Chemistry
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Class-11 Chapter -12 For CBSE - NCERT

Resonance Structure

There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of benzene. Its cyclic structure containing alternating `color{red}(C–C)` single and `color{red}(C=C)` double bonds shown is inadequate for explaining its characteristic properties.


As per the above representation, benzene should exhibit two different bond lengths, due to `color{red}(C–C)` single and `color{red}(C=C)` double bonds. However, as determined experimentally benzene has a uniform `color{red}(C–C)` bond distances of 139 pm, a value intermediate between the `color{red}(C–C)` single(154 pm) and `color{red}(C=C)` double (134 pm) bonds. Thus, the structure of benzene cannot be represented adequately by the above structure.

`color{red}("The resonance structures (canonical structures or contributing structures)")``color{red}("are hypothetical and individually do not")``color{red}(" represent any real molecule.")`

Another example of resonance is provided by nitromethane `color{red}((CH_3NO_2))` which can be represented by two Lewis structures, (I and II). There are two types of `color{red}(N-O)` bonds in these structures.

𝐇𝐨𝐰𝐞𝐯𝐞𝐫, 𝐢𝐭 𝐢𝐬 𝐤𝐧𝐨𝐰𝐧 𝐭𝐡𝐚𝐭 𝐭𝐡𝐞 𝐭𝐰𝐨 𝐍–𝐎 𝐛𝐨𝐧𝐝𝐬 𝐨𝐟 𝐧𝐢𝐭𝐫𝐨𝐦𝐞𝐭𝐡𝐚𝐧𝐞 𝐚𝐫𝐞 𝐨𝐟 𝐭𝐡𝐞 𝐬𝐚𝐦𝐞 𝐥𝐞𝐧𝐠𝐭𝐡 (𝐢𝐧𝐭𝐞𝐫𝐦𝐞𝐝𝐢𝐚𝐭𝐞 𝐛𝐞𝐭𝐰𝐞𝐞𝐧 𝐚 𝐍–𝐎 𝐬𝐢𝐧𝐠𝐥𝐞 𝐛𝐨𝐧𝐝 𝐚𝐧𝐝 𝐚 𝐍=𝐎 𝐝𝐨𝐮𝐛𝐥𝐞 𝐛𝐨𝐧𝐝). 𝐓𝐡𝐞 𝐚𝐜𝐭𝐮𝐚𝐥 𝐬𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐞 𝐨𝐟 𝐧𝐢𝐭𝐫𝐨𝐦𝐞𝐭𝐡𝐚𝐧𝐞 𝐢𝐬 𝐭𝐡𝐞𝐫𝐞𝐟𝐨𝐫𝐞 𝐚 𝐫𝐞𝐬𝐨𝐧𝐚𝐧𝐜𝐞 𝐡𝐲𝐛𝐫𝐢𝐝 𝐨𝐟 𝐭𝐡𝐞 𝐭𝐰𝐨 𝐜𝐚𝐧𝐨𝐧𝐢𝐜𝐚𝐥 𝐟𝐨𝐫𝐦𝐬 𝐈 𝐚𝐧𝐝 𝐈𝐈.

The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures. The difference in energy between the actual structure and the lowest energy resonance structure is called the `color{green}("resonance stabilisation energy")` or simply the `color{green}("resonance energy.")`

`color{red}("The more the number of important contributing structures,")``color{red}(" the more is the resonance energy.")`

The following rules are applied while writing resonance structures:
The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons.

Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others.
Q 3254634554

Write resonance structures of `CH_3COO^–` and show the movement of electrons by
curved arrows.

Solution:

First, write the structure and put unshared pairs of valence electrons on appropriate atoms. Then draw the arrows one at a time moving the electrons to get the other structures.
Q 3274634556

Write resonance structures of `CH_2=CH–CHO.` Indicate relative stability of the contributing structures.

Solution:


[`"I"` : Most stable, more number of covalent bonds, each carbon and oxygen atom has an octet and no separation of opposite charge `"II"`: negative charge on more electronegative atom and positive charge on more electropositive atom; `"III"`: does not contribute as oxygen has positive charge and carbon has negative charge, hence least stable].
Q 3284634557

Explain why the following two structures, `"I"` and `"II"` cannot be the major contributors
to the real structure of `CH_3COOCH_3.`


Solution:

The two structures are less important contributors as they involve charge separation. Additionally, structure `"I"` contains a carbon atom with an incomplete octet.

Resonance Effect

The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two `color{red}(π)`-bonds or between a `color{red}(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 `color{red}(R)` or `color{red}(M)` effect.

(i) `color{red}("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) `color{red}("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 :

`color{red}("+R effect: – halogen", –OH, –OR, –OCOR, –NH_2, –NHR, –NR_2, –NHCOR),`

`color{red}("– R effect" – COOH, –CHO, > C=O, – CN, –NO_2)`


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 `color{red}(π)`-electrons are delocalised and the system develops polarity.

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 `color{red}(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 `color{red}(E)` and the shifting of the electrons is shown by a curved arrow.

There are two distinct types of electromeric effect:

(i) `color{red}("Positive Electromeric Effect (+E effect) :")` In this effect the `pi`−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example :

(ii) `color{red}("Negative Electromeric Effect (–E effect):")` In this effect the `pi`- electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. For example:


`color{green}("When inductive and electromeric effects")` `color{green}(" operate in opposite directions, the electomeric effect predominates.")`

Hyperconjugation

Hyperconjugation is a general stabilising interaction. It involves delocalisation of `color{red}(sigma)` electrons of `color{red}(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`sigma` electrons of `color{red}(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 `color{red}(CH_3 overset(+)(C)H_2)` (ethyl cation) in which the positively charged carbon atom has an empty `p` orbital. One of the `color{red}(C-H)` bonds of the methyl group can align in the plane of this empty `p` orbital and the electrons constituting the `color{red}(C-H)` bond in plane with this `p` orbital can then be delocalised into the empty` p` orbital as depicted in Fig.



This type of overlap stabilises the carbocation because electron density from the adjacent `color{red}(sigma)` 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 hyperconjugation interaction and stabilisation of the cation. Thus, we have the following relative stability of carbocations :


Delocalisation of electrons by hyperconjugation in the case of alkene can be depicted as in Fig.


There are various ways of looking at the hyperconjugative effect. One of the way is to regard `color{red}(C—H)` bond as possessing partial ionic character due to resonance.

`color{green}("The hyperconjugation may also be regarded as no bond resonance.")`
Q 3204634558

Explain why `(CH_3)_3 overset(+)(C)` is more stable than `CH_3 overset(+)(C)H_2` and `overset(+)(C)H_3` is the least stable
cation.

Solution:

Hyperconjugation interaction in `(CH_3)_3 overset(+)(C)` is greater than in `CH_3 overset(+)(C) H_2` as the `(CH_3)_3 overset(+)(C)` has nine `C-H` bonds. In `overset(+)(C)H_3` vacant p orbital is perpendicular to the plane in which `C-H` bonds lie; hence
cannot overlap with it. Thus, `overset(+)(C)H_3` lacks hyperconjugative stability.
 
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