Organic Chemistry · Aldehydes and Ketones: Nucleophilic Addition Reactions

Nucleophilic Addition of HCN: Cyanohydrin Formation

8 min read

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Hydrogen cyanide (HCN) adds across the carbonyl of an aldehyde or ketone to give a — a molecule with both a hydroxyl and a (C≡N) on the same carbon:

$R 2 C = O + HCN ⇌ R 2 C (OH)(CN)$

The actual nucleophile is the , not HCN itself, so the reaction is run with a cyanide salt (NaCN or KCN) plus acid, or with a little base to generate CN⁻. Because HCN is a weak acid (pK_a ≈ 9.2), useful amounts of cyanide exist only near or above pH 9. The mechanism is standard two-step addition: CN⁻ attacks the carbonyl carbon, the π electrons move to oxygen forming an alkoxide, and the alkoxide picks up a proton, regenerating catalytic CN⁻.

Cyanohydrin formation is reversible, and for simple carbonyls the equilibrium often favors the starting material — so excess HCN or product removal drives it forward. The synthetic importance lies in the nitrile carbon, which adds a new carbon to the skeleton and can be converted into carboxylic acids (hydrolysis) or amino alcohols (reduction). The classic of sugars begins with cyanohydrin formation, extending an aldose chain by one carbon, and acetone cyanohydrin is an industrial intermediate in methyl methacrylate production.

Safety note: HCN and cyanide salts are highly toxic and may be fatal; cyanohydrin work belongs only in a properly equipped laboratory with trained supervision and a fume hood. This is educational discussion, not an instruction to perform the reaction.

Why this matters

Cyanohydrin chemistry is the cleanest example of carbon–carbon bond formation by nucleophilic addition. Adding CN⁻ lengthens a carbon chain by one carbon, and the nitrile is a versatile handle: hydrolysis gives α-hydroxy carboxylic acids, reduction gives 1,2-amino alcohols (important in medicinal chemistry), and Grignard addition to the nitrile gives ketones. The Kiliani–Fischer synthesis uses cyanohydrin intermediates to homologate sugars — how chemists built the higher sugars — and acetone cyanohydrin feeds the industrial route to methyl methacrylate (Plexiglas monomer). On exams, cyanohydrin problems test the site of nucleophilic attack, the pH/ control of CN⁻, and the transformations of the nitrile group.

The college version

Core Concepts

The nucleophile is cyanide ion, not HCN

HCN is a weak acid (pK_a ≈ 9.2); in neutral water it exists mostly as HCN, with only tiny amounts of CN⁻. The reaction therefore needs a source of cyanide: a salt such as NaCN (which dissociates fully) or a base strong enough to deprotonate some HCN. The Henderson–Hasselbalch relationship controls the ratio:

$pH = pK a + log [CN -] [HCN]$

At pH ≈ pKa ≈ 9.2, half the HCN is deprotonated. Practical preparations therefore use NaCN or KCN with a slight acid: the salt supplies the nucleophile, the acid the proton that finishes the addition.

Mechanism: attack, alkoxide, protonation

Step 1: cyanide's lone pair bonds to the carbonyl carbon while the C=O π electrons move onto oxygen, giving a tetrahedral alkoxide. Step 2: the alkoxide removes a proton from HCN (or water), giving the neutral cyanohydrin and regenerating CN⁻:

$R 2 C = O + CN - \to R 2 C (O -)(CN) HCN ⟶ R 2 C (OH)(CN) + CN -$

The overall stoichiometry is one HCN per carbonyl, but only catalytic CN⁻ is needed once the reaction is underway — nucleophilic addition in its purest form.

Equilibrium and how to drive it

Cyanohydrin formation is reversible, and for acetone the equilibrium constant is small, so simple mixing gives little product. Chemists push the equilibrium right with excess HCN, product removal, or a more reactive carbonyl (aldehydes beat ketones; electron-poor carbonyls react best). The reversibility is also useful: cyanohydrins act as masked carbonyls and can transfer HCN to other substrates.

The nitrile group is a synthetic handle

The C≡N triple bond undergoes several classic transformations:

Hydrolysis (aqueous acid or base) gives a carboxylic acid — so a cyanohydrin yields an α-hydroxy carboxylic acid, R₂C(OH)–COOH.
Reduction (LiAlH₄ or catalytic hydrogenation) converts C≡N to CH₂–NH₂, giving a 1,2-amino alcohol, R₂C(OH)–CH₂NH₂.
Grignard addition to the nitrile, followed by hydrolysis, gives a ketone — building still larger carbon skeletons.

The Kiliani–Fischer connection

An aldose treated with HCN forms a cyanohydrin with one more carbon; hydrolysis gives the aldonic acid, and reduction regenerates an aldose one carbon longer. This two-step homologation is the Kiliani–Fischer synthesis — the nucleophilic addition you have just learned, applied to a sugar aldehyde.

Common Confusions

Do Not Confuse	With	Difference
HCN as the nucleophile	CN⁻ as the nucleophile	HCN is the acid/proton source; CN⁻ is what attacks the carbonyl.
Cyanohydrin	Hydrate	Hydration adds H₂O → R₂C(OH)₂; HCN addition → R₂C(OH)(CN).
Nitrile (C≡N)	Isocyanide (N≡C)	In a nitrile the triple-bond carbon attaches to the rest of the molecule; in an isocyanide the nitrogen does.
Reversible HCN addition	Irreversible Grignard addition	CN⁻ addition is reversible; Grignard reagents add irreversibly.
pKa of HCN (≈9.2)	pH of the solution	pKa is a property of HCN; pH is the condition you control.
α-Hydroxy acid product	Amino alcohol product	Hydrolysis gives the α-hydroxy acid; reduction gives the amino alcohol.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine the carbonyl carbon is a door frame and cyanide (CN⁻) is a key that fits its lock. The key slides in, the door swings open and dumps its electrons onto the oxygen, and a proton helper (H⁺) holds the oxygen in place. The result is a cyanohydrin — a molecule with both an OH and a CN on the same carbon. It's like locking a trailer onto the molecule: the CN adds a whole new piece (one more carbon) you can later turn into other useful parts.

Worked example

Example 1: How much cyanide is available at a given pH?

The pH of a cyanohydrin preparation is buffered at 10.2. Given pK_a(HCN) = 9.2, what is [CN⁻]/[HCN]?

Use the Henderson–Hasselbalch equation:

$pH = pK a + log [CN -] [HCN]$

Substitute the values:

$10.2 = 9.2 + log [CN -] [HCN] \Rightarrow log [CN -] [HCN] = 1.0$

Take the antilog:

$[CN -] [HCN] = 10 1.0 = 10$

So at pH 10.2 there is ten times as much CN⁻ as HCN — roughly 91% of the total cyanide is nucleophilic. Compare pH 8.2 (one unit below the pKa), where the ratio is 0.1, only ~9% cyanide — showing why cyanohydrin reactions run under basic or salt-supplied conditions rather than at neutral pH.

Example 2: Planning a synthesis — one-carbon chain extension of benzaldehyde

Benzaldehyde (C₆H₅CHO, molar mass 106.12 g/mol) reacts with HCN to give mandelonitrile, C₆H₅CH(OH)CN (molar mass 133.15 g/mol). How many grams of mandelonitrile can form from 5.00 g of benzaldehyde with excess HCN?

The balanced equation has a 1:1 mole ratio, so first convert grams to moles:

$n(benzaldehyde) = 5.00 g 106.12 g/mol = 0.0471 mol$

Then convert moles of product to grams:

$m(mandelonitrile) = 0.0471 mol \times 133.15 g/mol = 6.27 g$

Check the units: g ÷ (g/mol) = mol; mol × (g/mol) = g. The carbon count confirms the extension: benzaldehyde (C₇) becomes mandelonitrile (C₈) — the nitrile adds exactly one carbon. If the lab recovered 5.4 g, the percent yield is:

$% yield = 5.4 g 6.27 g \times 100 % = 86 %$

The grams cancel, leaving a dimensionless percentage.

Key takeaways

Cyanohydrin = carbonyl + HCN → R₂C(OH)(CN); the attacking species is CN⁻, so cyanide salts or basic conditions are required.
HCN has pK_a ≈ 9.2 — use Henderson–Hasselbalch to predict the CN⁻/HCN ratio at any pH.
Mechanism: CN⁻ attack → tetrahedral alkoxide → protonation (catalytic CN⁻ is regenerated).
Aldehydes give cyanohydrins more readily than ketones; electron-withdrawing groups increase reactivity.
The reaction is reversible; excess HCN, product removal, or more reactive carbonyls drive it to completion.
The nitrile converts to: carboxylic acid (hydrolysis → α-hydroxy acid), primary amine (reduction → 1,2-amino alcohol), or ketone (Grignard + hydrolysis).
Kiliani–Fischer sugar synthesis starts with cyanohydrin formation to extend an aldose chain by one carbon.
HCN and cyanides are extremely toxic — educational discussion only.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

Write the general reaction for cyanohydrin formation from a ketone.
Show answer
R₂C=O + HCN ⇌ R₂C(OH)(CN), a cyanohydrin.
Why is cyanide ion, not HCN itself, the attacking nucleophile in this addition?
Show answer
Nucleophilic addition needs an electron-rich species to attack the electrophilic carbonyl carbon. Neutral HCN's lone pair is not usefully available, whereas CN⁻ is a strong nucleophile with a full negative charge on carbon.
At what pH is half of the HCN deprotonated, and why does that matter?
Show answer
At pH = pKa ≈ 9.2, where [CN⁻] = [HCN]. Below that pH, cyanide concentration drops sharply and the addition slows; that is why cyanohydrin reactions use cyanide salts or basic conditions.
Name two ways to drive the reversible cyanohydrin equilibrium toward product.
Show answer
Use excess HCN (Le Châtelier), remove the cyanohydrin product as it forms, or use a more reactive, electron-poor carbonyl.
What functional groups are obtained by (a) hydrolysis and (b) reduction of a cyanohydrin's nitrile?
Show answer
(a) Hydrolysis gives a carboxylic acid — with the OH still present, an α-hydroxy acid, R₂C(OH)COOH. (b) Reduction gives an amine — a 1,2-amino alcohol, R₂C(OH)CH₂NH₂.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Cyanohydrin: A compound with –OH and –CN on the same carbon, R₂C(OH)(CN).
Nitrile: A carbon–nitrogen triple bond, –C≡N.
Cyanide ion (CN⁻): The deprotonated form of HCN; the actual nucleophile.
pKa: The pH at which an acid is half-deprotonated.
Henderson–Hasselbalch equation: pH = pKa + log([base]/[acid])
α-Hydroxy acid: A carboxylic acid with an OH on the adjacent (α) carbon.
Kiliani–Fischer synthesis: A two-step chain extension of aldoses via cyanohydrin.
Reversible addition: An equilibrium reaction whose direction can be pushed by conditions.

Sources & references

openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.

In 30 seconds

Why this matters

The college version

Core Concepts

The nucleophile is cyanide ion, not HCN

Mechanism: attack, alkoxide, protonation

Equilibrium and how to drive it

The nitrile group is a synthetic handle

The Kiliani–Fischer connection

Common Confusions

The same idea, in plain words

Worked example

Example 1: How much cyanide is available at a given pH?

Example 2: Planning a synthesis — one-carbon chain extension of benzaldehyde

Key takeaways

Check yourself

Keep learning

Key vocabulary

Related lessons

Sources & references