SHRI VISHNU ENGINEERING COLLEGE FOR WOMEN::BHIMAVRAM (AUTONOMOUS) DEPARTMENT OF BASIC SCIENCE APPLIED CHEMISTRY/ENGINEERING CHEMISTRY UNIT-6 HAND OUT ON STEREOCHEMISTRY STEREO CHEMISTRY ISOMERS & ISOMERISM Compounds posses same molecular formula but differ from each other in physical or chemical properties and are called isomers and the phenomenon is called isomerism. ISOs = equal Meros = parts When isomerism is due to the difference in the arrangement of atoms within the molecule, without any reference to space, the phenomenon is referred to as “Structural Isomerism”. Structural Isomerism is of five types: 1. Chain Isomers. 2. Position Isomers. 3. Functional Isomers. 4. Metamerism. 5. Tautomerism. Chain Isomerism Isomers are chain isomers when two or more compounds have the same molecular formula but differ in the branching of carbon atoms. For example, we can represent C5H12 as three compounds: CH3CH2CH2CH2CH3– pentane Guess the remaining two structure possible by C5H12 Position Isomerism Isomers are position isomers when the two or more compounds differ in the position of the functional group or substituent atoms. For example, we can represent C3H7OH in two arrangements: CH3CH2CH2OH -Propan-1-ol CH3CHOHCH3 -Propan-2-ol Functional Isomerism Isomers are functional isomers when the two or more compounds have an identical molecular formula but differ in the functional group present. These isomers are the functional isomers. For example, we can represent C3H6O as a ketone and as an aldehyde. Aldehyde Ketone v) Tautomerism Stereo Isomerism Stereoisomerism is a phenomenon in which compounds have the same molecular formula but differ in the relative positioning or orientation of atoms in space. Stereoisomers are the compounds exhibiting stereoisomerism. We can further classify stereoisomerism into: Geometric Isomerism: it is shown by molecules in which their spatial positions are locked to each other due to the presence of a ring structure or a double bond. Optical Isomerism: Two or more compounds which have the same molecular arrangement but differ in the optical activity are optical isomers. Conformational isomerism Optical isomerism Types of Symmetry A meso compound or meso isomer is a stereoisomer with an identical or superposable mirror image i.e., a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereogenic centers, the molecule is not chiral Plane of symmetry A plane of symmetry is an imaginary plane that bisects a molecule into halves that are mirror images to each other. A symmetry plane can also be identified by its Cartesian orientation, e.g., (xz) or (yz). Center of symmetry or inversion center, abbreviated i. A molecule has a center of symmetry when, for any atom in the molecule, an identical atom exists diametrically opposite this center an equal distance from it..