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Benzoic Acid

C7H6O2 organic
Molar Mass
122.121 g/mol
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Properties

StateSolid at room temperature
ColorWhite crystalline needles
SolubilitySlightly soluble in cold water (3.4 g/L at 25°C), very soluble in hot water and organic solvents
Melting Point122°C
Boiling Point249°C

About Benzoic Acid

Benzoic acid is the simplest aromatic carboxylic acid — a benzene ring bonded directly to a carboxyl, with a pKa of 4.20 that places it slightly stronger than acetic acid (pKa 4.76) because the phenyl group withdraws electron density from the carboxylate by induction. The compound forms colorless needle-like crystals that are familiar to anyone who's run a teaching-lab calorimetry experiment, since pure benzoic acid is the certified primary standard for bomb-calorimeter calibration: its heat of combustion is known precisely (-3227 kJ/mol), and it burns cleanly without sooty residue, so a known mass burned in pure oxygen produces a reproducible thermal pulse against which the calorimeter's heat capacity is calibrated. Beyond the calibration role, benzoic acid and its sodium salt (sodium benzoate) are among the most-produced food preservatives, particularly effective against yeasts and molds in acidic foods (pH < 4.5). The mechanism depends on protonation: at low pH the un-ionized benzoic acid molecule diffuses through microbial cell membranes and dissociates inside the cell, where the proton release acidifies the cytoplasm and disrupts metabolism. The compound is also the synthetic precursor to benzoyl chloride (the standard reagent for installing benzoyl protecting groups on alcohols and amines), benzoate plasticizer esters used in polymer formulations, and various perfume ingredients. The natural occurrence in gum benzoin resin (where 'benzoic' originates as a name) and in cranberries explains the fruit's natural resistance to spoilage.

Where you'll encounter it

If you've drunk a soda containing sodium benzoate or eaten a brined vegetable preserved with benzoate, you've consumed the same chemistry that benzoic acid provides — the un-ionized acid form blocking microbial metabolism in the acidic food matrix. In an undergraduate physical chemistry lab, benzoic acid is the substance burned to calibrate a Parr bomb calorimeter on the first day of an enthalpy-of-combustion lab; the standard 1 g pellet releases about 26.4 kJ, which is a convenient size for the temperature pulse most commercial calorimeters are designed around. In a research synthesis, benzoic acid is occasionally the starting material for benzoyl chloride preparation by reaction with thionyl chloride, which then enters acyl chemistry on alcohols, amines, and carbon nucleophiles.

Common Uses

  • Sodium-benzoate food preservative for acidic foods and beverages
  • Primary standard for bomb-calorimetry heat-of-combustion calibration
  • Benzoyl chloride precursor for protecting-group and acyl chemistry
  • Benzoate plasticizer ester production for polymer formulations
  • Perfume and flavor ester building block in fragrance chemistry

Safety Information

Low toxicity — generally regarded as safe for food use at the low levels permitted for preservative use in beverages. Mild skin and eye irritant, with dust irritation in respiratory exposure during industrial handling. The one notable interaction is with ascorbic acid in the presence of trace metals: under heat or light, the combination can form trace benzene through a Cu/Fe-catalyzed decarboxylation, which is why beverage formulators specifically avoid pairing benzoate preservatives with high vitamin C content in transparent containers.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.

Constituent Elements

C — Carbon H — Hydrogen O — Oxygen

Frequently Asked Questions

What is the molar mass of benzoic acid?
122.121 g/mol. Sum 7(12.011) for the carbons, 6(1.008) for the hydrogens, and 2(15.999) for the two oxygens, giving 122.12. The number is worth memorizing for calorimetry — a 1.000 g sample of pure benzoic acid contains 8.19 mmol, which corresponds to a heat release of 26.43 kJ in pure oxygen and is the standard calibration pellet for most commercial bomb calorimeters.
Why is benzoic acid used as a food preservative?
The un-ionized acid form (HA, pKa 4.20) diffuses freely through microbial cell membranes; once inside the cell at neutral pH, it dissociates to release a proton, acidifying the cytoplasm and disrupting metabolism. The mechanism only works in acidic foods (pH < 4.5) where most of the benzoate is in the protonated HA form — at higher pH, the ionized A⁻ doesn't penetrate cell membranes and the antimicrobial activity drops sharply. That's why sodium benzoate works well in soft drinks, brined foods, and salad dressings (all naturally acidic) but is ineffective in neutral or alkaline foods.
Is benzoic acid soluble in water?
Modestly. Solubility is 3.4 g/L at 25 °C, low enough that benzoic acid behaves more like an organic solid than a salt at room temperature. Two changes raise solubility dramatically: heating to 100 °C bumps it to 56 g/L (the basis of the classic recrystallization-from-water purification protocol), and switching to organic solvents like ethanol or acetone gives essentially unlimited solubility because the hydrophobic phenyl ring solvates well in non-aqueous environments. The pH-dependence is also significant — at pH 7 with NaOH neutralization, the sodium benzoate form is freely soluble (>500 g/L) since the ionic compound dissolves like any other Na salt.