Nucleophilic Strength in SN2 Reactions Which of the following is a strong nucleophile in a protic solvent? O MeOH O F O EtOH O H2O O CN Save for Later
The Correct Answer and Explanation is:
The correct answer is CN⁻ (cyanide ion).
In SN2 (substitution nucleophilic bimolecular) reactions, the nucleophile plays a critical role in attacking the electrophilic carbon and displacing the leaving group. The strength of a nucleophile is influenced by the solvent used. In a protic solvent, which has hydrogen bonding capabilities (such as water or alcohols), the nucleophile interacts with the solvent molecules, and this can either stabilize or destabilize the nucleophile.
- Protic solvents (e.g., water, ethanol, methanol) form hydrogen bonds with nucleophiles. These interactions reduce the availability of the nucleophile to attack the substrate, as the nucleophile becomes more “solvated” or “encumbered” by the solvent.
- A strong nucleophile is one that is highly reactive and ready to attack the electrophilic carbon. In protic solvents, smaller anions (e.g., F⁻, OH⁻) tend to be more solvated, which reduces their nucleophilicity.
Now, considering the options:
- MeOH (methanol) and EtOH (ethanol) are both protic solvents, but their nucleophilic strength is reduced in the presence of a protic solvent due to hydrogen bonding, especially compared to nonpolar nucleophiles.
- F⁻ (fluoride) is a weak nucleophile in a protic solvent because it forms strong hydrogen bonds with the solvent, making it less reactive.
- H2O (water) is a very weak nucleophile in a protic solvent, as water molecules themselves are already very hydrogen-bonded and not readily available to donate electrons.
- CN⁻ (cyanide), despite being in a protic solvent, remains a strong nucleophile because it is a smaller, less solvated anion that can still effectively attack an electrophilic carbon. Cyanide ions are not as tightly solvated as smaller halides, making them more reactive and thus stronger nucleophiles in SN2 reactions.
Thus, CN⁻ is the strongest nucleophile in a protic solvent due to its minimal solvation and high reactivity.
