Acetic Acid
Ethanoic acid/Acetic acid
Acetic acid, also known as ethanoic acid, is
an organic chemical compound best recognized for giving vinegar
its sour taste and pungent smell. Pure water-free acetic acid (glacial acetic acid)
is a colorless hygroscopic liquid and freezes below 16.7 °C
(62 °F) to a colourless crystalline solid.
Acetic acid is corrosive, and its vapour is
irritating to eyes and nose, although it is aweak acid based on its ability
to dissociate in aqueous solutions.
Acetic acid is one of the
simplest carboxylic acids (the second-simplest, next to formic acid). It is an
important chemical reagent and industrial chemical that is used in the
production of polyethylene terephthalate mainly used in soft drink
bottles; cellulose acetate, mainly for photographic film;
and polyvinyl acetate for wood glue, as well as many synthetic fibers
and fabrics. In households diluted acetic acid is often used in descaling
agents. In the food industry acetic acid is used under the food
additive code E260 as an acidity regulator.
The global demand of acetic
acid is around 6.5 million tonnes per year (Mt/a), of which approximately
1.5 Mt/a is met by recycling; the remainder is manufactured
from petrochemical feedstocks or from biological sources.
The trivial
name acetic acid is the most commonly used and officially preferred
name by the IUPAC. This name derives from acetum, the Latin word for vinegar. The
synonym ethanoic acid is a systematic name that is sometimes used in
introductions to chemical nomenclature.
Glacial acetic acid is
a trivial name for water-free acetic acid. Similar to the German name Eisessig (literally,
ice-vinegar), the name comes from the ice-like crystals that form slightly
below room temperature at 16.7°C (about 62°F).
The most common and
official abbreviation for acetic acid
is AcOH or HOAc where Ac stands for
the acetyl group CH3−C(=O)−;. In the context of acid-base
reactions the abbreviation HAc is often used
where Ac instead stands for the acetate anion (CH3COO−),
although this use is regarded by many as misleading. In either case,
the Ac is not to be confused with the abbreviation for the chemical element actinium.
Acetic acid has
the empirical formula CH2O and the molecular formula C2H4O2. The latter is
often written as CH3-COOH, CH3COOH, or CH3CO2H to better reflect its structure.
The ion resulting from loss of H+ from acetic acid is
the acetate anion. The name acetate can also refer to
a salt containing this anion or
an ester of acetic acid.
Vinegar is as old as
civilization itself, perhaps older. Acetic acid-producing bacteria are present
throughout the world, and any culture practicing the brewing of beer or wine inevitably discovered vinegar as
the natural result of these alcoholic beverages being exposed to air.
The use of acetic acid in
chemistry extends into antiquity. In the 3rd century BC, the Greek philosopher Theophrastos
described how vinegar acted on metals to produce pigments useful in art,
including white lead ( lead carbonate) and verdigris, a
green mixture of copper salts including copper(II)
acetate. Ancient Romans boiled soured wine in lead pots
to produce a highly sweet syrup called sapa. Sapa was rich in lead
acetate, a sweet substance also called sugar of lead or sugar
of Saturn, which contributed to lead poisoning among the Roman
aristocracy. The 8th century Persian alchemist Jabir Ibn Hayyan (Geber)
concentrated acetic acid from vinegar through distillation.
In the Renaissance, glacial acetic acid was
prepared through the dry distillation of metal acetates. The 16th
century German alchemist Andreas Libavius
described such a procedure, and he compared the glacial acetic acid produced by
this means to vinegar. The presence of water in vinegar has such a profound
effect on acetic acid's properties that for centuries many chemists believed
that glacial acetic acid and the acid found in vinegar were two different
substances. The French chemist Pierre Adet proved them to be identical.
In 1847 the German
chemist Hermann Kolbe synthesised acetic acid from inorganic
materials for the first time. This reaction sequence consisted ofchlorination
of carbon disulfide to carbon tetrachloride, followed
by pyrolysis to tetrachloroethylene and aqueous chlorination
to trichloroacetic acid, and concluded with electrolytic reduction
to acetic acid.
Detail of acetic acid crystals
By 1910 most glacial acetic
acid was obtained from the "pyroligneous liquor" from distillation of
wood. The acetic acid was isolated from this by treatment with milk of
lime, and the resultant calcium acetate was then acidified with sulfuric acid to recover acetic
acid. At this time Germany was producing 10,000 tons of glacial acetic
acid, around 30% of which was used for the manufacture of indigo dye.
Acidity
The hydrogen (H) atom in
the carboxyl group (−COOH) in carboxylic acids such as acetic acid
can be given off as an H+ ion (proton), giving them their acidic character.
Acetic acid is a weak, effectively monoprotic acid in aqueous solution,
with a pKa value of 4.8. A 1.0 M solution (about the
concentration of domestic vinegar) has a pH of 2.4, indicating that merely 0.4%
of the acetic acid molecules are dissociated.
The crystal structure of
acetic acid shows that the molecules pair up into dimers connected
by hydrogen bonds. The dimers can also be detected in the vapour at
120 °C. They probably also occur in the liquid phase of pure acetic acid,
but are rapidly disrupted if any water is present. This dimerisation behaviour
is shared by other lower carboxylic acids.
Solvent
Liquid acetic acid is
a hydrophilic ( polar) protic solvent, similar to ethanol and water. With a moderate dielectric
constant of 6.2, it can dissolve not only polar compounds such as inorganic
salts and sugars, but also non-polar compounds such as
oils and elements such as sulfur and iodine. It readily mixes with many other
polar and non-polar solvents such as water, chloroform,
and hexane. This dissolving property and miscibility of acetic acid
makes it a widely used industrial chemical.
Chemical reactions
Acetic acid is corrosive to many metals including iron, magnesium, and zinc, forming hydrogen gas and metal salts
called acetates. Aluminium,
when exposed to oxygen, forms a thin layer ofaluminium oxide on its surface
which is relatively resistant, so that aluminium tanks can be used to transport
acetic acid. Metal acetates can also be prepared from acetic acid and an
appropriate base, as in the popular " baking soda +
vinegar" reaction. With the notable exception of chromium(II)
acetate, almost all acetates are soluble in water.
Acetic acid undergoes the
typical chemical reactions of a carboxylic acid, notably the
formation of ethanol by reduction, and
formation of derivatives such as acetyl chloride via nucleophilic
acyl substitution. Other substitution derivatives include acetic
anhydride; this anhydride is produced by loss of water from two
molecules of acetic acid. Esters of acetic acid can likewise be formed
via Fischer esterification, and amides can also be formed. When
heated above 440 °C, acetic acid decomposes to produce carbon dioxide and methane, or
to produce ketene and water.
Detection
Acetic acid can be detected
by its characteristic smell. A colour reaction for salts of acetic acid
is iron(III) chloride solution, which results in a deeply red colour that
disappears after acidification. Acetates when heated with arsenic trioxide
form cacodyl oxide, which can be detected by its malodorous vapors.
Acetic acid is an organic
acid with the formula CH3-COOH. Its functional group is carboxylic acid group.
Acetic acid is a
monocarboxylic acid because it contains only one "COOH"group. It has
a sour taste and pungent smell. It is the main component of vinegar. Vinegar is
typically 3-7% solution of acetic acid in water. Vinegar is mainly used as a
preservative in food and in the pickling of vegetables. Water free acetic acid
is known as glacial acetic acid.
What are the properties of
acetic acid?
- Acidic
character
- When
dissolved in water, acetic acid undergoes dissociation to form hydrogen
(H+) ion. Because of the release of a proton, acetic acid has an
acidic character. It turns blue litmus paper red, indicating that it is
acidic in nature. However, it is a weak acid because it does not
dissociate completely in aqueous solution.
- Reaction with
sodium bicarbonate
- Acetic acid
reacts with sodium bicarbonate to produce carbon dioxide.
On passing CO2 gas through lime
water, the lime water turns milky. The milky appearance of lime water is due to
the formation of solid calcium carbonate (CaCO3).
Common uses of acetic acid:
- Acetic acid
is used as coagulant in the manufacture of rubber.
- It is used in
the manufacture of various dye stuffs and perfumes.
- It is used in
the manufacture of rayon fibre.
- It is used as
a solvent.
Learning Outcomes
- Students
understand the chemical properties of acetic acid after this experiment.
- Students acquire
skills to perform & visualize the reaction of acetic acid with the
following.
- Water
- Litmus paper
- Sodium
bicarbonate
- Students will
be able to properly use glassware like thistle funnel, delivery tubes,
etc., in the real lab.
- Based on
acquired skills, the student will be able to analyze a given sample &
recognize it as acetic acid.
Acetic
acid
Acetic acid, systematically named ethanoic acid is a
colorless liquid organic compound with
the chemical formula CH3COOH
(also written as CH3CO2H or C2H4O2). When undiluted, it is sometimes
called glacial acetic acid. Vinegar is no less than 4%
acetic acid by volume, making acetic acid the main component of vinegar apart
from water. Acetic acid has a distinctive sour taste and pungent smell. In
addition to household vinegar, it is mainly produced as a precursor to polyvinyl acetate and cellulose acetate. It
is classified as a weak acid since
it only partially dissociates in solution, but concentrated acetic acid is
corrosive and can attack the skin.
Acetic acid is the second simplest carboxylic acid (after formic acid). It
consists of a methyl group attached
to a carboxyl group. It
is an important chemical reagent and
industrial chemical, used primarily in the production of cellulose acetate for photographic film, polyvinyl acetate for
wood glue, and
synthetic fibres and fabrics. In households, diluted acetic acid is often used
in descaling agents. In
the food industry,
acetic acid is controlled by the food additive code E260
as an acidity regulator and as a condiment. In biochemistry, the
acetyl group, derived from acetic acid, is fundamental to all forms of life.
When bound to coenzyme A, it
is central to the metabolism of carbohydrates and fats.
The global demand for acetic acid is about 6.5 million metric tons per
year (Mt/a), of which approximately 1.5 Mt/a is met by recycling; the
remainder is manufactured from methanol.[7] Vinegar is mostly
dilute acetic acid, often produced by fermentation and subsequent oxidation of ethanol.
Contents
Nomenclature
The trivial name acetic
acid is the most commonly used and preferred IUPAC name. The systematic name ethanoic
acid, a valid IUPAC name,
is constructed according to the substitutive nomenclature.[8] The name acetic
acid derives from acetum, the Latin word for vinegar, and is related to the
word acid itself.
Glacial acetic acid is a name for water-free (anhydrous)
acetic acid. Similar to the German name Eisessig (ice-vinegar),
the name comes from the ice-like crystals that form slightly below room
temperature at 16.6 °C (61.9 °F) (the presence of 0.1% water lowers
its melting point by 0.2 °C).
A common abbreviation for acetic acid is AcOH,
where Ac stands for the acetyl group CH
3−C(=O)−. Acetate (CH
3COO−) is abbreviated AcO−. The Ac is not to be confused with the abbreviation for the chemical element actinium.[10] To better reflect its structure, acetic acid is often written as CH
3–C(O)OH, CH
3−C(=O)OH, CH
3COOH, and CH
3CO
2H. In the context of acid-base reactions, the abbreviation HAc is sometimes used,[11]where Ac in this case is a symbol for acetate (rather than acetyl). Acetate is the ion resulting from loss of H+
from acetic acid. The name acetatecan also refer to a salt containing this anion, or an ester of acetic acid.
3−C(=O)−. Acetate (CH
3COO−) is abbreviated AcO−. The Ac is not to be confused with the abbreviation for the chemical element actinium.[10] To better reflect its structure, acetic acid is often written as CH
3–C(O)OH, CH
3−C(=O)OH, CH
3COOH, and CH
3CO
2H. In the context of acid-base reactions, the abbreviation HAc is sometimes used,[11]where Ac in this case is a symbol for acetate (rather than acetyl). Acetate is the ion resulting from loss of H+
from acetic acid. The name acetatecan also refer to a salt containing this anion, or an ester of acetic acid.
Properties
Acetic acid crystals
Acidity
The hydrogen centre in the carboxyl group (−COOH)
in carboxylic acids such as acetic acid can separate from the molecule by
ionization:
CH3CO2H ⇌ CH3CO2− + H+
Because of this release of the proton (H+), acetic acid has
acidic character. Acetic acid is a weak monoprotic acid. In
aqueous solution, it has a pKa value of 4.76. Its conjugate base is acetate (CH3COO−). A
1.0 M solution
(about the concentration of domestic vinegar) has a pH of 2.4, indicating that merely 0.4% of the acetic acid
molecules are dissociated. However, in very dilute (< 10−6 M)
solution acetic acid is >90% dissociated.
Cyclic dimer of acetic
acid; dashed green lines represent hydrogen bonds
In solid acetic acid, the molecules
form chains, individual molecules being interconnected by hydrogen bonds.[15] In the vapour at
120 °C (248 °F), dimers can be detected. Dimers also occur in the
liquid phase in dilute solutions in non-hydrogen-bonding solvents, and a
certain extent in pure acetic acid,[16] but
are disrupted by hydrogen-bonding solvents. The dissociation enthalpy of the dimer is
estimated at 65.0–66.0 kJ/mol, and the dissociation entropy at
154–157 J mol−1 K−1.[17] Other carboxylic
acids engage in similar intermolecular hydrogen bonding interactions.[18]
Solvent properties
Liquid acetic acid is
a hydrophilic (polar) protic solvent,
similar to ethanol and water. With
a moderate relative static permittivity (dielectric constant) of 6.2, it
dissolves not only polar compounds such as inorganic salts and sugars, but also non-polar compounds such as
oils as well as polar solutes. It is miscible with polar and non-polar solvents such as water, chloroform,
and hexane. With higher alkanes
(starting with octane),
acetic acid is not completely miscible, and its miscibility
declines with longer n-alkanes.[19] The solvent and miscibility properties
of acetic acid make it a useful industrial chemical, for example, as a solvent
in the production of dimethyl terephthalate.
Biochemistry
The acetyl group, formally derived from
acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it
is central to the metabolism of carbohydrates and fats. Unlike longer-chain carboxylic acids
(the fatty acids),
acetic acid does not occur in natural triglycerides.
However, the artificial triglyceride triacetin (glycerin
triacetate) is a common food additive and is found in cosmetics and topical
medicines.[20]
Acetic acid is produced and excreted by acetic acid bacteria, notably the genus Acetobacter and Clostridium acetobutylicum. These bacteria are found universally
in foodstuffs, water, and soil, and acetic acid is produced naturally
as fruits and other foods spoil. Acetic acid is also a component of the vaginal lubrication of humans and other primates, where it appears to serve
as a mild antibacterial agent.
Purification and
concentration plant for acetic acid in 1884
Acetic acid is produced industrially
both synthetically and by bacterial fermentation. About 75% of acetic acid made for use
in the chemical industry is made by the carbonylation of
methanol, explained below.[7] The biological route
accounts for only about 10% of world production, but it remains important for
the production of vinegar because many food purity laws require vinegar used in
foods to be of biological origin. Other processes are methyl formate
isomerization, conversion of syngas to acetic acid, and gas phase oxidation of
ethylene and ethanol.[22] Acetic acid is often
a side product of different reactions, i.e. during heterogeneous catalytic
acrylic acid synthesis[23][24][25] or fermentative
lactic acid production.[26] As of 2003–2005,
total worldwide production of virgin acetic acid[27] was
estimated at 5 Mt/a (million tonnes per year), approximately half of which
was produced in the United States. European production was
approximately 1 Mt/a and declining, while Japanese production was 0.7 Mt/a. Another
1.5 Mt were recycled each year, bringing the total world market to
6.5 Mt/a.[28][29] Since then the global
production has increased to 10.7 Mt/a (in 2010), and further; however, a
slowing in this increase in production is predicted.[30] The two biggest
producers of virgin acetic acid are Celaneseand BP Chemicals. Other major producers include Millennium Chemicals, Sterling Chemicals, Samsung, Eastman, and Svensk Etanolkemi.[31]
Most acetic acid is produced by
methanol carbonylation. In
this process, methanol and carbon monoxide react
to produce acetic acid according to the equation:
The process involves iodomethane as
an intermediate, and occurs in three steps. A catalyst, metal carbonyl, is
needed for the carbonylation (step 2).[32]
1.
CH3OH
+ HI → CH3I + H2O
2.
CH3I +
CO → CH3COI
3.
CH3COI
+ H2O → CH3COOH + HI
Two related processes for the
carbonylation of methanol: the rhodium-catalyzed Monsanto process, and
the iridium-catalyzed Cativa process.
The latter process is greener and more efficient[33] and
has largely supplanted the former process, often in the same production plants.
Catalytic amounts of water are used in both processes, but the Cativa process
requires less, so the water-gas shift reaction is suppressed, and fewer
by-products are formed.
By altering the process
conditions, acetic anhydride may
also be produced on the same plant using the rhodium catalysts.[34]
Prior to the commercialization of the Monsanto process, most
acetic acid was produced by oxidation of acetaldehyde. This
remains the second-most-important manufacturing method, although it is usually
not competitive with the carbonylation of methanol. The acetaldehyde can be
produced by hydration of acetylene. This was the dominant technology in the early 1900s.[35]
Light naphtha components
are readily oxidized by oxygen or even air to give peroxides,
which decompose to produce acetic acid according to the chemical equation,
illustrated with butane:
2 C4H10 + 5 O2 → 4 CH3CO2H +
2 H2O
Such oxidations require metal catalyst, such as the
naphthenate salts of manganese, cobalt, and chromium.
The typical reaction is conducted at temperatures and
pressures designed to be as hot as possible while still keeping the butane a
liquid. Typical reaction conditions are 150 °C (302 °F) and 55 atm.[36] Side-products may
also form, including butanone, ethyl acetate, formic acid,
and propionic acid. These side-products are also commercially
valuable, and the reaction conditions may be altered to produce more of them
where needed. However, the separation of acetic acid from these by-products
adds to the cost of the process.[37]
Under similar conditions and using similar catalysts as
are used for butane oxidation, the oxygen in air to produce acetic acid can oxidize acetaldehyde.[37]
2 CH3CHO + O2 → 2 CH3CO2H
Using modern catalysts, this reaction can have an acetic acid
yield greater than 95%. The major side-products are ethyl acetate, formic acid,
and formaldehyde, all
of which have lower boiling points than
acetic acid and are readily separated by distillation.[37]
In more recent times, chemical company Showa Denko,
which opened an ethylene oxidation plant in Ōita, Japan, in 1997, commercialized a cheaper
single-stage conversion of ethylene to acetic acid.[38] The process is
catalyzed by a palladium metal
catalyst supported on a heteropoly acid such
as silicotungstic acid. Similar process use the
same metal catalyst on silicotungstic acid and silica:[39]
C2H4 + O2 → CH3CO2H
It is thought to be competitive with methanol carbonylation for
smaller plants (100–250 kt/a), depending on the local price of ethylene.
The approach will be based on utilizing a novel selective photocatalytic
oxidation technology for the selective oxidation of ethylene and ethane to acetic
acid. Unlike traditional oxidation catalysts, the selective oxidation process
will use UV light to produce acetic acid at ambient temperatures and pressure.
For most of human history, acetic acid bacteria of the genus Acetobacter have
made acetic acid, in the form of vinegar. Given sufficient oxygen, these
bacteria can produce vinegar from a variety of alcoholic foodstuffs. Commonly
used feeds include apple cider, wine, and fermented grain, malt, rice, or potato mashes. The overall
chemical reaction facilitated by these bacteria is:
C2H5OH + O2 → CH3COOH + H2O
A dilute alcohol solution inoculated
with Acetobacter and kept in a warm, airy place will become vinegar
over the course of a few months. Industrial vinegar-making methods accelerate
this process by improving the supply of oxygen to the bacteria.[40]
The first batches of vinegar produced
by fermentation probably followed errors in the winemaking process.
If must is fermented at too
high a temperature, acetobacter will overwhelm the yeast naturally occurring
on the grapes. As the demand for vinegar
for culinary, medical, and sanitary purposes increased, vintners quickly
learned to use other organic materials to produce vinegar in the hot summer
months before the grapes were ripe and ready for processing into wine. This
method was slow, however, and not always successful, as the vintners did not
understand the process.[41]
One of the first modern commercial
processes was the "fast method" or "German method", first practiced
in Germany in 1823. In this process, fermentation takes place in a tower packed
with wood shavings or charcoal. The alcohol-containing
feed is trickled into the top of the tower, and fresh air supplied from
the bottom by either natural or forced convection. The
improved air supply in this process cut the time to prepare vinegar from months
to weeks.[42]
Nowadays, most vinegar is made in
submerged tank culture, first described in 1949 by Otto
Hromatka and Heinrich Ebner.[43] In this method,
alcohol is fermented to vinegar in a continuously stirred tank, and oxygen is
supplied by bubbling air through the solution. Using modern applications of
this method, vinegar of 15% acetic acid can be prepared in only 24 hours in
batch process, even 20% in 60-hour fed-batch process.[41]
Species of anaerobic bacteria, including members of the genus Clostridium or Acetobacterium can
convert sugars to acetic acid directly without creating ethanol as an
intermediate. The overall chemical reaction conducted by these bacteria may be
represented as:
C6H12O6 → 3 CH3COOH
These acetogenic bacteria
produce acetic acid from one-carbon compounds, including methanol, carbon monoxide, or a
mixture of carbon dioxide and hydrogen:
2 CO2 + 4 H2 → CH3COOH + 2
H2O
This ability
of Clostridium to metabolize sugars directly, or to produce acetic
acid from less costly inputs, suggests that these bacteria could produce acetic
acid more efficiently than ethanol-oxidizers like Acetobacter.
However, Clostridium bacteria are less acid-tolerant than Acetobacter.
Even the most acid-tolerant Clostridium strains can produce vinegar
in concentrations of only a few per cent, compared
to Acetobacter strains that can produce vinegar in concentrations up
to 20%. At present, it remains more cost-effective to produce vinegar
using Acetobacter, rather than using Clostridium and
concentrating it. As a result, although acetogenic bacteria have been known
since 1940, their industrial use is confined to a few niche applications.[44]
Uses
Acetic acid is a chemical reagent for the production of
chemical compounds. The largest single use of acetic acid is in the production
of vinyl acetate monomer,
closely followed by acetic anhydride and ester production. The volume of acetic
acid used in vinegar is comparatively small.[7][29]
Vinyl acetate monomer
The primary use of acetic acid is the production of vinyl acetate monomer
(VAM). In 2008, this application was estimated to consume a third of the
world's production of acetic acid.[7] The reaction consists
of ethylene and acetic acid
with oxygen over a palladium catalyst, conducted in the gas
phase.[45]
2 H3C−COOH + 2 C2H4 + O2 → 2
H3C−CO−O−CH=CH2 + 2 H2O
Vinyl acetate can be polymerized to polyvinyl acetate or
other polymers,
which are components in paints and adhesives.[45]
The major esters of
acetic acid are commonly used as solvents for inks, paints and coatings. The esters include ethyl acetate, n-butyl acetate, isobutyl acetate,
and propyl acetate. They
are typically produced by catalyzed reaction
from acetic acid and the corresponding alcohol:
Most acetate esters, however, are produced from acetaldehyde using
the Tishchenko reaction. In addition, ether acetates are used
as solvents for nitrocellulose, acrylic lacquers, varnish
removers, and wood stains. First, glycol monoethers are produced from ethylene oxide or propylene oxide with
alcohol, which are then esterified with acetic acid. The three major products
are ethylene glycol monoethyl ether acetate (EEA), ethylene glycol monobutyl
ether acetate (EBA), and propylene glycol monomethyl ether acetate (PMA, more
commonly known as PGMEA in semiconductor manufacturing processes, where it is
used as a resist solvent). This application consumes about 15% to 20% of
worldwide acetic acid. Ether acetates, for example EEA, have been shown to be
harmful to human reproduction.[29]
The product of the condensation of two molecules of acetic acid
is acetic anhydride. The
worldwide production of acetic anhydride is a major application, and uses
approximately 25% to 30% of the global production of acetic acid. The main
process involves dehydration of acetic acid to give ketene at 700–750 °C.
Ketene is thereafter reacted with acetic acid to obtain the anhydride:[46]
CH3CO2H → CH2=C=O + H2O
CH3CO2H + CH2=C=O → (CH3CO)2O
Acetic anhydride is an acetylation agent.
As such, its major application is for cellulose acetate, a
synthetic textile also
used for photographic film. Acetic anhydride is also a reagent
for the production of heroin and
other compounds.
Glacial acetic acid is an excellent polar protic solvent, as
noted above. It
is frequently used as a solvent for recrystallization to purify organic compounds.
Acetic acid is used as a solvent in the production
of terephthalic acid (TPA), the raw material for polyethylene terephthalate (PET). In 2006, about 20% of
acetic acid was used for TPA production.[29]
Acetic acid is often used as a solvent for reactions
involving carbocations, such
as Friedel-Crafts alkylation. For example, one stage in the
commercial manufacture of synthetic camphor involves a Wagner-Meerwein rearrangement of camphene to isobornyl acetate; here acetic acid acts both as a
solvent and as a nucleophile to
trap the rearranged carbocation.[47]
Glacial acetic acid is used in analytical chemistry for the
estimation of weakly alkaline substances such as organic amides. Glacial acetic
acid is a much weaker base than water, so the
amide behaves as a strong base in this medium. It then can be titrated using a
solution in glacial acetic acid of a very strong acid, such as perchloric acid.[48]
Acetic acid is used as part of cervical cancer screening in many areas in the developing world.[51] Acetic acid is
applied to the cervix and
if an area of white appears after about a minute the test is positive.[51]
It is an effective antiseptic when used as a 1% solution, with
broad spectrum of activity against streptococci, staphylococci, pseudomonas,
enterococci and others. It may be an option for skin infections caused by
pseudomonas resistant to typical antibiotics.
While diluted acetic acid is used in iontophoresis, no
high quality evidence supports this treatment in rotator cuff disease.
As a treatment for otitis externa, it
is on the World Health Organization's List of Essential Medicines, the most important medications needed
in a basic health system.[58]
Acetic acid has 349 kcal per 100 g.[59] Vinegar is typically
no less than 4% acetic acid by mass.[60][61][62] Legal limits on
acetic acid content vary by jurisdiction. Vinegar is used directly as a condiment, and
in the pickling of
vegetables and other foods. Table vinegar tends to be more diluted (4% to 8%
acetic acid), while commercial food pickling employs solutions that are more
concentrated. The proportion of acetic acid used worldwide as vinegar is not as
large as commercial uses, but is by far the oldest and best-known application.
Reactions
What does acetic acid react
with?
Acetic acid undergoes
the typical chemical reactions of a carboxylic acid. ...
Reduction of acetic acid gives ethanol. The OH group is
the main site of reaction, as illustrated by the conversion
of acetic acid to acetyl chloride.
What are the chemical properties of
acetic acid?
Physical properties: Pure acetic acid
is a colorless liquid with a strong, corrosive pungent odor. Its density is
1.05 g/mL, and boiling point is 118 °C. It has a characteristic sour
taste, and is highly miscible in water.
Acetic acid Formula
Acetic acid Formula
Acetic
acid is an organic acid which is the main component of vinegar. It is also
called glacial acetic acid, ethanoic acid or methane carboxylic acid.
Formula
and structure: The chemical formula of acetic acid is CH3COOH. Its
molecular formula is C2H4O2 and its molar mass is 60.05 g/mol. Acetic acid
is a simple carboxylic acid consisting of the methyl group (CH3) linked to the
carboxylic acid group (COOH). It can also be considered as the acetyl group
(CH3CO) linked to a hydroxyl group (OH). Its chemical structure can be written
as below, in the common representations used for organic molecules.
Occurrence: Acetic
acid is produced naturally in a dilute form (as vinegar), during the microbial
fermentation of sugars. It is also an important metabolic intermediate found in
most plants and animals.
Preparation: Acetic
acid is produced in commercial quantities by both bacterial fermentation and
chemical synthesis. The bacterial fermentation of alcoholic food sources (such
as wine, fermented grain, malt, rice, etc.) produces acetic acid by oxidation
of ethyl alcohol (C2H5OH).
C2H5OH
+ O2 → CH3COOH + H2O
Chemically,
it is produced by the reaction of methanol (CH3OH) with carbon monoxide in the
presence of rhodium-iodine catalyst.
CH3OH
+ CO + Rh/I2 → CH3COOH
Physical
properties: Pure acetic acid is a colorless liquid with a strong, corrosive
pungent odor. Its density is 1.05 g/mL, and boiling point is 118 °C. It has a
characteristic sour taste, and is highly miscible in water.
Chemical
properties: Acetic acid is a weak acid. As a carboxylic acid, it forms
typical derivatives such as acid chlorides, anhydrides, esters and amides. It
can be reduced (removal of oxygen or addition of hydrogen) to give ethanol.
When heated above 440 °C, it decomposes to give carbon dioxide and methane:
CH3COOH
→ CH4 + CO2
Uses: Acetic
acid is largely used in the food industry as vinegar, and as an acidity
regulator. It is also an important industrial reagent used in the production of
various chemicals such as polyvinyl acetate, cellulose acetate, metal acetates,
etc. It is also used as a solvent for paints and resins.
Health
effects/safety hazards: As a dilute solution (vinegar), acetic acid is
safe for consumption. However, its concentrated form is very corrosive to eyes,
skin and mucous membranes upon inhalation or contact, leading to severe
irritation and/or burns.
What is
Ethanoic Acid?
Ethanoic acid (CH3COOH) belongs to
the group of carboxylic acids and is commonly called as acetic
acid. It is slightly heavier than water with a density of 1.05
g/cm3. After adding 5-8% of acetic acid in water it becomes vinegar and is
mostly used as preservatives in pickles. Acetic acid is the common name for
Ethanoic acid.
Properties
of Acetic acid
Chemical formula
|
CH3COOH
|
Molecular Weight/ Molar
Mass
|
60.05 g/mol
|
Density
|
1.05 g/cm3
|
Boiling Point
|
118oC
|
Melting Point
|
16oC
|
What is
glacial acetic acid?
Ethanoic acid is also referred
to as glacial acetic acid because its melting point is 16oC.
Hence it often freezes in
the winter season when the climate is cold.
Reactions
of ethanoic acids:
·
Esterification
reaction:
·
When
carboxylic acid and alcohol react, the product formed is known as an ester. Below is an example of the formation
of an ester from the reaction of ethanoic acid with absolute ethanol in the
presence of an acid as a catalyst.
·
CH3COOH
+ CH3CH2OH →
CH3COOCH2CH3
·
(Ethanoic
acid) (Ethanol) (Esters)
·
Esters
have a sweet fruity smell. They are mainly used for making perfumes and
synthetic flavoring agents. The reaction of esters with alkalis gives carboxylic acid salt
and alcohol. This reaction is used in the making of soaps and the process is
called as saponification reaction.
·
CH3COOC2H5 +
NaOH →
C2H5OH + CH3COONa
·
Reaction
with a base:
·
Ethanoic
acid reacts with a base to give the salt and water just like other mineral
acids.
·
Reaction
with carbonates and hydrogen carbonates:
·
Carbon
dioxide, salt, and water are produced when ethanoic acid reacts with carbonates
and hydrogen carbonates. Sodium acetate is usually produced as a salt when
ethanoic acid reacts with sodium bicarbonate as shown in the reaction below:
CH3COOH + NaHCO3 → CH3COONa + H2O + CO2
Uses of
Acetic Acid
1. Ethanoic acid is widely used in many
industries.
2. Commercially it is used in the
manufacturing of esters, vinegar, and many polymeric materials.
3. Vinegar has been shown to reduce high
concentrations of blood sugar.
4. Used as an agent to lyse red blood
cells before white blood cells are examined.
5. Used as a solvent in the production of
camphor, ascent and cooking ingredient.
6. Used as a stop bath for photographic
emulsion development.
7. Farmers sometimes spray acetic acid on
livestock silage to fight fungal and bacterial growth.
Two
typical organic reactions of acetic acid
Acetic acid undergoes the typical chemical reactions of
a carboxylic acid. Upon treatment with a standard base, it converts to
metal acetate and water. With strong bases (e.g.,
organolithium reagents), it can be doubly deprotonated to give LiCH2CO2Li.
Reduction of acetic acid gives ethanol. The OH group is the main site of
reaction, as illustrated by the conversion of acetic acid to acetyl chloride.
Other substitution derivatives include acetic anhydride;
this anhydride is
produced by loss of water from two molecules of acetic
acid. Esters of acetic acid can
likewise be formed via Fischer esterification, and amides can be formed. When heated above
440 °C (824 °F), acetic acid decomposes to produce carbon dioxide and methane, or to produce ketene and water:
CH3COOH → CH4 + CO2
CH3COOH → CH2CO + H2O
Reactions with inorganic
compounds
Acetic acid is mildly corrosive to metals including iron, magnesium,
and zinc, forming hydrogen gas and salts
called acetates:
Mg + 2CH3COOH → (CH3COO)2Mg + H2
Because aluminum forms a passivating acid-resistant film of aluminum oxide, aluminum
tanks are used to transport acetic acid. Metal acetates can also be prepared
from acetic acid and an appropriate base, as in the popular "baking soda + vinegar" reaction:
NaHCO3 + CH3COOH → CH3COONa + CO2 + H2O
A colour reaction for
salts of acetic acid is iron(III) chloride solution, which results in a
deeply red colour that disappears after acidification.[67] A more sensitive test
uses lanthanum nitrate with iodine and ammonia to give a blue solution.[68] Acetates when heated
with arsenic trioxide form cacodyl oxide,
which can be detected by its malodorous vapours.[69]
Other derivatives
Organic or inorganic salts are produced from acetic acid. Some
commercially significant derivatives:
·
Palladium (II) acetate, used as a catalyst for organic
coupling reactions such as the Heck reaction.
Halogenated acetic acids are produced from acetic acid. Some
commercially significant derivatives:
·
Chloroacetic acid (monochloroacetic
acid, MCA), dichloroacetic acid (considered a by-product), and trichloroacetic acid. MCA is used in the manufacture
of indigo dye.
Amounts of acetic acid used in these other applications together
account for another 5–10% of acetic acid use worldwide
Vinegar
Vinegar was known early in
civilization as the natural result of exposure of beer and wine to air, because acetic
acid-producing bacteria are present globally. The use of acetic acid in alchemy
extends into the 3rd century BC, when the Greek philosopher Theophrastus described
how vinegar acted on metals to produce pigments useful in art,
including white lead (lead carbonate)
and verdigris, a
green mixture of copper salts
including copper(II) acetate. Ancient Romans boiled soured wine to produce a
highly sweet syrup called sapa. Sapa that was produced in
lead pots was rich in lead acetate, a
sweet substance also called sugar of lead or sugar of Saturn, which contributed to lead poisoning among
the Roman aristocracy.[70]
In the 16th-century German alchemist Andreas Libavius described
the production of acetone from
the dry distillation of
lead acetate, ketonic decarboxylation. The presence of water in vinegar has
such a profound effect on acetic acid's properties that for centuries chemists
believed that glacial acetic acid and the acid found in vinegar were two
different substances. French chemist Pierre Adet proved
them identical.
Crystallized acetic acid.
In 1845 German chemist Hermann Kolbe synthesized acetic acid from inorganic compounds for the first time. This reaction
sequence consisted of chlorination of carbon disulfide to carbon tetrachloride, followed by pyrolysis to tetrachloroethylene and aqueous chlorination to trichloroacetic acid, and concluded with electrolytic reduction to acetic acid.[72]
By 1910, most glacial acetic acid was obtained from the pyroligneous liquor, a
product of the distillation of wood. The acetic acid was isolated by treatment
with milk of lime, and
the resulting calcium acetate was
then acidified with sulfuric acid to
recover acetic acid. At that time, Germany was producing 10,000 tons of glacial acetic acid, around
30% of which was used for the manufacture of indigo dye.[70][73]
Because both methanol and carbon monoxide are
commodity raw materials, methanol carbonylation long appeared to be attractive
precursors to acetic acid. Henri Dreyfus at British Celanese developed
a methanol carbonylation pilot plant as early as 1925.[74] However, a lack of
practical materials that could contain the corrosive reaction mixture at the
high pressures needed
(200 atm or more) discouraged commercialization
of these routes. The first commercial methanol carbonylation process, which
used a cobalt catalyst, was
developed by German chemical company BASF in 1963. In 1968, a rhodium-based catalyst
(cis−[Rh(CO)2I2]−) was discovered that could operate efficiently at lower
pressure with almost no by-products. US chemical company Monsanto Company built
the first plant using this catalyst in 1970, and rhodium-catalyzed methanol
carbonylation became the dominant method of acetic acid production (see Monsanto process). In
the late 1990s, the chemicals company BP Chemicals commercialized the Cativa catalyst
([Ir(CO)2I2]−), which is promoted by iridium[75] for greater
efficiency. This iridium-catalyzed Cativa process is greener and more efficient[33] and
has largely supplanted the Monsanto process, often in the same production
plants.
Interstellar acetic acid was discovered in
1996 by a team led by David Mehringer[76] using the
former Berkeley-Illinois-Maryland Association array at the Hat Creek Radio Observatory and the former Millimeter Array located
at the Owens Valley Radio Observatory. It was first detected in the Sagittarius B2 North
molecular cloud (also known as the Sgr B2 Large Molecule Heimatsource). Acetic acid has the
distinction of being the first molecule discovered in the interstellar medium
using solely radio interferometers; in all previous ISM molecular discoveries made in the millimetre
and centimeter wavelength regimes, single dish radio telescopes were at least
partly responsible for the detections.
Health effects and safety
Concentrated acetic acid is corrosive to
skin.[77][78] These burns or blisters
may not appear until hours after exposure.
Prolonged inhalation exposure (eight hours) to acetic acid vapours
at 10 ppm can produce some irritation of eyes, nose, and throat; at 100 ppm
marked lung irritation and possible damage to lungs, eyes, and skin may result.
Vapour concentrations of 1,000 ppm cause marked irritation of eyes, nose and
upper respiratory tract and cannot be tolerated. These predictions were based
on animal experiments and industrial exposure.
In 12 workers exposed for two or more years to acetic acid
airborne average concentration of 51 ppm (estimated), produced symptoms of
conjunctive irritation, upper respiratory tract irritation, and hyperkeratotic
dermatitis. Exposure to 50 ppm or more is intolerable to most persons and results
in intensive lacrimation and irritation of the eyes, nose, and throat, with
pharyngeal oedema and chronic bronchitis. Unacclimatised humans experience
extreme eye and nasal irritation at concentrations in excess of 25 ppm, and
conjunctivitis from concentrations below 10 ppm has been reported. In a study
of five workers exposed for seven to 12 years to concentrations of 80 to 200
ppm at peaks, the principal findings were blackening and hyperkeratosis of the
skin of the hands, conjunctivitis (but no corneal damage), bronchitis and
pharyngitis, and erosion of the exposed teeth (incisors and canines).[79]
The hazards of solutions of acetic acid depend on the
concentration. The following table lists the EU classification of acetic acid solutions:[80]
Concentration
by weight |
Molarity
|
Classification
|
|
10–25%
|
1.67–4.16 mol/L
|
Irritant (Xi)
|
|
25–90%
|
4.16–14.99 mol/L
|
Corrosive (C)
|
|
>90%
|
>14.99 mol/L
|
Corrosive (C) Flammable (F)
|
Concentrated acetic acid can be ignited only with difficulty at
standard temperature and pressure, but becomes a flammable risk in temperatures
greater than 39 °C (102 °F), and can form explosive mixtures with air
at higher temperatures (explosive limits:
5.4–16%).
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