Q.Cyanide ion acts as an ambident nucleophile. From which end it acts as a stronger nucleophile in aqueous medium? Give reason for your answer.

The key idea here is that the cyanide ion ($\text{CN}^-$) is ambident — it can attack through either the carbon atom or the nitrogen atom. In aqueous medium, the carbon end is the stronger nucleophile.

Reasoning:

1. The cyanide ion is ambident: it has nucleophilic lone pairs on both carbon and nitrogen.
2. Nitrogen is smaller and more electronegative than carbon, making it the "harder" of the two sites (per the hard-soft acid-base framework) -- and hard, electronegative atoms hydrogen-bond more strongly with water. So in aqueous solution, the nitrogen end is more strongly solvated (hydrated) via hydrogen bonding than the carbon end. …

Why This Question Matters

The cyanide ion ($\ce{CN-}$) is a classic example of an ambident nucleophile — a species that has two different atoms capable of donating a pair of electrons. The ion is linear: $[\ce{:\!C#N:}]^{-}$, with a lone pair on carbon and a lone pair on nitrogen. In principle, either end could attack an electrophile. But in practice, the outcome depends heavily on the reaction medium. The question asks specifically about aqueous medium, which changes the story entirely.

The Core Concept: Solvation and Nucleophilicity

Nucleophilicity is about how readily a species donates its electrons. But in a protic solvent like water, the nucleophile must first desolvate — break its hydrogen bonds with surrounding water molecules — before it can attack. The stronger the solvation, the harder it is for the nucleophile to act. This is the key to understanding why carbon wins over nitrogen in water.

Step-by-Step Reasoning

1. Structure of the cyanide ion

The cyanide ion has a triple bond between carbon and nitrogen. Carbon is less electronegative (2.55) than nitrogen (3.04), so the negative charge is not evenly distributed. The carbon end carries a higher negative charge density because it is less able to pull electron density toward itself. Nitrogen, being more electronegative, stabilises the negative charge better — but that also means it is less "available" for nucleophilic attack.

2. Solvation in water

Water is a protic solvent — it can donate hydrogen bonds. The nitrogen end of $\ce{CN-}$ has a lone pair that is more exposed and more basic (in the Lewis sense), so it forms stronger hydrogen bonds with water molecules. The carbon end, with its lower electronegativity and more diffuse charge, is less strongly solvated.

3. The desolvation penalty

For the nitrogen end to attack an electrophile, it must first break several hydrogen bonds with water. This costs energy. The carbon end, being less solvated, has a much smaller desolvation penalty. Since nucleophilicity in solution is a kinetic property (rate of attack), the carbon end attacks faster.

4. Charge density and polarisability …

Method: Ambident Nucleophile Analysis — Hard-Soft Acid-Base (HSAB) Principle

Step 1: Identify the ambident nature of cyanide ion

The cyanide ion ($\text{CN}^-$) has two nucleophilic sites:

• The carbon end ($\text{C}^-$)
• The nitrogen end ($\text{N}^-$)

Both atoms carry lone pairs and can donate electrons.

Step 2: Apply the HSAB Principle

The HSAB principle states:

• Hard nucleophiles prefer hard electrophilic centres (small, highly charged, less polarisable).
• Soft nucleophiles prefer soft electrophilic centres (large, low charge, highly polarisable).

In aqueous medium:

• Water is a hard solvent — it strongly solvates hard species via hydrogen bonding.
• The nitrogen end of $\text{CN}^-$ is harder (more electronegative, less polarisable) than the carbon end.
• The carbon end is softer (more polarisable, less tightly held electrons).

Step 3: Analyse solvation effect in water

• In water, the nitrogen end gets heavily solvated by hydrogen bonds from water molecules. This reduces its nucleophilicity. …

Here’s a clear breakdown of the common mistakes students make on this question, along with how to avoid each.

Common Mistake #1: Confusing “Ambident” with “Ambiphilic”

The mistake:

Students often think “ambident” means the nucleophile can act as both a nucleophile and an electrophile. This is wrong.

Why it’s wrong:

• Ambident nucleophile = a species with two different nucleophilic atoms (e.g., CN⁻ has both C and N as possible attack sites).
• Ambiphilic = can act as both nucleophile and electrophile (different concept).

How to avoid:

Remember the root: ambi (both) + dent (teeth) → “two teeth” for attacking. Only nucleophilic character is in question here.

Common Mistake #2: Saying "Cyanide attacks from the nitrogen end" -- or attributing carbon's advantage to formal charge

The mistake:

Some students think N is more nucleophilic simply because it is more electronegative. Others argue carbon must be the stronger nucleophile because it "carries more negative charge" in the resonance hybrid.

Why it is wrong:

Electronegativity alone, or which atom carries more formal charge, does not decide nucleophilic strength in a given solvent -- SOLVATION matters just as much. In aqueous medium, the carbon end is the stronger nucleophile, but the real reason is that nitrogen (smaller, more electronegative -- "harder") gets strongly solvated by water's hydrogen bonding, which locks up its lone pair. Carbon (larger, more polarisable -- "softer") stays less solvated and more available.

How to avoid:

Think in terms of hardness and solvation, not just "which atom has more charge": nitrogen is harder and gets solvated more strongly in water, suppressing its nucleophilicity; carbon is softer, stays less solvated, and ends up the stronger nucleophile in aqueous medium.

Common Mistake #3: Ignoring the “aqueous medium” condition

The mistake:

Students give a general answer without specifying the solvent.

Why it’s wrong:

• In polar protic solvents (like water), the nucleophilicity order can change due to solvation.
• N is more solvated (H‑bonding with water) → its nucleophilicity is reduced.
• C is less solvated → remains more available for attack.

How to avoid:

Always mention solvent effect in your reasoning. Write:

In aqueous medium, the nitrogen end is heavily solvated by H‑bonding, reducing its nucleophilicity. The carbon end is less solvated and thus acts as the stronger nucleophile.

Common Mistake #4: Treating the Exemplar's own bond-stability reason and the solvation reason as rivals

The situation:

The Exemplar's printed hint for this question explains the carbon end's superiority by bond stability: attack through carbon is preferred because it leads to a C–C bond, which is more stable than the C–N bond that N-attack would create. Some students learn only the solvation argument and then hesitate to use (or wrongly "correct") the book's own reasoning; others quote the bond-stability line without addressing the "aqueous medium" condition the question asks about.

The right way to see it:

• NCERT's printed rationale — C-attack is preferred because the C–C bond formed is more stable than a C–N bond — is the expected, fully exam-acceptable answer. Use it with confidence.
• The solvation argument is the complementary kinetic picture that specifically answers the "in aqueous medium" part: the harder, more electronegative nitrogen end is more strongly hydrogen-bonded by water, which ties up its lone pair, while the softer, less-solvated carbon end stays more available to attack.

How to avoid: …