The resonance effect is defined as ‘the polarity produced in the
molecule by the interaction of two π-bonds or between a π-bond and lone
pair of electrons present on an adjacent atom’. The effect is
transmitted through the chain. There are two types of resonance or
mesomeric effect designated as R or M effect.
(i) Positive Resonance Effect (+R effect) In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. This electron displacement makes certain positions in the molecule of high electron densities. This effect in aniline is shown as :
(ii) Negative Resonance Effect (- R effect) This effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugated system. For example in nitrobenzene this electron displacement can be depicted as :
The atoms or substituent groups, which represent +R or -R electron displacement effects are as follows :
+R effect: – halogen, -OH, -OR, -OCOR, -NH2, -NHR, -NR2, -NHCOR, -R effect: – COOH, -CHO, >C=O, – CN,-NO2
The presence of alternate single and double bonds in an open chain or cyclic system is termed as a conjugated system. These systems often show abnormal behaviour. The examples are 1,3- butadiene, aniline and nitrobenzene etc. In such systems, the π-electrons are delocalised and the system develops polarity.
12.7.8 Electromeric Effect (E effect)
It is a temporary effect. The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only. It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E and the shifting of the electrons is shown by a curved arrow ( ). There are two distinct types of electromeric effect.
(i) Positive Eelctromeric Effect (+E effect) In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example :
(ii)Negative Electromeric Effect (-E effect) In this effect the π – electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. For example:
When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates.
12.7.9 Hyperconjugation
Hyperconjugation is a general stabilising interaction. It involves delocalisation of σ electrons of C-H bond of an alkyl group directly attached to an atom of unsaturated system or to an atom with an unshared p orbital. The σ electrons of C-H bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p orbital. Hyperconjugation is a permanent effect.
To understand hyperconjugation effect, let us take an example of CH3C+H2 (ethyl cation) in which the positively charged carbon atom has an empty p orbital. One of the C-H bonds of the methyl group can align in the plane of this empty p orbital and the electrons constituting the C-H bond in plane with this p orbital can then be delocalised into the empty p orbital as depicted in Fig. 12.4 (a).
This type of overlap stabilises the carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge.
In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hperconjugation interaction and stabilisation of the cation. Thus, we have the following relative stability of carbocations :
Hyperconjugation is also possible in alkenes and alkylarenes.
Delocalisation of electrons by hyperconjugation in the case of alkene can be depicted as in Fig. 12.4(b).
There are various ways of looking at the hyperconjugative effect. One of the way is to regard C-H bond as possessing partial ionic character due to resonance.
The hyperconjugation may also be regarded as no bond resonance.
Problem 12.19
Explain why (CH3)3C+ is more stable than and CH3C+H2 is the least stable cation.
Solution
Hyperconjugation interaction in (CH3)3C+ is greater than in CH3C+H2 as the has nine C-H bonds. In C+H3, vacant p orbital is perpendicular to the plane in which C-H bonds lie; hence cannot overlap with it. Thus, C+H3 lacks hyperconjugative stability.
12.7.10 Types of Organic Reactions and Mechanisms Organic reactions can be classified into the following categories:
(i) Substitution reactions
(ii) Addition reactions
(iii) Elimination reactions
(iv) Rearrangement reactions
You will be studying these reactions in Unit 13 and later in class XII.
(i) Positive Resonance Effect (+R effect) In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. This electron displacement makes certain positions in the molecule of high electron densities. This effect in aniline is shown as :
(ii) Negative Resonance Effect (- R effect) This effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugated system. For example in nitrobenzene this electron displacement can be depicted as :
The atoms or substituent groups, which represent +R or -R electron displacement effects are as follows :
+R effect: – halogen, -OH, -OR, -OCOR, -NH2, -NHR, -NR2, -NHCOR, -R effect: – COOH, -CHO, >C=O, – CN,-NO2
The presence of alternate single and double bonds in an open chain or cyclic system is termed as a conjugated system. These systems often show abnormal behaviour. The examples are 1,3- butadiene, aniline and nitrobenzene etc. In such systems, the π-electrons are delocalised and the system develops polarity.
12.7.8 Electromeric Effect (E effect)
It is a temporary effect. The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only. It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E and the shifting of the electrons is shown by a curved arrow ( ). There are two distinct types of electromeric effect.
(i) Positive Eelctromeric Effect (+E effect) In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example :
(ii)Negative Electromeric Effect (-E effect) In this effect the π – electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. For example:
When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates.
12.7.9 Hyperconjugation
Hyperconjugation is a general stabilising interaction. It involves delocalisation of σ electrons of C-H bond of an alkyl group directly attached to an atom of unsaturated system or to an atom with an unshared p orbital. The σ electrons of C-H bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p orbital. Hyperconjugation is a permanent effect.
To understand hyperconjugation effect, let us take an example of CH3C+H2 (ethyl cation) in which the positively charged carbon atom has an empty p orbital. One of the C-H bonds of the methyl group can align in the plane of this empty p orbital and the electrons constituting the C-H bond in plane with this p orbital can then be delocalised into the empty p orbital as depicted in Fig. 12.4 (a).
This type of overlap stabilises the carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge.
In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hperconjugation interaction and stabilisation of the cation. Thus, we have the following relative stability of carbocations :
Hyperconjugation is also possible in alkenes and alkylarenes.
Delocalisation of electrons by hyperconjugation in the case of alkene can be depicted as in Fig. 12.4(b).
There are various ways of looking at the hyperconjugative effect. One of the way is to regard C-H bond as possessing partial ionic character due to resonance.
The hyperconjugation may also be regarded as no bond resonance.
Problem 12.19
Explain why (CH3)3C+ is more stable than and CH3C+H2 is the least stable cation.
Solution
Hyperconjugation interaction in (CH3)3C+ is greater than in CH3C+H2 as the has nine C-H bonds. In C+H3, vacant p orbital is perpendicular to the plane in which C-H bonds lie; hence cannot overlap with it. Thus, C+H3 lacks hyperconjugative stability.
12.7.10 Types of Organic Reactions and Mechanisms Organic reactions can be classified into the following categories:
(i) Substitution reactions
(ii) Addition reactions
(iii) Elimination reactions
(iv) Rearrangement reactions
You will be studying these reactions in Unit 13 and later in class XII.
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