Question

With respect to the compounds I-V, choose the correct statement(s).

• A. The acidity of compound I is due to delocalization in the conjugate base.
• B. The conjugate base of compound IV is aromatic.
• C. Compound II becomes more acidic, when it has a -NO₂ substituent.
• D. The acidity of compounds follows the order I >IV>V>II>III.

Answer 1

Answer: (A), (B), (C)

Compound II becomes more acidic, when it has a -NO₂ substituent.

Explanation for correct option:

A. The acidity of compound I is due to delocalization in the conjugate base.

1. When at least three p orbitals overlap, this is called conjugation.
2. Let's examine this molecule and determine whether or not three p orbitals can overlap anywhere:
3. Because double bonds form on p-orbitals, they are the simplest way to recognize a p-orbital.
4. Option A is correct

B. The conjugate base of compound IV is aromatic.

1. Due to the conjugation of the p orbitals and the presence of delocalized p electrons, aromatic compounds contain a 'ring current' and are hence diamagnetic.
2. The ring with eight pi electrons cannot be aromatic,
3. Hence the conjugate base with six pi electrons is aromatic and should be more stable.
4. This suggests that cyclopentadiene is more acidic than cycloheptatriene
5. Option B is correct

C. Compound II becomes more acidic, when it has a $-N{O}_2$ substituent.

1. In the ortho or meta position, deactivating substituents like the nitro group $(-N{O}_2)$strip the aromatic ring and the carboxylate anion of their electron density.
2. As a result, the conjugate base's negative charge is stabilized, raising the acidity of the carboxylic acid.
3. Option C is correct

Explanation for incorrect option:

D. The acidity of compounds follows the order $I>IV>V>II>III$

1. Triphenylmethane (I) is acidic because resonance has stabilized the conjugate base.
2. Because the conjugate base of cyclopentadiene (IV) is aromatic, it is acidic.
3. Due to the nitro group's electron withdrawing nature, nitrobenzene is more acidic than benzene.
4. By -I effect, it will stabilize the conjugate base of benzene.
5. Option is incorrect then the order of acidity is $\mathrm{IV}>\mathrm{V}>\mathrm{I}>\mathrm{II}>\mathrm{III}$

Hence the correct statements are:

A. The acidity of compound I is due to delocalization in the conjugate base.

B. The conjugate base of compound IV is aromatic.

C. Compound II becomes more acidic, when it has a -NO₂ substituent.