An Introduction to Acetic Acid
Acetic acid is one of the commonly used domestic items which is generally used in all Indian households, it is more commonly known as Vinegar when used in a 5%-8% concentration. Acetic acid with the chemical formula of CH3COOH is also known as ethanoic acid. It is a colourless liquid that has a characteristic pungent smell. This organic compound is soluble in water and is more acidic than H₂O but significantly less acidic than mineral acids.
The structure of CH3COOH in the solid-state exists in a chain of molecules, and they are connected through intramolecular hydrogen bonds with each other. The undiluted version of acetic acid is also known as glacial acetic acid because it has a melting point of 16⁰ C and hence gets frozen in winter when the weather is cold.
The Structure, Properties, Preparation and Applications of Acetic Acid
Acetic acid is generally referred to as the second simplest form of a carboxylic acid-containing methyl group with a CH3 chemical formula and is connected to the carboxylic acid group (COOH). In a way, we can say that it is the acetyl group (CH3CO) connected to the hydroxyl group (OH).
The structure of Acetic acid reveals that it contains sp² hybridization in the molecule. It has been observed that in the solid state, there is a chain of molecules of acetic acid that are connected to each other via hydrogen bonds. The Acetic acid molecule has two carbon atoms, while one carbon atom is having a double bond with Oxygen, the other carbon atom has three bonded hydrogen molecules and both the carbon atoms are bonded to each other by a single bond.
Ethanoic acid behaves as a dimer while in liquid and vapour state because of intermolecular hydrogen bonding. Now, to draw the Lewis structure of acetic acid, firstly you have to figure out the number of valence electrons. As for this particular organic compound, carbon, oxygen, and hydrogen have 4, 6, and 1 valence electrons, respectively. Therefore the total number of valence electrons will be= 2(4) +2(6) +4(1) = 24. So, if you look at the Lewis structure, you will see that there is one double bond and six single bonds in the molecule. You will notice that each oxygen molecule contains two lone pairs of electrons.
Properties of CH3COOH
Chemical Properties:
Acidity (Reaction with metals, Alkalis)
Ethanoic acid is known to react with active metals like Na to liberate hydrogen. This reaction is similar to that of ethanol.
\[ CH_{3}COOH + Na \rightarrow CH_{3}COONa + H_{2} \]
Acetic acid reacts with NaOH to form salt and water.
\[ CH_{3}COOH + NaOH \rightarrow CH_{3}COONa + H_{2}O \]
CH3COOH also reacts with sodium carbonate and sodium hydrogen carbonate, which are weaker bases and release CO2.
\[ Na_{2}CO_{3} + 2 CH_{3}COOH \rightarrow 2 CH_{3}COONa + CO_{2} + H_{2}O \]
\[ CH_{3}COOH + NaHCO_{3} \rightarrow CH_{3}COONa + H_{2}O + CO_{2} \]
Acid Chlorides
It reacts with thionyl chloride and produces acetyl chloride.
\[CH_{3}COOH + SOCl_{2} \] → \[ CH_{3}COCl + HCl + SO_{2} \]
Acid Anhydride
Acetic acid reacts with an acid chloride in the presence of a base and releases acetic anhydride.
\[ CH_{3}CO_{2}H \rightarrow CH_{2}=C=O + H_{2}O \]
\[CH_{3}CO_{2}H + CH_{2}=C=O \rightarrow (CH_{3}CO)_{2}O\]
Ester
When alcohol reacts with any carboxylic acid, it produces esters. When acetic acid reacts with ethyl alcohol, it leads to the formation of ethyl ethanoate.
\[ CH_{3}COOH + C_{2}H_{5}OH \rightarrow CH_{3}COOC_{2}H_{5} + H_{2}O \]
CH3COOH as Solvent
Acetic acid is a polar protic solvent. It is an acid solvent as it can donate an H+ ion to the substance to be dissolved. It is often used in reactions like Friedel-Crafts Alkylation. Using it, terephthalic acid can be produced.
Acetic Acid Preparation
Ethanoic acid is prepared industrially primarily via carbonylation. This process takes place in the following steps
\[ CH_{3}OH + HI \rightarrow CH_{3}I + H_{2}O \]
\[ CH_{3}I + CO \rightarrow CH_{3}COI \]
\[ CH_{3}COI + H_{2}O \rightarrow CH_{3}COOH + HI \]
Other Methods
Acetaldehyde Oxidation
\[ 2 CH_{2}CHO + O_{2} \rightarrow 2 CH_{3}CO_{2}H \]
Under some conditions and using catalysts, acetic acid can be formed. In this reaction, several significant by-products are produced, for example, formic acid, ethyl acetate, etc.
Ethylene Oxidation
In the presence of palladium catalyst, we can get acetic acid from ethylene.
\[ C_{2}H_{4} + O_{2} \rightarrow CH_{3}CO_{2}H \]
Oxidative Fermentation
In the presence of sufficient oxygen, acetic acid bacteria can produce ethanoic acid from foodstuffs.
\[ C_{2}H_{5}OH + O_{2} \rightarrow CH_{3}COOH + H_{2}O \]
Anaerobic Fermentation
\[ C_{6}H_{12}O_{6} \rightarrow 3 CH_{3}COOH \]
Some anaerobic bacteria like Acetobacterium can transform sugar directly into acetic acid.
Apart from these methods, researchers are also trying to produce acetic acid through the oxidation of waste gases from industrial processes.
Baking Soda and CH3COOH Reaction
The reaction between sodium bicarbonate and acetic acid produces carbon dioxide gases. It is usually used in chemical volcanoes.
The overall chemical equation is as follows:
\[ NaHCO_{3}(s) + CH_{3}COOH (l) \rightarrow CO_{2}(g) + H_{2}O(l) + Na^{+}(aq) + CH_{3}COO^{-}(aq) \]
Where, s = solid, g = gas, l = liquid, aq = aqueous or water solution.
You can write this reaction in another way as well,
\[ NaHCO_{3} + HC_{2}H_{3}O_{2} \rightarrow NaC_{2}H_{3}O_{2} + H_{2}O + CO_{2} \]
However, this reaction is not responsible for the dissociation of the NaC2H3O2 in water.
The chemical reaction typically takes place in two steps. In which the first one is a double displacement reaction where the acetic acid of the vinegar reacts with sodium bicarbonate to produce carbonic acid and sodium acetate
\[ NaHCO_{3} + HC_{2}H_{3}O_{2} \rightarrow NaC_{2}H_{3}O_{2} + H_{2}CO_{2} \]
Since Carbonic acid is unstable, it goes through a decomposition reaction to release carbon dioxide gas
\[ H_{2}CO_{3} \rightarrow H_{2}O + CO_{2} \]
This is all about acetic acid, its physical and chemical properties. Focus on the reactions given here to prepare acetic acid and those displaying its chemical properties to understand its features well.
FAQs on Acetic Acid
1. What is acetic acid and what is its chemical formula?
Acetic acid, also known as ethanoic acid, is a simple carboxylic acid. It is a colourless liquid with a distinct pungent, sour smell. Its chemical formula is CH₃COOH, which indicates it consists of a methyl group (CH₃) attached to a carboxyl functional group (COOH).
2. What is the relationship between vinegar and acetic acid?
Vinegar is essentially a dilute aqueous solution of acetic acid. Typically, household vinegar contains about 5-8% acetic acid by volume, which gives it its characteristic sour taste and pungent aroma. The remainder of the solution is primarily water.
3. What are the key physical and chemical properties of acetic acid?
Acetic acid has several distinct properties as per the CBSE/NCERT curriculum for the 2025-26 session. Key properties include:
- Physical Properties: It is a colourless liquid, soluble in water, and has a pungent odour. Its melting point is 16.6°C.
- Chemical Properties: It behaves as a weak acid, reacting with active metals to liberate hydrogen gas, and with bases like NaOH to form salt and water. It also undergoes esterification when reacted with alcohols in the presence of an acid catalyst.
4. How is acetic acid prepared on an industrial scale?
The most common industrial method for preparing acetic acid is the carbonylation of methanol. In this process, methanol (CH₃OH) reacts with carbon monoxide (CO) in the presence of a catalyst, typically a rhodium or iridium complex, to produce acetic acid (CH₃COOH). Other methods include the oxidation of acetaldehyde and ethylene.
5. What are the primary uses of acetic acid in industry and daily life?
Acetic acid has a wide range of applications:
- As Vinegar: Used as a food preservative and flavouring agent in cooking.
- Industrial Solvent: It is used as a polar protic solvent in the production of terephthalic acid (a raw material for PET plastic).
- Chemical Synthesis: It serves as a precursor for producing vinyl acetate monomer (VAM), acetic anhydride, and various esters used in perfumes and paints.
6. Why is acetic acid considered a weak acid?
Acetic acid is classified as a weak acid because it only partially ionises or dissociates when dissolved in water. Unlike strong acids like hydrochloric acid (HCl) which dissociate completely to release all their hydrogen ions (H⁺), only a small fraction of acetic acid molecules release their proton. This equilibrium results in a lower concentration of H⁺ ions in the solution, making it less acidic.
7. What is 'glacial' acetic acid, and why is it given that name?
Glacial acetic acid is the name for pure, undiluted, or water-free acetic acid. It earns the name 'glacial' because it has a melting point of 16.6°C (about 62°F). In cooler laboratory environments, it can freeze and form solid crystals that resemble ice or a glacier, hence the descriptive name.
8. Why does acetic acid form a dimer structure in certain states?
In its liquid and vapour states, acetic acid molecules pair up to form a dimer. This occurs due to intermolecular hydrogen bonding. The hydroxyl group (-OH) of one molecule forms a hydrogen bond with the carbonyl oxygen (C=O) of a neighbouring molecule, and vice versa. This creates a stable, eight-membered ring structure that significantly affects its physical properties like boiling point.
9. How does the chemical structure of acetic acid explain its properties?
The structure of acetic acid (CH₃COOH) is key to its behaviour. The carboxyl group (-COOH) is the source of its acidity, as the hydrogen atom on the hydroxyl group can be donated as a proton (H⁺). The presence of the highly electronegative oxygen atoms makes the O-H bond polar and weak. This structure also allows for strong hydrogen bonding, explaining its solubility in water and its existence as a dimer.
10. What happens at a molecular level when acetic acid reacts with sodium bicarbonate?
When acetic acid (CH₃COOH) reacts with sodium bicarbonate (NaHCO₃), a two-step acid-base reaction occurs. First, acetic acid donates a proton to the bicarbonate ion, forming sodium acetate (CH₃COONa) and unstable carbonic acid (H₂CO₃). Because carbonic acid is unstable, it immediately decomposes into water (H₂O) and carbon dioxide (CO₂) gas, which is observed as vigorous fizzing or bubbling.
