Question

Suppose 1,4-dinitrobenzene is used rather than 1,4-dimethoxybenzene in the ELECTROPHILIC AROMATIC SUBSTITUTION with Sulfuric acid as catalyst and 2-methyl 2-propanol. Would you expect this reaction to occur at a faster or slower rate than the reaction you conducted? Justify your answer by drawing applicable resonance structures.

Answer 1

The electron-withdrawing nature of the nitro groups in 1,4-DNB deactivates the benzene ring towards electrophilic aromatic substitution, while the electron-donating nature of the methoxy groups in 1,4-DMB activates the benzene ring towards electrophilic aromatic substitution.

What are these reactions?

The reaction with 1,4-dinitrobenzene (1,4-DNB) would occur at a slower rate than the reaction with 1,4-dimethoxybenzene (1,4-DMB). This is because the nitro groups in 1,4-DNB are strong electron-withdrawing groups, which deactivate the benzene ring towards electrophilic aromatic substitution. In contrast, the methoxy groups in 1,4-DMB are electron-donating groups, which activate the benzene ring towards electrophilic aromatic substitution.

To understand this difference, let's consider the resonance structures of the two compounds.

1,4-Dinitrobenzene (1,4-DNB)

In 1,4-DNB, the nitro groups are meta directors, which means that they direct electrophilic substitution to the meta positions. This is because the nitro groups can withdraw electrons from the benzene ring, creating a partial positive charge on the meta positions. However, the nitro groups also reduce the electron density in the benzene ring, making it less reactive towards electrophilic attack.

Resonance structures of 1,4-DNB:

O=N=O

1,4-Dimethoxybenzene (1,4-DMB)

In 1,4-DMB, the methoxy groups are ortho and para directors, which means that they direct electrophilic substitution to the ortho and para positions. This is because the methoxy groups can donate electrons to the benzene ring, creating a partial negative charge on the ortho and para positions. Additionally, the methoxy groups increase the electron density in the benzene ring, making it more reactive towards electrophilic attack.

Resonance structures of 1,4-DMB:

OMe⁺

Therefore, the electron-withdrawing nature of the nitro groups in 1,4-DNB deactivates the benzene ring towards electrophilic aromatic substitution, while the electron-donating nature of the methoxy groups in 1,4-DMB activates the benzene ring towards electrophilic aromatic substitution. This is why the reaction with 1,4-DNB would occur at a slower rate than the reaction with 1,4-DMB.