Which of the following is not a nucleophile?

• A. ${\mathrm{CN}}_{\mathrm{-}}$
• B. ${\mathrm{NO}}_3\mathrm{-}$
• C. ${\mathrm{OH}}_{\mathrm{-}}$
• D. $H_{\mathrm{+}}$

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the definition of a nucleophile. A nucleophile is a species that donates an electron pair to form a new covalent bond. Typically, nucleophiles have lone pairs or π bonds and carry a negative charge or partial negative charge.
2. Step 2: Analyze each species given in the problem. For example, \(\mathrm{CN}^-\) has a lone pair on carbon and carries a negative charge, making it a good nucleophile.
3. Step 3: Consider \(\mathrm{NO_3}^-\) (nitrate ion). Although it carries a negative charge, its resonance stabilization and the nature of its orbitals make it less nucleophilic compared to others, but it can still act as a nucleophile under certain conditions.
4. Step 4: Look at \(\mathrm{OH}^-\) (hydroxide ion). It has a lone pair and a negative charge, making it a strong nucleophile.
5. Step 5: Examine \(\mathrm{H}^+\) (proton). Since it is positively charged and lacks lone pairs, it cannot donate an electron pair and therefore is not a nucleophile.