B.E. (Petroleum & Natural Gas) Stereochemistry By Engr. Asadullah Memon B.E. (Petroleum & Natural Gas)

Stereochemistry Stereochemistry: The study of the three-dimensional structure of molecules. Structural (constitutional) isomers: same molecular formula but different bonding sequence. Stereoisomers: same molecular formula, same bonding sequence, different spatial orientation.

Stereochemistry Stereochemistry is the study of the static and dynamic aspects of the three-dimensional shapes of molecules. Stereochemistry, a sub-discipline of chemistry, involves the study of the relative spatial arrangement of atoms with in molecules. It has long provided a foundation for understanding structure and reactivity. Stereochemistry plays an important role in determining the properties and reactions of organic compounds.

Stereochemistry (S)-ketamine (R)-ketamine anesthetic hallucinogen The properties of many drugs depends on their stereochemistry: (S)-ketamine (R)-ketamine anesthetic hallucinogen

Review of Isomerism

ISOMERS Isomers are compounds which have the same molecular formula (i.e. they have the same atoms), but differ in the way these atoms are arranged. There are four types of isomers: Constitutional isomers, Stereoisomers Configurational isomers, and Conformational isomers. Constitutional isomers are isomers where the atoms are linked together in a different skeletal framework and are different compounds. Stereo isomers contains same molecular formula, same bonding sequence, different spatial orientation. Configurational isomers are structures having the same atoms and bonds, but which have different geometrical shapes which cannot be interconverted without breaking covalent bonds. Configurational isomers can be separated and are different compounds with different properties. Conformational isomers are different shapes of the same molecule and cannot be separated.

Types of Stereoisomers Stereo isomers contains same molecular formula, same bonding sequence, different spatial orientation. Two types of stereoisomers: Enantiomers (Mirror image) two compounds that are nonsuperimposable mirror images of each other Diastereomers (Non-Mirror Image) Two stereoisomers that are not mirror images of each other Geometric isomers (cis-trans isomers) are one type of diastereomer.

Example of Stereoisomers The two different arrangements are mirror images of one another, but they cannot be superimposed. Any object that cannot be superimposed on its mirror image is called chiral, that is, it has the property of being right-handed or left-handed.

Examples of Enantiomers Molecules that have one carbon with 4 different substituents have a non-superimposable mirror image Enantiomers = non-superimposable mirror image stereoisomers Build molecular models to see this

Example of constitutional Isomer (Alkanes) Constitutional isomers are compounds which have the same molecular formula but have the atoms joined together in a different way. In other words, they have different carbon skeletons. Constitutional isomers have different physical and chemical properties. Alkanes of a particular molecular formula can exist as different constitutional isomers. For example, the alkane having the molecular formula C4H10 can exist as two constitutional isomers – the straight chain alkane (butane) or the branched alkane (2-methylpropane; Fig. 1). These are different compounds with different physical and chemical properties.

Example of Configurational isomers (Alkenes and cycloalkanes) Configurational isomers are structures having the same atoms and bonds, but which have different geometrical shapes which cannot be interconverted without breaking covalent bonds. Alkenes having identical substituents at either end of the double bond can only exist as one molecule. However, alkenes having different substituents at both ends of the double bond can exist as two possible isomers. For example, 1-butene (Fig.1a) has two hydrogens at one end of the double bond and there is only one way of constructing it. On the other hand, 2-butene has different substituents at both ends of the double bond (H and CH3) and can be constructed in two ways. The methyl groups can be on the same side of the double bond (the cis isomer; Fig. 1b), or on opposite sides(the trans isomer; Fig. 1c). The cis and trans isomers of an alkene are configurational isomers (also called geometric isomers) because they have different shapes and cannot interconvert since the double bond of an alkene cannot rotate. Therefore, the substituents are ‘fixed’ in space relative to each other. The structures are different compounds with different chemical and physical properties.

Example of CONFORMATIONAL Isomer (Alkanes) Conformational isomers are different shapes of the same molecule and cannot be separated. Conformational isomers arise from the rotation of C–C single bonds. There are many different shapes which a molecule like ethane could adopt by rotation around the C–C bond. However, it is useful to concentrate on the most distinctive ones (Fig). The two conformations I and II are called ‘staggered’ and ‘eclipsed’ respectively. In conformation I, the C–H bonds on carbon 1 are staggered with respect to the C–H bonds on carbon 2. In conformation II, they are eclipsed.