Methods of Preparation — Revision Notes

⚡ 30-Second Revision

Primary Alcohol Oxidation: — $R-CH_2OH \xrightarrow{KMnO_4/H^+} R-COOH$
Aldehyde Oxidation: — $R-CHO \xrightarrow{\text{Tollens' or } KMnO_4/H^+} R-COOH$
Nitrile Hydrolysis: — $R-C equiv N \xrightarrow{H_3O^+ \text{ or } OH^-/H^+} R-COOH$ (chain extension)
Grignard + $CO_2$: — $RMgX + CO_2 \xrightarrow{1. \text{Dry Ether}, 2. H_3O^+} R-COOH$ (chain extension)
Acyl Halide Hydrolysis: — $R-COCl + H_2O \rightarrow R-COOH + HCl$
Anhydride Hydrolysis: — $(RCO)_2O + H_2O \rightarrow 2R-COOH$
Ester Hydrolysis (Acidic): — $R-COOR' + H_2O \xrightarrow{H^+, \text{Heat}} R-COOH + R'-OH$ (reversible)
Ester Hydrolysis (Basic/Saponification): — $R-COOR' + OH^- \xrightarrow{\text{Heat}} R-COO^- \xrightarrow{H^+} R-COOH + R'-OH$ (irreversible)
Alkylbenzene Oxidation: — $Ar-R \xrightarrow{KMnO_4/H^+, \text{Heat}} Ar-COOH$ (requires benzylic H)

2-Minute Revision

Carboxylic acids can be prepared through several key routes. The most direct involves oxidation of primary alcohols or aldehydes. Strong oxidizing agents like acidified potassium permanganate ($KMnO_4/H^+$) or dichromate ($K_2Cr_2O_7/H^+$) convert primary alcohols directly to carboxylic acids.

Aldehydes are more easily oxidized and can be converted using these strong agents or milder ones like Tollens' reagent. Another important method is the hydrolysis of nitriles ($R-C equiv N$), which can be achieved under acidic or basic conditions with heat, yielding the carboxylic acid.

This method, along with the Grignard reaction with carbon dioxide ($RMgX + CO_2$ followed by acidic workup), is excellent for synthesizing carboxylic acids with one more carbon atom than the starting material.

Remember, Grignard reactions require anhydrous conditions. Finally, hydrolysis of carboxylic acid derivatives such as acyl halides, acid anhydrides, and esters also yields carboxylic acids. Acyl halides are most reactive, followed by anhydrides, then esters.

Esters can be hydrolyzed under acidic (reversible) or basic (irreversible, saponification) conditions.

5-Minute Revision

Mastering the methods of preparing carboxylic acids is crucial for NEET. Let's consolidate the knowledge:

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Oxidation of Primary Alcohols and Aldehydes:

* Primary alcohols ($R-CH_2OH$) require strong oxidizing agents like $KMnO_4/H^+$ or $K_2Cr_2O_7/H^+$ (with heat) for complete oxidation to carboxylic acids ($R-COOH$). * Aldehydes ($R-CHO$) are more easily oxidized. They can be converted to carboxylic acids using strong oxidizers or milder, selective agents like Tollens' reagent ($[Ag(NH_3)_2]^+OH^-$) or Fehling's solution. * *Example:* $CH_3CH_2OH \xrightarrow{KMnO_4/H^+} CH_3COOH$

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From Nitriles (Cyanides):

* Nitriles ($R-C equiv N$) undergo hydrolysis to form carboxylic acids. This reaction typically requires heating with either aqueous acid ($H_3O^+$) or aqueous base ($OH^-$), followed by acidification if a base is used. * This method is valuable for increasing the carbon chain length by one carbon atom. * *Example:* $CH_3C equiv N \xrightarrow{H_3O^+, \text{Heat}} CH_3COOH$

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From Grignard Reagents:

* Grignard reagents ($RMgX$) react with carbon dioxide ($CO_2$) in an anhydrous solvent (like dry ether) to form an intermediate carboxylate salt. Subsequent acidic hydrolysis ($H_3O^+$) yields the carboxylic acid ($R-COOH$). * This method also extends the carbon chain by one carbon atom. * *Example:* $CH_3MgBr + CO_2 \xrightarrow{1. \text{Dry Ether}, 2. H_3O^+} CH_3COOH$

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Hydrolysis of Carboxylic Acid Derivatives:

* **Acyl Halides ($R-COX$):** Most reactive, readily hydrolyzed by water to $R-COOH$ and $HX$. * **Acid Anhydrides ($(RCO)_2O$):** Hydrolyzed by water to $2R-COOH$. * **Esters ($R-COOR'$):** Less reactive.

Require heating with dilute acid ($H_3O^+$) or base ($OH^-$). Acidic hydrolysis is reversible; basic hydrolysis (saponification) is irreversible and yields a carboxylate salt, which needs acidification to get the free acid.

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Side-chain Oxidation of Alkylbenzenes:

* Alkyl groups attached to a benzene ring can be oxidized to a carboxyl group ($Ar-COOH$) using strong oxidizing agents like $KMnO_4/H^+$ and heat. * Crucially, this reaction requires at least one benzylic hydrogen atom (hydrogen on the carbon directly attached to the ring). * *Example:* Toluene ($C_6H_5CH_3$) $\xrightarrow{KMnO_4/H^+, \text{Heat}}$ Benzoic acid ($C_6H_5COOH$).

Key Takeaways: Pay attention to the specific reagents, reaction conditions (especially anhydrous conditions for Grignard), and whether the carbon chain length changes. Understand the relative reactivity of derivatives and the reversibility of certain hydrolysis reactions.

Prelims Revision Notes

Methods of Preparation of Carboxylic Acids (NEET Quick Recall)

1. Oxidation of Primary Alcohols & Aldehydes:

* **Primary Alcohols ($R-CH_2OH$):** Use strong oxidizing agents. * Reagents: Acidified $KMnO_4$ ($KMnO_4/H^+$), Acidified $K_2Cr_2O_7$ ($K_2Cr_2O_7/H^+$), Jones Reagent ($CrO_3/H_2SO_4$). * Conditions: Heat often required. * Product: $R-COOH$. * **Aldehydes ($R-CHO$):** Easier to oxidize than alcohols. * Reagents: Strong oxidizers (as above) OR mild oxidizers like Tollens' reagent ($[Ag(NH_3)_2]^+OH^-$), Fehling's solution (alkaline $Cu^{2+}$ tartrate). * Product: $R-COOH$.

2. From Nitriles (Cyanides):

* Starting Material: Nitriles ($R-C equiv N$). Often prepared from alkyl halides ($R-X$) via $S_N2$ with $KCN/NaCN$. * Reaction: Hydrolysis. * Conditions: Heat with aqueous acid ($H_3O^+$) OR aqueous base ($OH^-$) followed by acidification. * Product: $R-COOH$ (chain extension by one carbon). * Intermediate: Amide ($R-CONH_2$).

3. From Grignard Reagents:

* Starting Material: Grignard reagent ($RMgX$). Prepared from alkyl/aryl halide ($R-X$) and $Mg$ in dry ether. * Reaction: Nucleophilic addition to $CO_2$. * Conditions: 1. $CO_2$ (dry ice or gas) in dry ether, 2. Acidic hydrolysis ($H_3O^+$). * Product: $R-COOH$ (chain extension by one carbon). * Crucial: Anhydrous conditions for Grignard reagent stability.

4. Hydrolysis of Carboxylic Acid Derivatives:

* Reactivity Order: Acyl Halides > Acid Anhydrides > Esters > Amides. * **Acyl Halides ($R-COX$):** React readily with water (even cold) to give $R-COOH + HX$. * **Acid Anhydrides ($(RCO)_2O$):** React with water (sometimes heat) to give $2R-COOH$.

* **Esters ($R-COOR'$):** * Acidic Hydrolysis: $R-COOR' + H_2O \xrightarrow{H^+, \text{Heat}} R-COOH + R'-OH$ (Reversible). * Basic Hydrolysis (Saponification): $R-COOR' + OH^- \xrightarrow{\text{Heat}} R-COO^- + R'-OH$.

Requires subsequent acidification ($H^+$) to get $R-COOH$. (Irreversible). * **Amides ($R-CONH_2$):** Require vigorous acidic or basic hydrolysis (stronger conditions than esters) to yield $R-COOH$.

5. Side-chain Oxidation of Alkylbenzenes:

* Starting Material: Alkylbenzene ($Ar-R$). * Reagents: Strong oxidizing agents like $KMnO_4/H^+$ or $K_2Cr_2O_7/H^+$. * Conditions: Heat. * Product: $Ar-COOH$. * Key Condition: Requires at least one benzylic hydrogen (H on carbon directly attached to benzene ring). Example: Toluene $\rightarrow$ Benzoic acid; tert-Butylbenzene $\rightarrow$ No reaction.

Vyyuha Quick Recall

To remember the main methods of preparation, think: 'GOAN HE'

G — Grignard reaction with $CO_2$
O — Oxidation of alcohols/aldehydes
A — Alkylbenzenes (side-chain oxidation)
N — Nitrile hydrolysis
H — Hydrolysis of derivatives (Acyl halides, Anhydrides, Esters)