Chapter 6 Chem PDF - Lecture Notes on Isomerism - Dar Al Uloom University

College of Medicine – ‫كلية الطب‬ ISOMERISM College of Medicine – ‫كلية الطب‬ After this lecture, the student will be able to: } Define isomerism } Illustrate the concept of isomerism in organic chemistry } Define isomers } Define constitutional (structural) isomers } Explain how the properties of c...

College of Medicine – ‫كلية الطب‬ ISOMERISM College of Medicine – ‫كلية الطب‬ After this lecture, the student will be able to: } Define isomerism } Illustrate the concept of isomerism in organic chemistry } Define isomers } Define constitutional (structural) isomers } Explain how the properties of constitutional isomers differ } Define stereoisomerism } Explain chiral & a chiral carbon atom } Identify two types of stereoisomers } Explain the cis/trans isomers } Explain the enantiomers College of Medicine – ‫كلية الطب‬ You may have noticed that the structures of some hydrocarbons differ only in the positions of substituents or of multiple bonds Even though both compounds have the formula C4H10, their boiling points and other properties differ Because their structures are different, they are different substances College of Medicine – ‫كلية الطب‬ } } } Isomerism Isomers are compounds that have the same molecular formula but different molecular structures The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic chemistry College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ Butane and 2-methylpropane represent a category of isomers called constitutional isomers, or structural isomers Constitutional isomers are compounds that have the same molecular formula but the atoms are joined together differently College of Medicine – ‫كلية الطب‬ Constitutional isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivity's In general, the more highly branched the hydrocarbon structure is, the lower the boiling point of the isomer will be compared with less branched isomers College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ Because molecules are three-dimensional structures, molecules with the same molecular formula and with atoms joined in exactly the same order may still be isomers Stereoisomers are molecules in which the atoms are joined in the same order but the positions of the atoms in space are different (with different spatial arrangement) Two types of stereoisomers are Diastereomers and Enantiomers College of Medicine – ‫كلية الطب‬ Cis-trans isomers, also known as geometric isomers, have atoms joined in the same order but the spatial orientation of the groups differs. The most common example of cis-trans isomerism occurs in molecules with double bonds. College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ Two arrangements are possible for the methyl groups and hydrogen atoms with respect to the rigid double bond in 2-butene. Cis configuration Trans configuration College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ CHIRALITY A molecule that is not identical to its mirror image is said to be chiral (kyral, from the Greek cheir, meaning “hand”). Stereocenters, or chirality centers: carbon atom bonded to four different groups College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ CHIRALITY College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ The second category of stereoisomerism occurs whenever a central atom has four different atoms or groups attached. Most commonly the central atom is carbon Pairs of molecules that are mirror images and not superimposable are called enantiomers, or optical isomers  These molecules are examples of enantiomers College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ College of Medicine – ‫كلية الطب‬ Unlike other isomers, enantiomers have identical physical properties such as boiling points and densities Enantiomers do, however, behave differently when they interact with other molecules that have asymmetric carbons Many molecules in your body have asymmetric carbons, so each enantiomer can have a different effect on the body College of Medicine – ‫كلية الطب‬ How do the physical and chemical properties of stereoisomers differ? College of Medicine – ‫كلية الطب‬ How do the physical and chemical properties of stereoisomers differ? Cis-trans isomers have different physical and chemical properties from each other. Enantiomers do not have different physical properties, but they do behave differently when they interact chemically with other molecules College of Medicine – ‫كلية الطب‬ BIG IDEA Hydrocarbons with the same molecular formula but different molecular structures can have different properties