1. business and industrial
2. energy
3. oil

Module:2
Lecture:8 Benzoic acid
Dr. N. K. Patel
Module: 2
Lecture: 8
BENZOIC ACID
INTRODUCTION
Benzoic acid, C6H5COOH or C7H6O2 is a colourless crystalline solid and
a simple aromatic carboxylic acid. The name derived from gum benzoin,
which was the only source for benzoic acid for long time. The dry distillation of
gum benzoin was first described by Nostradamusin1556, and then by Alexius
Pedemontanusin1560 and Blaise de Vigenère in 1596.
In 1830 Pierre Robiquet and Antoine Boutron-Charlard produced
benzaldehyde through a variety of experiences based on amygdalin,
obtained from bitter almonds but they failed in working out a proper
interpretation of the structure of amygdalin that would account for it, and
thus missed the identification of the benzoyl radical C7H5O-. This last step was
achieved by Justus von Liebig and Friedrich Wöhler in 1832, who determined
the composition of benzoic acid.
Salts of benzoic acid are used as a food preservative. Salkowski
discovered the antifungal abilities of benzoic acid in 1875, which was used for
a long time in the preservation of benzoate-containing cloudberry fruits. The
salts and esters of benzoic acid are known as benzoates.
MANUFACTURE
Raw materials
Basis: 1000kg Benzoic acid from phthalic anhydride
Phthalic anhydride
1350kg
Catalyst
12.5kg
NPTEL
1
Module:2
Lecture:8 Benzoic acid
Dr. N. K. Patel
Reaction
Manufacture process
Caustic soda
Carbon dioxide
Benzoic acid
Neutralizer
Phthalic
anhydride
Catalyst
Column
Kettle
Steam
Steam
Waste
Figure: Manufacture of Benzoic acid from Phthalic anhydride
Block diagram of manufacturing process
Diagram with process equipment
Animation
The process consist of two steps, first is manufacture of phthalic
anhydride from naphthalene or o-xylene and second is manufacture of
benzoic acid from phthalic anhydride via oxidation.
Phthalic anhydride along with 2 – 6% catalyst consisting equal parts of
chromium and disodium phthalate were charged into kettle. The charge was
heated to 2000C and introduced 2 – 20parts of steam/hr below the surface of
NPTEL
2
Module:2
Lecture:8 Benzoic acid
Dr. N. K. Patel
mixture. Water, benzoic acid and phthalic acid was refluxed to the reaction
mass and carbon dioxide vented out. The reaction was continued until the
mixture contain less than 5% phthalic acid. Caustic soda was added to
reaction mixture for complete separation of benzoic acid from the
unreacted phthalic acid before distillation. Benzoic acid is separated from
reaction mixture by distillation. Catalyst was reused for several batches and
finally catalyst was recovered by dissolving the residue in a solution of soda
ash and filtering off the metal precipitate.
Alternative procedure
Benzoic acid may be prepared by passing vapours of phthalic
anhydride mixed with steam over a stationary catalyst maintained at a
temperature 380 – 4200C. A suitable contact mass mixture consists of an
equal amount of copper carbonate and calcium hydroxide uniformly
coated on pre-size pumice granules.
Raw materials
Basis: 1000kg Benzoic acid from toluene by air oxidation
Toluene
850kg
Air
60000Cu.ft.
Catalyst
Small quantity
Reaction
Manufacture process
Toluene was mixed with 0.1 – 0.3% cobalt naphthenate catalyst and
charged into a reactor. Air was sparged in the reactor until 40% of toluene
oxidized. Reaction mass was maintained at 148.90C temperature and
40 – 70psi pressure.
The reaction mixture was then sent for distillation after 40% conversion
where unreacted toluene and by-product benzaldehyde was removed. The
toluene was recycled back to process.
NPTEL
3
Module:2
Lecture:8 Benzoic acid
Recycle toluene
Dr. N. K. Patel
Vent
gases
Hot
water
Water
Water
Scrubber
Mixer
Water
Cooler
Toluene
Catalyst
Separator
Reactor
Distillation column
Water
Air
Benzoic acid
Filter
Heavy
ends
Figure: Manufacture of Benzoic acid from Toluene by air oxidation
Block diagram of manufacturing process
Diagram with process equipment
Animation
The bottom stream from the column was mixed with hot water to
extract benzoic acid. The aqueous layer was separated and cool to
precipitate benzoic acid, which was filtered and washed to get technical
grade product.
Advantages
 It gives chlorine free benzoic acid
 To get better yield
 Good control of by-products formation
NPTEL
4
Module:2
Lecture:8 Benzoic acid
Dr. N. K. Patel
Engineering problems
Design for efficient gas-liquid contacting and economic recovery of
organics fractions which contains water in the second stage are the two
main design problems.
Engineering aspect
 Preparation of catalyst
The catalyst is prepared by the direct addition of equivalent amounts
of chromium hydroxide and caustic soda to the reaction vessel.
PROPERTIES










Molecular formula
Molecular weight
Appearance
Odour
Boiling point
Melting point
Density
Flash point
Auto ignition temp
Solubility
: C7H6O2
: 122.12gm/mole
: Colourless crystalline solid
: Pungent
: 249.00C
: 121.70C
: 1.316gm/cm3
: 121.10C
: 571.10C
: Slightly soluble in water, soluble in
alcohol, chloroform, ether, benzene,
carbon disulfide, turpentine
USES
 Benzoic acid is used to determine the heat capacity of a bomb
calorimeter
 To produce benzoyl chloride and benzyl benzoate
 Benzoic acid and its salts are used as a food preservatives
 It is a constituent of Whitfield’s ointment which is used for the treatment
of fungal skin diseases like ringworm, tinea and athlete’s foot
 Major ingredient in both tincture of benzoin and Friar’s balsam which
are used as antiseptics and inhalant decongestants
 As expectorant and analgesic
NPTEL
5
Module:2
Lecture:8 Benzoic acid
Dr. N. K. Patel
 As medicinal, industrial preserving foods, fats, alkaloid solutions, fruit
juices, cosmetics, resin preparations, plasticizers, synthetic fibres and
intermediates, dyes as mordant in calico printing
 As curing tobacco
 As standard in volumetric and calorimetric analysis and pharmaceutics
aid
NPTEL
6