Subject Chemistry Paper No and Title Module No and Title Module Tag Paper 1: ORGANIC - I (Nature of Bonding Module 4: Applications of Electronic Effects CHE_P1_M4 PAPER 1: ORGANIC - I (Nature of Bonding
TABLE OF CONTENT 1. Learning outcomes 2. Introduction 3. Effect on Stability of carbon intermediates 3.1 Stability of carbocations 3.2 Stability of carbanions 3.3 Stability of carbon free radicals 4. Effect on Acidity 5. Effect on Basicity 5.1 Basicity of Aliphatic Amines 5.2 Basicity of aromatic Amines 5.3. Basicity of amides 6. Effect on electrophilic substitution in substituted benzenes 7. Summary PAPER 1: ORGANIC - I (Nature of Bonding
1. Learning Outcomes After studying this module you shall be able to: 2. Introduction Apply electronic effects to compare stability of carbon intermediates Of the three permanent effects, in most cases, the dominant one is mesomeric effect, followed by hyperconjugative and then inductive effect. One important exception is that in case of halogens attached to conjugated systems like benzene, -I is more dominant than +M. These electronic effects have a very vast application and impact on the various properties of the organic molecules/species. Let us try to understand effects of these electronic effects on the stability of carbon intermediates, effect on acidity and basicity. 3. Effect on Stability of carbon intermediates In order to understand the stability of any charged species, keep in mind that more is the delocalization of the charge, greater is the stability. Electron donating groups (+I, +M and +H) hence, stabilize electron deficient species like carbocation and free-radicals. On the other hand, Electron wthdrawing groups (-I and -M ) hence, stabilize electron rich species i.e, carbanions. 3.1 Stability of carbocations The carbocations are generally unstable due to electron deficiency at the positively charged carbon atom but any effect which decreases the positive charge on this carbon increases their stability. This order of their relative stability is This can be explained as follows. The +I effect of the alkyl group pushes two electrons towards the electron deficient carbon which may be summarised as 1 o < 2 o < 3 o. However carbocations like allyl and benzyl are stable due to resonance. Thus, PAPER 1: ORGANIC - I (Nature of Bonding
Allyl and benzyl carbocations have slightly greater stability than secondary carbocations but are less stable than tertiary carbocations as phenyl and CH 2 = CH groups are not good electron donating groups as is evident from the fact that C 6H 5COOH and CH 2= CH - COOH are stronger acids than acetic acid. Substituted allyl and substituted benzyl carbocations have greater stability than tertiary carbocations. This is because of both resonance and inductive effect. Vinyl carbocations, on the other hand, are least stable. Thus the overall stability is in the following order: 3.2 Stability of carbanions The carbanions are unstable due to a negative charge on carbon and any factor which increases this negative charge makes them more unstable. Therefore, the groups with +I effect decreases their stability. Hence the order of their relative stabiltiy is the reverse of carbonium ions and is as follows: due to the destabilising +I effect of alkyl groups. However, allyl and benzyl carbanions are as usual more stable due to resonance. Thus, Carbanion is more stable than due to electron withdrawing -I effect of the chloro group. 3.3 Stability of carbon free radicals Free radicals are unstable due to unpaired electron and electron deficiency at the carbon atom and electron donating effect increases their stability. This order of their relative stability is PAPER 1: ORGANIC - I (Nature of Bonding
This is due to the increase in the number of +I effect of the alkyl group. This order may be summarised as 1 o < 2 o < 3 o. However carbon free radicals like allyl and benzyl are stable due to resonance. Thus, Allyl and benzyl carbocations have greater stability than tertiary, secondary and primary free radicals.. Thus the overall stability is in the following order: Let us extend this understanding for the comparison of carbocations which are joined to hetero atoms/ groups with the help of following examples. 1. Stability of following carbocations (A, B and C) Of the three carbocations, let us compare the differentiating groups amongst them which are CH 3, -OCH 3 and CHO respectively. Let us write what effect they are exerting on the carbocation centre? In A, the group are CH 3 is exerting both +I and +H effect, of which +H is dominant effect. In B, the group -OCH 3 is exerting both -I and +M effect, of which +M is dominant effect. In C, the group are CHO is exerting I effect only. Note that it is not exerting M effect as it is joined to a carbocation and not conjugated system. It cannot withdraw electrons by M as there are no electrons further! Clearly the most stable is B (because of +M), followed by A (because of +H) and C is least stable (because of I). PAPER 1: ORGANIC - I (Nature of Bonding
2. Stability of following carbanions(a, B and C) Of the three carbanions, let us compare the differentiating groups amongst them which are CH 3, -OCH 3 and CHO respectively. Let us write what effect they are exerting on the carbanion centre? In A, the group are CH 3 is exerting +I effect only. Note that it is not exerting +H effect as it is joined to a carbanion. In B, the group -OCH 3 is exerting -I effect only. Note that it is not exerting +M effect, as the next carbon is already electron rich, how can it give the electron pair? In C, the group are CHO is exerting I and M effects, of which M is dominant. Clearly the most stable is C (because of -M), followed by B (because of I) and A is least stable (because of +I). 3. Stability of following carbocations (A, B, C, D and E) Before we analyse the effects of the groups, remember that : Whenever two groups in a phenyl ring are - At ortho or para positions, they exert all effects (M, H and I) on each other - At meta position, they exert only inductive effect on each other (and not mesomeric or hyperconjugative) as they are not in conjugated positions The differentiating groups have been highlighted and the effects they are exerting on the carbocation centre are shown as below PAPER 1: ORGANIC - I (Nature of Bonding
Hence the order of stability is B>C>A>E>D 4. Effect on Acidity In a molecule having an electronegative atom joined directly to H atom, the acidity is affected by the following two factors: 1. Ease of Deprotonation: Higher the electronegativity of the electronegative atom, higher is the acidity. The higher electronegativity ensures easy deprotonation. 2. Stability of conjugate base (or the ion being formed): The conjugate base must be stable. Higher electronegativity helps in the stability of the anion. Both these conditions are enhanced by introducing electron withdrawing groups (-M, -H or I )and decreased by electron donating groups (+I, +M, +H). Remember that the second condition is more dominant one than the first to arrive to the conclusion. Hence if two are opposing, rely on the second one. e.g., Comparison of acidity of phenol, and o, m & p-nitrophenols PAPER 1: ORGANIC - I (Nature of Bonding
First of all identify the differentiating groups amongst all. In the framework of phenol, the nitro groups at different positions are differentiating groups, which are responsible for different acidities of these molecules. Write the effect that they are exerting as depicted below. Comparing the effects, we can say that phenol is least acidic as all the o, m and p-nitrophenols have electron withdrawing nitro groups. Of the o,m and p-nitrophenols, the m-nitrophenol is weakest as it has I effect of nitro being exterted on the phenoxide ion to stabilize it further. Of the o- and p-nitrophenols, the o-nitro-phenol has intramolecular H-bonding and hence will have difficulty in deprotonation as compared to the p-nitrophenol. Hence o-nitrophenol is slightly weaker acid than p-nitrophenol Hence the overall acidity order is Phenol < m-nitrophenol < o-nitrophenol < p-nitrophenol 2. Comparison of acidity of phenol, and o,m & p-cresols Comparing the effects, we can say that phenol is most acidic as all the o,m and p-cresols have electron donating methyl groups. Of the o,m and p-cresols, the m-cresol is stronger as it has +I effect of methyl being exterted on the phenoxide ion to destabilize it as compared to +H effect in ortho and para cases. There is no intra-molecular H-bonding here. PAPER 1: ORGANIC - I (Nature of Bonding
Of the o- and p-cresols, the +H effect is equally effective in both cases, but + I effect is more at ortho than at para position. + I effect shall decrease the acidity, hence, o-cresol is weaker acid than p-cresol. Hence the overall acidity order is o-cresol < p-cresol < m-cresol < Phenol Substituted benzoic acids To compare the acidities of substituted benzoic acids, we first need to understand ortho effect in brief. In ortho-substituted benzoic acids, due to the presence of a bulky substituent, the COOH becomes non-planar to the phenyl ring and resonance stops as the p-orbitals are not aligned properly. Due to this non-planarity, most of the ortho-substituted benzoic acids are stronger acids than meta or para isomers and also even the unsubstituted benzoic acid irrespective of the nature of the groups. Hence to arrive to the conclusion for comparing the acidities of substituted benzoic acids, mark the ortho unsubstituted benzoic acid as strongest acid. Then compare the effects of the differentiating groups as done in the case of substituted phenols. For example, Structure Name Effects of differentiating groups Order of acidity Benzoic o-toluic acid o-toluic acid o-toluic acid acid None Ortho effect +I only +H and +I 2 1 (Most acidic) 3 4 (least acidic) 5. Effect on Basicity 5.1 Basicity of Aliphatic Amines In order to understand this, focus on the electronegative atom having at least one lone pair or negative charge over it. PAPER 1: ORGANIC - I (Nature of Bonding
1. the atom having the lone pair of electrons (here, nitrogen atom) should be less electronegative, so that the lone pair is readily donated. 2. After accepting the proton, the conjugate acid should be stabilized. Both the above conditions, in general, are enhanced by electron donating groups (+I, +M). Hence, electron donating groups increase the basicity; and electron withdrawing groups (-I, -M) decrease the basicity. e.g. Alkylamines are stronger bases than ammonia. Aliphatic amines of all three classes have K b values of about 10-3 to 10-4 (0.001 to 0.0001); they are thus somewhat stronger bases than ammonia (K b = 1.8 x 10-5 ). This can be explained in terms of an electron donating inductive effect of the alkyl groups. Alkyl groups, by their electron donating effect, increase the electron density of nitrogen and hence, make the lone pair of nitrogen more easily available to be given to acids. Further, the electron donating alkyl group(s) stabilises the alkyl ammonium ion formed and, thus, shifts the equilibrium in the forward direction. H R N + H + H R releases electrons: makes unshared pair more available H R N H + H R releases electrons: stabilizes ions, increases basicity 5.2 Basicity of aromatic Amines Arylamines are weaker bases than ammonia This can be explained in terms of the electron withdrawing resonance and inductive effect of the aryl groups, which decrease the electron density of nitrogen and hence, make the lone pair of nitrogen less available to be given to acids. Aromatic amines have K b values of 10-9 or less, they are thus weaker bases than ammonia(k b = 1.8 x 10-5 ). PAPER 1: ORGANIC - I (Nature of Bonding
Effect of substituents on basicity of aromatic amines Substituents on the ring have a marked effect on the basicity of aromatic amines. K b of aniline = 4.2 x 10-10 An electron releasing substituent like CH 3 increases the basicity of aniline and an electronwithdrawing substituent like X or -NO 2 decreases the basicity. NH + 2 NH 3 + H + G G G releases electrons: stabilizes cation, increases basicity G = NH 2 OCH 3 CH 3 NH 2 + H + G + NH 3 G G withdraws electrons: destabilizes cation, decreases basicity G = NH 3 + NO 2 SO 3 COOH X A given substituent affects the basicity of an amine and the acidity of a carboxylic acid in opposite ways. This is to be expected, since basicity depends upon ability to accommodate a positive charge, and acidity depends upon ability to accommodate a negative charge. 5.3. Basicity of amides Amides are less basic than amines This is due to the delocalization of lone pair of nitrogen on more elecronegative oxygen atom. For resonance coplanarity of p orbitals is essential. Therefore, in amides where coplanarity is not possible, resonance stabilization cannot occur. For example, in the beta-lactam antibiotics (penicillins), the bicyclic ring structure does not allow the amide N-C=O atoms to exist in the same plane. Thus resonance delocalizationof nitrogen atom s non-bonding electrons is not possible and these amides are more basic (and more reactive toward nucleophiles) than 'normal" amides. PAPER 1: ORGANIC - I (Nature of Bonding
6. Effect on electrophilic substitution in substituted benzenes Ortho/meta/para directive influence in electrophilic substitution of substituted benzene is affected by the electronic effects of the substituent. The electron donating groups where the stronger effect is +M / +H / +I are ortho and para directing towards electrophilic substitution. On the other hand, electron withdrawing groups by -M / -I are meta directing towards electrophilic substitution. All ortho / para directing groups are activating except halogens. On the other hand, all electron withdrawing groups are deactivating. 7. Summary PAPER 1: ORGANIC - I (Nature of Bonding
Greater is the delocalization of the charge, greater is the stability. Electron donating groups (+I, +M and +H) hence, stabilize electron deficient species like carbocation and free-radicals. On the other hand, Electron wthdrawing groups (-I and -M ) hence, stabilize electron rich species i.e, carbanions. In a molecule having an electronegative atom joined directly to H atom, the acidity is affected by Ease of Deprotonation and Stability of conjugate base. Both these conditions are enhanced by introducing electron withdrawing groups (-M, -H or I )and decreased by electron donating groups (+I, +M, +H). Remember that the second condition is more dominant one than the first to arrive to the conclusion. Hence if two are opposing, rely on the second one. Hence, electron donating groups (+I, +M, +H) increase the basicity; and electron withdrawing groups (-I, -M) decrease the basicity. An electron releasing substituent like CH 3 increases the basicity of aniline and an electron-withdrawing substituent like X or -NO 2 decreases the basicity. Ortho/meta/para directive influence in electrophilic substitution of substituted benzene is affected by the electronic effects of the substituent. All ortho / para directing groups are activating except halogens. On the other hand, all electron withdrawing groups are deactivating. PAPER 1: ORGANIC - I (Nature of Bonding