Electronic Effect: Stability of Carbocations Bonding & Stereochemistry

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Electronic Effect: Stability of Carbocations | Bonding & Stereochemistry | Organic Chemistry

Learning Outcomes

After studying this lesson, you shall be able to:

  • Introduction
  • Effect on Stability of carbon intermediates
  • Stability of carbocations

Comparison of Carbocations

Let us extend this understanding for the comparison of carbocations which are joined to hetero atoms/groups with the help of following examples.

Comparison of Carbocations
  • 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 center? 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) .
Comparison of Carbocations

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 center?
  • 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) .
Stability of Carbanions

Stability of Following Carbocations (A, B, C, D and E)

Before we Analyse the effects of the groups, remember that:

Stability of Carbocations
  • 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 hyper conjugative) as they are not in conjugated positions.
  • The differentiating groups have been highlighted and the effects they are exerting on the carbocation center are shown.
  • Hence the order of stability is B > C > A > E > D

Effect on Acidity

Effect on Acidity

In a molecule having an electronegative atom joined directly to H atom, the acidity is affected by the following two factors:

Ease of Deprotonation

Higher the electronegativity of the electronegative atom, higher is the acidity. The higher electronegativity ensures easy deprotonation.

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.

Comparison of Acidity of Phenol, and O, M & P-Nitrophenols

First, 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.

Comparison of Acidity of Phenol
  • 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 exerted 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.

Comparison of Acidity of Phenol, and O, M & P-Cresols

Comparison of Acidity of Phenol
  • 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 exerted 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.
  • 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 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.

MCQs

1. Electronegativity is defined as the power of an atom in a molecule to________

(a) Repel electrons towards itself

(b) Attract electrons towards itself

(c) Expand itself

(d) All of the mentioned

Ans: (b)

2. Arrange the following groups in the order of decreasing effect.

(a)

(b)

(c)

(d)

Ans: (a)

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