Sodium cyanide (NaCN) is a versatile reagent that serves primarily as a source of the cyanide ion (CN⁻), one of the strongest nucleophiles in organic chemistry. When dissolved in a reaction mixture, NaCN splits into Na⁺ and CN⁻, and it’s that cyanide ion that does the real work. Depending on the reaction partner, CN⁻ can attack carbon centers to form new carbon-carbon bonds, add to carbonyl groups, or even act as a catalyst that temporarily reverses the polarity of a functional group.
NaCN as a Nucleophile in Substitution Reactions
The most common role of NaCN in organic chemistry is as a nucleophile in SN2 (bimolecular nucleophilic substitution) reactions. The cyanide ion attacks a carbon atom that bears a leaving group, such as a chloride or bromide, and displaces that leaving group in a single concerted step. The carbon on the cyanide forms a new bond to the substrate carbon while the leaving group departs from the opposite side.
This backside attack inverts the stereochemistry at the carbon center, which matters when the substrate is chiral. The reaction works best on primary and secondary alkyl halides, where there’s less steric crowding for the incoming CN⁻ to reach its target. The product is a nitrile (R-CN), a functional group that contains a carbon-nitrogen triple bond.
What makes this reaction so useful is that it extends the carbon chain by exactly one carbon atom. If you start with a five-carbon alkyl halide and react it with NaCN, you get a six-carbon nitrile. That nitrile can then be converted into other functional groups. Heating it in acidic or basic water hydrolyzes the nitrile first to an amide intermediate and then to a carboxylic acid. This two-step sequence (substitution followed by hydrolysis) is a classic way to convert an alkyl halide into a carboxylic acid with one additional carbon.
Addition to Aldehydes and Ketones
When NaCN reacts with an aldehyde or ketone, the cyanide ion attacks the electrophilic carbonyl carbon in a nucleophilic addition reaction. The electron pair from the carbon-oxygen double bond shifts onto the oxygen, creating an alkoxide intermediate. Protonation of that oxygen (typically by HCN in the reaction mixture) gives a cyanohydrin, a molecule with both an OH group and a CN group on the same carbon.
This reaction is reversible, so conditions matter. A common approach uses NaCN with a small amount of acid to generate HCN slowly in solution. Fast addition of strong acid to a cyanide salt is actually counterproductive because it can release HCN gas too quickly without giving the equilibrium time to favor product formation. Aldehydes react more readily than ketones because they have less steric hindrance around the carbonyl carbon.
Cyanohydrins are valuable intermediates. Like any nitrile, the CN group can be hydrolyzed to a carboxylic acid, giving you an alpha-hydroxy acid. Mandelic acid, used in skincare products, can be made this way from benzaldehyde.
Cyanide as a Catalyst in Benzoin Condensation
One of the more surprising roles of cyanide is as a catalyst rather than a reagent that gets incorporated into the final product. In the benzoin condensation, two molecules of benzaldehyde combine to form benzoin, a molecule with both a hydroxyl group and a ketone on adjacent carbons. Cyanide makes this happen through a concept called umpolung, which means “polarity reversal.”
Normally, the carbonyl carbon in an aldehyde is electrophilic. It wants to react with nucleophiles, not with another electrophilic carbonyl. The cyanide ion solves this problem in a clever sequence. First, CN⁻ attacks the carbonyl carbon of one benzaldehyde molecule, forming a cyanohydrin-like intermediate. This intermediate undergoes a proton shift that converts what was originally an electrophilic carbon into a nucleophilic one. That newly nucleophilic carbon then attacks the carbonyl of a second benzaldehyde molecule, forming the carbon-carbon bond of the product. Finally, cyanide departs and is free to catalyze another cycle.
This mechanism, first described by Arthur Lapworth in 1903, was a landmark in understanding how catalysts can fundamentally change the reactivity of a molecule. The cyanide ion is regenerated at the end, so only a small amount is needed.
Building Amino Acids via the Strecker Synthesis
NaCN plays a central role in one of the oldest known methods for making amino acids, the Strecker synthesis, discovered in 1850. The reaction combines an aldehyde, ammonia (or an amine), and cyanide in a one-pot sequence.
The mechanism starts with ammonia reacting with the aldehyde to form an imine (a carbon-nitrogen double bond). The cyanide ion then attacks the electrophilic carbon of that imine, producing an alpha-aminonitrile. Hydrolysis of the nitrile group under acidic or basic conditions converts it to a carboxylic acid, yielding the final amino acid. In practice, solid NaCN is preferred over gaseous HCN because it is easier and safer to handle, and solid ammonium chloride typically replaces free ammonia for the same reason.
The Strecker synthesis remains relevant today. Researchers have used chiral amines with NaCN to produce amino acids with specific stereochemistry. One example achieved 90% enantiomeric excess in the synthesis of alanine using a chiral amine auxiliary.
Gold Extraction in Industrial Chemistry
Outside the organic chemistry lab, the largest industrial use of NaCN is in gold mining. Gold is famously unreactive, but cyanide is one of the few substances that dissolves it. In the cyanidation process, crushed ore is treated with a dilute NaCN solution in the presence of oxygen and water. The overall reaction converts metallic gold into a soluble gold-cyanide complex:
4 Au + 8 CN⁻ + O₂ + 2 H₂O → 4 Au(CN)₂⁻ + 4 OH⁻
Each gold atom coordinates with two cyanide ions, forming the water-soluble dicyanogold(I) ion. This dissolved gold can then be recovered from solution, typically by adsorption onto activated carbon or by precipitation with zinc dust. The process, based on equations published by Elsner in 1846, is still the dominant method for extracting gold from low-grade ores worldwide.
Why Cyanide Is Such a Good Nucleophile
The cyanide ion’s effectiveness comes from its electronic structure. The carbon atom carries a lone pair of electrons and a negative charge, making it strongly attracted to electron-poor carbon centers. CN⁻ is also relatively small, so it can access sterically crowded sites more easily than bulkier nucleophiles. It sits high on standard nucleophilicity scales, outperforming common nucleophiles like hydroxide and acetate ions in SN2 reactions.
Another useful feature is that cyanide is an ambident nucleophile, meaning it can potentially bond through either its carbon or nitrogen atom. In the vast majority of organic reactions with NaCN, bonding occurs through carbon, giving nitriles (R-CN). Under certain conditions, particularly with silver cyanide or in reactions with very “hard” electrophiles, nitrogen bonding can occur, producing isonitriles (R-NC). When you see NaCN in a reaction scheme, you can almost always assume the product is a nitrile.
Safety and Toxicity
NaCN is extremely toxic. The cyanide ion blocks cytochrome c oxidase, the final enzyme in the cellular energy production chain. This prevents cells from using oxygen, causing rapid tissue suffocation even when blood oxygen levels are normal. The estimated lethal oral dose of HCN in humans is roughly 1.5 mg per kilogram of body weight, meaning a very small amount can be fatal.
Contact with acids is especially dangerous because it releases hydrogen cyanide gas, which is rapidly absorbed through the lungs. NaCN also absorbs moisture from air, forming a syrupy solution. NIOSH sets the workplace exposure limit at 5 mg/m³ as a time-weighted average, and handling requires full skin and eye protection with immediate access to emergency eyewash and drench showers.
Cyanide waste from reactions must be chemically neutralized before disposal. The standard method is alkaline chlorination using sodium hypochlorite (bleach). The hypochlorite oxidizes cyanide first to cyanate, a far less toxic compound, then further hydrolysis breaks cyanate down into ammonia and carbonate. The process requires an alkaline pH around 10.5, temperatures above 18°C, and takes about 15 minutes for the initial oxidation step.