Preview Organic Chemistry, 2nd Edition by Bhupinder Mehta, Manju Mehta (2015)

Preview Organic Chemistry, 2nd Edition by Bhupinder Mehta, Manju Mehta (2015) Preview Organic Chemistry, 2nd Edition by Bhupinder Mehta, Manju Mehta (2015) Preview Organic Chemistry, 2nd Edition by Bhupinder Mehta, Manju Mehta (2015) Preview Organic Chemistry, 2nd Edition by Bhupinder Mehta, Manju Mehta (2015) Preview Organic Chemistry, 2nd Edition by Bhupinder Mehta, Manju Mehta (2015)

ORGANIC CHEMISTRY SECOND EDITION BHUPINDER MEHTA Associate Professor Department of Chemistry Swami Shraddhanand College University of Delhi and MANJU MEHTA Associate Professor Department of Chemistry Maitreyi College University of Delhi ORGANIC CHEMISTRY, Second Edition Bhupinder Mehta and Manju Mehta © 2015 by PHI Learning Private Limited, Delhi All rights reserved No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher ISBN-978-81-203-5126-4 The export rights of this book are vested solely with the publisher Ninth Printing (Second Edition) July, 2015 Published by Asoke K Ghosh, PHI Learning Private Limited, Rimjhim House, 111, Patparganj Industrial Estate, Delhi-110092 and Printed by Mohan Makhijani at Rekha Printers Private Limited, New Delhi-110020 To all our Family Members for their affection and our loving daughter Ananta and son Sarthak for being patient and supportive Table of Contents Preface About the Cover Image Acknowledgements About the Authors What This Book Is About Organic Molecules: Structure, Bonding and Properties 1.1 ORGANIC CHEMISTRY—An introduction 1.2 Electronic structure and chemical bonding in organic compounds 1.3 LEWIS STRUCTURE and Chemical Bonds 1.3.1 Electronegativity 1.3.2 Ionic Bond 1.3.3 Covalent Bond 1.3.4 Atomic Radius, van der Waals Radius, Bond Length, and Bond Angle 1.3.5 Formal Charge 1.3.6 Bond Polarity and Dipole Moment 1.4 Concept of Hybridization and Covalent Bonding 1.4.1 sp3 Hybridization 1.4.2 sp2 Hybridization 1.4.3 sp Hybridization 1.5 Writing the structural Formula for Organic Molecules 1.6 CONCEPT OF RESONANCE (Mesomerism) 1.7 INTERMOLECULAR FORCES (van der Waals Forces) 1.7.1 Melting Point and Boiling Point 1.8 Purification and Identification of Organic Compounds 1.9 Classification of Organic Compounds 1.10 ISOMERISM in Organic Molecules 1.10.1 Constitutional Isomers (Formerly Structural Isomers) 1.10.2 Resonance versus Tautomerism 1.11 ACIDS AND BASES 1.11.1 Bronsted and Lowry Definition 1.11.2 Lewis Definition Exercises** Answers to selected exercises IUPAC Nomenclature of Organic Compounds 2.1 Introduction 2.2 IUPAC Nomenclature 2.2.1 Rules for Naming the Organic Compounds 2.3 Selected Examples Of Monofunctional And Polyfunctional Organic Compounds 2.3.1 Writing the Structure of an Organic Compound from its IUPAC Name 2.4 COMMON ERRORS IN WRITING IUPAC NAMES Exercises* Answers Stereochemistry 3.1 INTRODUCTION 3.2 Configurational Isomerism 3.2.1 Concept of Chirality [Asymmetry] 3.2.2 Chirality in Organic Molecules: Enantiomers and Diastereoisomers 3.2.3 Fischer Projection 3.2.4 Number of Stereoisomers of a Compound 3.3 Optical Activity 3.4 Absolute Configuration (R And S Configuration) 3.4.1 Assigning R and S Configuration 3.4.2 Relative Configuration (D- and L- Nomenclature) 3.4.3 Chirality in a Molecule with no Stereogenic (Chiral) Centre 3.5 GEOMETRICAL ISOMERISM 3.6 CONFORMATIONS 3.6.1 Conformations of Ethane 3.6.2 Conformations of Propane 3.6.3 Conformations of Butane 3.7 cycloalkanes: conformations and Geometrical Isomerism 3.7.1 Conformations of Cyclohexane 3.7.2 Conformations of Monosubstituted Cyclohexane 3.7.3 Conformations of Disubstituted Cyclohexane Answers to selected exercises Fundamentals of Organic Reactions 4.1 Electronic Displacements 4.1.1 Inductive Effect 4.1.2 Electromeric Effect 4.1.3 Resonance Effect [or Mesomeric Effect] 4.1.4 Hyperconjugation (No bond resonance) 4.2 REACTIVE INTERMEDIATES 4.2.1 Carbocations 4.2.2 Carbanions 4.2.3 Free Radicals 4.2.4 Carbene 4.2.5 Nitrene 4.3 ReAgent Types 4.3.1 Electrophiles and Nucleophiles 4.4 Types of Reactions 4.5 CHEMICAL ENERGETICS 4.5.1 Thermodynamics and Kinetics of Chemical Reactions 4.5.2 Chemical Equilibrium 4.5.3 Rate of Reaction 4.5.4 Energy Diagrams (or Energy Profile) of Chemical Reactions 4.6 STERIC EFFECT 4.7 Solvents in Organic Reactions 4.8 Organic Compounds as Acid and Bases EXeRCISES EXPLORE MORE (Set-I) lkanes and Cycloalkanes 5A.1 Introduction 5A.1.1 Physical Properties 5a.2 Preparation of Alkanes 5A.2.1 Catalytic Hydrogenation of Alkenes and Alkynes 5A.2.2 From Haloalkanes (Alkyl halides) 5A.2.3 From Carbonyl Compounds (Aldehydes and ketones) 5A.2.4 From Sodium Salt of Carboxylic Acids 5A.3 Chemical Properties of Alkanes 5A.3.1 Halogenation 5A.3.2 Nitration 5A.3.3 Sulfonation 5A.3.4 Chlorosulfonation 5A.3.5 Oxidation Reactions 5A.3.6 Other Reactions 5A.4 Petroleum 5A.4.1 Petrochemicals 5A.4.2 Coal 5B.1 Introduction 5B.2 Strain in Ring Compounds: Baeyer’s Strain Theory 5B.3 Preparation of Cycloalkanes 5B.4 Chemical Properties Of Cycloalkanes 5B.4.1 Halogenation 5B.4.2 Catalytic Hydrogenation 5B.4.3 Effect of Heat 5B.4.4 Reaction with Hydrogen Halides Selected Solved Examples Exercises Alkenes 6.1 Introduction 6.1.1 Physical Properties 6.2 Preparation Of Alkenes 6.2.1 Reduction of Alkynes: Formation of cis and trans Alkenes 6.2.2 Elimination Reactions [Saytzeff’s and Hofmann’s rule] 6.2.3 Other Methods 6.3 Chemical Properties of Alkenes 6.3.1 Stability of Alkenes 6.3.2 Electrophilic Addition Reactions 6.3.3 Free Radical Addition Reaction 6.3.4 Oxidation Reactions 6.3.5 Allylic Substitution Reactions 6.3.6 Polymerization Selected Solved Examples EXERCISES Alkadienes 7.1 Introduction 7.2 Buta-1,3-Diene 7.2.1 Molecular Orbital Picture of Buta-1,3-diene 7.3 Preparation of Buta-1,3-diene 7.4 Chemical Properties of Buta-1,3-diene 7.4.1 Electrophilic Addition Reactions 7.4.2 Free Radical Addition Reactions 7.4.3 Diels–Alder Reaction [Cycloaddition Reaction] 7.4.4 Reduction and Oxidation Reactions 7.4.5 Polymerization 7.5 Isoprene (2-MethylButa-1,3-diene) 7.5.1 Preparation 7.5.2 Chemical Properties 7.6 Chloroprene (2-chlorobuta-1,3-diene) Exercises Alkynes 8.1 Introduction 8.1.1 Physical Properties 8.2 PREPARATION OF ALKYNES 8.3 CHEMICAL PROPERTIES OF ALKYNES 8.3.1 Addition of Hydrogen 8.3.2 Electrophilic Addition Reactions 8.3.3 Nucleophilic Addition Reactions 8.3.4 Reactions Involving Acetylenic Hydrogens 8.3.5 Polymerization Reactions 8.3.6 Isomerization (Acetylene Allene Rearrangement) 8.3.7 Oxidation Reactions Selected Solved Examples EXERCISES Concepts of Aromaticity, Benzene and its Derivatives A Concepts of Aromaticity 9A.2 STRUCTURE OF BENZENE 9A.2.1 Kekule Structure 9A.2.2 Resonance Structure 9A.2.3 Orbital Picture of Benzene 9A.3 RESONANCE ENERGY: STABILITY OF BENZENE 9A.4 HUCKEL’S RULE AND AROMATICITY 9A.5 Aromaticity in BENZENE AND other CYCLIC systems 9A.5.1 Aromaticity and the Three Membered Ring Systems 9A.5.2 Aromaticity and Four Membered Ring Systems 9A.5.3 Aromaticity and Five Membered Ring Systems 9A.5.4 Aromaticity and Six Membered Ring Systems 9A.5.5 Aromaticity and Seven Membered Ring Systems 9A.5.6 Aromaticity and Eight Membered Ring Systems 9A.5.7 Aromaticity and Annulenes 9A.5.8 Aromaticity and Other Ring Systems B Benzene and its Derivatives 9B.1.1 Coal Tar: Source of Aromatic Hydrocarbons 9B.2 Nomenclature of aromatic compounds This is the most stable conformation of cyclohexane as it is free from torsional strain All the twelve hydrogens are in staggered state as evident form Newman projection The bond angle is nearly 109.5° and thus, it is free from angle strain also Boat conformation Twisting about carbon–carbon single bond of the chair form results in the formation of boat conformation Boat conformer is free from angle strain However, in boat conformation the hydrogens are in eclipsed state, which causes torsional strain in the molecule Along with this the hydrogen at C1 and C4 are close to each other and experience van der Waals repulsion known as flagpole interaction The torsional strain and flagpole interaction make boat conformation less stable compared to chair conformation Twist boat conformation The boat conformation is flexible and a slight twist about the bond reduces the torsional as well as flagpole interactions which makes the twist boat conformation a little more stable than the boat conformation Half chair conformation This is the least stable conformation of cyclohexane because carbon atoms at one end of the ring are planar The stability order of different conformations of cyclohexane is: Chair conformation >> twist boat conformation > boat conformation > half chair conformation The conformational anaylsis of cyclohexane is given in Fig 3.16 that follows Fig 3.16 Conformational analysis of cyclohexane Axial and equatorial hydrogens in chair conformation In the chair conformation of cyclohexane, all the twelve hydrogens are not equivalent The chair conformation has two types of hydrogens, axial (a) and equatorial (e) Six hydrogens are present perpendicular to the plane of the ring and are termed as axial hydrogens while the remaining six hydrogens project out sideways, along the plane of the ring and are termed as equatorial hydrogens Each carbon has one axial and one equatorial hydrogen which point in opposite directions The three axial hydrogens are perpendicular in upward direction and three axial hydrogens are perpendicular in downward direction The axial hydrogens point alternatively in upward and downward directions in accordance to the vertices of the cyclohexane ring If the carbon (vertex) of the chair conformation is in upward direction, the axial hydrogen will also be in upward direction, however the equatorial hydrogen will be in downward direction, however the equatorial hydrogen will be in downward direction A flip in chair conformation interconverts the axial and equatorial hydrogens 3.7.2 Conformations of Monosubstituted Cyclohexane In the replacement of hydrogen of a cyclohexane ring by a substituent, the substituent can occupy either an axial or an equatorial position For example, methylcyclohexane can be represented by following two chair conformations: If methyl group occupies an axial position, it is close to axial hydrogens at C3 and C5 (as C5 and C3 are equidistant from C1, these positions are also referred to as and 3′) The van der Waals repulsion occurs due to steric crowding of methyl group and two hydrogens This causes transannular strain The transannular strain is the strain produced in a ring due to steric repulsions This effect occurs due to axial substituents and is known as 1,3diaxial interaction A substituent at equatorial position does not experience any such repulsion as equatorial bonds project out sideways The Newman projection of axial and equatorial conformers of methylcyclohexane clearly depicts the stability of equatorial conformers The equatorial and axial conformers exist in equilibrium However, equatorial conformer is more stable due to absence of 1,3-diaxial interactions Thus, a substituent prefers to occupy an equatorial position With an increase in the bulkier substituent (bigger group), the ratio of equatorial conformer increases considerably and a very little of axial conformer is present at equilibrium 3.7.3 Conformations of Disubstituted Cyclohexane Two substituents present on same carbon The substituent present in a cyclohexane ring prefers to occupy an equatorial position In case two different substituents are present on same carbon, a bulkier substituent occupies equatorial position For example in 1ethyl-1-methylcylohexane, a conformation with ethyl group at equatorial position and methyl group at axial position is more stable than a conformation with methyl group at equatorial position and ethyl group at axial position Two substituents present on different carbons However, if two substituents are present at two different carbons then depending upon the position occupied by each substituent, the different conformers are designated as: equatorial–equatorial(e,e); equatorial– axial(e,a); axial–equatorial(a,e); and axial–axial(a,a) If two similar substituents are present, (e,a) and (a,e) represent the same conformers Cis- and Trans- isomerism in disubstituted cyclohexanes Disubstitued cyclohexanes exhibit cis–trans isomerism due to restricted rotation in cyclic system There are three possible disubstituted cyclohexanes: (i) 1,2-disubstituted cyclohexanes, (ii) 1,3-disubstituted cyclohexanes, and (iii) 1,4-disubstituted cyclohexanes Cyclohexane exists in non-planar chair conformation A planar form of cyclohexane (that is, hexagon) is used for a convenient representation of cis–trans isomers If two substituents are present on the same side, it represents a cis- isomer Thus, cis- and trans- forms of 1,2-, 1,3- and 1,4dimethylcyclohexane are represented in the following manner: Representing cis- and trans- isomers in chair conformation In chair conformations, the two substituents may occupy (e,e), (e,a), (a,e), and (a,a) conformation The substituents may be attached through an axial or an equatorial bond The equatorial and axial bonds may point in upward or downward direction • If both the bonds, through which substituents are attached, point in same direction, that is, both upward or both downward, the conformation represents a cis- isomer • If both the bonds, through which substituents are attached, point in opposite directions, that is, one upward and one downward, the conformation represents a trans- isomer The cis–trans isomerism and the stability of conformers in disubstituted cyclohexanes is explained by considering the examples of 1,2- , 1,3- and 1,4-dimethylcyclohexane as follows: 1,2-Dimethylcyclohexane Different chair conformations possible for 1,2-dimethylcyclohexane are: In the above conformations, (a,a) and (e,e) represent trans- isomers, whereas (a,e) or (e,a) represent cis- isomers 1,3-Dimethylcyclohexane Different chair conformations possible for 1,3-dimethylcyclohexane are: In these conformations, (a,a) and (e,e) represent cis- isomers, whereas (a,e) or (e,a) represent trans- isomers 1,4-Dimethylcyclohexane Different chair conformations possible for 1,4-dimethylcyclohexane are as follows: In the conformations above, (a,a) and (e,e) represent trans-isomer whereas (a,e) or (e,a) represent cis-isomers Stability of conformers in dimethylcyclohexanes In 1,2-, 1,3- and 1,4-dimethylcyclohexane, the stability of conformers is decided on the basis of the following: (i) The (e,e) conformer is the most stable and the most preferred conformation because of the absence of 1,3-diaxial interactions (ii) The (a,a) conformer is the least stable because of 1,3-diaxial interactions that are experienced by both methyl groups (iii) The (a,e) or (e,a) conformers are more stable than (a,a) but less stable than (e,e) because one of the methyl group at axial position experiences 1,3-diaxial interactions Exercises* Classify the following as ‘chiral’ and ‘achiral’ (a) Scissors (b) Shoe (c) Hammer (d) Nail (e) Screw (f) T-shirt (g) Foot (h) Fork (i) Nose In the following compounds, indicate the stereogenic (or chiral) centre by putting an asterisk mark over it (a) CH3CH(Br)CH2OH (c) CH3CH(Cl)CH(Br)CH3 (b) C6H5CH(OH)CH3 (d) CH3CH2CH(Cl)CH(CH3)CH3 What are stereoisomers? Define the terms enantiomers and diastereomers Comment on the physical and chemical properties of enantiomers and diastereomers What is optical activity? What is the necessary condition for a molecule to be optically active? What are meso compounds? Are the meso compounds optically active or inactive? Explain your answer through a suitable example What is general formula for determining the number of stereoisomers possible for a given compound? Indicate the number of stereoisomers possible for each of the following: (a) CH3CH(NH2)COOH (b) CH3CH2CH(OH)CH2CH3 (c) CH3CH2CH(Br)CH(Cl)CH3 (d) CHOCH(OH)CH(OH)CH(OH)CH2OH Draw the Fischer projections for all possible stereoisomers of: (a) C6H5CH(NH2)COOH (b) HOOCCH(CH3)CH(CH3)CH2OH 10 In accordance with the sequence rules, assign the decreasing order of priority to following groups/atoms (a) –CH3, –OH, –NH2, Cl (b) –Br, –CH2Br, –CH2CH3, —SH (c) –Cl, –Br, –F, –CH2I (d) –CH==CH2, –CH2CH==CH2, —CH2CH2CH3, —COOH (e) –Cl, –COCH3, CONH2, COCl (f) –COOH, –CH2COOH, –CH(Cl)COOH, —CH3 (g) –OCH3, –OH, –NH2, –F (h) –H, –D, –CH3, –T (i) –CH2CH3, –CH3, –CH2OH, –H (j) –CH2OH, –CH2COOH, –CH3, –H 11 Assign the R and S configuration to the following: 12 What are conformations? How does it differ from configuration? 13 Draw the Newman projection for different conformations possible for butane Give the conformational analysis for butane 14 What is dihedral angle? What is the dihedral angle in staggered and eclipsed Newman projections of ethane? 15 What are ‘skew’ conformations? Can all the skew conformations be termed as ‘gauche’ conformations? 16 What you understand by the following terms: (a) Torsional strain (b) Angle strain (c) Ring strain 17 Draw the conformations for cyclopropane Comment on the low stability of cyclopropane 18 Draw the different conformations for cyclohexane and arrange them in increasing order of stability 19 In monosubstituted cyclohexanes, why does a substituent prefer to occupy an equatorial position? 20 Draw the chair conformations of: (a) cis-1,2-dibromocyclohexane (b) trans-1,3-dimethylcyclohexane (c) cis-1,3-dichlorocyclohexane (d) cis-cyclohexane-1,3-diol (e) trans-1,4-diethylcyclohexane (f) cis-1-propyl-4-methylcyclohexane 21 In cyclohexane-1,3-diol, the (a,a) conformation is more stable compared to (e,e) conformation Explain 22 Designate ‘E’ & ‘Z’ to the following double bonded compounds: Answers to selected exercises Chiral: (a), (b), (e), (g) Achiral: (c), (d), (f), (h), (i) 2n where n = number of chiral centres (a) (b) None, its achiral (c) (d) 10 (a) Cl > OH > NH2 > CH3 (b) Br > SH > CH2Br > CH2CH3 (c) Br > Cl > F > CH2I (d) COOH > CH==CH2 > CH2CH==CH2 > CH2CH2CH3 (e) Cl > COCl > CONH2 > COCH3 (f) –CH(Cl)COOH > COOH > CH2COOH > CH3 (g) F > OCH3 > OH > NH2 (h) CH3 > T > D > H (i) –CH2OH > –CH2CH3 > –CH3 > H (j) CH2OH > CH2COOH > CH3 > H 11 (a) S (b) R (c) R (d) R (e) 2R, 3R (f) 22 (a) E (b) E (c) E (d) Z (e) Z * Answers to selected exercises are given at the end ... us BHUPINDER MEHTA MANJU MEHTA About the Authors Dr BHUPINDER MEHTA and Dr MANJU MEHTA (husband and wife) Enjoying the moments after accomplishing the venture of second edition Bhupinder Mehta. .. Delhi ORGANIC CHEMISTRY, Second Edition Bhupinder Mehta and Manju Mehta © 2015 by PHI Learning Private Limited, Delhi All rights reserved No part of this book may be reproduced in any form, by mimeograph.. .ORGANIC CHEMISTRY SECOND EDITION BHUPINDER MEHTA Associate Professor Department of Chemistry Swami Shraddhanand College University of Delhi and MANJU MEHTA Associate Professor