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Stability of carbocations, free radicals, and carbanions

In general, electron-releasing groups increase the stability of carbocation and electron-withdrawing groups decrease their stability.


Acidic strength of organic acids

Alcohols: Ethanol > Isopropanol > t-Butanol
Phenols and substituted phenols: Phenols are stronger acids than alcohols.
Phenol > m-Methylphenol > p-Methylphenol > o-Methylphenol
p-Nitrophenol > o-Nitrophenol > m-Nitrophenol > Phenol
The cumulative effect of three nitro groups in the 2,4,6 positions is shown by the fact that 2,4,6-trinitrophenols (picric acid) is a very strong acid.
All the halophenols are stronger acids than phenol, and so it follows that the –I effect of halogen atom is greater than its +R effect. Thus, the order of acidity of halophenols is as follows:
Iso-halophenol > m-Halophenol > p-Halophenol > Phenol.
m-Methoxyphenol > Phenol > o-Methoxy phenol > p-Methoxy phenol
Aliphatic carboxylic acids: The aliphatic carboxylic acids are much stronger acids than phenols and alcohols. The presence of electron-withdrawing substituents in simple aliphatic acids increases their acidity, while the electron-releasing substituents have reverse effect.
Some other trends of acidity are:
HCO2H > CH3CO2H > CH3CH2CO2H > …
HO2C – CO2H > HO2C – CH2 – CO2H > HO2C – CH2CH2 – CO2H > …
These trends reflect that in any homologous series, when we move from lower members to higher members, the acidity decreases.
Aromatic carboxylic acids: It has been pointed out that replacement of the hydrogen atom in formic acid by an alkyl group weakens the strength of the acid. The greater the +I effect of the R-group, the weaker is the acid. Phenylacetic acid, PhCH2CO2H, is stronger than acetic acid and therefore the phenyl group has an overall –I effect. On the other hand, benzoic acid is weaker than formic acid. In this case, the phenyl group has an overall releasing effect (which is smaller than that of methyl group). These apparently contradictory results may be explained as follows. When the carboxyl group is directly attached to the nucleus, the resonance effect (+R) overcomes the –I effect (in phenylacetic acid, the phenyl group is insulated from the carboxyl group by a CH2 group and so the +R effect is not possible):
Description: 39949.png
This prevents, to a large extent, the lone pair on the O atom of the OH group from entering into resonance with the CO group. The result is a smaller positive charge on the O atom of the OH group and so proton release is more difficult than in formic acid. The fact that benzoic acid is stronger than acetic acid means that [–I + (+R)] < [+I + (+H)] of the methyl group.
The same arguments may be applied to ionized benzoic acid.
Description: 39959.png
The ortho effect: Steric effect is shown by ortho substituents. Nearly all o-substituted benzoic acids are stronger than benzoic acid due to this ortho effect irrespective of the polar nature of the o-substituent.
As we have seen that benzoic acid is a resonance hybrid and so the carboxyl group is coplanar with the ring. An o-substituent tends to prevent this coplanarity. Thus, resonance is diminished (or prevented), and hence the O atom of the OH group has a greater positive charge, resulting in increased acid strength. It follows from this that the greater the steric inhibition of resonance, the stronger is the acid. Support for this is the following order of strengths of substituted benzoic acids:
2,6-di-Me > 2-t-bu > 2-Me
Here again, if we consider the stability of the anion, steric inhibition of resonance prevents the =R effect of the ring coming into operation, and since this weakens acid strength, its absence results in increased acid strength.
o-Hydroxybenzoic acid (salicylic acid) is far stronger than the corresponding m- and p-isomers. Steric inhibition of resonance cannot explain this very large increase, since the corresponding methoxybenzoic acids all have almost similar strengths. The carboxylate ions of o-hydroxybenzoic acid is stabilized by intramolecular hydrogen bonding while such hydrogen bonding is not feasible in o-methoxybenzoic acid and support for this is given by the following order of acid strength:
2,6-di-OH > 2-OH > Benzoic acid
It can be seen that two hydrogen bonds would be expected to bring about more stabilization than one hydrogen bonds.
Description: 39969.png
Including ortho effect, the order of acidity of some substituted benzoic acids is given as follows:
  1. o-Nitrobenzoic acid > p-Nitrobenzoic acid > m-Nitrobenzoic acid > Benzoic acid
  2. o-Methylbenzoic acid > Benzoic acid > m-Methylbenzoic acid > p-Methylbenzoic acid.
  3. o-Methoxybenzoic acid > m-Methoxybenzoic acid > Benzoic acid > p-Methoxybenzoic acid
A CH2 group flanked between two electron-withdrawing groups has acidic hydrogen. Aldehydic group is more electron withdrawing than ketonic group, which is more electron attracting than ester groups. Thus, the order of acidity of carbon acids is as follows:
Description: 39981.png
CHCl3 is more acidic than CHF3 because the conjugate base of CHCl3, i.e., Description: 39990.png is stabilized by –I effect of Cl’s as well as by pπ–dπ delocalization (which is absent in : CF3 due to the absence of d-orbital in F).
For the same reason, Description: 40014.png is more acidic than Description: 40008.png.
Basic strength of organic bases: Strength of bases is related to the ease of accepting a proton, which in turn depends on the availability of electron pair on the nitrogen atom (or some other basic atom). The more is the availability of electron pair, the more easily the proton will be accepted and more will be the basic strength. If we compare the basicities of NH3, MeNH2, Me2NH, and Me3N, then at a glance it would seem like NH3 < MeNH2 < Me2NH < Me3N is the basicity order. However, the result is quite different in aqueous media. The correct order is Me2NH > MeNH2 > Me3N > NH3.
Tetraalkylammonium salts, e.g., R4NI, on treatment with moist silver oxide (AgOH) yield basic solutions comparable in strength with the mineral alkalies. This is readily understandable as R4NOH formed is completely ionized to give R4N and free OH.
The effect of introducing electron-withdrawing groups, e.g., Cl and NO2, close to the basic center decreases the basicity due to their electron-withdrawing inductive effect. Thus, the amine tris(trifluoro methyl) amine is found to be virtually non-basic due to the presence of three powerful electron-withdrawing CF3 groups. The amides are also found to be only very weakly basic in water because of the –I and –R effects of RCO group which makes the electron pair very slightly available on nitrogen atom.
In aniline, owing to resonance, the lone pair of electrons on the nitrogen atom is less available or co-ordinating with a proton and at the same time, small positive charge on the nitrogen atom would tend to repel a proton. Alternatively, since there are more resonating structures possible for aniline itself than for the cation C6H5N +H3, the former will be stabilized with respect to the latter. Aniline is a weaker base than ammonia or cyclohexylamine. It is because of the fact that the electron pair on nitrogen is involved in delocalization, making it less available for donation.
Diphenylamine is even a weaker base than aniline due to the presence of another phenyl group, and triphenylamine (Ph3N) is not basic at all by any means.
Introduction of alkyl group (such as Me) on the nitrogen atom of aniline results in small increase in the basic strength.
C6H5NH2 < C6H5NH2Me < C6H5NMe2
Unlike such introduction in aliphatic amines, this small increase in basic strength is progressive, indicating that cation stabilization through hydrogen-bonded solvation, here has less influence on the overall effect.
The order of basicity of nitroanilines is
C6H5NH2 > m-NO2C6H4NH2 > p-NO2C6H4NH2 > o-NO2C6H4NH2

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