-R effect in benzene causes anti-aromaticity?

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In summary, the conversation discusses the formation of nitrobenzene through the -R effect, where a double bond is formed between N and C and a + charge appears on the ortho and para position. However, this leads to a planar conjugated ring with 4 π electrons, which is anti-aromatic and unstable. The participants request an explanation for this contradiction.
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Mcp
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Consider any -R effect showing group like NO2 in place of anyone of the hydrogen of the benzene ring, here we get nitrobenzene.

We know that after showing -R effect a double bond will be formed between N and C and one of the π bond will no longer be present in the ring and it's place will be taken by + charge on ortho and para position which will be in conjugation with the remaining two π bonds in the ring as can be seen from the diagram below.

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However the problem is that, in the canonical forms after resonance there will be 4 π electrons in a planar conjugated ring, which by definition is anti aromatic. But this should not be true because we know that resonance increases stability but anti aromatic hydrocarbons are unstable.

Can anyone give an explanation for the same.
 
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  • #2
None of the canonical forms has a ring of 4 pi electrons. They have six pi electrons in a non-cyclic conjugated system (including the C=N bond).
 

Related to -R effect in benzene causes anti-aromaticity?

1. What is the -R effect in benzene?

The -R effect in benzene is a phenomenon in which the presence of substituents on the benzene ring can alter its aromaticity. This effect is observed when a substituent with a lone pair of electrons is attached to the benzene ring, causing a decrease in the delocalization of electrons and a decrease in the overall aromaticity of the molecule.

2. How does the -R effect cause anti-aromaticity in benzene?

The -R effect causes anti-aromaticity in benzene by destabilizing the delocalization of electrons in the ring. This destabilization is due to the presence of a substituent with a lone pair of electrons, which disrupts the circular flow of electrons in the ring. As a result, the molecule becomes less stable and exhibits anti-aromatic properties.

3. What are some common substituents that can cause the -R effect in benzene?

Some common substituents that can cause the -R effect in benzene include halogens (such as chlorine, bromine, and fluorine), nitro groups, and carboxylic acids. These substituents have a lone pair of electrons that can interact with the delocalized electrons in the benzene ring and disrupt its aromaticity.

4. How does the -R effect affect the reactivity of benzene?

The -R effect can significantly affect the reactivity of benzene. Due to the decreased aromaticity caused by the presence of substituents, benzene becomes more reactive towards electrophilic substitution reactions. This is because the destabilization of the delocalization of electrons makes the benzene ring more susceptible to attack by electrophiles.

5. Can the -R effect be reversed in benzene?

Yes, the -R effect can be reversed in benzene. This can be achieved by introducing a strong electron-withdrawing group (such as a carbonyl group) onto the benzene ring. This group can withdraw electrons from the ring, restoring its delocalization and aromaticity. As a result, the anti-aromaticity caused by the -R effect is reversed.

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