Which of the following species is most likely to react as a nucleophile rather than as a base?

• A. ${\mathrm{OH}}_2-$
• B. ${\mathrm{Br}}_{\mathrm{-}}$
• C. ${\mathrm{CH}}_3-$
• D. ${\mathrm{NH}}_2-$

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the difference between a nucleophile and a base. A nucleophile donates an electron pair to an electrophile to form a bond, while a base abstracts a proton (H+). Both nucleophilicity and basicity involve electron pair donation, but their preferences differ based on the reaction context.
2. Step 2: Analyze the species given: \(\mathrm{OH_2^-}\) (hydroxide ion), \(\mathrm{Br^-}\) (bromide ion), \(\mathrm{CH_3^-}\) (methyl anion), and \(\mathrm{NH_2^-}\) (amide ion). Consider their charge density, electronegativity, and stability.
3. Step 3: Recognize that strong bases like \(\mathrm{CH_3^-}\) and \(\mathrm{NH_2^-}\) have high charge density and are more likely to abstract protons rather than attack electrophilic centers, making them stronger bases than nucleophiles.
4. Step 4: Consider \(\mathrm{Br^-}\), a larger, less basic ion with a more diffuse charge and higher polarizability, which makes it more likely to donate its electron pair to an electrophile (nucleophilic behavior) rather than abstract a proton (basic behavior).
5. Step 5: Conclude that among the given species, \(\mathrm{Br^-}\) is most likely to act as a nucleophile rather than as a base due to its lower basicity and higher nucleophilicity.