Acetone cyanohydrin (ACH) is the liquid α-hydroxy nitrile produced by base-catalysed condensation of acetone with hydrogen cyanide. The substance is identified by CAS Registry Number 75-86-5, EC number 200-909-4, and transport designation UN 1541, Class 6.1, Packing Group I. Its molecular formula is C4H7NO, with molar mass 85.11 g/mol, density 0.932 g/cm³ at 20 °C, and normal boiling point near 95 °C. The technical product is supplied as a stabilised, clear to pale yellow liquid; acid stabilisation is required because ACH reversibly dissociates to acetone and hydrogen cyanide. Harmonised classification under Regulation (EC) No 1272/2008 includes acute toxicity statements H300, H310, and H330, together with supplemental hazard EUH032 for acid-induced release of very toxic gas. Unlike acrylonitrile, which contains a conjugated nitrile group, ACH carries both a tertiary hydroxyl group and the nitrile on the same carbon; this structural feature controls its reactivity and its hazardous decomposition profile.

Production integrates ACH directly with captive hydrogen cyanide generation. Hydrogen cyanide from the Andrussow oxidation of methane and ammonia, or from acrylonitrile by-product recovery, is absorbed into a cooled acetone stream containing a controlled basic catalyst. Because hydrogen cyanide boils at 26 °C, the reaction and absorption sections are sealed, vented to caustic scrubbing, and designed for vapour containment. The crude cyanohydrin stream is then neutralised with mineral acid and adjusted to the stabilised pH window before storage.

Why Does Residual Alkalinity Govern Storage Stability More Than Thermal Stress?

The dominant stability variable for ACH is residual alkalinity because the retro-cyanohydrin pathway is specifically catalysed by bases. At pH ≤ 4, free cyanide ion concentration is suppressed and the dissociation to acetone and hydrogen cyanide remains kinetically limited under ordinary storage conditions. As pH rises above 4–5, the equilibrium and rate both shift toward hydrogen cyanide evolution. Thermal stress accelerates this process, but a closed container held at pH 2.5 is less hazardous at 35 °C than an improperly neutralised container at 15 °C carrying caustic residues from a transfer line or cleaning procedure. Commercial storage practice therefore requires acid-stabilised product, with pH typically controlled at 2.0–3.5, and inhibitor verification before bulk transfer. Plant transfer systems use PTFE-lined piping or austenitic stainless steel such as 316L; copper-containing alloys and unlined steel are avoided because cyanide can form copper complexes and hydrogen embrittlement can occur at weld zones. Forced ventilation and pressure-relief equipment are specified for intermediate bulk containers and storage tanks because liberated hydrogen cyanide is both toxic and flammable over a wide range. UN 1541 stabilised material is not thermally self-reactive under routine transport; however, contamination with caustic soda, ammonia, alkanolamines, or cyanide-producing biocatalysts can generate self-heating and pressurisation. Published safety data sheets from major producers define the maximum recommended storage temperature as 30–35 °C and require segregation from foodstuffs, oxidising agents, and concentrated mineral acids. A minor thermal excursion of a stabilised container is generally less severe than an equivalent pH excursion because the base-catalysed mechanism has an immediate effect on cyanide liberation.

Vapour-phase exposure is controlled by the free hydrogen cyanide content rather than by the ACH assay alone. In equilibrium, a small fraction of free hydrogen cyanide remains in the liquid and partitions into the headspace. Closed-loop sampling systems, nitrogen padding, and local exhaust ventilation are therefore used on production lines handling stabilised ACH. The free hydrogen cyanide limit in the liquid phase is set to maintain the headspace concentration below occupational exposure limits during normal transfer operations. Batch-to-batch variation in stabiliser distribution is a known production issue when neutralisation columns are operated below design temperature; acidic carryover or localised pH gradients can produce non-representative certificate-of-analysis values.

In the dominant commercial use, ACH is converted to methyl methacrylate by treatment with concentrated sulfuric acid at 98–100 wt%. The first-stage conversion to methacrylamide sulfate is strongly exothermic and is carried out in staged continuous stirred-tank or loop reactors with split acid addition and high-alloy heat exchangers. Reactor temperature is typically controlled in the 80–120 °C range; localised over-temperature reduces selectivity and can regenerate hydrogen cyanide and acetone. The resulting methacrylamide sulfate is then hydrolysed and esterified with methanol in the presence of water and additional sulfuric acid to yield methyl methacrylate, with ammonium bisulfate as the major co-product. The ammonium bisulfate stream is routed to sulfuric acid recovery or disposed through permitted deep-well injection; this sulfate burden is a defining feature of the ACH route compared with the C4 oxidation route from isobutylene or tert-butanol and the ethylene-based propionaldehyde route. Vendor-specific selectivity and acid ratio are proprietary, but published data confirm that excess sulfuric acid above the stoichiometric requirement shifts the first-stage equilibrium toward methacrylamide sulfate and suppresses unreacted ACH carryover. ACH is also consumed in smaller volumes for methacrylic acid, methacrylamide, and selected α-hydroxy nitrile synthons. In these specialty applications, the product is generally selected when a non-salt liquid cyanide equivalent is required in a neutral or acidic non-aqueous medium.

Because the ACH route uses cyanide chemistry and acid hydrolysis rather than selective oxidation, plant layout includes dedicated HCN detection, emergency scrubbers charged with aqueous sodium hydroxide, and closed-loop recovery of reactor off-gas. The sulfate handling system is designed to prevent crystallisation in transfer lines; line plugging and salt deposition are the dominant maintenance bottlenecks on commercial MMA units.

Specifications and Analytical Control Points for Stabilised Technical Grade

Commercial ACH is supplied as a stabilised technical liquid rather than as a pure analytical standard. Producer specifications commonly include assay, free hydrogen cyanide, water, acetone, pH, and appearance. The values below are representative of internationally traded technical grade; exact limits are producer-specific and are set out in the safety data sheet and certificate of analysis.

Analytically, ACH assay is generally measured by gas chromatography or by hydrolysis followed by cyanide determination. Free hydrogen cyanide is measured by producer-specific titrimetric or ion-selective electrode methods because the equilibrium between ACH, acetone, and hydrogen cyanide is matrix-sensitive. pH is measured on the as-received product or on a diluted aqueous phase under controlled ionic strength. These methods are not uniform across all producers; therefore, the certificate of analysis must be read with its method annotations and sampling conditions.

ACH is often compared with sodium cyanide and acrylonitrile because the three chemicals are cyanide-bearing industrial substances. The relevant distinction is cyanide availability under process conditions, not total nitrogen content. ACH releases hydrogen cyanide through retro-cyanohydrin cleavage in the presence of bases or under thermal abuse; sodium cyanide releases hydrogen cyanide upon acidification or contact with weak acids; acrylonitrile retains its nitrile group during normal storage and is primarily a monomer and chemical intermediate, not a free-cyanide source in the same sense.

Compared with hydrogen cyanide itself, ACH is a liquid at ambient temperature and has a much lower vapour pressure, but this apparent handling advantage is conditional on acid stabilisation. Hydrogen cyanide is transported as UN 1051, Class 6.1 and Class 3, whereas ACH is transported as UN 1541, Class 6.1. The lower vapour pressure of ACH does not eliminate cyanide inhalation risk; it transfers the risk from the primary container to the stabiliser-controlled equilibrium and downstream processing steps.

When Acetone Cyanohydrin Replaces Inorganic Cyanide Salts in Liquid-Phase Synthesis

Replacement of sodium cyanide or potassium cyanide with ACH is feasible only when the reaction medium is neutral or acidic and when sodium or potassium salts interfere with product quality. The substitution is common in methacrylate chemistry because ACH supplies both the cyanide fragment and the acetone-derived carbon skeleton. In alkaline media, ACH is not a drop-in replacement for sodium cyanide because the rate of cyanide release becomes difficult to control and the liberated hydrogen cyanide creates a vapour hazard. The process design response is to add ACH into a buffered or acidified reactor with sufficient cooling to remove the heat of reaction and to route all vents through a caustic scrubber. Vessel materials are selected from 316L stainless steel, PTFE-lined steel, or equivalent acid-resistant alloys; copper and brass components are excluded. For non-methacrylic cyanohydrin reactions, published data for ACH-specific substitution effects is limited, and compatibility with solvents, Lewis acids, and anhydrous conditions must be verified in a calorimetric screening study before scale-up. ACH is therefore not a general cyanide source; it is a specialty intermediate whose safe use depends on maintaining the stabilised pH window and controlling the dissociation equilibrium.