Electronic Effects, Acid-Base Strengthand Reaction Intermediates

Organic Chemistry – Some Basic Principles and Techniques ELECTRONIC EFFECTS, ACID-BASE STRENGTH AND REACTION INTERMEDIATES ELECTRONIC EFFECTS The effect which appears due to electronic distribution is called electronic effect. Classification : (a) It can also be defined as polarisation of one bond caused by polarisation of adjacent bond. (b) It is also called transmission effect. (c) It causes permanent polarisation in molecule, hence it is a permanent effect (d) The displacement of electrons takes place due to difference in electronegativity of the two atoms involved in the covalent bond. (e) The electrons never leave their original atomic orbital. (f) Its magnitude decreases with distance and it is almost negligible after 3rd carbon atom. Inductive Effect 1. The normal C–C bond has no polarity as two atoms of same electronegativity (EN) value are connected to each other. Hence the bond is nonpolar. Consider 2. a carbon chain in 1-chloro butane, Here due to more EN of Cl atom C – Cl bond pair is slightly displaced towards Cl atom hence creating partial negative ( –) charge over Cl atom and partial positive (+) charge over C1 atom. Now since C1 is slightly positive, it will also cause shifting of C1 – C2 bond pair electrons towards itself causing C2 to acquire small positive charge. Similarly C3 acquires slightly positive charge creating an induction of charge in carbon chain. Such an effect is called inductive effect. (g) The inductive effect is always operative through  bond, does not involve  bond electron. Types of inductive effect : (a) – I Effect : The group which withdraws electron cloud is known as – I group and its effect is called – I effect. Various groups are listed in their decreasing – I strength as follows. > – > – > – NO2 > –SO2R > –CN > – CHO > – COOH > – F > – Cl > – Br > – I > – OR > – OH > – C  CH > – NH2 > – C6H5 > – CH = CH2 > – H. Diagram showing I effect (b) + I effect : The group which release electron cloud is known as + I group and effect + I effect. The arrow shows electron withdrawing nature of – Cl group. Thus inductive effect may be defined as a permanent displacement of  bond pair electrons due to a dipole. (Polar bond) Some important points are: > > – C(CH3)3 > – CH (CH3)2 > – CH2 – CH3 > – CH3 > – D > – H The hydrogen atom is reference for + I and – I series. The inductive effect of hydrogen is assumed to be zero. Organic Chemistry – Some Basic Principles and Techniques (f) (g) Ex.56 Since – NO2 is – I group it pulls or withdraws electron from cyclohexane ring making it electron deficient (h) (i) 3. Applications of Inductive effect : (a) In deciding acidic strength of aliphatic carboxylic acids. Let us consider effect of COOH & – COO– in carbon chain + I effect  Basic strength (presence of + I groups increases basic character) Ex.57 The Bronsted - Lowry Definition of Acids and Bases Ex.58 Due to e¯ donating nature of carbon chain has become partially negative but – COOH is – I group therefore carbon chain has become partially positive. Ex.59 (a) CH3 CH2 According to the Bronsted - Lowry theory : An acid is a substance that can donate (or loose) a proton, and a base is a substance that can accept a proton. Acidic strength is directly related to stability of conjugate base –I stabilises conjugate base amd +I destabilises conjugate base. CH2 (b) (c) – I effect  Acidic strength (presence of – I groups increases acidic character) Ex.60 (I) CH2 CN (II) (III) H3C  H2C  H2C  CH  COOH | Cl CH3  CH2  CH  CH2  COOH | Cl CH3  CH  CH2  CH2  COOH | Cl (IV) CH3 – CH2 – CH2 – CH2 – COOH (d) Acid strength order I  II  III  IV Explanation : (e) We know that – I effect increases acidic strength. But it is distance dependent effect so where the – I group is nearest to – COOH, It exert strong effect and makes acid stronger. Organic Chemistry – Some Basic Principles and Techniques Ex.61 (I)O2N – CH2 – COOH Ex.65 (II) F – CH2 – COOH Stability : CH3  < CH3 < CH3 (III) H3CO – CH2 – COOH CH2  < CH3 CH3 (IV) CH3 – CH2 – COOH (f) Stability of carbanion stabilised by –I effect and destabilised by +I effect. Acid strength order I  II  III  IV Explanation : Since NO2 has strong – I effect its influence will make corresponding acid strongest (– I effect  acid character). > > > (b) Acidic Character of Alcohol : Greater - effect results in greater acidic character. Similarly greater Resonance & Mesomeric effect +  effect lowers acidic character. 1. Definition : Resonance is the phenomenon in which two or more structures involving in identical position of atom, can be written for a particular species, all those possible structures are known as resonating Ex.62 (a) Acidity : – CH2 – OH < – CH2 – structures or canonical structure. OH < – CH2 – OH < – CH2 – OH (b) Acidity : CH3 – OH > CH3 CH – OH > CH2 – OH > – OH (c) Dipole moment : Greater  effect results in greater dipole moment. Resonating structure are only hypothetical but they all contribute to a real structure which is called resonance hybrid. The resonance hybrid is more stable than any resonating structure. The P.E. difference between the most stable resonating structure and resonance hybride is called resonace energy. The stability of molecule is directly proportional to resonance energy. Ex.63  : CH3NO 2 > CH3COOH > CH3F > CH3OH (d) Stability of carbocation : Carbocations are electron deficient species and they are stabilised by + effect and destablised by –  effect. Because +  effect tends to decrease the positive charge and –  effect tends to increases the positive charge on (carbocation) Ex.64 Fig.1  Stability : CH3  < CH3 CH2  < (CH3 )2 C H < (CH3 )3 C (e) Stability of carbon free radical : Carbon free radical are stabilised by + I effect. The most stable resonating structure contribute maximum to the resonance hybrid and less stable resonating structure contribute minimum to resonance hybrid. Organic Chemistry – Some Basic Principles and Techniques Ex.66 (I) (II) (III)  (IV) (VI) The resonance energy of a resonance hybrid is the difference between the theoritical and experimental value of heat of hydrogenation of the compound.  (V) Fig.2  Catalyst + H2     – CH = CH2  + (–28.6 Kcal/mol) Accordingly, 2. Resonance Hybrid :- It is the actual structure of the species without violating the rules of covalence maxima for the atoms.  3. Resonance Energy:- The difference in energy between the hybrid and the most stable canonical structure is referred as the resonance energy. (as shown in Fig.2) O || CH3  CH  CH  C  CH3 .. (b) H2 N CH  CH  C  N (a) (b) + 3 × 28.6 = (–85.8 Kcal/mole) Therefore, benzene has [–85.8 – (–49.8)] Kcal/mol less energy than expected for a typical compound with three double bonds. Hence resonance energy of benzene molecule is– 85.8 + 49.8 = –36.0 Kcal/ mole. let us see resonating structures of given molecules Write resonating structure for each of the following molecules : Ex.67 (a) Catalyst + 3H2    Organic Chemistry – Some Basic Principles and Techniques (c) (c) (d) CH3 – O – CH = CH – (d) (e) (e)  .. (f) CH3  C  O : (f) (g) (g) (h) (h) (i) (i) (j) (j) (k) (k) (l) CH2 = C = O (l) (m) (m)   C H2 – C  O –2  N– N  N 3 Organic Chemistry – Some Basic Principles and Techniques The Rules of Resonance 4. Relative stability of the canonical form (a) All the canonical forms (resonating structure) (a) Nonpolar (uncharged) structure are most must have proper lewis structure. For instance none stable. Charge separation decreases stability. of them may have a carbon atom with five bonds. Separating opposite charges requires energy. Therefore, structures in which opposite charges are separated have greater energy (lower stability) than those that have no charge separation. (b) The positions of the nuclei of the atoms must remain the same in all of the structures. Structures 3 is not a resonance structure of 1 or 2, for example, because in order to form it we would have to move a hydrogen atoms and this is not permitted : (b) Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid. Ex.68 (II is more stable than I) (c) All atoms taking part in the delocalisation must lie in a plane so that orbitals overlaping become Ex.69 parallel to each other.  (II is more stable than I)   (c) Structures with more covalent bonds are more stable than other structures i.e.   z (d) All canonical forms must have the same number of unpaired electron. (e) The energy of actual molecule is lower than of any form. Therefore delocalisation is a stabilizing phenomenon. (f) All canonical forms do not contribute equally to the true molecule. The more stable structure is the greater contribution to its resonance hybrid. (d) Structure that carry negative charge on a more electronegative atom and positive charge on less electronegative atom are comparatively more stable. Organic Chemistry – Some Basic Principles and Techniques 7. Types of Mesomeric Effect : This is of two types (a) + m effect Ex.70  is more stable than ) Ex.71 5. (a) Positive Mesomeric effect (+ m effect) : When the group donates electron to the conjugated system it shows + m effect.   is more stable than ) Mesomeric effect : Mesomeric effect is defined as permanent effect of  electron shifting from multiple bond to atom or from multiple bond to single bond or from lone pair to single bond. Ex.74 (II) (III) Ex.72  and  are resonating structures of Cl – CH = CH2. This effect mainly operates in conjugated system of double bond. So that this effect is also known as conjugate effect. Ex.73 (IV) 6 Conjugation:- A given atom or group is said to be in conjugation with an unsaturated system if:(i) It is directly linked to one of the atoms of the multiple bond through a single bond. (ii) It has π bond, positive charge, negative charge, odd electron or lone pair electron. (a) CH2 = CH – CH = CH2 (Conjugation between C = C and C = C) (V) (b) (Conjugation between +ve charge and C = C) (c) (Conjugation between lone pair and C = C)  (d) CH2 = CH – CH2 (Conjugation between odd electron and C = C) (e) (Conjugation between negative charge and C = C) (b) – m effect (I) Organic Chemistry – Some Basic Principles and Techniques Ex.77 (VI) Ex.78 By drawing resonating structures of following molecules, Judge whether the group attatched to ring exerts + m or – m effect. Groups which shows '+ m' effect are : , – > – NH2 > – NHR > – NR2 > – OH > – OR > – NH  C  R > – O  C  R > –Ph, – CH = CH2 || O || O >–F> Sol. (a) O || – NH – C – CH3 (+ m group) – Cl > – Br > – I, – N = O. (b) Negative Mesomeric Effect (– m effect) : When the group withdraws electron from the conjugated system, it shows – m effect Groups which can show – m effect are – NO2 > – C  N > – SO3H > – CHO > C  R || O > C  OH || O Ex.75 Ex.76 (b) (– m effect group) : Organic Chemistry – Some Basic Principles and Techniques Note: 8. Difference between Inductive and Mesomeric effects:1. When a +m group and –m group are at metapositions with respect to each other then they are Inductive effect Mesomeric effect not in conjugation with each other, but conjugation (1) It is found in (1) It is found in with benzene ring exists. saturated and unsaturated compounds unsaturated compounds. (2) It involves partial shifting of sigma electrons. etc. 2. +m group increases electron density in benzene ring while –m group decreases electron density in the benzene ring. (3) The electron pair is slightly displaced from its position and thus partial charges are developed. (4) It is transmitted over a quite short distance. The effect becomes negligible after third atom in the chain (distance dependent). 9. Ex.79 Write electron density order in the following compound. especially having conjugated system. (2) It involves complete shifting of pi-electrons of pi-bonds or lone pair of electrons. (3) The electron pair is completely transferred and thus full positive and negative charges are developed. (4) It is transmitted from one end to other end of the chain provided conjugation is present. It is distance independent. Important points : (a) If any -bond has more than one  bond in conjugation, then only one  bond will take part in delocalisation. CH2  CH – C  CH (a) Ans. III > I > II > IV (b) Ans. I > II > III > IV  Out of two  bonds only one  bond will take part in delocalisation. (b) If any conjugate position has more than one lone pair then only one lone pair will take part in the delocalisation.  Out of the two lone pair ’s only one will take part in delocalisation. (c) If any conjugate position has  bond and any of the positive charge, negative charge, odd electron, lone pair electrons then only  bond will take part in delocalisation. (c) Ans.IV > I > III > II  Nitrogen has  bond as well as lone pair, but only  bond of nitrogen will take part in delocalisation. Organic Chemistry – Some Basic Principles and Techniques (d) Electrons of negative charge or lone pair 5. Hyperconjugation in toluene behave as 2 electrons if it is in conjugation to  bond. Behaves as 2 es  Behaves as 2 es Hyperconjugation 1. Hyper conjugation : It is delocalisation of sigma electron with p-orbital. Also known as  conjugation or no bond resonance. It may takes place in alkene, alkynes, carbocation, free radical, benzene nucleus. Necessary Condition : Presence of at least one hydrogen at saturated carbon which is  with respect to alkene, alkynes, carbocation, free radical, benzene nucleus. 2. (a) The effect of electron displacement due to this type of resonance is called hyperconjugative effect. (b) Since canonical forms of this resonance may not contain any bond between H and C so hyperconjugation is also known as no bond resonance. (c) These resonating structures only suggest that * There is some ionic character between C – H bond. * Carbon - carbon double bond acquires some single bond character. Hyperconjugation in alkene (d) Number of no bond resonating structures due to hyperconjugation = Number of -hydrogens (In aliphatic systems) 3. Hyperconjugation in carbocation Ex.80 4. Hyperconjugation in radical 6. Organic Chemistry – Some Basic Principles and Techniques (d) Dipole moment : Since hyperconjugation causes Applications of hyperconjugation the development of charge, it also affects the dipole (a) Stability of Alkenes :- More is the number of moment of the molecule. hyperconjugative structures more stable is the alkene. "More alkylated alkenes are more stable". Ex.83 (I) CH2 = CH2 < CH3 – CH = CH2 Stability of alkenes  no. of hyperconjugative structures. Example (Dipole moment) (II) H – CH = O < CH3 – CH – –H < CH3 – CH = (Dipole moment) (e) Stability of carbocation : (b) Heat of hydrogenation : Greater the number of  hydrogen results greater stability of alkene. Thus greater extent of hyperconjugation results lower value of heat of hydrogenation Stability of alkenes  no. of hyperconjugative structures  More hyperconjugation more is the stability. Ex.84 (a) < CH3 (b) CH3 – 1 – HHydrogenat ion < CH3 CH3 < (CH3)3 > CH3 – CH2 – CH2 – > > – (due to resultant of inductive effect and hyperconjugation) Ex.81 CH2 = CH2 > CH3 – CH = CH2 > CH3 – CH = CH – CH3 (HHydrogenation) (c) Bond Length : Bond length is also affected by hyperconjugation (f) Stability of free radical : Greater the number of -hydrogen results greater stability of carbon free radical Ex.85 (a) < CH3 – < CH3 – – CH3 < CH3 – Ex.82  (i) Bond length of C(II) – C(III) bond is less than normal C–C bond. (ii) Bond length of C(II) – C(I) bond is more than normal C=C bond. (iii) C–H bond is longer than normal C–H bond. (b) CH3 – > CH3 – CH2 – > > (due to resultant of inductive effect and hyperconjugation, both operates in same direction) ACIDS AND BASES Organic Chemistry – Some Basic Principles and Techniques Relative acidity of hydrocarbons (a) The Bronsted - Lowry Definition of Acids and Bases A ccor di ng to the Br onsted - Lowry theory, an acid is a substance that can donate (or loose) a proton, and a base is a substance that can accept a proton. Let us consider, an example of this concept, the reaction that occurs when gaseous hydrogen chloride dissolves in water : Being most electronegative the sp hybridised carbon atom of ethyne polarizes its C – H bond to the greatest extent causing its H to be most positive therefore ethyne is most acidic hence HC  CH > H2C = CH2 > H3C – CH3 Acidity of Phenols When phenol ionises the phenoxide ion is more stabilised by resonance than the unionised phenol. In phenol unlike charges are spread and so it is less stable. Hydrogen chloride, a very strong acid, transfers its proton to water. Water acts as a base and accepts the proton. The products that result from this reaction are a hydronium ion (H3O+) and a chloride ion (Cl–). The molecule or ion that forms when an acid loses its proton is called the conjugate base of that acid. (The chloride ion is the conjugate base of HCl). The molecule or ion that is formed when a base accepts a proton is called the conjugate acid of that base. (b) The Lewis Definition of Acids and Bases Lewis proposed that acids are electron pair acceptors and bases are electron pair donors. For example aluminiumchloride, reacts with ammonia in the same way that a proton donor does. Using curved arrows to show the donation of the electron pair of ammonia (the Lewis base), we have the following examples: Groups which are – I, – m increases acidic character of phenol of effectively dispersing negative charge of phenoxide ion. Alternatively + I and + m groups decreases acid strength. Cl¯ + H – Ex.86 acid strength order :I > II > IV > V > III Organic Chemistry – Some Basic Principles and Techniques Now Acidity of carboxylic acids step 1, III will be least acidic as it has no dispersion + H of negative charge (No mesomerism). step 2 since – I, – m group will increase acid strength, Nitrophenol will be most acidic followed by phenol, step 3 Amongest cresol and methoxyphenol, (i) (I) exists as two equivalent cannonical methoxyphenol has +m effect of – OCH3 which structures I(A) and I(B). This ion is resonance increases e¯ density hence decrease acidic strength stablised and resonance hybrid structure is I(C). Ex.87 acid strengh order : I > III > II > IV Step 1 : Notice that CH3 have + I effect so all methylphenols (cresols) are less acidic than phenol (I). Step 2 : Now amongest cresols p- and o- CH3 are increasing the e¯ density due to their hyper conjugation but ortho isomer has viable + I effect also, which will help in destabilising phenoxide ion therefore o- is least acidic. Since at meta position only + I works it as least e¯ density amongst the cresol  (ii) ion is more stable due to resonance, hence carboxylic acids are acidic in nature. (iii) Electron withdrawing group (–I effect) stablises the anion and hence, increases acidic nature. Ex.89 F – CH2 – COOH > Cl – CH2COOH > Br – CH2COOH > I – CH2COOH Ex.88 acid strength order : II > IV > III > I > > Step 1 In nitrophenols – I effect of NO2 will help to Ex.90 increase acidic strength hence phenol is least acidic amongst all nitrophenols Cl – CH2COOH > CH3COOH Step 2 Only – I effect is applicable in meta nitrophenol it will be number three. Now – o, – p have both – I and – m effect of NO2 group over OH and in this particular case para isomer is more acidic (iv) Electron releasing group (+ I effect) destablises the anion and hence decreases acidic nature. than ortho since H is trapped by NO2 group. Organic Chemistry – Some Basic Principles and Techniques Ex.91 HCOOH > CH3COOH > CH3 – CH2 – COOH Ex.92 > > Ortho effect It is common observation that generally ortho substituted benzoic acids are highly acidic as compared to their isomers and benzoic acids itself. This is called ortho effect in benzoic acid. However exceptions are seen. Ka order = ortho > para > meta > benzoic acid (2) G = (–I > +m) – Cl, Br, F, I acid strength order : II > IV > III > I Explanation the ortho isomer will be most acidic due to ortho effect . Ex.93 (1) (G = –m, –I) Ka order = ortho > meta > para > benzoic acid (3) G = (+m > –I) ..... OCH3 Ka order = meta  ortho > benzoic acid > para (4) G = (+I, H.C.) ........ R (Alkyl group) Ka order = ortho > benzoic acid > meta > para Comparison between two geometrical isomers Ex.94 1. Maleic acid Fumaric acid Organic Chemistry – Some Basic Principles and Techniques BASES Aliphatic Bases Ease of protonation is the basic nature Increasing strength of nitrogenous bases is related to readiness with which they are prepared to take up protons. Consider the following molecules , , , By visiting + I effect of methyl in above example we may expect basic nature as NH3 < MeNH2 < Me2NH < Me3N (Which is not true always) This is due the fact that basic strength of an amine in water is determined not only by ease of electron donation (protonation) of N atom but also by the extent to which cation so formed can undergo SOLVATION and become stabilised by H atom attatched to N atom greater is possibility of solvation via H bonding by water. Alkyl groups are hydrophobic and inhibits H bonding hence solvation. > Now K1m > K1f Since the conjugate base is stabilised by intramolecular H bonding. But K2f > K2m Since in maleate ion, after donation of two groups faces each other and makes system unstable. In fumarate ion this repulsion is minimised. Thus, due to two opposite effect viz. solvation of cation and + I effect. The jumbled order comes to be Me2NH > Me – NH2 > Me3N > NH3 It is also interesting to note that in gas phase basicity order of amines follows the usual order of Me3N > Me2NH > MeNH2 > NH3 Since only + I effect works and no solvation effect persists. Organic Chemistry – Some Basic Principles and Techniques Steric effect of ortho-substituted group Similarly : On the same reasons ethyl amines and other amine follows the following order for basic (ortho effect) : strength: Et3N > Et2NH > EtNH2 > NH3 H (gas phase)  Et2NH > Et3N > EtNH2 > NH3 (a) Ortho-substituted anilines are mostly weaker (water as solvent) bases than aniline itself. Aromatic amines (Ph – H2) or Anilines (b) Ortho-substituent causes steric hinderance to solvation in the product (conjugate acid i.e. cation). (c) The small groups like –NH2 or –OH do not experience (SIR) due to small size. When the lone pair lies in conjugation with a multiple bond, it resides in ‘2p’ atomic orbital, so that system can get resonance stabilisation. Aniline is a weaker base than NH3 because it has delocalised lone pair. Ex.97 (1) G = (–M, –); NO2 (a) (b) Ex.95 Which of them is strong base (c) In pyrole lone pairs are involved in resonacne therefore it is less basic. But in pyridine lone pairs are in perpendicular plane of  orbitals therefore no t involved in resonance Ex.96 Which of them is stronger base (d) (Aniline > m > p > o). (2) G = (–); CCl3 (Aniline > p > m > o). (3) G = (– > + m); Cl CH3 – NH2 Since ease of donation of lone pair of N is basicity, CH3 – NH2 is more basic due to + I effect of – CH3 group. Aryl amine aniline is very less basic since lone pair of N is involved in resonance. (Aniline > p > m > o) Only (–) decides the order. Organic Chemistry – Some Basic Principles and Techniques (4) G = (+, HC); If R = –CH3 (Toluidines) The other possibility is that the bond breaks so that each fragments takes away one of the electrons of the bond. This process, called homolysis (Gr. homothe same + lysis), produces fragments with unpaired electrons called radicals. Heterolysis of a bond normally requires that the bond be polarised. (5) G = (+m > –); Kb order : p > Aniline > o > m Polarisation of a bond usually result from different electronegatives of the atoms joined by the bond. The greater the difference in electronegativity, the greater the polarisation. In the given instance, atom B is more electronegative than A. Even with a highly polarised bond, heterolysis rarely occurs without assistance, because heterolysis requires separation of oppositively charged ions but oppositively charged ions attract each other, their separation requires considerable energy. Often, heterolysis is assisted by a molecule with an unshared pair that can form a bond to one of the atoms : REACTION INTERMEDIATE Dissociation of a Covalent Bond Homolysis and Heterolysis fission of Covalent Bonds Formation of the new bond furnishes some of the energy required for the heterolysis. Reactions of organic compounds always involve the Carbanion making and breaking of covalent bonds. A covalent Definition : A carbon intermediate which contain bond may break in two fundamentally different ways. three bond pair and a negative charge on it, is called The bond may break so that one fragment takes carbanion. away both electrons of the bond, leaving the other fragment with an empty orbital. This kind of cleavage, called heterolysis (Gr: hetero-different + lysisloosening or cleavage), produces charged fragments Ex.98 , Methyl carbanion or ions. The bond is said to have broken heterolytically: Ex.99 CH3 – Ethyl carbanion Ex.100 (CH3)2 Isopropyl carbanion Organic Chemistry – Some Basic Principles and Techniques Hybridisation : Hybridisation of carbanion may be Keto and Enol Tautomers sp3, sp2 & sp. The keto and enol forms of carbonyl compounds Hybridisation Example sp3 are constitutional isomers, but of a special type, because they are easily interconverted in the presence of traces of acids and bases, chemists use a special term to describe this type of constitutional isomerism. Interconvertible keto and enol forms are said to be tautomers, and their interconversion is called tautomerization. , CH3 – sp2 H 2C = sp , CH2 = CH – HC  Stability of carbanion : Carbanions are stabilised by electron withdrawing effect as (i) – I effect (ii) – m effect (iii) Delocalisation of charge Carbanions are Lewis bases. In their reactions they seek a proton or some other positive centre to which they can donate their electron pair and thereby neutralize their negative charge. Under most circumstances, we encounter keto - enol tautomers in a state of equilibrium. (The surfaces of ordinary laboratory glassware are able to catalyze the interconversion and establish the equilibrium). For simple monocarbonyl compounds such as acetone and acetaldehyde, the amount of the enol form present at equilibrium is very small. The greater stability of the following keto forms of monocarbonyl compounds can be related to the greater strength of the carbon-oxygen  bond compared to the carbon - carbon  bond ( ~ 364 versus ~ 250 kJ mol–1): Keto Form Acetaldehyde Ex.101 > > > (Stability order) Ex.102   CH2  CH – C H2 < C6H5 – C H2 <  •• ( C 6 H5 )3 C Acetone (Stability increasing order.) TAUTOMERISM Definition : Tautomerism is a phenomenon by which a single compound exists in two or more readily interconvertible structures that differ in the relative positions of at least one atomic nucleus, generally hydrogen. Cyclohexanone Enol Form Organic Chemistry – Some Basic Principles and Techniques In compounds whose molecules have two carbonyl Nitro-Acinitro system : groups separated by one – CH2 – group (called The acidic nature of the nitro compounds gives rise -dicarbonyl compounds), the amount of enol to the belief that the nitro compounds exist in two present at equilibrium is far higher. For example, forms, a more stable or normal nitro form and the pentane-2, 4 -dione exists in the enol form to an less stable acinitro form. extent of 76% in the liquid. The stability of the nitro form as compound to the acinitro form due to stabilised by resonance. Ex.103 The greater stability of the enol form of -dicarbonyl compounds can be attributed to stability gained through resonance stabilization of the conjugated double bonds and (in a cyclic form) through hydrogen bonding: Nitroso-Isonitroso system : Like primary and secondary nitro compounds, primary and secondary nitroso compounds also exhibit tautomerism with their more stable isonitroso or oxime form. Ex.104 , The conversion of a carbonyl compound into its enol is called enolization. Lactum-Lactim system : Imine - Enamine system : Among these two tautomers, enamines are stable only when there is no hydrogen on the nitrogen, otherwise the imine form predominates. Organic Chemistry – Some Basic Principles and Techniques Process of interconversion : (a) Base-catalyzed enolization involves the intermediate of an enolate ion, and is thus a consequence of the acidity of the -hydrogen. (c) Exchange of -hydrogens from deuterium as well as racemization at the -carbon are catalyzed not only by bases but also by acids due to the phenomenon of tautomerisation. Protonation of the enolate anion by water on the carbon gives back the carbonyl compound. Protonation on oxygen gives the enol. Notice that the enolate ion is the conjugate base of both the carbonyl compound and the enol. (b) Acid-catalyzed enolization Carbocation Definition : A carbon intermediate which contain Involves the conjugate acid of the carbonyl three bond pair & a positive charge on it is called compound. Recall that this ion has carbocation carbocation. characteristics. Loss of the proton from oxygen gives back the starting carbonyl compound ; loss of the proton from the -carbon gives the enol. Notice , Methyl carbocation that an enol and its carbonyl isomer have the same Ex.105 conjugate acid. CH 3 – Ethyl carbocation Organic Chemistry – Some Basic Principles and Techniques Hybridisation : Carbocation may be sp2 & sp hybridised Hybridisation Example Ex.106 sp2 sp H 2C = > > , HC  Stability : Carbocation are stabilised by (i) + I effect (iii) Hyperconjugation (ii) + m effect t-Butyl carbocation has +I effect of three Me – groups and also Hyperconjugation effect which makes it most stable. Carbocations are electron deficient. They have only six electrons in their valence shell, and because of this, carbocations are Lewis acids. In this way they are like BF3 and AlCl3. Most of the carbocations Ex.107 > > are short-lived and highly reactive, they occur as intermediates in some organic reactions. Carbocations react with Lewis bases or ions that In Benzyl cation, Extensive Resonance is seen which can donate the electron pair, that they need to achieve stabilises . a stable octet of electrons (i.e., the electronic In Ethyl carbocation + I and Hyperconjugation of configuration of a noble gas): Me – group stabilizes carbocation, in vinyl carbocation stability decreases rapidly since carbon of 'CH2' is sp2 hybridized which is slightly more electronegative hence acts as – I atom which increases (+) charge. Because carbocations are electron seeking reagents, chemists call them electrophiles. Electrophiles are reagents which in their reactions seek the extra electrons that will give them a stable valence shell of electrons. All Lewis acids, including protons, are electrophiles. By accepting an electron pair, a proton achieves the valence shell configuration of helium; carbocations achieve the valence shell configuration of neon. Rearrangement : Whenever an Intermediate carbocation is formed in reaction it rearranges to a more stable one. Not all carbocations rearrange but only those which can produce more stable species can only rearrange. Consider the following reactions. (a) This reaction involves formation of carbocation but no rearrangement. Organic Chemistry – Some Basic Principles and Techniques Free radical (b) Ionic reactions are those in which covalent bonds break heterolytically and in which ions are involved as reactants, intermediates, or products. Another broad category of reaction mechanisms that involve homolysis of covalent bonds with the production of intermediates possessing unpaired electrons called radicals (or free radicals): (c) Table : 1 Carbocation rearrangement can also occur by ring expansion.   > Ex.108    H3 C – C H – CH3  H3 C – C H2  C H3 One very stable carbocation reported is (Stability order) cyclopropylmethyl carbocation. This unique stabilisation is seen in this case of three member ring Ex.109 (C H ) C > (C H ) C H > C H – C H 6 5 3 6 5 2 6 5 2 only.  (more stable than Benzyl) cyclopropyl methyl carbocation > CH2  CH – C– CH3 (Stability order) | CH3 Organic Chemistry – Some Basic Principles and Techniques Divalent Carbon intermediates : Carbenes Nitrenes Definition : There is a group of intermediates in The nitrogen analog of carbenes are nitrenes. They which carbon forms only two bonds. These neutral are very reactive since in them octet of N is divalent carbon compounds are called carbenes. incomplete. Most carbenes are highly unstable compounds that In nitrenes only one valencies of N are satisfied. are capable of only fleeting existence. Soon after carbenes are formed, they usually react with another molecules. The reactions of carbenes are especially Benzyne interesting because, in many instances, the reactions The benzene ring has one extra C – C  bond in show a remarkable degree of stereospecificity. benzyne Methods of preparation of carbene : CHCl3 + : CCl2 CH2I2 + Zn : CH2 CH2N2 N2  + : CH2 CH2 = C = O : CH2 + CO Clearly, we can see that the newly formed  bond cannot enter in resonance with other  orbitals of ring. since it is in perpendicular plane. Types of carbene Singlet Triplet Table : 2 Shape Bent Linear Hybridisation sp2 sp Nature of reaction stereospecific None State Excited state Ground state Magnetic Diamagnetic Paramagnetic Nature Diamagnetic Diradical It is also important to note that hybridisation of each carbon involved in 'Benzynic bond' is sp2 since the overlap between these orbitals is not so much effective.