What Is 2-Hydroxybenzonitrile?
2-Hydroxybenzonitrile - widely traded as salicylonitrile or 2-cyanophenol - is an aromatic nitrile that carries a hydroxyl group and a nitrile group on adjacent (ortho) positions of the same benzene ring. That single structural detail defines the whole product. The two functional groups are close enough to interact, which is why the compound behaves differently from its meta and para isomers in solubility, acidity and downstream cyclisation chemistry, and why it is purchased as a functionalised building block rather than as a bulk solvent or additive.
Its identity parameters are fixed: CAS Registry Number 611-20-1, molecular formula C7H5NO, molecular weight 119.12 g/mol. It is a crystalline solid at ambient temperature with a mild phenolic odour, and it is the direct nitrile analogue of salicylaldehyde and of salicylic acid. Chemically the molecule presents two independent handles - a phenolic O–H that can be etherified or esterified, and a nitrile carbon that can be hydrolysed, reduced or cyclised - which is exactly why it appears in so many multi-step routes.
It should not be confused with the salts and esters of salicylic acid. 2-Hydroxybenzonitrile is a nitrile; on hydrolysis it gives salicylic acid, but until that transformation is carried out the product is an industrial intermediate with its own hazard profile, its own specification range and its own handling requirements.
Identity and Physicochemical Data
The table below lists the parameters that determine drying, crystallisation, packaging and analytical acceptance limits for a crystalline nitrile of this type.
| Parameter | Typical value / range | Why it matters in practice |
|---|---|---|
| CAS Registry Number | 611-20-1 | Primary identity reference for certificates, inventories and customs |
| Molecular formula | C7H5NO | One nitrile function plus one phenolic function |
| Molecular weight | 119.12 g/mol | Molar-charge and yield calculations downstream |
| Appearance | White to off-white crystalline solid, becoming pale yellow on ageing | Yellowing signals oxidation or phenolic impurity build-up |
| Melting range | Approximately 92–95 °C | A narrow range confirms identity and purity far more cheaply than a full assay |
| Solubility | Sparingly soluble in cold water; soluble in ethanol, methanol, acetone, chloroform and ether; more soluble in hot water | Governs recrystallisation solvent selection and aqueous work-up losses |
| Acidity | Phenolic O–H noticeably more acidic than phenol itself, because the ortho nitrile withdraws electron density | Allows clean extraction into dilute alkali and back-extraction on acidification |
| Volatility | Low; high boiling for its molecular weight | Purification is normally by vacuum distillation or recrystallisation, not by steam stripping |
| Nitrogen chemistry | Nitrile nitrogen; hydrolysis yields salicylic acid, reduction yields amine-type products | Defines the downstream product families reachable in one or two steps |
| Storage stability | Stable when kept dry, cool and closed away from acids, bases and oxidising agents | Acid or alkali contact plus moisture drives hydrolysis during storage |
Two practical observations follow from these data. First, because the compound is a solid with a melting range close to the temperature of a warm warehouse in summer, storage temperature control is not cosmetic: caking and partial melting followed by re-solidification can seal a drum liner to the wall and make discharge difficult. Second, the phenolic hydroxyl means that aqueous streams leaving a process are alkaline-extractable, so wastewater treatment should be designed for a phenolic load rather than for a simple nitrile load.
Isomers and Why the Ortho Position Changes Everything
Hydroxybenzonitriles exist as three positional isomers, and they are not interchangeable. The ortho arrangement in CAS 611-20-1 permits intramolecular interaction between the hydroxyl and the nitrile, the meta and para isomers do not. The distinction is decisive in ligand design, in cyclisation chemistry and in analytic method development, because HPLC or GC separation of the three isomers is a routine requirement in any specification for the ortho product.
| Isomer | CAS Registry Number | Structural difference and effect |
|---|---|---|
| 2-Hydroxybenzonitrile (salicylonitrile) | 611-20-1 | Hydroxyl ortho to nitrile; enables intramolecular hydrogen bonding and direct cyclisation to benzoxazole or benzisoxazole rings |
| 3-Hydroxybenzonitrile | 873-62-1 | Meta relationship; no intramolecular interaction, different melting behaviour and different downstream ring closures |
| 4-Hydroxybenzonitrile | 767-00-0 | Para relationship; highest symmetry, distinct physical properties and separate chemical uses |
For a buyer, the implication is concrete: the specification for CAS 611-20-1 must state an individual limit for the other two isomers or at minimum require them to be reported. A total purity figure obtained by titration or by a non-specific method cannot distinguish the ortho product from a mixture of isomers, and a downstream cyclisation step that depends on the ortho arrangement will simply fail on an isomeric impurity that a loose specification allowed through.
Grades and Specification Ranges
Commercial 2-hydroxybenzonitrile is supplied mainly as a technical grade for industrial synthesis and as a higher-purity grade for pharmaceutical and electronic-material applications. The parameters that separate the two are the isomer limit, the residual water, the residual solvent and the trace-metal profile.
| Specification item | Technical grade | High-purity grade |
|---|---|---|
| Assay (2-hydroxybenzonitrile, HPLC against external standard) | ≥ 98.0% w/w | ≥ 99.5% w/w |
| Isomeric hydroxybenzonitriles | Reported individually | ≤ 0.2% combined |
| Salicylamide / salicylic acid | ≤ 0.5% combined | ≤ 0.2% combined |
| Water (Karl Fischer) | ≤ 0.5% w/w | ≤ 0.2% w/w |
| Melting range | Reported, typically 92–95 °C | Narrow range with defined minimum |
| Residue on ignition / ash | ≤ 0.1% | ≤ 0.05% |
| Appearance | White to pale yellow crystalline solid | White crystalline solid, uniform particle size |
| Packaging | 25 kg net fibre or kraft drum with polyethylene liner; 500–1000 kg big bags for bulk | Smaller lined drums, often with an inner sealed bag |
A tight melting range is the cheapest and most informative release test for this material. A melting range that is wide or that starts below the specified minimum indicates either residual solvent, an isomeric impurity or absorbed moisture, all three of which can usually be traced back to the final drying and crystallisation stage rather than to the reaction itself.
Manufacturing Routes and Process Positioning
Two industrially established route families dominate commercial supply. In the first, an ortho-functionalised aromatic aldehyde is converted to its oxime and the oxime is then dehydrated to the nitrile. In the second, the corresponding aromatic amide - salicylamide - is dehydrated to the nitrile. Both families start from well-established salicylic-acid or phenol-derived feedstocks, and both finish with a purification step, normally vacuum distillation or recrystallisation, that determines the final isomer and colour profile.
From a supply-chain and qualification standpoint, four points deserve attention:
• Feedstock linkage. Cost tracks the salicylate feedstock complex rather than the nitrile market itself, so price movements upstream in phenol and salicylic acid chemistry are the useful leading indicators.
• Dehydration reagent choice. Different dehydration systems leave different impurity fingerprints - residual amide, carboxylic acid from over-hydrolysis, or traces of the reagent itself. The certificate should identify what is controlled, not just a total purity number.
• Isomer formation. Isomeric nitrile content is normally controlled at the reaction and crystallisation stage; a supplier that cannot report individual isomer levels is a supply risk for any ortho-specific application.
• Colour and drying. The final drying step sets both residual water and colour. Vacuum drying at moderate temperature preserves a white product; aggressive thermal drying tends to give a yellow cast that buyers of optical-grade material will reject.
None of these steps requires unusual equipment, which is why commercial supply is broad and why the differentiators between suppliers are consistency, documentation and traceability rather than process novelty.
Quality Control, Analytics and Impurity Limits
A practical release programme for 2-hydroxybenzonitrile rests on five measurements:
• Assay by HPLC. Reverse-phase HPLC with UV detection against an external standard of known purity is the standard assay technique for aromatic nitriles of this type. It resolves the ortho, meta and para isomers and reports the main component as a percentage of the dried basis.
• Isomer and related-substance profile. The same chromatographic method, run against an isomer mixture standard, quantifies 3- and 4-hydroxybenzonitrile as individual peaks; the hydrolysis products salicylamide and salicylic acid are quantified in a second run or in the same gradient.
• Water content. Karl Fischer titration performed in line with GB/T 6283 is the reference method. Water is the main driver of hydrolysis on storage and the main cause of a depressed melting range.
• Melting range. Determined by the capillary method, the melting range is the fastest in-house identity and purity screen and is normally run on every incoming lot.
• Residual solvents and ash. Where a pharmacopoeial-grade material is required, residual solvents are controlled against the limits of the applicable ICH Q3C class, and elemental impurities are controlled using a risk-based approach consistent with USP general chapters on elemental impurities. Ash and residue on ignition provide a cheap cross-check for inorganic carry-over from the dehydration step.
Sampling should follow GB/T 6678, with a composite drawn from several positions in the container because crystalline materials segregate by particle size during transport - fines concentrate at the surface and coarse crystals settle, so a single-point scoop sample is not representative.
Applications: Pharmaceutical and Agrochemical Intermediates
The commercial importance of 2-hydroxybenzonitrile lies in its two orthogonal functional groups, which allow chemists to build complexity selectively.
• Nitrogen heterocycle construction. The ortho hydroxyl and nitrile can be combined in cyclisation reactions to give benzoxazole and benzisoxazole nuclei, which are recurring frameworks in pharmaceutical and agrochemical discovery. This is the single most important structural reason for choosing the ortho isomer.
• Phenolic etherification and esterification. Alkylation or acylation of the hydroxyl gives O-substituted benzonitriles, which are then carried forward through the nitrile to amides, amidines or amines. This sequence is common when a lipophilic ether substituent is needed on an aromatic ring that also carries a nitrogen function.
• Hydrolysis to salicylic acid. Controlled hydrolysis converts the nitrile to the carboxylic acid and yields salicylic acid, the platform from which salicylate actives such as salicylamide and acetylsalicylic acid are manufactured. Positions in the salicylate value chain should therefore be read as an upstream downstream relationship rather than as a direct equivalence between the nitrile and the finished medicine.
• Agrochemical intermediates. Nitrile-bearing aromatic intermediates are used in crop-protection manufacture, where the nitrile group serves as a precursor to the amide or carboxylic acid function carried in the final active substance. The compound is a manufacturing intermediate; it is not itself a formulated crop-treatment product and is not applied as one.
• Ligands and complexing agents. The adjacent hydroxyl and nitrile make the molecule a convenient precursor for Schiff-base and salicylaldimine-type ligands and their metal complexes, which are widely studied in catalysis and coordination chemistry.
A note on mode of action and toxicology: the nitrile function is not the same as a free cyanide, and hydrolysis or metabolic cleavage converts the molecule to salicylate-type products. Claims that this compound acts as a neurotoxic insecticide should be treated with caution; it is supplied and controlled as an industrial intermediate, and its downstream uses are described in terms of the synthetic steps it enables.
Applications: Specialty Chemicals, Optical and Materials Uses
Beyond pharmaceutical and agrochemical synthesis, the ortho-hydroxy nitrile motif is used in several materials-oriented applications.
• Optical and photochromic systems. Benzoxazole and related heterocycles built from ortho-substituted benzonitriles appear in fluorescent whitening and optical-material chemistry, where the planarity and conjugation of the resulting ring system control the absorption and emission behaviour.
• Dye and pigment intermediates. The phenolic hydroxyl provides a coupling or chelation site, and O-substituted benzonitriles are used as intermediates for colorants and for metal-complexing dyes.
• Polymer and resin additives. Aromatic nitriles serve as precursors to nitrile-substituted monomers, cross-linkers and stabilisers; the phenolic function allows the molecule to be anchored into a polymer backbone or onto a filler surface.
• Corrosion and metal-treatment chemistry. Ortho-functionalised aromatic nitriles and their hydrolysis products are used in formulations for metal-surface treatment, where the phenolic and nitrile groups chelate onto metal ions.
• Analytical and reference uses. Because it is a crystalline, well-defined solid with a sharp melting range, the compound is also used as a reference material and as a substrate in method-development and kinetics studies of nitrile hydrolysis.
Health, Safety, Storage and Transport
2-Hydroxybenzonitrile is a solid aromatic nitrile and should be handled with the controls appropriate to that class.
• Health hazards. The substance is typically classified as harmful if swallowed (Acute Toxicity Category 4) and as an eye and skin irritant. Dust generation should be controlled, and no eating, drinking or smoking is permitted in the handling area.
• Combustion products. Burning nitriles release hydrogen cyanide and nitrogen oxides, so fire-fighting must be carried out from a protected position with self-contained breathing apparatus. In a fire, keep containers cool with water spray from a distance.
• Engineering and PPE. Local exhaust ventilation at charging and packing stations, dust-tight transfer equipment, chemical goggles, chemically resistant gloves and protective clothing. Respiratory protection is used for tasks that generate dust outside an enclosure.
• Incompatibilities. Strong oxidising agents, strong acids and strong bases. Acid or alkali plus moisture hydrolyses the nitrile; oxidisers can react exothermically with the phenolic ring.
• Storage. Keep containers closed in a cool, dry, well-ventilated area, protected from direct sunlight and from temperatures high enough to soften or melt the product. A typical shelf life is 24 months in unopened, correctly stored packaging, subject to re-test of assay, melting range and water before use.
• Transport. The product is normally shipped as a solid environmental/health hazard under the applicable national and international rules, with the UN number, packing group and hazard labels taken from the product safety data sheet. Where the substance or its formulation meets the criteria for a hazardous good, ADR and IMDG provisions for the assigned class apply, and the shipping papers must match the safety data sheet exactly.
• Spill and waste. Sweep or vacuum spilled solid into labelled containers without generating dust clouds, avoid washing into drains, and dispose of residues through a licensed waste route for nitrile-bearing chemical waste. Do not allow acidic or alkaline wash water to stand in contact with the material.
Regulatory and Compliance Notes
As an industrial chemical, 2-hydroxybenzonitrile is governed by inventory, hazard-communication and transport frameworks rather than by a single product-specific regulation. The compliance file for an incoming lot should contain a safety data sheet structured per ISO 11014 with GHS/CLP classification, transport classification consistent with the applicable modal regulations, evidence of inventory listing for the destination market such as TSCA and IECSC, and, where the material is supplied into the European Union, REACH registration evidence with the correct registration number quoted by the supplier.
Two application-driven points are worth separating. First, the substance is an industrial intermediate and not a food or flavouring ingredient; it must not be treated as a permitted flavouring substance unless a specific listed clearance applies. Second, for cosmetic or pharmaceutical use the relevant framework applies to the finished material and to its quality grade, not to the industrial intermediate: cosmetic ingredients are governed by the EU cosmetic regulation and any applicable INCI naming, while pharmaceutical use requires a grade controlled against a pharmacopoeial monograph or an equivalent in-house specification.
Workplace exposure control is based on airborne dust limits combined with ventilation and personal protection; buyers should obtain the supplier's recommended exposure guidance and reconcile it with the occupational limits applicable at their own site before the first delivery is accepted.
Sourcing and Incoming Inspection Checklist
The following checks catch nearly all of the commercial risks associated with this product:
1. Identity - CAS 611-20-1, molecular formula C7H5NO and molecular weight 119.12 g/mol confirmed against the label and certificate.
2. Isomer control - 3- and 4-hydroxybenzonitrile reported individually, with an agreed combined limit for ortho-specific applications.
3. Assay and hydrolysis products - HPLC assay within limit, with salicylamide and salicylic acid reported separately.
4. Physical quality - melting range within the specified window, colour white to off-white, and no caking, clumping or visible discolouration of the inner bag.
5. Moisture and ash - Karl Fischer water and residue on ignition within the agreed limits.
6. Packaging and documentation - intact lined drums or big bags with correct net weight, batch numbers matching across label, certificate and shipping documents, safety data sheet in the required language, and a retained sample kept for the agreed period.
7. Consistency over time - statistical review of certificates across several deliveries, because for an intermediate used in a registered route an unannounced shift in impurity profile is a larger risk than a single out-of-specification result.
FAQ
Q: What is 2-hydroxybenzonitrile and what is its CAS number?
2-Hydroxybenzonitrile, also called salicylonitrile or 2-cyanophenol, is CAS 611-20-1 with the molecular formula C7H5NO and a molecular weight of 119.12 g/mol. It is a crystalline aromatic solid with a hydroxyl group and a nitrile group in adjacent positions on the same benzene ring.
Q: Do the different hydroxybenzonitrile isomers have different CAS numbers?
Yes, and they are not interchangeable. The ortho isomer is CAS 611-20-1, the meta isomer is 3-hydroxybenzonitrile CAS 873-62-1, and the para isomer is 4-hydroxybenzonitrile CAS 767-00-0. Specifications for the ortho product should require the other two isomers to be reported or individually limited.
Q: What are the typical appearance and melting range of the ortho isomer?
The commercial product is a white to off-white crystalline solid with a melting range of approximately 92–95 °C. A widening or depression of that range usually indicates residual solvent, absorbed moisture or an isomeric impurity, and it is the quickest incoming check available.
Q: Which analytical methods are used to release 2-hydroxybenzonitrile?
Assay and the related-substance profile are determined by reverse-phase HPLC with UV detection against an external standard, water by Karl Fischer titration in accordance with GB/T 6283, and identity by capillary melting range. Where a pharmacopoeial grade is required, residual solvents and elemental impurities are additionally controlled under the applicable ICH and pharmacopoeial guidance.
Q: How should 2-hydroxybenzonitrile be stored and handled?
Store closed in a cool, dry, well-ventilated area away from strong acids, strong bases and oxidising agents, and protect from direct sunlight and from heat that could melt the solid. Use local exhaust ventilation during charging and packing, with goggles, gloves and protective clothing. A 24-month shelf life in unopened containers is typical, with assay, melting range and water re-tested before use.
Q: Is hydrolysis or exposure to moisture a problem during storage?
It can be. Water in combination with acid or alkali converts the nitrile group towards salicylamide and salicylic acid, which depresses the melting range, raises the acid value of the material and can change the outcome of a downstream step. Keeping the material dry, neutral and cool, and confirming the water content on receipt, is the practical way to control that risk.