Methods of Preparation of Carboxylic Acid with Reactions and Equations
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Before jumping to the preparation of carboxylic acid, it is important to know what carboxylic acid is and the nature of carboxylic acid. Carboxylic acid is an organic compound in which a carbon atom is doubly bonded to an oxygen atom and with a hydrogen atom with a single bond. The carboxyl group (COOH) gets its name because of the carbonyl group (C=O) and a hydroxyl group. Carboxylic acids are much more acidic in nature than other compounds containing a hydroxyl group. The general formula for carboxylic acid can be denoted as CnH2n+1COOHGenerally, carboxylic acids contain one carboxyl group, but Sometimes two or more groups are also attached. These acids are further called dicarboxylic acids, tricarboxylic acids, and so on. It is a very useful product and used in many different industries.
Structure of Carboxylic Acid
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Methods of Preparation of Carboxylic Acids
The preparation of carboxylic acids can be done using various methods. Some of them are listed below-
1. Preparation of Carboxylic Acid From the Grignard Reagent
The preparation of carboxylic acid can be done using Grignard reagents chemical reaction. It can be simply done by first reacting to Grignard reagent with dry ice (crushed) or solid CO2. In this step, salts of carboxylic acids are formed, after that, acidification of these salts with mineral acids gives corresponding carboxylic acids.
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2. Preparation of Carboxylic Acid From Alcohol
It is very easy to prepare carboxylic acids from alcohol. Alcohols easily undergo oxidation reactions using oxidizing agents such as potassium permanganate (KMnO4), chromium trioxide (CrO3–H2SO4–), and potassium dichromate (K2Cr2O7). These strong oxidizing agents make alcohol undergo an oxidation reaction twice and convert alcohol into carboxylic acids directly.
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3. Preparation of Carboxylic Acid From Aldehyde
The preparation of carboxylic acids can also be done using mild oxidizing agents also. Alcohols, when oxidized using a mild oxidizing agent such as Tollen’s reagents [Ag(NH3)2+OH−] and manganese dioxide (MnO2), undergo oxidation reaction only once and form corresponding aldehydes. When these aldehydes further undergo oxidation reaction using mild oxidizing agents only give carboxylic acid.
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4. Preparation of Carboxylic Acid From Ester
When ester undergoes acid hydrolysis, it leads to the formation of carboxylic acids. In the first step of basic hydrolysis of ester, it produces carboxylates. On further acidification, it gives corresponding carboxylic acids. Also, it is necessary to carry out hydrolysis of esters with mineral acids or alkali to produce a carboxylic acid.
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5. Preparation of Carboxylic Acid From Acyl Halides and Anhydrides
When acid chlorides undergo hydrolysis with water, it gives carboxylic acids. Another way to produce carboxylic acid from acid chlorides is by making it undergo hydrolysis with an aqueous base. This reaction produces carboxylate ions, on further acidification of which will give corresponding carboxylic acids. Anhydrides, on the other side, simply undergo hydrolysis with water and give corresponding carboxylic acid.
6. Preparation From Nitriles and Amides
On the hydrolysis of nitriles, it forms amides. These amides chemically react in the presence of catalysts such as H+ or OH- to give carboxylic acids. In a similar way, amides can directly react to produce carboxylic acid in the presence of H+ or OH- catalyst.
7.Preparation From Alkylbenzenes
Alkylbenzenes undergo oxidation reactions to form aromatic carboxylic acids. When the alkyl benzene compound goes under oxidation with acidic or alkaline potassium permanganate or chromic acid it gives carboxylic acid, which is aromatic in nature. In this process, the side products of oxidation reaction depend upon the alkyl groups, whether it is primary or secondary. The tertiary alkyl group remains unaffected in this reaction.
Conversion of Carboxylic Acid to Alcohol
Conversion of carboxylic acid to alcohol is a very simple process. It is done by reduction of carboxylic acid using reagents such as Lithium tetrahydridoaluminate or dry ether (LiAlH4). The -COOH group in carboxylic acid turns into -CH2OH which is a primary alcoholic group.
Uses of Carboxylic Acids
Carboxylic acids are very useful in our daily lives. They act as an important ingredient in the manufacturing of many common goods. Some of the common uses are stated below-
Manufacturing of soaps
It is very useful in the food industry. It is used in the production of soft drinks also.
Used in the production of vinegar.
It is very useful in the pharmaceutical industry also. It is an important ingredient used in the production of drugs such as aspirin etc.
It is also used in the rubber industry.
These acids are used in the production of dye, perfumes, etc.
FAQs on Preparation of Carboxylic Acids and Their Synthesis Methods
1. What are the main methods of preparation of carboxylic acids?
The main methods of preparation of carboxylic acids include oxidation of alcohols or aldehydes, hydrolysis of nitriles, hydrolysis of esters, and carbonation of Grignard reagents. Common laboratory and industrial methods are:
- Oxidation of primary alcohols: RCH2OH + [O] → RCOOH (using KMnO4 or K2Cr2O7).
- Oxidation of aldehydes: RCHO + [O] → RCOOH.
- Hydrolysis of nitriles: RCN + 2H2O + HCl → RCOOH + NH4Cl.
- Hydrolysis of esters: RCOOR′ + H2O → RCOOH + R′OH (acidic conditions).
- Grignard reaction with CO2: RMgX + CO2 → RCOOMgX → RCOOH (after acid hydrolysis).
2. How do you prepare carboxylic acids from primary alcohols?
Carboxylic acids are prepared from primary alcohols by strong oxidation using agents like KMnO4 or K2Cr2O7. The reaction proceeds through an aldehyde intermediate:
- Step 1: RCH2OH → RCHO
- Step 2: RCHO → RCOOH
Example: CH3CH2OH + 2[O] → CH3COOH + H2O. This method is widely used in laboratory synthesis of carboxylic acids.
3. How are carboxylic acids prepared from nitriles?
Carboxylic acids are prepared from nitriles by acidic or basic hydrolysis followed by acidification. In acidic medium, the general reaction is:
RCN + 2H2O + HCl → RCOOH + NH4Cl
- Nitrile is heated with dilute HCl or H2SO4.
- An amide intermediate forms first.
- Further hydrolysis gives the carboxylic acid.
This method increases the carbon chain length by one carbon atom.
4. What is the preparation of carboxylic acids from Grignard reagents?
Carboxylic acids are prepared from Grignard reagents by reaction with carbon dioxide followed by acid hydrolysis. The reaction steps are:
- RMgX + CO2 → RCOOMgX
- RCOOMgX + H3O+ → RCOOH + MgXOH
This method is important because it forms a new C–C bond and increases the carbon chain by one carbon atom.
5. How are carboxylic acids prepared by oxidation of aldehydes?
Carboxylic acids are prepared by oxidation of aldehydes using mild or strong oxidizing agents such as KMnO4, K2Cr2O7, or Tollens’ reagent. The general reaction is:
RCHO + [O] → RCOOH
- Aldehydes are easily oxidized compared to ketones.
- No change in carbon chain length occurs.
Example: CH3CHO + [O] → CH3COOH.
6. How do you prepare carboxylic acids from esters?
Carboxylic acids are prepared from esters by hydrolysis under acidic or basic conditions. In acidic hydrolysis:
RCOOR′ + H2O ⇌ RCOOH + R′OH (in presence of dilute H2SO4)
- Reaction is reversible in acidic medium.
- In basic medium (saponification), RCOO-Na+ is formed, which on acidification gives RCOOH.
This method is commonly used in both laboratory and industry.
7. What is the preparation of carboxylic acids from acid chlorides?
Carboxylic acids are prepared from acid chlorides by hydrolysis with water. The reaction is:
RCOCl + H2O → RCOOH + HCl
- This reaction is rapid and often vigorous.
- It is used when pure carboxylic acid is required.
Acid chlorides themselves are more reactive derivatives of carboxylic acids.
8. How are aromatic carboxylic acids prepared from alkyl benzenes?
Aromatic carboxylic acids are prepared from alkyl benzenes by strong oxidation of the side chain using hot alkaline KMnO4. The reaction converts any alkyl side chain with a benzylic hydrogen into –COOH:
C6H5CH3 + 3[O] → C6H5COOH + H2O
- Toluene forms benzoic acid.
- The side chain is completely oxidized regardless of its length.
9. Why are ketones not easily oxidized to carboxylic acids?
Ketones are not easily oxidized to carboxylic acids because oxidation requires cleavage of a strong C–C bond adjacent to the carbonyl group. Unlike aldehydes, ketones lack a hydrogen attached to the carbonyl carbon, making mild oxidation difficult. Under very strong oxidizing conditions (e.g., hot KMnO4), ketones may break into smaller carboxylic acids, but this involves carbon–carbon bond cleavage.
10. What is an example of laboratory preparation of acetic acid?
Acetic acid can be prepared in the laboratory by oxidation of ethanol using acidified potassium dichromate. The balanced reaction is:
3CH3CH2OH + 2K2Cr2O7 + 8H2SO4 → 3CH3COOH + 2Cr2(SO4)3 + 2K2SO4 + 11H2O
- Ethanol is first oxidized to acetaldehyde.
- Further oxidation gives acetic acid (CH3COOH).
