Which of the following species can act as the strongest base?

RO^-

The correct answer is option 2. \(RO^-\).

To determine which species can act as the strongest base, we need to consider their ability to accept protons (H⁺). The stronger the base, the more readily it can accept a proton. Here’s an analysis of each species:

1. \( \text{HO}^- \) (Hydroxide ion):

This is a strong base. It readily accepts a proton to form water (\( \text{H}_2\text{O} \)).

2. \( \text{RO}^- \) (Alkoxide ion):

This is also a strong base. Alkoxides like \( \text{RO}^- \) (where \( R \) is an alkyl group) are stronger bases than hydroxide ions because the alkyl group is less electronegative than the hydrogen in hydroxide ion, making the alkoxide ion more reactive.

3. \( \text{C}_6\text{H}_5\text{O}^- \) (Phenoxide ion):

The phenoxide ion is less basic compared to hydroxide and alkoxide ions. The negative charge on the oxygen is stabilized by resonance with the aromatic ring, which makes it less eager to accept a proton compared to \( \text{HO}^- \) and \( \text{RO}^- \).

4. \( \text{-OC}_6\text{H}_4\text{NO}_2 \) (o-Nitrophenoxide ion):

This species is even less basic due to the presence of the nitro group (\( -NO_2 \)) attached to the aromatic ring. The nitro group is a strong electron-withdrawing group and stabilizes the negative charge on the oxygen through resonance and inductive effects, which further reduces its basicity.

Ranking of Basicity

Strongest Base: \( \text{RO}^- \) (Alkoxide ion) – Alkoxides are typically stronger bases than hydroxides.

Weakest Base: \( \text{-OC}_6\text{H}_4\text{NO}_2 \) (o-Nitrophenoxide ion) – The nitro group significantly reduces the basicity by stabilizing the negative charge.

Therefore, the strongest base among the given options is \( \text{RO}^- \) (alkoxide ion).