double bonds where the cis-trans terminology does not work: having different substituents on the carbons If priority groups are on same side of double bond = Z isomer If priority groups are on opposite sides of double bond = E isomer This convention is much more useful than cis-trans

Stereoisomers: Optical Isomers (enantiomers) - Two molecules have same structural formula but are mirror images (not superimposable on one another). - This only occurs when there is a chiral* carbon present, a carbon bonded to four different substituents. The chiral carbon is also known as asymmetric or a stereocenter (hence, stereoisomers)

Stereoisomers: Optical Isomers (enantiomers) Two superimposable forms are called enantiomers An equal mixture of these are called racemic mixture Two isomers with the exact same structural formulas Chiral carbon has four different substituents Different mirror images are not superimposable Therefore, NOT the same moleculez

Stereoisomers: Optical Isomers Diastereomers are a type of isomer where some chiral centers have different configurations (but not all chiral centers) Therefore, not mirror images of each other

Stereoisomers: Optical Isomers Properties Diastereomers usually differ in physical/chemical properties Enantiomers have the same physical/chemical properties EXCEPT: Optically different Reactivity with other chiral carbons

Stereoisomers: Optical Isomers Enantiomer differences: optically different Polarimeter is an instrument to determine the manner in which an optical isomer rotates light One enantiomer will rotate the light one direction, the other will rotate the opposite direction A racemic mixture (equal amounts of both isomers) will effectively rotate both evenly, so the cancel each other and not by optically active

Stereoisomers: Optical Isomers Enantiomer differences: RXTs w/ chiral molecules If a racemic mixture of a compound RXTs w/ 1 enantiomer of a different compound Different products produced Different properties, so can be separated This is called resolution