[Chemistry Class Notes] on Acetic Acid Pdf for Exam

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

1. Chemical Properties:

Acidity (Reaction with metals, Alkalis)

1. 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} ]

2. Acetic acid reacts with NaOH to form salt and water. 

[ CH_{3}COOH + NaOH rightarrow CH_{3}COONa + H_{2}O ]

3. 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

1. [ CH_{3}OH + HI rightarrow CH_{3}I + H_{2}O ]

2. [ CH_{3}I + CO rightarrow CH_{3}COI ]

3. [ 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.