126432165 Mechanism in Advance Organic Chemistry

7/29/2019 126432165 Mechanism in Advance Organic Chemistry

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This pageintentionally leftblankCopyright 2008, New Age International (P) Ltd., PublishersPublished by New Age International (P) Ltd., PublishersAll rights reserved.No part of this ebook may be reproduced in any form, by photostat, microfilm,xerography, or any other means, or incorporated into any information retrievalsystem, electronic or mechanical, without the written permission of the publisher.All inquiries should be emailed to [email protected] FOR ONE WORLDNEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS4835/24, Ansari Road, Daryaganj, New Delhi - 110002Visit us at www.newagepublishers.comISBN (13) : 978-81-224-2647-2PREFACEFor a very long time the need was felt by graduate and postgraduatestudents ofChemistry of almost all colleges and of I ndian Universities for a book dealingwithadvanced mechanistic organic chemistry written in understandable languageand

with suitable examples which can be easily grasped to make their concept clear.Besides students, this has also been the requirement of teachers teaching advancedorganic chemistry.Till about 1959 there appears to be the only book by E.S. Gould (Structure andMechanism of Organic Chemistry) but the examples mentioned in it are so difficultat several places that they elude the comprehension of even teachers,not to talkof students. Around sixties appeared the book by J erry March (AdvancedOrganic

Chemistry, Reactions, Mechanism and Structure). I t was definitely a much betteradvance over that of Gould, but it has been made so bulky that its cost has becomeprohibitive. I t adores the racks and shelves of libraries. I n view of the above difficultiesof teachers and students, the present book has been brought out.Some of its special chapters are the Pericyclic Reactions, which includes Cheletropic,Electrocyclic, Sigmatropic and Cycloaddition reactions. The concept of Stereochemistryand Conformation deserve special attention not because they cater to the needs of

higher students, but they are immensely useful for candidates trying for UGC andCSI R sponsored competitive examinations, but also those preparing for Union PublicService Commission and State Public Service Commission Exams. The candidateswill find the chapters immensely useful and is sure to rouse interestin them in

knowing more about mechanistic chemistry.Spectroscopy is another topic, a good knowledge of which is expected from any good


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chemist. The spectroscopy finds immense applications in the identification of severalunknown compounds in the field of research and medicine. Here one has to be verycareful in elucidating the correct structure of entities. Moreover in every examination(competitive or regular) one has to show his skill in the area of spectroscopy andtherefore chapters on spectroscopy giving a clear and lucid account is also noteworthy.(v)To the best of my knowledge, a book written by an I ndian author and covering allthe chapters covered in it is not known.Part I deals with chapter involving typical complex organic reactions. Part I Ideals

with various aspects of spectroscopy and the use of this technique in identificationof compounds. Miscellaneous reagents used in organic synthesis are discussed inPart I I I .The author is immensely grateful to Prof. R.P. Rastogi for writing the forward forthis volume.R.P. Narain

(vi)FOREWORDThe present book deals with mechanistic principles of organic reactionsin an

exhaustive and systematic manner. I t would be equally useful for thestudy ofcomplex reactions where inorganic reactions are also involved in the reaction networkincluding biochemical reactions. The author had long teaching and researchexperience in Gorakhpur University. The subject matter has been clearly presented.The book is divided into three parts. Part I deals with typical com

plex organicreactions such as (i) reactions involving carbocations and carbanions, (ii) Pericyclicand electrocyclic reactions and (iii) Sigmatropic and Chelotropic reactions. This partalso includes material useful for characterization of products from structural pointof view such as Geometrical isomerism, Stereochemistry and Conformation. Part IIis concerned with spectroscopic methods of structure determination suchas U.V.,I .R., N.M.R. and Mass spectroscopy, which is quite useful from the angle of

identification and characterization of reaction intermediates. I n partI I I , detailedtreatment of commonly used oxidizing and reducing agents along with involvedreaction mechanism has been presented.Treatment of each part is comprehensive and thorough. The book wouldbe usefulfor teaching at post-graduate level and also as reference book for undergraduatelevel. I t would be extremely useful for reference for research scientists in co


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nnectionwith organic synthesis, reaction mechanism of complex reactions involving inorganicand organic reactions and also bio-chemical reactions.R.P. Rastogi(vi i )This pageintentionally leftblankCONTENTSPreface (v)Foreword (vii)PART ITYPICAL COMPLEX ORGANIC REACTIONSChapter 1. Carbocations (Carbonium Ions) 311Nomenclature 3Structure and Stability 4Generation of Carbocations 8Reactions of Carbocations 8 Reactions with Nucleophiles Elimination Reactions Rearrangement Reactions Addition to Unsaturated SystemsNon-Classical Carbocations 10Generation of a Non-classical Carbocations 11

Chapter 2. The Carbanion Chemistry 1220Stability 12Generation of Carbanion 13 Abstraction of Hydrogen by a Base From Unsaturated CompoundsReactions of Carbanions with Electrophiles 15 Substitution Reactions of Alkyl Anions Addition Reactions of Alkyl Anions Reactions of Carbanians with Electrophiles Elimination Reactions Oxidation Reactions of CarbanionsRearrangement Reactions 19 I ntermolecular Rearrangements

I ntermolecular Additions I ntramolecular Eliminations Sigmatropic Carbanion Rearrangement(ix)(x)Chapter 3. Theory of Concerted Reactions 2129What are Concerted Reactions ? 21 Concerted Mechanism Stepwise MechanismStereoselective and Stereospecific Reactions 23Theory of Concerted Reactions 24Homo and Lumo Structures 25 Ethylene

Butadiene SystemFrontier Orbital Approach 28 Conjugated TrieneChapter 4. Pericyclic Reactions 3054I ntroduction 30Cycloaddition Reactions 31 Classification Classification based on the Number of ElectronsGeneral Rule for Pericyclic Reactions 32What is Symmetry Allowed and Symmetry Forbidden Reactions 33


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Woodward Hoffmann Rule Bond involved in Cycloaddition Reaction Some more Example of 2 + 2 Cycloaddition Photochemical Cycloaddition 2 + 3 Cycloaddition 2, 1 Cycloaddition 4 + 2 Cycloaddition Diel -Alder ReactionScope of Diel -Alder Reaction 44 Stereochemi try of Endo and Exo Addition Orientation (Regio electivity) Diene Reactivity Dienophile Reactivity I ntra Molecular Diel -Alder ReactionChapter 5. Electrocyclic Reaction 5570I ntroduction 55Thermal Electrocyclic Reaction 57 2 Systems 4 System 6 System(xi )Electrocyclic Reaction of Radicals 67Metal Catalysed Electrocyclic Reactions 68Photochemical Electrocyclic Reactions 69Cha ter 6. Sigmatro ic Rearrangements 7193

I ntroduction 71 NomenclatureThe Homo of Allylic Radicals 72 Hydrogen MigrationsOther Views about Sigmatro ic Transition State 74 Formation of a Cyclic Transition State Structure Frontier Orbital A roach Some Exam les of Hydrogen Shifts Migrations in Cyclo ro ane rings Migrations of Atoms or Grou s other than HydrogenSelection Rules 79 Shifts in Neutral S ecies Shifts in Charged S ecies

Shifts in Electron Deficient Centers A lication of FMO MethodCo e and Claisen Rearrangements 83 Co e Rearrangement Oxy-Co e Rearrangement Claisen Rearrangement Some Other Exam lesAbnormal Claisen Rearrangement 91 [2, 3] Sigmatro ic RearrangementPhotochemical Rearrangements 92 The Ene ReactionsCha ter 7. Cheletro ic Reactions 94100I ntroduction 94

Symmetry Allowed Cycloaddition of SO2to a diene

Cheletro ic Reactions involving 4 Electrons Addition of Carbenes or Nitrenes Reactions involving 6 Electrons Cycloaddition and Elimination Reactions involving 8 Electroncs(xi i )Cha ter 8. Geometrical Isomerism 101120


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General I ntroduction 101 Com ounds having Double Bonds Relative Stabilities of I somersNomenclature of I somers 105Determination of Configuration of Geometrical I somers 106 Physical Methods of Determination of ConfigurationsStability and I nter Conversion of cis-trans I somers 111 Pre aration of cis-trans I somers by Physical and Chemical MethodsStereochemistry of Addition to Double Bonds 114 Cis Addition Trans AddtionStereochemistry of Cyclic Com ounds 120Cha ter 9. Basic Conce ts of Stereochemistry 121157I m ortance of Sterochemistry 121Asymmetry and Chirality 123 Diasteroisomers Pro erties of DiastereoisomersWhat Com ound dont show O tical Activity 125 Plane of Symmetry Centre of Symmetry Alternating Axis of SymmetryNomenclature of O tical I somers 129 Fischer ProjectionsStereoisomerism in Cyclic Com ounds 137

Methods of Determining Absolute Configuration 139Asymmetric Synthesis 144 Asymmetric Transformation Mechanism of Asymmetric Synthesis Absolute Asymmetric SynthesisStereoselective and Stereos ecifc Reactions 148Resolution of Racemic Mixtures 149Racemisation 152 Mechanism of Racemisation Walden I nversion Mechanism of Walden I nversion(xi i i )Cha ter 10. Conformation 158194

Conformation and Conformational Analysis 158 Conformation of Ethane Conformation of Pro ane Conformation of Butane Ecli sed and Staggered Forms Ring Strains in CycloalkanesPrinci les of Conformation 165 Rotation about Bonds involving carbon aatoms in a state of s2hybridisation

Methods for Determining Confirmation 167Conformational Asymmetry 172Conformations of Three to Five Memmbered Rings 173

Stereochemistry of Cyclic Com ounds 175 Cyclo ro ane Rings Cyclobuane Rings Cyclo entane rings Cyclohexane Rings Monosubstituted Cyclohexanes Disubstituted Cyclohexanes 1, 3 Disubstituted CyclohexanesConformational Energies of Substitutents 185Conformation and Chemical Reactivity 185


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Six Membered Heterocyclic Rings 187Cyclooctane and Cyclononane 188Cyclodecane 189Conce t of I Strain 190Bayers Strain Theory 190Limitations of Strain Theory 192Fused Ring Systems 193Stability of Ring Systems 194Cha ter 11. Some Novel Reactions in OrganosiliconChemistry 195209Ex edited Organic Synthesis with Organosilicon Com ounds 195Creation of a Bond 196Protection of Grou s 197Activation 197Some A lications of Organosilicon Com ounds to Organic Synthesis 198 Why Silicon Com ounds are Reactive(xi v) Formation and Cleavage of Bonds to Silicon Organosilicon Protecting Grou s Electro hilic Substitution Reactions Synthesis of Olefins Reactive I ntermediates in Organosilicon Chemistry Silylenes Silenes

Silanones Disilenes Silyl Amino Silyl Radicals Thermolysis Photolysis, Pyrolysis and Electron TransferPART IISPECTROSCOPYCha ter 12. General Considerations 211213General Consideration 211S ectrosco y 212 Ty es of Molecular S ectraLasers, I m ortance and Uses 213Cha ter 13. Ultraviolet S ectrosco y 214224

Ultraviolet and Visible S ectrosco y 214Ultraviolet S ectrosco y 216 Origin of U.V. Absor tion S ectra Colour in Organic Com ounds Absor tion in Organic Molecules Energy Levels in Different Molecular Orbitals Difference between n * and * transition Choice of Solvent and Solvent EffectsA lication of U.V. S ectrosco y 223Cha ter 14. Infrared Absor tion S ectrosco y 225241I ntroduction 225Origin of I nfrared S ectra 227 Molecular Vibrations

Ty es of Vibrations in Polyatomic Molecules 228(xv) Vibrational Degrees of Molecule Factors I nfluencing Vibrational FrequenciesI nstruments 236 Source of Radiation Sam ling Techniques Finger Print RegionA lications of S ectrosco y 240 I R S ectra of Some Com ounds


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Cha ter 15. Nuclear Magnetic Resonance S ectrosco y 242263I ntroduction 242Hydrogen Nuclei 245Chemical Shifts and Number of Signals 245 I nstrumentation Ex ression of Chemical Shifts Acetylene in Magnetic Field Benzene in Magnetic Field Factors I nfluencing Chemical Shift Solvents Used S litting of Signals and S in-s in Cou ling Origin of Multi let Peaks Cou ling Constants Vicinal Cou ling I nter renation of NMR S ectraA lications of NMR S ectrosco y 26013C NMR S ectrosco y 26231P Phos horus 26311B Boron 263Cha ter 16. Mass S ectrometry and Lasers 264271I ntroduction 264

A lications of Mass S ectrometry 264Some I m ortant Points in Mass S ectrometry 265 Mass S ectrometer Princi le of O eration I nter retation of Some S ectra Mass S ectrum of Benzene, of 2,2,4 Trimethyl PentaneSome Binding Princi les for finding the Base Peaks 270 Use of a S ark Sam le(xvi )PART IIIMISCELLANEOUS REAGENTS USED

IN ORGANIC SYNTHESESCha ter 17. Some Oxidising Agents And their Mechanism 275284Ozone 275

Lead tetra-acetate 276Periodic Acid 278Osmium Tetraxide 279Selenium Dioxide 281Perbenzoic Acid 282Cha ter 18. Some Reducing Agents and their Mechanism 285297Lithmium Aluminium Hydride 285Sodium BoroHydride 288Metal/Liquid Ammonia 290Some Metal Catalysts used in Hydrogenation 292Raney Nickel Catalysts 293Platinum and Platinum Black 294Palladium 295

Aluminium I so ro oxide 296Cha ter 19. Miscellaneous Com ounds 288314Diazomethane 298Liquid Ammonia 300N-Bromosuccinimide 304Trifluoroacetic Acid 307Trifluroacetic Anhydride 308Sodium Amide (Sodamide) 309Dimethyl Sul hoxide (DMSO) 311Boron Trifluoride 313


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Index 315-317PART-ITYPICAL COMPLEXORGANIC REACTIONSThis ageintentionally leftblank

CHAPTERCarbocations (Carbonium Ions)The carbonium ions are ositively charged highly reactive intermediatesand have six electronsin the outer shell of central carbon. They may be re resented asC RHH+C H

HRCRHH+120+So they are very reactive toward any nucleo hilic reagent i.e., one that will donate a air ofelectrons to carbon to form a fourth covalent bond. Being ositivelycharged they are stabilized

inductively by electron-donating substances and rendered less stable byelectron-withdrawingsubstituents.Since carbon of a carbonium ion is in an s2hybrid, the ion air is lanar with a vacant orbital(as in ethylene) which is er endicular to the lane of three covalent bonds to the carbon.NOMENCLATUREAs early as 1902 Gomberg suggested that the word carbonium was ina ro riate for suchs ecies because onium signifies a covalency greater than that of the neutral atom.But the nomenclature

went on till about 1964 when George Olah and his coworkers found evidence by mass s ectrosco ythe existence of a ositively charged intermediate, theCH5+ion. It was su osed to have been

formed by the attack of H+on CH bond to give the s ecies


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CH5+also called the methanonium

ion.CH H + H3

H C3HH+CH3H2+It has a three centre, two electron bond. In this the formal covalency of carbon is five ratherthan three.On this basis Olah ro osed that the word carbonium ion should be reserved for such enta-

34 MECHANI SMS I N ADVANCED ORGANI C CHEMI STRYcoordinated s ecies, while other ositively charged s ecies originally called carbonium ions be giventhe name carbenium ions. Olah also suggested the nomenclature carbocations to encom ass boththe ty e of s ecies and the IUPAC has acce ted this ro osal.Although entacovalent ions are much rarer than other ositively charged ions, the general wordcarbocation is used for all such s ecies. It is interesting to notethat another s ecies, the ethanoniumion2 5

C Hhas also been re orted in the gas hase by IR s ectrosco y.

So now the word carbocation is used to cover all such electron deficient and ositively chargeds ecies, although some authors still refer to use the word carboniumions.The difference between carbenium and carbonium ions is summarized as follows:Carbocation ro erty Carbenium ions Carbonium ionsExam leCH

3

CCH3CH3CH5Number of covalent 3 5


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bonds to carbonNo. of electrons 6 8in outer shellElectron deficiency electron not electrondeficient deficientThe sim lest exam le of carbenium ion isCH3which is so unstable that it is rarely formed even

in gaseous hase. The more stable ones have been re ared in solutionand in some cases even in the

solid state. When in solutions they are formed in olar solvents where they getsolvated. They may alsobe associated with a negative ion called gegenion. The ion airs areformed in non olar solvents.STRUCTURE AND STABILITYThe carbocations being reactive intermediates have short life and so they are unstable. But theyhave been rendered stable after being tra ed in strongest su er acidswhich are mixtures of

fluorosul huric and antimony entafluoride dissolved in SO2or chlorofluorosul huric acid SO

2CIF.Under these conditions their structure and stability have been correlated.In general, the greater the resonance and hy er-conjugation the greateris the stability of

carbocation. The stability also de ends on the field strengths. The following exam les illustrate thisoint.1. Stability de ending on the number of canonical formsWe know that among the sim le carbocations the order of stability istertiarycarbocation

secondarycarbocationrimarycarbocation6 MECHANI SMS I N ADVANCED ORGANI C CHEMI STRYCH2CH CH2CH2

CH CH2allyl cations(Two canonical structures)So the order of reactivity is:CH H CH6 5 2> CH =CHCH2 2


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> CH CH CH3 22and the above is also their stability.Now lest us study the stability of tri henyl and henylalkyl cations.The tri henylmethyl carbocation shows its stability because the ositivecharge on the carbon

is distributed uniformly over a number of structures.CCCCetc.CThus a number of canonical structures is ossible. Like tri henylmethylcation, di henylmethyl

cation is also very stable. In some cases the carbocations are so st

able that their salts have beenisolated in the solid state. The solid salt of tri henylmethyl cation with boronfluoride as4 6 5 3(C H ) C BF Ais available commercially. Arylmethyl cations are further stabilized ifthey have electron donatinggroups in ortho and para positions.2. The stability increases when the positive charge is in conjugation with a doublebondThis is due to increased delocalization of the positive charge over t

wo atoms instead of beingconcentrated at one.RRCRCRRRCRC

RCRRRCRCRC


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RRC3. The presence of a heteroatom adjacent to the cationic centre andbearing anunshared pair of electrons, like nitrogen, oxygen or halogen increases stabilitySuch ions are stabilized by resonanceC RRO....CH3R CRO..CH3

CARBOCATI ONS (CARBONI UM I ONS) 7Simple acyl cations have been prepared in solution and even in the solid state. It has been foundthat acetyl cationCH CO3is as stable as ter butyl cation. Their canonical forms are represen

ted as:C R R C O..

O....It has been shown that although the positive charge is located on cation, this structure contributesmore than that containing a triple bond.Therefore in general the carbocation stability is increased due to resonance.4. Conjugation between the bent orbitals and p orbitals of the cationIn this connection the case of cyclopropylalkyl cation and substitutedcyclopropyl cations isinteresting. Cyclopropylmethyl cation has been found to be more stablethan benzyl cation and the

stability increases with each addition of cyclopropyl groups. ThusC CH3CH3CHC


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DimethylcyclopropylcationDicyclopropylcationThricyclopropylcationThis increased stability has been explained between the bentorbitals of cyclopropyl rings and the vacant p orbital of the cationcarbon. The vacant p orbital lies parallel to C2C3bond of the

cyclopropane ring and not perpendicular to it. Thus the geometrybecomes similar to that of a cyclopropane ring conjugated with anoblefinic bond.The structure of carbocation has been measured by13C nmr shift of the carbon atom carryingthe positive charge. The shift agrees with the electron density on this carbon.The following table given by Olah and J. Denovan (J. Amer. Chem. Soc. 1977.90.5026) showsthat substitution of an ethyl for a methyl group or a methyl for hy

drogen causes a downfield shiftshowing that the central carbon becomes somewhat more positive.Table 1.1Ions Chemical shift Temperature C(C2H5)2C CH

3139.4 20(CH3)2CC2H5139.2 60

(CH3)3C135.4 20HC(OH)2


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+17.0 30C(OH)3+28.0 50The presence of hydroxy group decreases the positive character of thecentral carbon.Vacant p orbitalC2C38 MECHANI SMS I N ADVANCED ORGANI C CHEMI STRYGENERATION OF CARBOCATIONSFollowing are some of the methods used to generate carbocations.1. By direct ionisation: The direct ionisation of alkyl halides givescarbocations.R X R + XSolvent

(x = Cl, Br, I)The process is accelerated due to the presence of metal ions likeAg

.2. Form alkenes: A proton or other positive species adds to one atomof an unsaturated system

leaving a positive charge on the adjacent carbon.C Z + H C Z H 3. Decomposition of an alkyl diazonium ions:R NR + N2N

4. By protonation of alcohols: The protonated alcohols give carbocations on decomposition.R OH + H ....R OH..H

R + H O25. By reaction of acyl halides in presence of anhydrous aluminium chl

oride: The FrielelCrafts acylation reactions came under this heading.R COCl RCO AlCl4

RCO + AlCl4


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AlCl3anhydrousREACTIONS OF CARBOCATIONSThe carbocations being highly reactive intermediates of very short lifereact further without

being isolated and give stable products. In general they give the following reactions:1. Reaction with nucleophiles2. Elimination of a proton3. Rearrangements4. Addition to unsaturated systems.1. Reaction with nucleophilesThe carbocations react with species, the nucleophilesR + Y R Y Y = OH, halide ion or may be even a neutral species.CARBOCATI ONS (CARBONI UM I ONS) 9CH3C HCH

3+ BrCH3C HCH3BrCH3CCH

3CH3+..OH..H CH3C

H CH3CH3OCH3CCH3


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CH3HOH + H2. Elimination of a protonIn this process the elimination of a proton results in the formationof an alkene. Thus thedehydration of alcohols gives rise to alkene in presence of conc. H2SO4.R CH2CH2OH....RCH2

CH2OH....H O

2R CH2CH2H

or R C CH2RCH = CH2HIt mayrearrangeRCH CH32cation1cation

H HSo carbocations can adopt two pathways to give the stable product.3. RearrangementsAs shown in the example due to rearrangement 1 carbocation rearrangesto 2 cation. Thisrearrangement may involve the migration of a hydrogen with its pair of electron when it is calledthe hydride shift or it may involve the migration of an alkyl group


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with its bonding electrons inwhich case it is called the alkyl shift. They hydride shift is givenin the above example. The alkyl

shift may be represented asC C HRC CRHH H4. Addition to unsaturated systemsA carbocation may add to an alkene producing a new positive charge at a new position as shownin the following example.10 MECHANI SMS I N ADVANCED ORGANI C CHEMI STRYR CHCH2+ C R CH CH2C

CH3CH3CH3 CH3CH3CH3

Formed by any of the two methods, the new carbocation reacts furtherto stabilize itself. Thisgenerally happens either by reacting with a nucleophile or by the loss of a proton.If the new cation formed by union with an alkene further combines with an alkene, this willresult in the formation of a polymer.A number of reactions have been explained on the basis of generationof carbocations. Theexamples include the Friedel Crafts alkylation and arylation reactions.Besides pinacol pinacolne rearrangement, Beckmann rearrangement and Wagner Merwein rearrangement areother examples.

NON CLASSICAL CARBOCATIONS OR BRIDGED CARBOCATIONSThe carbocations so far studied are called classical carbocations in which the positive chargeis localized on one carbon atom or delocalized by resonance involvingan unshared pair of electronsor a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non classicalcarbocation the positive charged is delocalized by double or triple bond that is not in the allylicposition or by a single bond. These carbocations are cyclic, bridged


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ions and possess a three centrebond in which three atoms share two electrons. The examples are 7 norbornenyl cation, norbornylcation and cyclopropylmethyl cation.12347561(a)HH1(b) 1(c)(1)56432

1

CH2CHCH2CH2CH2

CHCH2CH7 norbornenyl cation7norbornylcation22(a) 2(b)cyclopropylmethyl

cation33(a) 3(b)CH2CH2

CH2


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CH2HCARBOCATI ONS (CARBONI UM I ONS) 11It we accept the existence of bridged ions, the question to be answered is why should suchions be formed in preference to classical carbocations in any particular reaction. One reasonableanswer is that when several intermediates are possible, the most stable one is the one likely to beformed. Since charge is most diffuse in the bridged ion than in the classical ion, the bridged structurewould be expected to be more stable than the classical structure.GENERATION OF NON CLASSICAL CARBOCATIONSThe non classical carbocations can be generated if proper substrate ischosen. For example thenorbornyl carbocation can be generated by the departure of a leavinggroup from an exosubstituedsubstrateX routeX route

exo ub titutednorbornyl compund44(a) 4(b)YThi i called the route to a non-cla ical carbocation becau e of the participation of a bond. If a bond is involved then it is called a route. Many chs aruge that the structurewritten from 7-norbornenyl cation are not non-classical carbocations because they are not canonicalforms but real structures and there is ra id EQUILIBRIUM between them.Distinction between neighbouring grou artici ation and non-classical carbocati

onIt is im ortant to distinguish between neighbouring grou artici ationand non-classicalcarbo-cation. A non-classical carbocation can be formed due to articiation of several s ecies asa neigh-bouring grou .Some of these neighbouring grou s are1. C = C as a neighbouring grouHere we have a c = c grou attached to a carbon atomwhich is adjacent to be carbon atom where nucleo hilicsubstitution can occur and during the course of the reactionbecomes bonded of artially bonded to the reaction centre toform a non-classical or bridged ion (Fig. 1 to 1(c)). Thus the rate

and/or the stereochemistry maybe affected. This ex lains why the acetolysis of 5 is 1011times faster than that of 5(a), because it

involves the formation of a non-classical carbocation2. The carbon-carbon single bond as a neighbouring grouThis has been ex lained in Fig. (2 2(b)) and also in Fig. (3 to 3(b)).3. The 2-nor-bornyl systemThis is given in Fig. (2 to 2(b)).


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5(a)H X5X H12 MECHANI SMS I N ADVANCED ORGANI C CHEMI STRY12

CHAPTERThe Carbanion ChemistryThe carbanions are formed during heterolytic fission of a CX bond inwhich the carbon atomis more electronegative than X.R C XHHR CHH

X + :The shared air of electron goes to the carbon atom which acquires an octet with a negativecharge and X goes out as a ositive ion. The above s ecies containing the carbon with the octet anda negative charge is called a carbanion.So, it is a tricovalent carbon with a air of electrons and thegeometry has been found to be yramidal resembling ammonia andamines in which the unshared air occu ies one a ex of thetetrahedron.Therefore, the central carbon atom is in a state of s

3 hybridi-sation, but when it is stabilized by declocatization, the hybridizationbecomes, s2for effective resonance.

STABILITYBeing electron-rich it is a very reactive intermediate and forms thebasis of many im ortantorganic synthesis. It can combine with a great variety of electro hiles e.g., alkyl halides and ketonesand can form com ounds having carbon-carbon bonds in high yields.R C O

RCRRROR X R R + X

R


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a carbanionR = alkylX = halogen.CRRRCarbanionTHE CARBANI ON CHEMI STRY 13Because of the unshared electron airs, it can also function as a base and can abstract a rotonto form a conjugate acid.R3C H + B : R3C + BH The benzyl carbanion has been found to be most stable because of theextensive delocalization

of the negative charge over the various resonating structures.CH2

CH2CH2CH2The stability becomes greater due to the resence of electron attracting grou s like NO2, CN

or carbonyl group in the molecule and it decreases due to electron releasing groups.CH2N COOCH2C NOO

The stability of the carbanions has been found to be in the orderbenzyl > vinyl > phenyl > C2H5> n propyl > isobutyl > neopentyl.

The effect of groups on the stability of carbanions when present in position is in the order.NO > R C > SO > CN CONH > h logens > H > R


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2 2 2OGENERATION OF CARBANIONSThe c rb nions h ve been gener ted s intermedi tes in l rge numberof re ctions.In recent ye rs new types of c rb nions h ve been prep red which rep ired with differentmet ls nd this h s found the b sis for the prep r tion of m ny novel org nomet llic compounds.Some of these met ls include group IA, IIA met ls besides luminium,zinc nd c dmium.( ) Abstr ction of hydrogen by b seIn org nic compounds h ving CH bonds, the c rbon tom being more electroneg tivewill pullthe bonding electron tow rds itself nd so hydrogen c n go out s proton. Therefore, in presenceof b se this H will be ttr cted by the b se nd c rb nion will be obt ined. This cidic hydrogenwill be bstr cted by b ses like N OH, t-BuOor lkyl lithium where the lkyl r dic l will beh ve

like b se.14 MECHANI SMS I N ADVANCED ORGANI C CHEMI STRYA simple ex mple of such bstr ction of proton is the form tion of t

riphenylmethyl c rb nionby N NH2in presence of liquid mmoni .

Ph C H + N NH3 2Ph C N + NH3 3lig. NH3

In the Grign rd re gent, the lkyl r dic l cts like c rb nion sshown by its re ctions withCO2le ding to the form tion of c rboxylic cids

O C O + RMgX O C O MgX+RHOHRCOOH + MgOHX

C rboxylic cidTherefore deproton tion is method used most often for the prep r tion of reson nce ndinductively st bilized c rb nions.R H + M RM + H2Dimethyl sulphide, Me2S (with estim ted pK v lue s high s 49 in the medium of DMSO)loses proton with lkyl lithium, the ide h s been put forw rd th t ny type of


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non-met l tom bondedto c rbon tom will m ke proton tt ched to th t c rbon tom cidic to be repl ced by strongb ses like butyl lithium.Deproton tion of very we kly cidic CH cids is ccomplished by met ll tion withorg nolithiumnd org nosodium compounds.(b) From Uns tur ted CompoundsThe c rb nions c n be gener ted from uns tur ted compounds either by reduction or byddition.(i) M ny uns tur ted compounds t ke up two electrons (reduction) fromctive met ls toform dic rb nions. It is illustr ted s follows:C C + 2M M C C MThe nion from one electron tr nsfer is prob bly n intermedi te. Simple lkenes do not formsuch st ble di nions, but some conjug ted deriv tives do form di nionss with cyclooct tetr ene

forming di nion with lk li met ls.+ 2 N 2 2 N+Benzene ( rom tic) does not yield its nti rom tic di nion nd n phth lene reduces only to its

nion r dic l.(ii) By ddition:The ddition of nucleophile to n uns tur ted C = C bond gener tes c rb nion.THE CARBANI ON CHEMI STRY 15R+ NuNuR+ Nu

Nu N NREACTIONS OF CARBANIONS WITH ELECTROPHILESThese re ctions re of two types:( ) Substitution, nd(b) Addition.1. Substitution re ctions of lkyl (sp3) nionsThe c rb nions re involved in number of substitut ion re ctions. The ex mple re theform tion of mono nd di lkyl m lonic esters.H

HCCOOEtCOOEtOEtH CCOOEtCOOEt


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RBr RHCCOOEtCOOEt+ BretcRCCOOEtCOOEtC rb nion intermedi teSimil rly with other re ctive methylene groups like diketones we getmono and dialkylatedderivatives.HHCCOOEtCOOEtOEtH C

COCH3RBr RHC etc.Car anion intermediateCOCH3Br

COCH3COCH

3diketoneIn such reactions the leaving group is a halide ion ut many othergroups such as sulphonates,sulphates or alkoxides have also een used. A ond is formed etweenR and some non-metallic ormetallic element.Virtually all elements except no le gases can e onded to car on.Some su stitution reaction are as follows:(i)YHRRH + Y

(ii)In (i) there is an a straction of a proton y a strong organometallic ase like BuLi ecause theyare readily availa le. It is an extremely important reaction ecause it is usedto generate other anions.2. Addition Reactions of Alkyl (Sp3) Anions


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The formation of an aldol y the union of two molecules of aldehydein presence of alkaliesis an important example under this heading. The formation of acetaldolfrom acetaldehyde is as

follows:16 MECHANI SMS I N ADVANCED ORGANI C CHEMI STRYCH CHO + HCH CHO3 2OHCH CHOH CH CHO3 2H O2CH CH CH CHO3Acetaldol hydroxy utraldehydeThe mechanism is as follows:CH CHO3OHCH CHO + H O2 2a car anion

The car anion then attracts the second molecule of aldehyde.CH3C HOCH CHO3CH3C H

OCH CHO2H O2CH3C HOHCH CHO2Acetaldol

The acetaldol again having hydrogen tom to the ldehyde repe ts theprocess nd long ch inpolymers re formed.Here the c rb nion obt ined form the first molecule dds t the ldehydic c rbon tom of thesecond molecule.Other re ctions involving the ddition of c rb nions re re ctions likePerkins re ction, Cl isen

condens tion, benzilic cid re rr ngement nd Mich el ddition.3. Re ctions of carbanions with Electro hiles


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The ty e carbanions undergo most of the reactions given by alkyl anion . An example i+ FeCl3Fe4. Elimination Reaction of CarbanionThe elimination reaction of carbanion have been put under differentcla e . The e are:(a) elimin tions(b) eliminations(c) eliminations(d) eliminationsTHE CARBANI ON CHEMI STRY 17Besi es these cycloeliminations are also known. Many of elimination reactions procee rapi lyat room temperature but are slow at ry ice temperatures. Thus permitting carbanions containinggoo leaving groups to be use in substitution.Out of the above the elimination is most common. These eliminationsresult in the formationof alkenes and alkynes. When phenylethyl romide is heated with an alcoholic solution of an alkalifirst a car anion is formed y the loss of a proton followed y the

loss of a halide ion andsimultaneous formation of a dou le ond.Ph CHHCH2BrOHPh CH CH2Br Ph CH CH

2+ BrThe presence of a positively char ed su stituent on the c rbon tompromotes the cidityof hydro en.5. Oxidation Reactions of Car anionsOne electron oxidation of monocar anions leads to car on radicals andtwo electron oxidationives car ocations. In most of these oxidations, the mechanism is not known, thou h pro ress isein made on some mechanisms. But there appears to e a parallelism

etween ase stren th andease of oxidation of car anions.(i) Oxidation of Car anions to Hydroperoxides, Alcohols and KetonesThe car anions take up O2and these take up protons to ive hydroperoxides in ood yields.

But ecause they are explosive in nature, they are not usually isolated and on reduction with sodiumsulphite on trialkyl phosphite ive alcohols. Alcohols can also e prepared via hydroperoxy moly -


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denum complexes and alkyl oranes. These reactions are summarized as follows:O2RMoO Py5RO Mo2CIB

126432165 Mechanism in Advance Organic Chemistry

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