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1014 General Organic Chemistry Chapter 23...

1014 General Organic Chemistry Chapter 23 General Organic Chemistry Carbon is an essential element of organic Bond Four - Three - Two - compounds, it has four electrons in its outer most shell. One - Two - Bond angle 109.5 120 180 According to the ground state electronic Geometry Tetrahedra Trigonal Linear configuration of carbon, it is divalent. Tetravalency of l planar carbon can be explained by promoting one of the 2s 2 - % s-character 25 or 1/4 33.33 or 50 or 1/2 electrons to the unocupied 2 p z 1 atomic orbital. 1/3 The four valencies of carbon atom are similar and (2) Determination of hybridisation at different they are symmetrically arranged around the carbon carbon atoms : It can be done by two methods, atom. According to Le Bell and Van’t Hoff the four valencies of carbon do not lie in one plane. They are (i) First method : In this method hybridisation directed towards the corners of a regular tetrahedron can be know by the number of   bonds present on with carbon atom at the centre and the angle between that particular atom. any two valencies is 109o28. Number of – bond/s 0 1 2 Hybridisation in Organic Compounds 3 2 Type of hybridisation sp sp sp (1) The process of mixing atomic orbitals to form Examples : a set of new equivalent orbitals is termed as hybridisation. There are three types of hybridisation, O 3 || (i) sp hybridisation (involved in saturated (i) CH 3  CH  CH  C  CH 3 organic compounds containing only single covalent      bonds), sp 3 sp 2 sp 2 sp 2 sp 3 (ii) sp 2 hybridisation (involved in organic (ii) CH 2  C  CH 2 compounds having carbon atoms linked by double    bonds) and sp sp 2 sp 2 (iii) sp hybridisation (involved in organic (iii) CH 3  CH  CH  CH 2  C  N compounds having carbon atoms linked by a triple       bonds). sp 3 sp 2 sp 2 sp 3 sp sp Table : 23.1 Type of sp3 sp2 sp (iv) HC  C  CH  CH 2 hybridisation     Number of 1s and 3p 1s and 2p 1s and 1p sp sp sp 2 sp 2 orbitals used Number of Nil One Two  In diamond carbon is sp3 hybridised and in unused graphite carbon is sp 2 hybridised. p-orbitals  General Organic Chemistry 1015 (ii) Second method : (Electron pair method) of the decreasing order of s orbital character in the three hybrid orbitals. ep = bp + lp; where ep = electron pair present in sp 3  sp 2  sp hybrid orbitals , bp = bond pair present in hybrid orbitals (ii) Electronegativity of different orbitals Number of bp = Number of atoms attached to (a) Electronegativity of s-orbital is maximum. the central atom of the species (b) Electronegativity of hybrid orbital  % s- character in hybrid orbitals Central atom First atom H H Orbital sp sp 2 sp 3 C=C % s - character 50 33. 33 25  s-character in decreasing order and electroneg ativity in decreasing order Second atom H H Thus sp-hybrid carbon is always electronegative bp = 3 Third atom in character and sp 3 - hybrid carbon is electropositive in character. sp 2 -hybrid carbon can behave as Central atom 1 2 electropositive (in carbocation) as well as H  C  C H 2 1 electronegative (in carbanion) in character.  H   bp  2 CH 3  CH 2 CH 2  CH 3 bp = 3 sp 2 sp Number of lp’s can be determined as follows, (a) If carbon has  - bonds or positive charge or odd electron, than lp on carbon will be zero. Electronegative Electropositive (b) If carbon has negative charge, then lp will be carbon having carbon positive charge equal to one. (c) Electronegativities of different hybrid and Number of electron pairs (ep) tells us the type of unhybrid orbitals in decreasing order is as follows hybridisation as follows, s  sp  sp 2  sp 3  p  ep 2 3 4 5 6 % s - character and electroneg ativity in decreasing order. Type of sp sp 2 sp 3 sp 3 d sp 3 d 2 hybridisation (iii) Bond length variation in hydrocarbons % of s orbital character Example :   1 1 (i) CH 2  CH (ii) CH 2  CH     C  C bond length C  H bond length bp  2 bp  2 Table : 23.2 lp  0 lp  1 ep  2, sp ep  3, sp 2 Bond type Bond Bond type Bond   (C – H) length (C – C) length (iii) CH 2  C  CH 3 (iv) CH  C |  sp 3  s 1.112Å sp 3  sp 3 (al 1.54 Å bp 1 CH 3 lp  1 (alkanes) kanes) bp  3 ep  2, sp lp  0 sp 2  s 1.103Å sp 2  sp 2 (al 1.34Å ep  3, sp 2 (alkenes) kenes)  (v) CH 3  CH  CH 3 sp  s 1.08Å sp  sp (alk 1.20Å  (alkynes) bp  3 ynes) lp  1 ep  4 , sp 3 (iv) Bond strength in hydrocarbons : The shorter is the bond length, the greater is the compression (3) Applications of hybridisation between atomic nuclei and hence greater is the strength (i) Size of the hybrid orbitals : Since s - orbitals of that bond. are closer to the nucleus than p - orbitals, it is reasonable to expect that greater the s character of an Table : 23.3 orbital the smaller it is. Thus the decreasing order of Bond Bond Bond type Bond the size of the three hybrid orbitals is opposite to that type (C – energy (C – C) energy  1016 General Organic Chemistry H) (kcal/mole) (kcal/mole) (1) Tertiary alkyl halides having bulky groups form tertiary carbocation readily when hydrolysed sp 3  s 104 sp 3  sp 3 80 – 90 because of the presence of the three bulky groups on (in (in the carbon having halogen. alkanes) alkanes) Steric strain around this Steric strain is CH 3 carbon released sp  s 2 106 sp  sp 2 2 122 – 164 |  (More strained species) (less strained (in (in H 3 C  C  Cl  H 3 C  C  CHspecies) 3 | | alkenes) alkenes) CH 3 CH 3 sp  s 121 sp  sp 123 – 199 (in (in (2) Primary alkyl halide having quaternary  - alkynes) alkynes) carbon does not form transition state because of the steric strain around  -carbon by the  -carbon. To (v) Acidity of hydrocarbons release the strain it converts into carbocation. (a) Hydrogen present on electronegative carbon Bulky CH group 3 is acidic in nature. | (b) Acidity of hydrogen is directly proportional to CH 3  C CH 2  Cl | the electronegativity of the atom on which hydrogen is CH 3 Strained carbon present. due to bulky group present around this Thus carbon. H O  H  NH 3  CH  CH    (3) Steric strain inhibits the resonance. This phenomenon is known as steric inhibitions of Electronegativity of the atoms Acidity of compounds in decreasing order resonance. (c) Acidity of hydrocarbon  % of s-character Electronic displacement in covalent bonds CH  CH CH 2  CH 2 CH 3  CH 3 It is observed that most of the attacking reagents % s-character 50 33.33 always possess either a positive or a negative charge, 25 therefore for a reaction to take place on the covalent pKa 25 44 50 bond the latter must possess oppositely charged s- character and acidity in decreasing order centres. This is made possible by displacement (partial 1 or complete) of the bonding electrons. The electronic Acidity  Ka and Acidity  (pKa   log Ka) pKa displacement in turn may be due to certain effects, Order of acidic nature of alkynes is, some of which are permanent and others are temporary. The former effects are permanently HC  CH  HC  C  CH 3 operating in the molecule and are known as The relative acidic character follows the order; polarisation effects, while the latter are brought into H2O  ROH  HC  CH  NH 3  CH 2  CH 2  CH 3  CH 3 play by the attacking reagent and as soon as the attacking reagent is removed, the electronic Obviously, the basic character of their conjugate displacement disappears; such effects are known as the bases follows the reverse order, i.e., polarisability effects. CH 3 CH 2  CH 2  CH   NH 2  HC  C   RO   HO  Electronic displacement Steric effect On account of the presence of bulkier groups at Polarisation Polarisability the reaction centre, they cause mechanical interference effect effect and with the result the attacking reagent finds it (permanent) (temporary) difficult to reach the reaction site and thus slows down the reaction. This phenomenon is called steric Inductiv Mesomeri Hyperconjugati Inductomer Electromeri hinderance or steric effect. e effect c effect ve effect ic effect c effect  General Organic Chemistry 1017 Inductive effect or Transmission effect (ii) Reactivity of alkyl halide : + I effect of methyl group enhances – I effect of the halogen atom (1) When an electron withdrawing (X) or by repelling the electron towards tertiary carbon atom. electron-releasing (Y) group is attached to a carbon CH 3 CH 3 chain, polarity is induced on the carbon atom and on the substituent attached to it. This permanent polarity H3C C X  H3C CH X is due to displacement of shared electron of a covalent CH 3 bond towards a more electronegative atom. This is  CH 3 CH 2 X  CH 3 X called inductive effect or simply as I – effect. Tertiary > Secondary > Primary > Methyl C  C  C  C Non polar      (iii) Relative strength of the acids : C  C  C  C X (a) Any group or atom showing +I effect      C  C  C  C Y decreases the acid strength as it increases the negative charge on the carboxylate ion which holds the hydrogen (2) Carbon-hydrogen bond is taken as a standard firmly. Alkyl groups have + I effect. of inductive effect. Zero effect is assumed for this bond. Thus, acidic nature is, Atoms or groups which have a greater electron withdrawing capacity than hydrogen are said to have–I HCOOH  CH 3 COOH  C 2 H 5 COOH  C3 H 7 COOH  C4 H 9 COOH effect whereas atoms or groups which have a greater +I effect increases, so acid strength decreases electron releasing power are said to have +I effect. Formic acid, having no alkyl group, is the most  N H 3  NO 2  CN  SO 3 H  CHO  CO  COOH  COCl  COOR acidic among these acids. > CONH 2  F  Cl  Br  I  OH  OR  NH 2  C 6 H 5  H (b) The group or atom having – I effect increases –I power of groups in decreasing order with respect to the the acid strength as it decreases the negative charge on reference H the carboxylate ion. Greater is the number of such atoms or groups (having – I effect), greater is the acid ter. alkyl > sec. alkyl > pri. alkyl > CH 3  H strength. + I power in decreasing order with respect to the Thus, acidic nature is, reference H CCl 3 COOH  CHCl 2 COOH  CH 2 ClCOOH  CH 3 COOH + I power  number of carbon in the same type of Trichloro acetic acid Dichloro acetic acid Monochloro acetic acid Acetic acid alkyl groups (– Inductive effect increases, so acid strength increases) CH 3  CH 2  CH 2  CH 2   CH 3  CH 2  CH 2   (c) Strength of aliphatic carboxylic acids and CH 3  CH 2  benzoic acid + I power in decreasing order in same type of R COOH C6 H 5 COOH alkyl groups    I group  I group (3) Applications of Inductive effect (i) Magnitude of positive and negative charges : Hence benzoic acid is stronger acid than aliphatic Magnitude of +ve charge on cations and magnitude of carboxylic acids but exception is formic acid. Thus, –ve charge on anions can be compared by + I or – I groups present in it. HCOOH > C6 H 5 COOH > RCOOH ve Acid strength in decreasing order  Magnitude of charge 1  Decreasing order of acids :    I power of the group.  I power of the group NO2CH 2COOH  FCH 2COOH  ClCH 2COOH  BrCH 2COOH.  Magnitude of ve charge F3C  COOH  Cl3C  COOH  Br3C  COOH  I3C  COOH. 1    I power of the group.  I power of the group CH 3 OH  CH 3 CH 2 OH  (CH 3 ) 2 CHOH  (CH 3 )3 COH Methyl Ethyl Iso propyl Tert butyl alcohol Alcohol alcohol alcohol  1018 General Organic Chemistry As compared to water, phenol is more acidic (–I (viii) Stability of carbonium ion :+I effect tends effect) but methyl alcohol is less acidic (+I effect). to decrease the (+ve) charge and –I effect tends to OH  H  OH > CH 3 OH increases the +ve charge on carbocation. Phenol Water Methyl alcohol (CH 3 )3 C   (CH 3 )2 CH   CH 3 CH 2  CH 3 (vi) Relative strength of the bases (Basic nature of  NH 2 ) (ix) Stability of carbanion : Stability of carbanion increases with increasing – I effect. The difference in base strength in various amines can be explained on the basis of inductive effect. The +I CH 3  CH 3 CH 2  (CH 3 )2 CH   (CH 3 )3 C  effect increases the electron density while –I effect Resonance effect or mesomeric effect decreases it. The amines are stronger bases than NH 3 (1) The effect in which  electrons are as the alkyl groups increase electron density on transferred from a multiple bond to an atom, or from a nitrogen due to + I effect while ClNH 2 is less basic due multiple bond to a single covalent bond or lone pair (s) to –I effect. “So more is the tendency to donate electron of electrons from an atom to the adjacent single pair for coordination with proton, the more is basic covalent bond is called mesomeric effect or simply as nature, i.e., more is the negative charge on nitrogen M-effect. In case of the compound with conjugated atom (due to +I effect of alkyl group), the more is basic system of double bonds, the mesomeric effect is nature”. transmitted through whole of the conjugated system and thus the effect may better be known as conjugative Thus, the basic nature decreases in the order; effect. (C 2 H 5 )2 NH  CH 3 CH 2 NH 2  CH 3 NH 2  NH 3  ClNH 2 (2) Groups which have the capacity to increase Diethy l Ethy l Methy l Ammonia Chloro amine amine amine amine the electron density of the rest of the molecule are said to have  M effect. Such groups possess lone pairs of The order of basicity is as given below; electrons. Groups which decrease the electron density Alkyl groups (R– Relative base strength of the rest of the molecule by withdrawing electron ) pairs are said to have M effect, e.g., CH 3 R2 NH  RNH 2  R3 N  NH 3 (a) The groups which donate electrons to the double bond or to a conjugated system are said to have C2 H 5 R2 NH  RNH 2  NH 3  R3 N  M effect or R effect. (CH 3 )2 CH RNH 2  NH 3  R2 NH  R3 N  M effect groups :.... (CH 3 )3 C NH 3  RNH 2  R2 NH  R3 N  Cl,  Br ,  I,  N H 2 ,  NR 2 ,OH ,OR,SH ,OCH 3 , S R..  The relative basic character of amines is not in total accordance with inductive effect (t  s  p) but it is (b) The groups which withdraw electrons from the double bond or from a conjugated system towards in the following order: Secondary > Primary > Tertiary. itself due to resonance are said to have M effect or The reason is the steric hindrance existing in the t- R effect. amines. M effect groups :  In gas phase or in aqueous solvents such as chlorobenzene etc, the solvation effect, i.e., the O || stabilization of the conjugate acid due to H -bonding  NO 2 ,C  N ,  C ,CHO ,COOH ,SO 3 H are absent and hence in these media the basicity of amines depends only on the +I effect of the alkyl group (3) The inductive and mesomeric effects, when thus the basicity of amines follows the order : present together, may act in the same direction or 3 o  2 o  1 o  NH 3. oppose each other. The mesomeric effect is more powerful than the former. For example, in vinyl (vii) Basicity of alcohols : The decreasing order chloride due to – I effect the chlorine atom should of base strength in alcohols is due to +I effect of alkyl develop a negative charge but on account of mesomeric groups. effect it has positive charge. (CH 3 )3 COH  (CH 3 )2 CHOH  CH 3 CH 2 OH  CH 3 OH (3 o ) (2 o ) (1 o )  General Organic Chemistry 1019.. .. (2) Structural requirements for : Cl  CH  CH 2  : Cl  CH  CH 2 hyperconjugation.... (i) Compound should have at least one sp 2 - Application of mesomeric effect : It explains, hybrid carbon of either alkene alkyl carbocation or (1) Low reactivity of aryl and vinyl halides, alkyl free radical. (2) The acidic nature of carboxylic acids, (ii)  -carbon with respect to sp 2 hybrid carbon (3) Basic character comparison of ethylamine and aniline, should have at least one hydrogen. (4) The stability of some free radicals, If both these conditions are fulfilled then carbocations and carbanions. hyperconjugation will take place in the molecule. Difference between Resonance and (iii) Hyperconjugation is of three types Mesomerism : Although both resonance and mesomerism represent the same phenomenon, they (iv) Resonating structures due to differ in the following respect : Resonance involves all hyperconjugation may be written involving “no bond” types of electron displacements while mesomerism is between the alpha carbon and hydrogen atoms. noticeable only in those cases where a multiple bond is  in conjugation with a multiple bond or lone pair of H H |  electron. H  C  CH  CH 2  H  C  CH  C H 2  | | Example : H H (i) H H   |  | ..  H C  CH  C H 2  H  C  CH  C H 2 H 2 C  CH  CH  CH 2  H 2 C  CH  CH  C H 2 | H H .... :O :O: (v) Number of resonating structures due to the..  || || hyperconjugation = Number of  -hydrogens + 1. (ii) R  C  O  H  R  C  O  H.... Applications of hyperconjugation Both (i) and (ii) are the examples of mesomerism (1) Stability of alkenes : Hyperconjugation and resonance effect. Let us consider the following explains the stability of certain alkenes over other..   alkenes. example H Cl :  H Cl. Such an electron Stability of alkenes  Number of alpha hydrogens..  Number of resonating structures displacement is the example of resonance only (not the CH 3  CH  CH 2  CH 3  CH 2  CH  CH 2  CH 3  CH  CH  CH 2 mesomerism). | CH 3 Hyperconjugative effect Stability in decreasing order (1) When a H  C bond is attached to an (2) Carbon-carbon double bond length in unsaturated system such as double bond or a benzene alkenes : As we know that the more is the number of ring, the sigma () electrons of the H  C bond interact resonating structures, the more will be single bond or enter into conjugation with the unsaturated system. The interactions between the electrons of  systems character in carbon-carbon double bond. (multiple bonds) and the adjacent  bonds (single (3) Stability of alkyl carbocations : Stability of H  C bonds) of the substituent groups in organic alkyl carbocations  number of resonating structures  compounds is called hyperconjugation. The concept of number of alpha hydrogens. hyperconjugation was developed by Baker and Nathan and is also known as Baker and Nathan effect. (4) Stability of alkyl free radicals : Stability of alkyl free radicals can be explained by In fact hyperconjugation effect is similar to hyperconjugation. Stability depends on the number of resonance effect. Since there is no bond between the resonating structures.  -carbon atom and one of the hydrogen atoms, the hyperconjugation is also called no-bond resonance. (5) Electron releasing (or donating) power of R in alkyl benzene : CH 3  (or alkyl group) is R group,  1020 General Organic Chemistry ortho-para directing group and activating group for  Cl Cl electrophilic aromatic substitution reaction because of |  Cl  C  CH  CH 2  Cl  C  CH  CH 2  the hyperconjugation. | | Cl Cl The electron donating power of alkyl group will depends on the number of resonating structures, this Cl Cl  |  |  depends on the number of hydrogens present on - Cl C  CH  CH 2  Cl  C  CH  CH 2 carbon. The electron releasing power of some groups | Cl Cl  are as follows, The meta directing influence and the deactivating CH 3 CH 3 | effect of CX 3 group in electrophilic aromatic CH 3   CH 3  CH 2   CH   CH 3  C  substitution reaction can be explained by this effect. CH 3 | CH X X X X 3 | |  |  |  Increasing inductive effect X  C X X C X X C X X C X | || || || Electron donating power in decreasing order due  + to the hyperconjugation.  (6) Heat of hydrogenation : Hyperconjugation decreases the heat of hydrogenation. Inductomeric effect (7) Dipole moment : Since hyperconjugation Inductomeric effect is the temporary effect which causes the development of charges, it also affects the enhances the inductive effect and it accounts only in dipole moment in the molecule. the presence of an attacking reagent. The increase in dipole moment, when hydrogen of Example, formaldehyde (  2.27 D) is replaced by methyl group, H i.e., acetaldehyde (  2. 72 D) can be referred to H hyperconjugation, which leads to development of HO  H  C  Cl  HO............ C............ Cl charges. H H H H H H | |  H H  C  O , H  C  CH  O  H  C  CH  O Cl  (  2.27 D) | |  HO  C  H H H (  2. 72 D) H (8) Orienting influence of alkyl group in o, p - In methyl chloride the –I effect of Cl group is further increased temporarily by the approach of positions and of CCl 3 group in m -position : Ortho- hydroxyl ion. para directing property of methyl group in toluene is partly due to I effect and partly due to Electromeric effect hyperconjugation. (1) The phenomenon of movement of electrons from one atom to another in multibonded atoms at the Reverse Hyperconjugation : The phenomenon of demand of attacking reagent is called electromeric hyperconjugation is also observed in the system given effect. It is denoted as E-effect and represented by a below, curved arrow ( ) showing the shifting of electron X pair. |    C  C  C ; where X  halogen A  B  E A B : Reagent | In such system the effect operates in the reverse direction. Hence the hyperconjugation in such system is (2) (i)When the transfer of electrons take place known as reverse hyperconjugation. towards the attacking reagent, the effect is called  E effect. The addition of acids to alkenes.  General Organic Chemistry 1021 C C  H   C  C.. | A : B  A  B H Free radical  (ii) The factor which favours homolysis is that CH 3 CH  CH 2  H  CH 3  C H  CH 3 Propene the difference in electronegativity between A and B is Since, CH 3 group is electron donating, the less or zero. electrons are transferred in the direction shown. (iii) Homolysis takes place in gaseous phase or in The attacking reagent is attached to that atom on the presence of non polar solvents (CCl 4 , CS 2 ) , peroxide, which electrons have been transferred. UV light, heat ( 500 o C) , electricity and free radical. (ii) When the transfer of electrons takes place (iv) Mechanism of the reaction in which away from the attacking reagent, the effect is called homolysis takes place is known as homolytic  E effect. Example, The addition of cyanide ion to mechanism or free radical mechanism. carbonyl compounds. (2) Heterolytic bond fission or heterolysis C  O  CN   C  O (i) In heterolysis, the covalent bond is broken in | such a way that one species (i.e., less electronegative) CN is deprived of its own electron, while the other species The attacking reagent is not attached to that gains both the electrons. atom on which electrons have been transferred.   (3) Direction of the shift of electron pair : The A : B  A:  B carbanion carbocation direction of the shift of electron pair can be decided on Thus formation of opposite charged species takes the basis of following points. place. In case of organic compounds, if positive charge (i) When the groups linked to a multiple bond are is present on the carbon then cation is termed as similar, the shift can occur in either direction. carbocation. If negative charge is present on the (ii) When the dissimilar groups are linked on the carbon then anion is termed as carbanion. two ends of the double bond, the shift is decided by the (ii) The factor which favours heterolysis is direction of inductive effect. greater difference of electronegativities between A In the case of carbonyl group, the shift is always and B. towards oxygen, i.e., more electronegative atom. (iii) Mechanism of the reaction in which   C  O  C O : heterolysis takes place is known as heterolytic mechanism or ionic mechanism. In cases where inductive effect and electromeric effect simultaneously operate, usually electrometric (iv) The energy required for heterolysis is always effect predominates. greater than that for homolysis due to electrostatic forces of attraction between ions. Cleavage (fission or breaking) of covalent bonds Reaction Intermediates Breaking of covalent bond of the compound is known as bond fission. A bond can be broken by two Short lived fragments called reaction intermediates result from homolytic and heterolytic ways, bond fission. The important reaction intermediates are (1) Homolytic bond fission or Homolysis free radicals, carbocations, carbanions, carbenes, (i) In homolysis, the covalent bond is broken in benzyne and nitrenes. such a way that each resulting species gets its own electron. This leads to the formation of odd electron species known as free radical. Table : 23.4 Characteristi Free radical Carbocation Carbanion Carbene c  1022 General Organic Chemistry Nature Neutral having odd electron Positive charge on C Negative charge on Neutral, divalent C with 2 unshared electrons Hybridisation sp2 sp2 sp3 (non- (i) sp2 (singlet) conjugated) (ii) sp (triplet) sp2 (Conjugated) Structure Planar Planar Pyramidal/Planar (i) Planar (singlet) (ii) Linear (triplet) Magnetism Paramagnetic Diamagnetic Diamagnetic (i) Diamagnetic (ii) Paramagnetic Stability...      Triplet > singlet order Ph3 C  Ph2 CH  Ph CH 2  Ph3 C  Ph2 CH  PhCH 2  Ph3 C  Ph2 CH   . CH 2  CH  CH 2  PhCH 2  Allyl CH 2  CH  CH 2  3 o  2 o   .. 3 o  2 o  1o  CH 3 CH 2  1o  2 o  3 o 1  CH 2  CH 2  CH o Benzyne (2) There is possibility of two spin states for nitrenes depending on whether the two non-bonding (1) 1, 2-Didehydrobenzene, C6 H 4 and its electrons (the normal nitrogen lone pair remains derivatives are called benzyne or arynes and the paired) have their spins paired or parallel. simplest member is benzyne... (2) It is neutral reaction intermediate derived R –. N. These two are lone pair of from benzene ring by removing two substituents, of  electrons ortho positions, one in the form of electrophile and These two may be paired or other in the from of nucleophile leaving behind two unpaired electrons to be distributed between two orbitals. (3) In general nitrenes obey Hunds rule and the ground state triplet with two degenerate sp -orbitals Abnormal  bond containing a single electron each. R–N 2 Two sp -orbitals ouside the ring sp-Triplet nitrene (4) Nitrenes can be generated, in situ, by the (3) Benzyne intermediate is aromatic in following methods, character. (4) When halobenzene is heated with sodamide (i) By action of Br2 in presence of a base on a 1 o formation of benzyne takes place. amide (Hofmann-bromamide reaction), Cl O  O O ||  ||  ||.. R  C  NH 2  R  C  NHBr  R  C  N  Br Br2 / NaOH OH   NaNH 2 1 o Amide  H 2O..   (5) (i) It behaves as dienophile and gives Diels- Alder reaction with diene. O ||.. O  C  N  R   (ii) It reacts with strong nucleophile like NH 2   R  C  N   Rearrangement  Br.. Isocy anate  NH2 NH2 KOH  H    R  NH 2  K 2 CO 3  N H 2   (Hydrolysis) 1 o Amine  (ii) By decomposition of azides in presence of.. heat or light. Nitrenes (R – N : ).. ....  or h ν R  N  N  N :  R  N :  N  N (1) The nitrogen analogous of carbenes are called Alkylazide Alkylnitrene nitrenes.  General Organic Chemistry 1023.. (ii) Neutral nucleophiles : It can be classified (iii) Unsubstituted nitrene (H  N :) can be into two categories : obtained by photolysis of (or by passing electric (a) Neutral covalent compound, in which central discharge through) NH 3 , N 2 H4 or N 3 H. atom has complete octet, has at least one lone pair of electrons and all atoms present on central atom should Attacking reagents not be electronegative, is neutral nucleophile. The fission of the substrate molecule to create............ centres of high or low electron density is influenced by N H 3, R  N H 2 , R2 N H , R3 N , N H 2  N H 2 (Nitrogen attacking reagents. Most of the attacking reagents can nucleophile) be classified into two main groups....... Electrophiles or electrophilic reagents and H  O H, R  O H, R  O R (Oxygen Nucleophiles or nucleophilic reagents....... (1) Electrophiles : Electron deficient species or nucleophiles) electron acceptor is an electrophile....... H  S  H , R  S  H , R  S  R (Sulphur nucleophiles) It can be classified into two categories :...... (i) Charged electrophiles : Positively charged........ species in which central atom has incomplete octet is P H 3 , R P H 2 , R2 P H , R3 P (Phosphorus called charged electrophile. nucleophiles)  O   H , X , R, N , N  O, S O3 H (b) Organic compound containing carbon, carbon O.. multiple bond/ bonds behaves as nucleophile. All cations are charged electrophiles except Alkenes, Alkynes, Benzene,  cations of IA, IIA group elements, Al    and NH 4 CH 2  CH  CH  CH 2 , CH 2  CH  C  CH (ii) Neutral electrophiles : It can be classified (iii) Ambident nucleophiles : Species having two into three categories, nucleophilic centres out of which, one is neutral (a) Neutral covalent compound in which central (complete octet and has at least one lone pair of atom has incomplete octet is neutral electrophile, electrons) and the other is charged (negative charge)..... behaves as ambident nucleophile BeCl 2 , BH 3 , ZnCl 2 , AlX 3, FeX 3 , CH 3 , CH 2 , CX 2 O .. ..   (b) Neutral covalent compound in which central C  N , O  N  O, O  S  OH atom has complete or expended octet and central atom  O has unfilled –d-sub-shell is neutral electrophile, SnCl 4 , SiCl 4 , PCl 5 , SF6 , IF7  Organometallic compounds are nucleophiles.  Nucleophiles are Lewis bases. (c) Neutral covalent compound in which central atom is bonded only with two or more than two Organic compounds which behave as an electronegative atoms is called neutral electrophile. electrophile as well as a nucleophile : Organic BeCl 2 , BX 3 , AlX 3 , FeX 3 , SnCl 4 , PCl 3 ; compound in which carbon is bonded with electronegative atom (O, N, S) by multiple bond/bonds.. PCl 5 , NF3 , C X 2 , CO 2 , SO 3 , CS 2 , behaves as electrophile as well as nucleophile : O O O O Cl 2 , Br2 and I2 also behave as neutral || || || || R  C  H , R  C  R, R  C  OH , R  C  Cl , electrophiles. O O Electrophiles are Lewis acids. || ||   R  C  OR, R  C  NH 2 , R  C  N , R  N  C (2) Nucleophiles : Electron rich species or electron donors are called nucleophiles. Nucleophiles  During the course of chemical reaction can be classified into three categories : electrophile reacts with nucleophile. (i) Charged nucleophiles : Negatively charged  Strong Lewis acid is stronger electrophile species are called charged nucleophiles.   CO 2  N O 2  S O 3 H. Stronger is an acid, weaker is its       H , O H , R  O, C H 3 , X , S H , R  S conjugated base or weaker is the nucleophile.  1024 General Organic Chemistry Examples : HF  H 2 O  NH 3  CH 4     I Br Cl F          F   OH   NH 2  CH 3 Increasing basicity  Increasing order of nucleophilicity. Decreasing leaving ability Types of organic reactions (iii) The leaving power of some nucleophilic It is convenient to classify the numerous groups are given below in decreasing order, reactions of the various classes of organic compound O O O into four types, ||  ||  ||   Substitution reactions,  Addition reaction, CF3  S  O  Br S  O  CH 3 SO  || || ||  Elimination reactions,  Rearrangement O O O reactions, O O ||  ||  Substitution reactions C 6 H 5  S  O  CH 3  S  O || || Replacement of an atom or group of the substrate O O by any other atom or group is known as substitution O O reactions.   ||     ||  Substituting or  I  Br  CF3  C  O  H O  Cl  F  CH 3  C  O Examples : Leaving group attacking (iv) In these reactions leaving group of the group substrate is replaced by another nucleophile. If reagent CH 3  CH 2  Br  NaOH  CH 3  CH 2 OH  NaBr is neutral then leaving group is replaced by negative Ethyl bromide Ethyl alcohol part of the reagent. Negative part of the reagent is (Bromine atom is replaced by hydroxyl group) always nucleophilic in character.      E  Nu Types of substitution reactions : On the basis of R  L    R  Nu  L ; R  L  Nu  R  Nu  L the nature of attacking species substitution reactions (v) In S N reactions basicity of leaving group are classified into following three categories, should be less than the basicity of incoming (1) Nucleophilic substitution reactions nucleophilic group. Thus strongly basic nucleophilic group replaces weakly basic nucleophilic group of the (2) Electrophilic substitution reactions substrate.  (3) Free radical substitution reactions  Example : R  Cl  OH  R  OH  Cl.....(A) ( NaOH ) (1) Nucleophilic substitution reactions   Basicity of OH is more than Cl hence OH replaces (i) Many substitution reactions, especially at the  saturated carbon atom in aliphatic compounds such as Cl as Cl. alkyl halides, are brought about by nucleophilic   R  OH   R  Cl  OH......(B) Cl reagents or nucleophiles. ( HCl )    R  X  OH   R  OH  X Basicity of Cl is less than OH , hence Cl will not Substrate Nucleophil e Leaving group  replace OH as OH hence reaction (B) will not occur. Such substitution reactions are called (vi) Unlike aliphatic compounds having nucleophilic substitution reactions, i.e., S N reactions (S nucleophilic group as leaving group, aromatic stands for substitution and N for nucleophile). compounds having same group bonded directly with (ii) The weaker the basicity of a group of the aromatic ring do not undergo nucleophilic substitution reaction under ordinary conditions. substrate, the better is its leaving ability. The reason for this unusual reactivity is the Leaving power of the group  1 presence of lone pair of electron or  bond on the key Basicity of the group atom of the functional group. Another factor for the low reactivity is nucleophilic character of aromatic HI  HBr  HCl  HF Example :          ring. Decreasing acidity (vii) The S N reactions are divided into two classes, S N 2 and S N 1 reactions. Table : 23.5 Distinction between SN2 and SN1 reactions  General Organic Chemistry 1025 Factors SN2 Reactions SN1 Reactions   Number of steps R : L  R   : L Slow One: R : L  : Nu R : Nu  : L Two: (i) (ii) R   : Nu   Fast  R : Nu Reaction rate and order Second order: First order: Rate  [Substrate] [Nucleophile] or Rate  [Substrate] or Rate = K1 [RL] Rate = K 2 [RL ][: Nu ] Molecularity Bimolecular Unimolecular   TS of slow step     : Nu    C   : L : Nu    C    L    Nu : Reacting nucleophile The nucleophile attacks the carbon of The nucleophile can attack the carbon of the the substrate exclusively from the substrate both from the back and front sides back side. although the back side attack predominates. Stereochemistry Complete inversion of configuration Inversion and retention takes place. takes place. Reactivity order of alkyl Methyl>1°>2°>3°halides. 3°>2°>1° > methyl halides. (I  Br  Cl  F) halides (I  Br  Cl  F) Rearrangement No rearranged product is formed Rearranged products can be formed. (except for allylic). Nature of nucleophiles Favoured by strong and high Favoured by mild and low concentration of concentration of nucleophiles. nucleophiles. Polarity Favoured by solvents of low polarity. Favoured by solvents of high polarity. Reaction rate determining By steric hindrance.  By electronic factor (stability of R ). factor Catalysis Not catalysed by any catalyst (phase Catalysed by Lewis and Bronsted acids, e.g., transfer).  Ag , AlCl 3 , ZnCl 2 and strong HA. (2) Electrophilic substitutions reactions : of reaction is 1, it is written as S E 1 (unimolecular)and Electrophilic substitution involves the attack by an if the order is 2, it is S E 2 (Bimolecular). electrophile. It is represented as SE (S stands for substitution and E stands for elctrophile). If the order SE1 Reaction mechanism : Electrophilic substitution in SE2 Reaction mechanism : Electrophilic aliphatic compounds are very rare; some of the important substitution is very common in benzene nucleus examples are: (aromatic compounds) in which -electrons are (i) Replacement of the metal atom in an organometallic highly delocalized and an electrophile can attack   this region of high electron density. compound by hydrogen : R  M  H R  H  M In all electrophilic aromatic substitution  reactions, it involves:   MgBr H e.g., CH 3  CH 2  MgBr   CH 3  CH 2  CH 3  CH 3   Step 1. The formation of an electrophile, E, i.e.,    CH 3  CH 2  MgBr  H  Br CH 3  CH 2  H CH 3  CH 3  MgBr2 In halogenation; Cl  Cl  FeCl 3 Cl  Fe Cl 4 CH 3  CH 2 Na  C6 H6 CH 3  CH 3  C6 H5 Na In nitration; (ii) Decarboxylation of silver salt of carboxylic acid by means    HNO 3  2 H 2 SO 4 NO 2  2 HSO 4  H 3 O of bromine:       In sulphonation; 2 H 2 SO 4 SO 3  HSO 4  H 3 O R3 C  C  OAg  Br  Br R3 C  C  O  Br  Br  Ag || || In Friedel-crafts reaction; O O   R  Cl  AlCl 3 R  AlCl 4 R3C  Br  CO2  AgBr   (iii) Isotopic exchange of hydrogen for deuterium or tritium: RCOCl  AlCl 3 RCO  AlCl 4 H E Step 2. The electrophile H E theHaromatic attacks E    E Benzene + H E E  1026 General Organic Chemistry   ring to form carbonium ion (or arenium ion) RH D⇋ RDH which is stabilized by resonance.   R  H T ⇋ R T H Step 3. Carbonium ion loses the proton to form substitution product. The bromination of benzene in the presence of FeBr 3 is a example of electrophilic substitution reaction. Similarly, Nitration, sulphonation and Friedel-Crafts reaction…..etc., in benzene nucleus are the other examples of electrophilic substitution reactions. (3) Free radical substitution reactions : Free CH 2  CO radical substitution reactions involves the attack by a CH 3  CH  CH 2  N  Br   CCl 4 free radical. These reactions occurs by free radical Propene CH 2  CO mechanism which involves Initiation, Propagation and NBS Termination steps. Examples, CH 2  CO (i) Chlorination of methane : The chlorination of Br  CH 2  CH  CH 2  NH methane in the presence of ultraviolet light is an Ally lbromide CH 2  CO examples of free radical substitution. Succinimide CH 4  Cl 2  CH 3 Cl  HCl UV Addition reactions Methane light Methylchloride These reactions are given by those compounds (ii) Arylation of aromatic compounds (Gomberg which have at least one  bond, reaction) : The reaction of benzene diazonium halide O || with benzene gives diphenyl by a free radical i.e., ( C  C ,C  C , C , C  N ). In such substitution reaction. reaction there is loss of one  bond and gain of two  C6 H 5  H  C6 H 5 N 2 X   C6 H 5  C6 H 5  N 2  HX Alkali bonds. Thus product of the reaction is generally more Benzene diazonium halide Diphenyl stable than the reactant. The reaction is a spontaneous reaction. (iii) Wurtz reaction : Ethyl bromide on treatment Types of addition reactions : Addition reactions with metallic sodium forms butane, ethane and can be classified into three categories on the basis of ethylene by involving free radical mechanism. the nature of initiating species. (iv) Allylic bromination by NBS (N- (1) Electrophilic additions Bromosuccinimide) : NBS is a selective brominating (2) Nucleophilic additions agent and it normally brominates the ethylenic compounds in the allylic (CH 2  CH  CH 2 ) position. (3) Free radical additions This type of reaction involving substitution at the alpha (1) Electrophilic addition reactions carbon atom with respect to the double bond is termed (i) Such reactions are mainly given by alkenes Allylic subs