Why a benzene ring sometimes gives -I effect?

Why is No2 is more electron withdrawing then CN?

The benzene ring (C6H6) is often considered electron-donating or electron-releasing due to its high electron density. However, in certain cases, it can also exhibit an electron-withdrawing effect, often referred to as the -I (minus I) effect.

The -I effect occurs when a substituent or functional group attached to the benzene ring withdraws electron density away from the ring, leading to a decrease in electron density and making the ring less electron-rich.

There are two main reasons why a benzene ring can exhibit the -I effect:

1. Inductive Effect: The inductive effect occurs when the electronegativity difference between the substituent and benzene ring causes the electrons to be more strongly held or withdrawn by the substituent. Groups with high electronegativity, such as halogens (e.g., Cl, Br), nitro groups (NO2), and carbonyl groups (e.g., C=O), can exhibit an -I effect by pulling electrons away from the ring through sigma bonds. This reduces the electron density on the benzene ring, resulting in a net electron withdrawal.

2. Mesomeric Effect: The mesomeric effect (also known as resonance effect) occurs when the substituent delocalizes or shares electrons with the benzene ring through pi bonds. Certain groups, such as carbonyl groups and nitro groups, have resonance structures that involve the donation or withdrawal of electrons to or from the benzene ring. In the case of benzene with a nitro group (NO2), the nitro group's resonance structures contribute to a strong electron-withdrawing effect.

Regarding the second part of your question, why NO2 is more electron-withdrawing than CN, it can be attributed to the fact that the nitro group (NO2) has both a strong inductive and mesomeric effect. It contains an electronegative oxygen atom and a highly electron-withdrawing nitro group, which make it an effective electron acceptor.

On the other hand, CN (cyanide) mainly exhibits an -I effect due to the highly electronegative carbon atom and the partial negative charge on nitrogen. However, the mesomeric effect of CN is weaker compared to NO2. Therefore, NO2 is generally considered more electron-withdrawing than CN due to its combined inductive and mesomeric effects.