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.
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.