General Organic Chemistry-II

General Organic Chemistry-II Applications of electronic effects: Th1: Reaction intermediates Reactive, shortlived, high energy, unstable species, those are formed in the course of organic reactions are called reaction intermediates. Reaction intermediates generally formed after bond breaking and before bond formation. A covalent bond can get cleaved by: (i) Heterolytic cleavage (ii) Homolytic cleavage. Homolytic fission of covalent bonds: The bond may break in such a way that each fragment takes away one of the electrons of the bond. This process is called homolysis, produces fragments with unpaired electron called radicals. Heterolytic fission of covalent bonds: The bond breaks in such a way that one fragment takes away both electrons of the bond, leaving the other fragment with an empty orbital. This kind of cleavage called heterolysis, produces charged fragments or ions. Heterolysis of a bond normally requires polarized bond. 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. Section (A) : Carbanions D1: A carbon intermediate which contain three bond pair and a negative charge on it, is called carbanion. Hybridisation: Hybridisation of carbanion may be sp3, sp2 or sp. Hybridisation Example , CH3– sp3 sp2 H2C= sp HC , , CH2=CH– , , , 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. ⎯⎯ → General Organic Chemistry-II ⎯⎯ → Examples of stability order: Ex. (a) CH3 CH 3 1 (b) CH  C .. 1 .. CH2 CH2 CH 3 CH2 2 CH3 CH3 C CH3 (Stability order: 1>2>3>4) CH3 4 3 (Stability order: 1 > 2 > 3) .. .. (c) (Stability order:1 > 2 > 3) 1 2  (d) CH2 = CH – C H2 1 3  ••  C6H5 – CH2 (Stability order : 1 < 2 < 3) 2 3 Rearrangement: Generally carbanions do not undergo rearrangement. (C6H5 )3 C Section (B) : Carbon free radicals D2: Free radical: An uncharged intermediate which has three bond pair and an unpaired electron on carbon. Homolysis of covalent bond results into free radical intermediates possess the unpaired electrons. It is generated in presence of sun light, peroxides or high temperature Note: (i) It is neutral species with odd e–. (ii) It is paramagnetic in nature due to odd e–. (iii) Rearrangement is not observed generally. (iv) Carbon atom having odd electron is in sp2 hybridised state (v) Any reaction if it is carried out in the presence of sunlight, peroxide or high temperature it generally proceeds via free radical intermediate.  Ex. Stability of free radical: It is stabilised by resonance, hyperconjugation and + I groups. (a) < CH3– < CH3– –CH3 < CH3– (Stability order) (b) CH3– > CH3–CH2– > > (Stability order) (Due to resultant of inductive effect and hyperconjugation, both operates in same direction) • • • (c) (C6H5 )3 C• > (C6H5 )2 CH > C6H5 – CH2 > CH2 = CH – C– CH3 | CH3 (Stability order) General Organic Chemistry-II Section (C) : Carbocations D3: A carbon intermediate which contain three bond pair & a positive charge on it is called carbocation. Hybridisation: Carbocation may be sp2 or sp hybridized. Hybridisation Example sp2 sp , , H2C= , HC , Carbocations are electron deficient. They have only six electrons in their valence shell, and because of this, carbocations act as Lewis acids. Most of the carbocations are short-lived and highly reactive, they occur as intermediates in some organic reactions. Carbocations react with Lewis bases or ions that can donate the electron pair that they need to achieve a stable octet of electrons (i.e., the electronic configuration of a noble gas): ⎯⎯ → ⎯⎯ → Because carbocations are electron seeking reagents, chemists call them electrophiles. 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. Stability: Carbocations are stabilised by (i) +M effect (ii) Delocalisation of charge (iii) Hyperconjugation (iv) +I effect General stability order: > > > Ex. > > > > > > t-Butyl carbocation has +I effect of three Me–groups and also Hyperconjugation effect which makes it most stable. Ex. > > In Benzyl cation, extensive resonance is seen which stabilises C. In Ethyl carbocation +I and hyperconjugation of 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 group which increases (+) charge density. General Organic Chemistry-II Rearrangement of carbocations: (i) Whenever an Intermediate carbocation is formed in reaction it may rearranges. Only those carbocation will rearrange which can produce more stable species. It can be done either by (i) Shifting of H, alkyl, aryl, bond (1, 2 shifting) (ii) Ring expansion (more strained ring to less strained ring) (iii) Ring contraction Shifting of H, alkyl, aryl, (1, 2 shift) 1,2 H (a) ⎯⎯⎯⎯ → (b) ⎯⎯⎯⎯→ Shifting 1,2 Me Shifting 1,2 Ph ⎯⎯⎯⎯→ (c) Shifting – + CH2 (ii) 1,2 Bond ⎯⎯⎯⎯⎯ → Ring expansion: Shifting (iii) 1,2 Bond ⎯⎯⎯⎯⎯ → Ring contraction: Shifting Note: Cyclopropylmethyl cation: In solvolysis of simple primary cylcopropylmethyl systems the rate is enhanced because of the participation by the -bonds of the rings. The ion that forms initially is an unarranged cyclopropylmethyl cation that is symmetrically stabilized, that is, both the 2, 3 and 2, 4bonds help stabilize the positive charge. Cyclopropyl group stabilizes on adjacent positive charge even better than a phenyl group. 3  1 2 4  ~ 48%  ~ 5% ~ 47% This special stability, which increases with each additional cyclopropyl group, is a result of conjugation between the bend orbitals of the cyclopropyl rings's and the vacant P-orbital of cationic carbon.  Q. Rearrange (if applicable) the following carbocations into more stable form: CH3 (a) + CH3 CH3 + –– – CH3 (d) (c) CH3 – C – CH – CH3 + CH3 (b) (e) (f) General Organic Chemistry-II D4: Carbon free radical Carbocation Carbanion Shape trigonal planar trigonal planar Pyramidal Hybridisation sp2 sp2 sp3 No. of electrons in outermost shell 7 6 8 Carbenes (Divalent Carbon intermediates) : There is a group of intermediates in which carbon forms only two bonds. These neutral divalent carbon species are called carbenes. Most carbenes are highly unstable that are capable of only fleeting existence. Soon after carbenes are formed, they usually react with another molecules. Methods of preparation of carbene : CHCl3 + – ZnI2 → : CH2 CH2I2 + Zn ⎯⎯⎯⎯ –HCl ⎯⎯⎯→ : CCl2  h or  CH2N2 ⎯⎯⎯⎯ ⎯ → N2 + : CH2 Types of carbene D5:  CH2 = C = O ⎯⎯ → : CH2 + CO Singlet Triplet Shape Bent Linear Hybridisation sp2 sp Nature of reaction stereospecific None State Excited state Ground state Magnetic Diamagnetic Paramagnetic Nature Paired electrons Diradical Nitrenes: The nitrogen analog of carbenes are nitrenes. They are very much reactive since in them octet of N is incomplete. In nitrenes only one valencies of N are satisfied. D6: Benzyne : The benzene ring has one extra C–C  bond in benzyne Clearly, we can see that the newly formed  bond cannot enter in resonance with other  orbitals of ring. Since it is in perpendicular plane. It is also important to note that hybridisation of each carbon involved in 'Benzynic bond' is sp2 since the overlap between these sp2 hybrid orbitals is not so much effective. General Organic Chemistry-II Section (D) : Basic strength Th2. Bases D7: D8: (a) Arhenius base: Base is a substance that can donate (or loose) OH– ions in H2O. (b) The Bronsted Lowry definition of acids and bases: 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 : D9: (c) 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. ⎯⎯ → Cl¯ + H– ⎯⎯ → (d) Basicity (Kb): Kb = [NH4+ ] [OH− ] [NH3 ] pKb = – log Kb Basicity order in periodic table: (1) Basic strength decreases down the group, (2) Basic strength decreases along the period because electron neagativity increases, so electron donor tendency decreases. 1.1 Aliphatic bases: (1) On the basis of +I effect basic strength of amines should be 3º > 2º > 1º > NH 3 But this order is applicable only when the amines are in gaseous state or in case of non-polar aprotic solvent. (2) Basic strength of nitrogeneous compound depends upon the hybridised state of nitrogen R–CH2 –CH2 –NH2 > R –CH2 –CH = NH > R–CH = CH–NH2 > R–CN     sp3 (localised) sp2 (localised) sp2 (delocalised) sp (3) More electronegative atom (–I group) will decrease the basic strength CH3–CH2–CH2–NH2 > CH2=CH–CH2–NH2 > H–CC–CH2–NH2 (4) Cyclic amines are more basic than acyclic amines of same nature > General Organic Chemistry-II (5) Amidines are more basic in nature because their conjugate acid are more stable due to resonance. ⎯⎯⎯ → H Nitrogen (X) is more basic than nitrogen (Y). Th3. Basic strength of aromatic amines and substituted anilines: (a) Aniline: Lone pair of aniline lies in conjugation with a multiple bond, it resides in ‘2p’ atomic orbital, so that it can get resonance stabilisation and hence, basic strength decreases. So, Aniline is a weaker base than NH3. – – H H N: –    A.O. Diagram:  – S.F. : H   N 2 sp hybrid orbital (localised lone pair) H H  (b) Pyridine (C5H5N) : 6- electrons, aromatic Lone pair of N in pyridine is localised so it is more basic than aniline. (c) Pyrrole (C4H5N:) : 6- electrons, aromatic  (lone pair in 2p A.O.)  •• N–H (sp2) S.F. :  A.O. Diagram:   N–H Complete delocalisation of e — Lone pair of N in pyrrole is delocalised in the aromaticity so it is very less basic than aniline. > > – – N  :NH2  H H – (4e–) + 3 N ––– sp H H Non-aromatic – – NH3 Aromatic – + N H Aromatic  N H H (d) Ortho substituted anilines : Electron releasing groups (ERG) +M, HC, + increases the Kb and Electron withdrawing groups (EWG) –m, – decreases the Kb – – – – – Steric effect of ortho-substituent in aniline (ortho effect) : H + H–N–H H2N: G G H (conjugate acid) ⎯⎯⎯ → General Organic Chemistry-II (i) Ortho-substituted anilines are mostly weaker bases than aniline itself. (ii) Ortho-substituent causes steric hinderance to solvation in the product (conjugate acid i.e. cation). (iii) The small groups like –NH2 or –OH do not experience (SIR) due to small size. Ex. (a) G = (–M, –); NO2 – NH2 (Aniline > m > p > o). (b) G = (–); CCl3 NH2 – – – – NH2 NH2 NH2 – CCl3 Only (–) decides the order. (c) G = (– > + m); Cl NH2 NH2 Cl (Aniline > p > m > o). CCl3 NH2 NH2 – – – – – – CCl3 NH2 NH2 – – (Aniline > p > m > o) Cl – – Only (–) decides the order. (d) G = (+, HC); If R = –CH3 (Toluidines) NH2 – – Cl CH3 – – m + CH3 w + HC → more do min ating (e) G = (+M > –); Kb order : p > Aniline > o > m General basicity order : Th4. Solvent effect in bases : The trend is not regular in the aqueous state as evident by their pKb values given in Table. Name of amine pKb Name of amine pKb Methanamine 3.38 N,N-Diethylethanamine 3.25 N-Metheylmethanamine 3.27 Phenylmethanamine 4.70 N, N-Dimethylmethanamine 4.22 Aniline 9.38 Ethanamine 3..29 N-Methylaniline 9.30 N-Ethylethanamine 3.00 N,N-Dimethylaniline 8.92 Table : pKb Values of Amines in Aqueous Phase (Ref. NCERT) General Organic Chemistry-II In the aqueous phase, the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules. The greater the size of the ion (Alkyl groups are hydrophobic and inhibits H bonding and solvation.), lesser will be the solvation and the less stabilised is the ion. The order of solvation of ions are as follows: OH2 | + R N > OH2 R | | | | > R | H +| N > R H R 20 H OH2 0 3 Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. On the basis of above two sequences, we can say that the basic strength of amines is the combined effect of inductive effect, steric hindrance and solvation. R2NH > RNH2 > R3N > NH3 (if R = CH3) ......... (1) R2NH > R3NH > RNH2 > NH3 (if R = C2H5) ..........(2) Th5. Reactions of bases : (i) Nature of aqueous solution: Amines combine with water to form alkyl ammonium hydroxides. This gives hydroxide ions in solution, thus the aqueous solution of amines is basic in nature. + RNH2 + HOH RNH3OH– [RNH3]+ + OH– 1º Amine + R2NH + HOH R2 NH2OH– [R2NH2]+ + OH– 2º Amine The aqueous solution of amines behave like NH 4OH and gives the precipitate of ferric hydroxide with ferric chloride. (ii) 3RNH3OH + FeCl3 ⎯⎯→ Fe(OH)3 + 3RNH3Cl Brown ppt. Aliphatic and aromatic amines form salt because of their basic nature: + – RNH2 + H2O ⎯⎯→ R NH3 OH Alkylammoniumhydroxide + – RNH2 + HCI ⎯⎯→ RNH3 CI Alkylammoniumchloride + RNH2 + H2SO4 ⎯⎯→ ( R NH3 )2 SO4–2 Similarly we get + – C6H5NH2 + H2O ⎯⎯→ C6H5 NH3 O H Aniliniumhydroxide + – C6H5NH2 + HCI ⎯⎯→ C6H5 NH3 CI or C6H5NH2 : HCI Aniliniumchloride Anilinehydrochloride Salts of amines are ionic compounds and hence water soluble. General Organic Chemistry-II Section (E) : Acidic strength Th6. Acids D10: D11: (a) Arhenius acid: An acid is a substance that can donate (or loose) a proton in H 2O. (b) The Bronsted Lowry definition of acids and bases: 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 : 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 (H 3O+) 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. D12: (c) The Lewis definition of acids and bases Lewis proposed that acids are electron pair acceptors and bases are electron pair donors. ⎯⎯ → (d) Acid dissociation constant (Ka) : RCOOH + H2O RCOO– + H3O+ Keq = [RCOO− ] [H3O+ ] [RCOOH] [H2O] Ka = [RCOO− ] [H3O+ ] [RCOOH] pKa = – log Ka 6.1 Relative acidity of hydrocarbons : 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 hydrocarbon. HC  CH > H2C = CH2 > H3C – CH3 6.2 Acidity of phenols : The phenoxide ion is more stabilised by resonance than the unionised phenol. Groups which are –I, –m increases acidic character of phenol because effectively dispersing the negative charge of phenoxide ion. Alternatively + I and + m groups decreases acid strength. Ex. (a) Ans. Acid strength order : I > II > IV > V > III General Organic Chemistry-II Sol. Step-1. III will be least acidic as it has no dispersion of negative charge (No delocalisation of negative charge). Step-2. Since –I, –m group will increase acid strength, Nitrophenol will be most acidic followed by phenol, Step-3. Amongst cresol and methoxyphenol, methoxyphenol has +M effect of –OCH3 which increases e– density hence decrease acidic strength Ex. (b) Ans. Sol. 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 amongst 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. Ex. (c) Ans. Sol. Acid strength order: II > IV > III > I Step-1 : In nitrophenols –I effect of NO2 will help to increase acidic strength hence phenol is least acidic amongst all nitrophenols 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 than ortho since H is trapped by NO2 group. Table : pKa values of some phenols and Ethanol. (Ref. NCERT) Compound Formula pKa Compound Formula o-Nitrophenol o–O2N–C6H4–OH 7.2 o-Cresol o-CH3–C6H4–OH pKa 10.2 m-Nitrophenol m–O2N–C6H4–OH 8.3 m-Cresol m-CH3C6H4–OH 10.1 p-Nitrophenol p-O2N–C6H4–OH 7.1 p-Cresol p-CH3–C6H4–OH 10.2 Phenol C6H5–OH 10 Ethanol C2H5OH 15.9 From the above data, you will note that phenol is million times more acidic than ethanol. 6.3 Acidity of carboxylic acids : Conjugate base of carboxylic acid exists as two equivalent cannonical structures (A) and (B). This ion is resonance stablised and resonance hybrid structure is (C).  Electron withdrawing group (–M, –I effect) increases acidic nature. Electron releasing group (+M, +I effect) decreases acidic nature. General Organic Chemistry-II Ex. (a) F–CH2–COOH > Cl–CH2COOH > Br–CH2COOH > I–CH2COOH Cl Cl (b) Cl – C – COOH > Cl – CH – COOH > Cl–CH2COOH > CH3COOH Cl (c) HCOOH > CH3COOH > CH3–CH2–COOH COOH > CH2 – COOH COOH CH – COOH COOH 2 The effect of the following groups in increasing acidicty order is [Ref. NCERT] Ph < I < Br < Cl < F < CN < NO2 < CF3 Thus, the following acids are arranged in order of decreasing acidity (based on pK a values) : CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC–CH2COOH > FCH2COOH > ClCH2COOH > BrCH2COOH > HCOOH > ClCH2CH2COOH > C6H5COOH > C6H5CH2COOH > CH3COOH > CH3CH2COOH (d) COOH > CH2 6.4 Comparison between two geometrical isomers : Maleic acid : Fumaric acid : Now K1m > K1f Since the conjugate base is stabilised by intramolecular H bonding. But K2f > K2m Since in maleate ion, after donation of H two –COO groups faces each other and makes system unstable. In fumarate ion this repulsion is minimised. 6.5 Acidic strength of substituted benzoic acids : Formic acid is more acidic than benzoic acid while phenyl acetic acid is more acidic than acetic acid. HCOOH > PhCOOH > > Electron withdrawing group attached to benzene ring will increase the acidic strength while electron releasing group decreases acidic strength. If electron donating group present at para position than it is always less acidic than benzoic acid. > > > > Also it is less acidic then meta substituted benzoic acid. > > On the other hand if e– withdrawing group is present at meta position then it is more acidic than benzoic acid. General Organic Chemistry-II > > > > Th7. Ortho effect : D13. It is common observation that generally ortho substituted benzoic acids are more acidic as compared to their isomers and benzoic acids itself. This is called ortho effect (which is combined effect of steric hindrance, crowding & electronic effect) in benzoic acid. However exceptions are seen. Ex. (a) > (b) > > > > > COOH (c) > COOH > > NO2 NO2 (d) > (e) > > (f) > > > > > OH NO2 (g) OH NO2 NO2 > > O2N NO2 (picric acid) OH O2N NO2 NO2 NO2 General Organic Chemistry-II Section (F) : Feasible reactions of acids and bases Th8. Reaction of acid with salt: (1) NaX Salt of Weak acid + HY ⎯→ Strong Acid NaY + HX Remark: A stronger acid displaces the weaker acid from weak acid metal salt. The weaker acid is released out as a gas or liquid or precipitates out as a solid. The weaker acid cannot displace the stronger acid from the salt. 1. 2 NaCl + H2SO4 ⎯→ Na2SO4 + 2HCl 2. Na2SO4 + 2HCl ⎯→ No reaction 3. CH3COONa + CH3SO3H ⎯→ CH3COOH + CH3SO3Na (feasible) 4. CH3COONa + PhOH ⎯→ PhONa + CH3COOH (not feasible) Section (G) : Tautomerism Th9. Tautomerism D14: 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. These two isomers remain in dynamic equilibrium and can be isolated also give different lab test. Conditions : 1. Usually present in the following functional groups 2. Basic need for its existence is attachment of these groups with the sp 3 hybridised C-atom having atleast one hydrogen atom as – To get tautomer of above structures -hydrogen atom is shifting to more electronegative atom attached to double bond (i.e. hydrogen atom from 1st atom to 3rd atom) and double bond is developed between 1,2-atom from 2,3-atom. This can be represented as: These two forms (remain in equilibrium) are called tautomers of each other. The interconvertibility of tautomers is a chemical reaction which involves making and breaking of bonds. Ex. O || (a) CH3 − C − CH3 ( I) (c) OH | CH2 = C − CH3 ( II) (b) (d) General Organic Chemistry-II Th10. Keto-enol tautomerisation : 10.1 (A) Base-catalyzed enolization : Protonation of the carbanion 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 : Involves the conjugate acid of the carbonyl compound. Recall that this ion has 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 that an enol and its carbonyl isomer have the same conjugate acid. 10.2 % Enol content in the carbonyl compounds : (a) For monocarbonyl % Enol is very less (b) Enol content increases with increase in the stability of enol by resonance, hyperconjugation, hydrogen bonding etc. Decreasing order of enol content for above carbonyl compounds is : 3 > 2 > 1. (c) For a carbonyl compound having active methylene group percentage of enol content will be more because enolic form has intramolecular H-bonding and also it will be stabilised by resonance. General Organic Chemistry-II (d) If active methylene group is more acidic then enol content will be more. For example in acetyl acetone enolic content is 75–76% while it is 7–8% in acetoacetic ester because ester group shows less electron withdrawing nature than keto group. (e) Percentage of enol content is more in non-polar media while % of keto form is more in polar media. Th11. Racemisation and D-exchange : Exchange of -hydrogen 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. 1. (D-exchange) 2. 3. Th12. Other examples of tautomers (not to be done in class only for the reference of students) (a) Imine-Enamine system : R2CH – CR = NR Imine Among these two tautomers, enamines are stable only when there is no hydrogen on the nitrogen, otherwise the imine form predominates. (b) Nitroso-oxime system: Like primary and secondary nitro compounds, primary and secondary nitroso compounds also exhibit tautomerism with their more stable isonitroso or oxime form. NO | CH3 – C – CH3 ; CH3 – CH – CH3 || 2-Nitrosopropane NOH Acetoneoxime General Organic Chemistry-II (c) Nitro-Acinitro system: The acidic nature of the nitro compounds gives rise to the belief that the nitro compounds exist in two forms, a more stable or normal nitro form and the less stable acinitro form. R – CH2 | N O O R – CH || N HO O Nitroform Acinitroform The stability of the nitro form is more as compared to the acinitro form because it is stabilised by resonance. O OH C 6H 5 – CH = N C6H5 – CH2 – N O O Acinitroform Nitroform (d) Lactam-Lactim system : O O N | H O O || C N OH OH | C N NH ; SO2 SO2 CHECK LIST Definitions (D) D1 : Carbanion D2 : Free radicals D3 : Carbocation D4 : Carbenes (Divalent carbon intermediates) D5 : Nitrenes D6 : Benzyne D7 : Arhenius base D8 : Bronsted base D9 : Lewis definition of acids and bases D10 : Arhenius acid D11 : Bronsted acid D12 : Lewis definition of acids and bases D13 : Ortho effect D14 : Tautomerism Theories (Th)               Th1 : Reaction intermediates Th2 : Th3 : Bases Basic strength of aromatic amines and substituted anilines Th4 : Solvent effect in bases Th5 : Reactions of bases Th6 : Acids Th7 : Ortho effect Th8 : Reactions of acids with salts Th9 : Tautomerism Th10 : Keto-enol tautomerisation Th11 : Racemisation and D-exchange Th12 : Other examples of tautomers            