Note for Engineering Chemistry - EC By Mallikarjun Baldha

Topics ( 4 )

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UNIT-4. STEREOCHEMISTRY, REACTION MECHANISM & DRUGS STEREOCHEMISTRY: ➢ The study of the three-dimensional structure of molecules ➢ Stereochemistry plays an important role in determining the properties and reactions of organic compounds ➢ The properties of many drugs depend on their stereochemistry ➢ Enzymes are capable of distinguishing between stereoisomers ISOMERISM: The compounds having the same molecular formula but showing different properties is known as isomerism. Such compounds are called as isomers. The following flow chart shows different types of isomerism. ISO MERS CO NSTITUTIO NAL (STRUCTURAL) ISO MERISM STEREO ISO MERISM CO NFIGURATIONAL ISO MERS CHAIN ISOMERISM PO SITION ISO MERISM GEO METRICAL ISO MERISM CO NFORMATIO NAL ISO MERS O PTICAL ISOMERISM FUNCTIO NAL GROUP ISO MERISM CIS ENANTIO MERS METAMERISM TRANS DIASTEREO MERS TAUTO MERISM ISOMERS: Compounds that have the same molecular formula but do not have identical structures. I.B.TECH ENGINEERING CHEMISTRY 1BM ACE ENGINEERING COLLEGE

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STRUCTURAL (CONSTITUTIONAL) ISOMERS: Isomers have same molecular formula but have different arrangements of atoms within the molecule. Structural isomerism can be further classified as: 1. Chain isomerism Chain isomerism is a type of structural isomerism where the isomers have same molecular formula but they differ in the order in which the carbon atoms are bonded to each other. 2. Position isomerism Position isomerism is a type of structural isomerism where the main carbon skeleton is same but they differ in the position of functional group attached to it. 3. Functional group isomerism Functional group isomerism is a type of structural isomerism where isomers have same molecular formula but differ in functional group. 4. Metamerism Metamerism is a type of structural isomerism where the isomers have same molecular formula but differ due to the different number of carbon atoms or alkyl groups on either side of functional group ( i.e., -O-,-S-, -NH-, -C(=O)-). I.B.TECH ENGINEERING CHEMISTRY 2BM ACE ENGINEERING COLLEGE

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5. Tautomerism Tautomerism is a special type of structural isomerism where the isomers stays in dynamic equilibrium with each other by simple proton transfer in an intramolecular fashion. STEREOISOMERS: Compounds having same molecular formula, same sequence of bonding of atoms but differ in their three-dimensional orientation of atoms (or) groups in space are called stereoisomers. This phenomenon is called as stereoisomerism. Stereoisomerism are of two types: (i) Geometrical isomerism - (ii) Optical isomerism. l. Geometrical Isomerism: Isomerism that arises out of difference in the spatial arrangement of atoms (or) groups about the doubly bonded carbon atoms is called Geometrical isomerism. These isomers are not mirror images of each other(Diastereomers). Cis-trans isomerism: Differ in the arrangement of their atoms in space (cannot interconvert)- Alkenes. The E-Z convention for Cis-Trans Isomers: I.B.TECH ENGINEERING CHEMISTRY 3BM ACE ENGINEERING COLLEGE

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II. Optical isomerism: Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have non-superimposable mirror images. Optical activity: when a substance rotates the plane of plane polarized light. Plane polarized light – light that has been passed through a Nicol prism or other polarizing medium so that all of the vibrations are in the same plane. Polarimeter – an instrument used to measure optical activity. dextrorotatory – when the plane of polarized light is rotated in a clockwise direction when viewed through a polarimeter. (+) or (d) levorotatory – when the plane of polarized light is rotated in a counter-clockwise direction when viewed through a polarimeter. (-) or (l) Configuration: The arrangement in space of the four different groups about a chiral center. How do we show configurations? Br F Br Cl H Cl F “wedge” formulas I.B.TECH ENGINEERING CHEMISTRY H Fischer projections “cross structures” 4BM ACE ENGINEERING COLLEGE