We've all tasted pickles at some point in our lives! That sharp tanginess has always piqued my curiosity. I noticed that it is a transparent sour liquid called vinegar that attracts the tastes of all.
But have you ever wondered what vinegar's chemical composition is, and why it is used in pickles?
Ethanoic acid is what it is.
It is known to kill and thus terminate the growth of microorganisms in pickles, making it the magical ingredient that makes pickles last longer! In the eighth century, a Persian alchemist named Jabir Ibn Hayyan (Geber) produced concentrated acetic acid from vinegar through distillation.
TABLE OF CONTENTS
Ethanoic acid, colloquially known as acetic acid, is an organic compound and the second simplest form of carboxylic acid following formic acid (HCOOH). The IUPAC name of acetic acid is ethanoic acid and its molecular formula can be written as C2H4O2 or CH3COOH.
Ethanoic acid is sour to taste, colourless and has a pungent odour. The molecular weight of ethanoic acid is 60.05 g mol-1. Vinegar is a solution of acetic acid in water and it is formed after adding 5-8% of acetic acid by volume, in water. Vinegar is mostly used as a preservative in pickles. This acid has a variety of commercial applications, and its global demand is approximately 6.5 million metric tons per year.
Commercially, acetic acid is produced by a chemical method involving the carbonylation of methanol. However, the entire process of production of ethanoic acid can be divided into two broad categories: Chemical Method and Fermentation.
In this process, methanol and carbon monoxide are made to react with each other in the presence of a Rhodium-based catalyst at a temperature of 150-200OC and a pressure of 30-50 bar. The major product formed is acetic acid, along with by-products like formic acid or formaldehyde.
CH3OH + HI → CH3I + H2O
CH3I + CO → CH3COI
CH3COI + H2O → CH3COOH + HI
Acetaldehyde is oxidised at 150OC and 55 bar pressure in the presence of a chromium-based catalyst to form acetic acid.
2 CH3CHO + O2 → 2CH3COOH
Additionally, a direct conversion process of ethylene to acetic acid can also be performed at high temperatures and pressure in the presence of lead-platinum or lead catalyst.
C2H4 + O2 CH3COOH
Hydrocarbons obtained from petroleum industries, for example, light naptha can also be used to produce acetic acid in the presence of cobalt or chromium acetate catalysts. They are readily oxidised by oxygen or even air to give peroxides, which decompose to give acetic acid.
2C4H10 + 5O2 → 4CH3COOH + 2H2O
Fermentation is mainly used for the production of vinegar from alcoholic foodstuff. In this, the agricultural feed, apple cider, wine, fermented grain, potato mash, rice, and malt is first treated with yeast, followed by acetic acid bacteria (Acetobacter) to form the end-product of acetic acid. There are three fermentation processes known and used, namely Orleans, Tinkle and Submerged.
C2H5OH + O2→ CH3COOH + H2O
Ethanoic acid is also generated from an anaerobic fermentation by certain bacteria that have the ability to directly convert sugar into acetic acid.
Ethanoic acid is a weak acid. However, it reacts with a variety of compounds to form useful end-products.
CH3COOH ⇌ CH3COO- + H+
This sodium ethanoate can be used in the production of methane gas by making it react with soda lime.
Ethanoic acid in the liquid state is a hydrophile (readily dissolves in water) loves water and hence easily dissolves in water and is a polar protic solvent. A mixture of acetic acid and water is similar to a mixture of ethanol and water. In this way, acetic acid and water mixtures are comparable to ethanol and water mixtures. Ethanoic acid has a dielectric constant of 6.2 and hence can dissolve polar inorganic salts as well as non-polar compounds, such as oils, and non-polar solutes. Acetic acid also forms miscible mixtures with hexane, chloroform, and several oils. However, it does not form miscible mixtures with long-chain alkanes (such as octane). Hexane, chloroform, and a number of oils can all be mixed with acetic acid. But it is not miscible with long-chain alkanes (such as octane).
The desirable solvent properties of acetic acid, along with its ability to form miscible mixtures with both polar and non-polar compounds, make it a very important industrial solvent. Acetic acid is a very significant industrial solvent due to its advantageous solvent characteristics and capacity to mix miscibly with both polar and non-polar chemicals. It is widely used in the industrial preparation of dimethyl terephthalate. It is extensively utilised in the production of dimethyl terephthalate (DMT).
Q1. A liquid was mixed with ethanol and a drop of concentrated H2SO4 was added. A compound with a fruity smell was formed. The liquid was:
Answer: Esters have a fruity smell. We know carboxylic acid mixed with alcohol in presence of
conc. H2SO4 gives ester. Hence, the liquid is acetic acid. So, option C) is the correct answer.
Q2. Consider the following reaction.
It is an example of:
A. Nucleophilic substitution reaction
B. Nucleophilic addition only
C. Nucleophilic addition followed by elimination reaction
D. 1,2-Elimination only
Answer: The given reaction is an esterification reaction. In an esterification reaction, first, a nucleophilic addition reaction takes place followed by the elimination of a water molecule. S, option C) is teh correct answer.
Q3. When ethanoic acid is treated with aqueous NaHCO3 , CO2 is liberated. The C of CO2 comes from
D. None of the above
CH3COOH + NaHCO3 ⟶ CH3COO-Na+ + H2CO3
When ethanoic acid reacts with aqueous sodium bicarbonate, sodium acetate and carbonic acid are formed. Carbonic acid further gives water and CO2. Therefore, the C of CO2 comes from bicarbonate. So, option D) is the correct answer.
Q4. Formic acid can be distinguished from acetic acid because formic acid:
A. Releases H2 with sodium
B. Reduces ammoniacal AgNO3
C. Gives ester with alcohol
D. Turns red litmus to blue
Option A): Formic acid cannot be distinguished from acetic acid as all carboxylic acids react with sodium to release H2.
Option B): Formic acid cannot be distinguished from acetic acid as all carboxylic acids react with alcohols to give esters
Option D): Formic acid cannot be distinguished from acetic acid cannot be distinguished as all carboxylic acids turn blue litmus red.
Option C): When ammoniacal AgNO3 is heated with an aldehyde, it forms a silver mirror. HCOOH (formic acid) contains both an aldehyde and carboxylic group, so it gives Tollens test and the silver complex [Ag(NH3)2]+ gets reduced to give a silver mirror. Whereas acetic acid does not give Tollens test (reduces ammoniacal AgNO3). So, option C) is the correct answer.
In the following set of reactions, product D is:
So, option D) is the correct answer.
Q5. How can ethanol and ethanoic acid be differentiated?
Answer: The difference between ethanol and ethanoic acid depending on their physical and chemical properties is given in the table below.
At room temperature, ethanol is always a liquid, and it has a melting point of 156 K and a boiling point of 351 K. Ethanol has a melting point of 156 K and a boiling point of 351 K, and it is always a liquid at ambient temperature.
Ethanoic acid is also referred to as glacial acetic acid because its melting point is 16oC. It often freezes in the winter season when the climate is cold.
Because ethanoic acid has a melting point of 16oC, it is also known as glacial acetic acid. In the winter, when the weather is cold, it frequently freezes.
Ethanol has a pleasant odour
Ethanoic acid has a smells like vinegar-like smell.
Ethanol does not react with carbonates or hydrogen carbonate bicarbonates and carbonates.
Ethanoic acid reacts with sodium hydrogen carbonate bicarbonate as shown in the reaction below.
CH3COOH + NaHCO3 → CH3COONa + H2O + CO2
What type of acid is ethanoic acid?
Answer: CH3COOH is a weak acid. It undergoes complete dissociation only when it is reacted with a strong base. Hydrochloric acid is a much stronger acid than acetic acid. Acetic acid is a weak acid because it only partially dissociates into its constituent ions when dissolved in water.
What is the harmful effect of acetic acid?
Answer: Exposure to more concentrated solutions of acetic acid When we are exposed to high concentrations of acetic acid that is around (>25%), it can cause corrosive damage. Breathing vapours with high levels of acetic acid can cause irritation of the eyes, nose and throat, cough, chest tightness, headache, fever and confusion. A cough, chest tightness, headache, fever, and confusion can all result from breathing in high-acetic-acid vapours, as might irritation of the eyes, nose, and throat.
What is glacial acetic acid?
Answer: An undiluted solution of acetic acid is commonly referred to as glacial acetic acid. It forms crystals which appear like ice at temperatures below 16.6oC. It has a wide range of applications as a polar protic solvent. In the field of analytical chemistry, glacial acetic acid is widely used in order to estimate substances that are weakly alkaline.
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