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Aromaticity and Stereochemistry

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Aromaticity and Stereochemistry Lecture-1 Department of Chemistry INTRODUCTION Aromaticity and Stereochemistry Dr. L. Satish K. Achary Assistant Professor Syllabus for Organic Chemistry Unit- I: Aromaticity and Stereochemistry Aromaticity and anti-aromaticity, Hückel’s rule, anti-aromaticity, y-aromaticity, homo-aromaticity, Cyclodextrins, catenanes and rotaxanes. Stereochemistry: Conformational analysis of cycloalkanes, effect of conformation on reactivity. Elements of symmetry, Chirality of organic molecules with or without chiral centres, molecules with more than one chiral center, projection formulae (i) Fischer (ii) Sawhorse (iii) Newman (iv) Flying Wedge; Homotopic, enantiotopic and diastereotopic atoms, groups and faces. Stereospecific and stereoselective synthesis. Asymmetric synthesis. Optical activity in the absence of chiral carbon. Unit- II: Structure and Reactivity Structure and Reactivity: Types of mechanisms, types of reactions, thermodynamic and kinetic requirements, Hammond postulate, Curtin-Hammett principle, transition states and intermediates, methods of determining mechanisms, isotopic effects. Generation, structure, stability and reactivity of carbocations, carbanions, free radicals, carbenes and nitrenes. Effect of structure on reactivity. The Hammett equation and linear free energy relationship (sigmarho) relationship, Taft equation. 16-12-2020 2 www.youtube.com/c/powerupwithpowerpoint Unit-III: Aliphatic Nucleophilic and Electrophilic Substitution Aliphatic Nucleophilic Substitution: The SN2, SN1, mixed SN1 and SN2 and SET mechanisms. Reactivity effects of substrate structure, attaching nucleophile, leaving group and reaction medium. The neighbouring group mechanism. The SNi mechanism. Nucleophilic substitution at an allylic, aliphatic trigonal and a vinyl carbon. Aromatic Nucleophile Substitution: The SNAr, SN1, benzyne and SRN1 mechanisms. Reactivity; effect of substrate structure, leaving group and attacking nucleophile. Aliphatic Electrophilic Substitution: Bimolecular mechanisms: SE1, SE2 and SEi. The SE1 mechanism, electrophilic substitution accompanied by double bond shifts. Effect of substrates, leaving group and the solvent polarity on the reactivity. Aromatic: The arenium ion mechanism, orientation and reactivity, energy profile diagrams. The ortho/para ratio, ipso attack, orientation in other ring systems. Quantitative treatment of reactivity in substrates and electrophiles. Unit-IV: Addition reactions Addition to Carbon-Carbon Multiple Bonds: Mechanistic and stereochemical aspects of addition reactions involving electrophiles, nucleophiles and free radicals, regio- and chemo-selectivity, orientation and reactivity. Addition to cyclopropane ring. Hydroboration. Addition to Carbon-Hetero Multiple Bonds: Mechanism of metal hydride reduction of saturated and unsaturated carbonyl compounds, acids, esters and nitriles. Addition of Grignard reagents, organozinc and organolithium reagents to carbonyl and unsaturated carbonyl compounds. 16-12-2020 3 www.youtube.com/c/powerupwithpowerpoint Unit-V: Free Radical and Elimination Reactions Free Radical Reactions: Types of free radical reactions, free radical substitution mechanism, mechanism at an aromatic substrate, neighbouring group assistance. Reactivity for aliphatic and aromatic substrates at a bridgehead. Reactivity in the attaching radicals. The effect of solvents on reactivity. Elimination Reactions: The E2, E1 and E1cB mechanisms and their spectrum. Orientation of the double bond. Reactivity; effects of substrate structures, attaching base, the leaving group and the medium. 16-12-2020 4 www.youtube.com/c/powerupwithpowerpoint Books Jagdamba Singh and L.D.S Yadav Advanced Organic Chemistry, Pragati prakashan. G. S. Zweifel and M. H. Nantz. “Modern Organic Synthesis-An Introduction”,W. H. Freeman and Company, 2006. March’s Advanced Organic Chemistry Reactions, Mechanisms, And Structure Wiley-interscience A John Wiley & Sons, Inc., Publication. Trost B. M. and Fleming I. Comprehensive Organic Synthesis, Pergamon Press. 16-12-2020 5 www.youtube.com/c/powerupwithpowerpoint Aromaticity 16-12-2020 3 C. V. Raman Global University, Odisha Contents 01 Introduction 02 Aromatic antiaromatic and non-aromatic 03 Huckel's rule 04 Annulene 05 Homoaromaticity 06 Some examples 16-12-2020 1 C. V. Raman Global University, Odisha Introduction Aromaticity Aromatic Non-aromatic Antiaromatic 16-12-2020 2 C. V. Raman Global University, Odisha Conditions Cyclic and Planar Conjugated π system sp2 or sp hybridized ring carbon Ring atom with unhybridized p-orbital in continuous overlapping. Huckel’s rule Energy and stability 16-12-2020 2 C. V. Raman Global University, Odisha Cyclic and Planar 16-12-2020 10 C. V. Raman Global University, Odisha Conjugated π system 16-12-2020 11 C. V. Raman Global University, Odisha sp2 or sp hybridized ring carbon 16-12-2020 12 C. V. Raman Global University, Odisha Ring atom with unhybridized p-orbital in continuous overlapping 16-12-2020 13 C. V. Raman Global University, Odisha Huckel’s rule If the compound is cyclic, planar, fully conjugated & posses (4n +2) π electrons, then it is an aromatic compound. If the compound is cyclic, planar, fully conjugated & posses 4n π electrons, then it is an anti-aromatic compound. where n is any integer like 1,2,3,…….. 16-12-2020 14 C. V. Raman Global University, Odisha Aromatic Antiaromatic Non-aromatic Cyclic and Planar Cyclic and Planar Cyclic/Acyclic Conjugated π system Conjugated π system Planar/Non-planar sp2 or sp hybridized sp2 or sp hybridized May follow 4n+2 or 4n ring carbon ring carbon rule Ring atom with Ring atom with Conjugate/Non-conjugate. unhybridized p-orbital in unhybridized p-orbital in continuous overlapping. continuous overlapping. 4n+2 π system 4n π system More energy than aromatic and less than Low energy and high High energy and low antiaromatic stability stability 16-12-2020 4 C. V. Raman Global University, Odisha N.A A.A A A.A A A N.A A.A A 16-12-2020 N.A A.A 16 A C. V. Raman Global University, Odisha Cyclooctatetraene is supposed to be antiaromatic but because of the nonplanar structure, it is non-aromatic. 16-12-2020 17 C. V. Raman Global University, Odisha Tropone is aromatic due to electron withdrawing nature of carbonyl group and system will be conjugated with positive charge of carbonyl carbon with 6 pi-electrons. 16-12-2020 18 C. V. Raman Global University, Odisha Pyrrole and Pyridine 16-12-2020 19 C. V. Raman Global University, Odisha 16-12-2020 20 https://chem.libretexts.org/Courses/Purdue/Purdue%3A_Chem_26200%3A_Organic_Chemistry_II_(Wenthold)/Chapter_14.__Aromaticity/1C. V. Raman Global University, Odisha 4.04%3A_MO_description_of_aromaticity 16-12-2020 21 https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(McMurry)/C. V. Raman Global University,15% 3OdishaA_Benzene_and_Aromaticity/ 15.06%3A_Aromatic_Heterocycles-_Pyridine_ Furan 16-12-2020 22 C. V. Raman Global University, Odisha Annulene Annulenes are monocyclic compounds with alternative single and double bonds. [4n] annulene = Antiaromatic [4n+2] annulene = Aromatic 16-12-2020 23 C. V. Raman Global University, Odisha Annulene [10] annulene is supposed to be aromatic but because of close proximity between the 2H’s ring can’t remain in one plane. [10] annulene is non-aromatic and called as puckering of the ring. 16-12-2020 24 C. V. Raman Global University, Odisha Homoaromatic In a molecule, if delocalization of π cloud or π conjugation by passes sp3 atoms of ring & possess Aromaticity called homoaromatic molecule. 16-12-2020 25 C. V. Raman Global University, Odisha 16-12-2020 26 C. V. Raman Global University, Odisha Cyclodextrins Cyclodextrins are macrocyclic oligosaccharides containing at least six D-(+)- glucopyranose units attached by α (1-4) bonds. One of the important feature of cyclodextrins is their ability to form inclusion complexes with a variety of compounds, by entrapping their molecules inside the cyclodextrin cavity, which act as a host. The smallest cyclodextrin is α-cyclodextrin which has a diameter about double to that of 18-crown-6. 16-12-2020 27 C. V. Raman Global University, Odisha The complex formed results- Increased solubility Increased dissolution rate Increased stability Decreased volatility Cyclodextrin are relatively hydrophobic central cavity and hydrophilic outer surface. 16-12-2020 28 C. V. Raman Global University, Odisha 16-12-2020 29 https://www.youtube.com/watch?v=yDyhBEUKqcU C. V. Raman Global University, Odisha Types of Cyclodextrin α -cyclodextrin: 6 membered sugar ring molecule Relatively irritating after Intramuscular injection Binds lipids. 16-12-2020 30 C. V. Raman Global University, Odisha Types of Cyclodextrin β-cyclodextrin: 7 membered sugar ring molecule Relatively less irritating after Intramuscular injection Binds cholesterol. 16-12-2020 31 C. V. Raman Global University, Odisha Types of Cyclodextrin γ-cyclodextrin: 8 membered sugar ring molecule 16-12-2020 32 C. V. Raman Global University, Odisha 16-12-2020 33 C. V. Raman Global University, Odisha Characteristics- It is a white in colour Practically odorless Fine crystalline powders Slightly sweet taste It is chemically inert 16-12-2020 34 C. V. Raman Global University, Odisha Synthesis- The production of cyclodextrin is relatively simple and involves treatment of ordinary starch with a set of enzymes. Commonly cyclodextrin glucosyltransferase (CGTase) is employed along with α- amylase. First starch is liquefied either by heat treatment or using α-amylase,
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