Benzaldehyde: Structure, Molecular Weight, Grades and Uses

The short answer

Benzaldehyde is the simplest aromatic aldehyde. Structure C₆H₅CHO, molecular formula C₇H₆O, CAS 100-52-7, molecular weight 106.12 g/mol, boiling point 178 °C. It is a colourless to pale yellow oily liquid with a strong bitter almond odour.

Two things define it commercially. It is the backbone of almond and cherry flavour and a large family of fragrance and pharmaceutical intermediates. And it oxidises in air faster than almost any other bulk aromatic, which makes storage and packing a real part of the specification rather than an afterthought.

Structure, molecular weight and identifiers

The aldehyde group sits directly on the ring, so the carbonyl and the aromatic system are conjugated. That conjugation stabilises the molecule, which is why benzaldehyde does not undergo the aldol self-condensation that an aliphatic aldehyde would — it has no alpha hydrogen — and why it takes part in the classic Cannizzaro and benzoin reactions instead.

Physical properties

Note the density above 1. Benzaldehyde is heavier than water, so in a two-phase system it forms the lower layer — the opposite of most aromatic solvents and an easy mistake to make during work-up.

Why benzaldehyde turns into benzoic acid

Left in contact with air, benzaldehyde autoxidises to benzoic acid. The reaction is a free radical chain: oxygen abstracts the aldehyde hydrogen to give an acyl radical, that adds oxygen to form a peroxy acid, and the peroxy acid oxidises another molecule of benzaldehyde. Because each cycle generates the radical that starts the next, the reaction accelerates once it begins.

What you see is white crystalline solid in the drum or on the closure, a rising acid value, and a flattened almond note in the finished flavour. Light and trace metals both catalyse it.

The controls that actually work:

• Exclude air. Nitrogen blanket on the drum or tank, and the container kept as full as practical. Headspace is where the reaction lives.
• Exclude light and metal. Amber glass, lined or stainless steel; not bare mild steel, and not copper.
• Stabilise where permitted. Technical and fragrance grades are frequently supplied with a small antioxidant addition. Food and pharmacopoeial grades often cannot be, so they are shipped under nitrogen and have a shorter practical shelf life.
• Buy to consumption. A part-used drum deteriorates far faster than a sealed one. Order in a pack size you will finish.

Check the acid value on receipt and again before use. It is the single most informative number on a benzaldehyde certificate.

Natural versus synthetic benzaldehyde

Benzaldehyde occurs naturally in bitter almond, apricot, peach and cherry kernels, bound as the glycoside amygdalin. Crushing and enzymatic hydrolysis release benzaldehyde, glucose and hydrogen cyanide — which is why crude bitter almond oil has to be purified before food use, and why the cyanide-free grade is specified explicitly.

Natural and synthetic benzaldehyde are the same molecule. No assay, chromatogram or spectrum will separate them. What separates them is provenance, and the analytical proof of provenance is isotopic: carbon-14 content distinguishes recently-grown biological carbon from petrochemical carbon, and site-specific isotope ratio analysis distinguishes botanical routes from one another.

If your label claims natural flavour, you need a natural-source certificate and the isotope data behind it, and you should expect to pay a multiple of the synthetic price for a chemically indistinguishable product. If it does not, synthetic is the rational choice.

How benzaldehyde is made

Two industrial routes dominate, both starting from toluene.

• Chlorination and hydrolysis. Toluene is chlorinated at the side chain to benzal chloride, which is hydrolysed to benzaldehyde. High yielding, but the product can carry trace chlorinated impurities, which is why chlorine-free benzaldehyde is a distinct and separately specified grade.
• Direct oxidation. Toluene is oxidised in the liquid or vapour phase. It avoids chlorine entirely but the aldehyde is an intermediate on the way to benzoic acid, so selectivity and conversion have to be traded off against each other.

Natural benzaldehyde is produced by hydrolysing amygdalin from kernels, or by retro-aldol cleavage of cinnamaldehyde obtained from cassia oil — the latter being the usual commercial source of natural material.

Commercial grades and what separates them

Scapex Exports supplies benzaldehyde 99+% and the related acetal, benzaldehyde dimethyl acetal, which is used where the free aldehyde would be too reactive or too unstable in the finished formulation.

What benzaldehyde is used for

• Flavour. Almond, cherry, marzipan and stone-fruit notes in confectionery, bakery, beverages and pharmaceutical syrups. It is one of the highest-volume single flavour chemicals in use.
• Fragrance. An almond top note in its own right and a building block for the cinnamic and heliotropin families.
• Chemical intermediate. Route to cinnamaldehyde and cinnamic acid by Perkin condensation, to benzyl alcohol and benzoic acid by Cannizzaro, to benzoin, mandelic acid and benzyl benzoate, and to a range of dye and agrochemical intermediates.
• Pharmaceutical synthesis. Starting material in several established syntheses, including chloramphenicol and ephedrine chemistry.
• Aniline dyes. Long-standing use in triphenylmethane dye manufacture.
• Analytical and laboratory. Derivatising reagent and a standard substrate in aldehyde chemistry teaching and method development.

Handling, storage and export documents

• Classification. Benzaldehyde appears in the dangerous goods lists as UN 1990, Class 3, packing group III, notwithstanding a closed-cup flash point of about 63 °C. Confirm the classification in force for your mode and destination with your forwarder before booking, because treatment varies between regimes.
• Health. Harmful if swallowed; irritating to skin and eyes. Use nitrile gloves, splash goggles and local exhaust ventilation. The odour is detectable far below any harmful level, so smelling it is not in itself an exposure alarm.
• Fire. Combustible liquid. Keep away from heat, sparks and oxidising agents. Peroxy intermediates formed during autoxidation add a further reason to avoid heating aged material.
• Materials. Stainless steel, lined mild steel, HDPE and amber glass. Avoid bare mild steel, copper and copper alloys, which catalyse oxidation and discolour the product.
• Packing. 200 to 220 kg lined or stainless drums, 25 kg carboys, or amber glass for reagent quantities, nitrogen blanketed.
• Regulatory. Benzaldehyde is a monitored chemical in several jurisdictions because of its position in certain pharmaceutical syntheses. A compliant supplier will ask for end use and consignee details, and some destinations require an import permit or end-use declaration.

An export consignment should carry a certificate of analysis stating assay, acid value, colour (APHA), refractive index, specific gravity, water and chlorinated compounds; a 16-section GHS safety data sheet; a dangerous goods declaration where the destination requires one; commercial invoice and packing list; certificate of origin; and for food grade the FCC or FEMA reference with heavy-metal limits and any Halal, Kosher or allergen declarations required.

Enquire with a specification

Benzaldehyde is supplied by Scapex Exports as a manufacturer and supplier from qualified Indian producers, in technical, chlorine-free, food and flavour grades, nitrogen blanketed and packed for the shelf life your process needs, with certificate of analysis, safety data sheet and full export documentation.

Send the grade, assay, acid value limit, chlorine requirement, natural or synthetic status, packing and destination port together with the end use, and you will get a quotation written against that specification rather than a generic one.

Frequently asked questions