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Bsc Chemistry Subject Chemistry Paper No and Title Paper 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No and Title Module 4: Applications of Electronic Effects Module Tag CHE_P1_M4 CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects 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 CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects 1. Learning Outcomes After studying this module you shall be able to: Apply electronic effects to compare stability of carbon intermediates 2. Introduction 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 1o < 2o < 3o. However carbocations like allyl and benzyl are stable due to resonance. Thus, CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects Allyl and benzyl carbocations have slightly greater stability than secondary carbocations but are less stable than tertiary carbocations as phenyl and CH2 = CH groups are not good electron donating groups as is evident from the fact that C6H5COOH and CH2= 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 CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects This is due to the increase in the number of +I effect of the alkyl group. This order may be summarised as 1o < 2o < 3o. 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 – CH3, -OCH3 and –CHO respectively. Let us write what effect they are exerting on the carbocation centre? In A, the group are –CH3 is exerting both +I and +H effect, of which +H is dominant effect. In B, the group -OCH3 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). CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects 2. Stability of following carbanions(A, B and C) Of the three carbanions, let us compare the differentiating groups amongst them which are –CH3, -OCH3 and –CHO respectively. Let us write what effect they are exerting on the carbanion centre? In A, the group are –CH3 is exerting +I effect only. Note that it is not exerting +H effect as it is joined to a carbanion. In B, the group -OCH3 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 CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects 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 CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects 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. CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 4: Applications of Electronic Effects 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.
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