Q.Write chemical reactions to affect the following transformations:

Concept: Oxidation Reactions

The core idea is the controlled oxidation of alcohols and aldehydes to carboxylic acids, and the oxidative cleavage of alkenes and alkyl side-chains on aromatic rings.

1. Butan-1-ol → Butanoic acid A primary alcohol is oxidised to a carboxylic acid using a strong oxidising agent like acidified potassium dichromate ($\ce{K2Cr2O7/H+}$) or potassium permanganate.

   $\ce{CH3CH2CH2CH2OH ->[K2Cr2O7/H+, \Delta] CH3CH2CH2COOH}$

2. Benzyl alcohol → Phenylethanoic acid Benzyl alcohol is first converted to benzyl chloride (using $\ce{PCl5}$ or $\ce{SOCl2}$), then treated with $\ce{KCN}$ to form benzyl cyanide, followed by acidic hydrolysis.

   $\ce{C6H5CH2OH ->[PCl5] C6H5CH2Cl ->[KCN] C6H5CH2CN ->[H3O+, \Delta] C6H5CH2COOH}$

3. 3-Nitrobromobenzene → 3-Nitrobenzoic acid The textbook's route: form the Grignard reagent in dry ether, bubble in $\ce{CO2}$ (carboxylation), then acidify.

   $\ce{Br-C6H4-NO2 ->[Mg, \text{dry ether}] BrMg-C6H4-NO2 ->[1.\ CO2][2.\ H3O+] HOOC-C6H4-NO2}$

4. 4-Methylacetophenone → Benzene-1,4-dicarboxylic acid Both the methyl group and the acetyl group are oxidised to carboxylic acids using a strong oxidising agent like alkaline $\ce{KMnO4}$ under heat. $\ce{CH3-C6H4-COCH3 ->[KMnO4/OH-, \Delta] HOOC-C6H4-COOH}$ …

Oxidation reactions are the backbone of converting alcohols and aldehydes into carboxylic acids. The principle is simple: a primary alcohol ($\ce{-CH2OH}$) can be oxidized first to an aldehyde ($\ce{-CHO}$) and then to a carboxylic acid ($\ce{-COOH}$). An aldehyde itself is easily oxidized to the acid. For aromatic compounds, any alkyl group attached to the benzene ring can be fully oxidized to a carboxyl group ($\ce{-COOH}$) under strong conditions, regardless of the chain length. This is because the benzylic carbon (the one directly attached to the ring) is the most reactive, and the entire side-chain is chopped down to a single carboxyl carbon.

Let’s go through each transformation step by step.

1. Butan-1-ol to butanoic acid Butan-1-ol is a primary alcohol. To get the carboxylic acid, we need to push the oxidation all the way. A common reagent is acidified potassium dichromate ($\ce{K2Cr2O7/H2SO4}$). The reaction proceeds via butanal as an intermediate, but under these conditions, the aldehyde is not isolated — it gets further oxidized to butanoic acid.

2. Benzyl alcohol to phenylethanoic acid Benzyl alcohol is $\ce{C6H5CH2OH}$. Oxidation gives benzaldehyde ($\ce{C6H5CHO}$) first, then benzoic acid ($\ce{C6H5COOH}$). But the target is phenylethanoic acid ($\ce{C6H5CH2COOH}$), which has a two-carbon side-chain. Direct oxidation of benzyl alcohol will give benzoic acid, not phenylethanoic acid. So we need a different strategy. The classic method is to first convert benzyl alcohol to benzyl chloride (using $\ce{SOCl2}$ or $\ce{PCl5}$), then react with $\ce{KCN}$ to form benzyl cyanide ($\ce{C6H5CH2CN}$), followed by acidic hydrolysis to get phenylethanoic acid.

3. 3-Nitrobromobenzene to 3-nitrobenzoic acid Here we have a bromine atom on the benzene ring, and we want to replace it with a carboxyl group. The textbook converts the aryl halide to a Grignard reagent and then carboxylates it: treat 3-nitrobromobenzene with magnesium in dry ether to form the arylmagnesium bromide, pass in carbon dioxide (the Grignard adds to $\ce{CO2}$ to give the magnesium carboxylate), and finally acidify to release the free acid.

4. 4-Methylacetophenone to benzene-1,4-dicarboxylic acid This molecule has two oxidizable groups: a methyl group at the 4-position and an acetyl group ($\ce{-COCH3}$) at the 1-position. Under strong oxidation (e.g., alkaline $\ce{KMnO4}$ or $\ce{K2Cr2O7/H+}$ with heat), both side-chains are oxidized to carboxyl groups. The methyl becomes $\ce{-COOH}$, and the acetyl group (which is already partially oxidized) also becomes $\ce{-COOH}$. The product is terephthalic acid (benzene-1,4-dicarboxylic acid). $\ce{CH3-C6H4-COCH3 (para) ->[KMnO4/OH-, \Delta] HOOC-C6H4-COOH}$ …

Method: Controlled Oxidation Using Strong Oxidising Agents

The core concept is that primary alcohols and aldehydes can be oxidised to carboxylic acids, while alkyl side chains on aromatic rings can be fully oxidised to carboxyl groups. The most common reagent for these transformations is acidified potassium dichromate ($\ce{K2Cr2O7/H2SO4}$) or alkaline potassium permanganate ($\ce{KMnO4}$). Below are the stepwise reactions.

(i) Butan-1-ol → Butanoic acid

Reagent: Acidified $\ce{K2Cr2O7}$ (or $\ce{KMnO4/H+}$)

Steps:

1. Primary alcohol ($\ce{-CH2OH}$) is first oxidised to an aldehyde (butanal).
2. The aldehyde is further oxidised to the carboxylic acid.

Reaction:

Detailed:

(ii) Benzyl alcohol → Phenylethanoic acid

Reagent: $\ce{PCl5}$ (or $\ce{SOCl2}$), then $\ce{KCN}$, then $\ce{H3O+}$ — chain extension via cyanide

Steps:

1. Convert benzyl alcohol ($\ce{C6H5CH2OH}$) to benzyl chloride with $\ce{PCl5}$ or $\ce{SOCl2}$.
2. Substitute with $\ce{KCN}$ to get benzyl cyanide ($\ce{C6H5CH2CN}$) — this adds the extra carbon.
3. Hydrolyse the nitrile ($\ce{H3O+}$, heat) to phenylethanoic acid.
   • Direct oxidation cannot work: it gives benzoic acid, one carbon short.

Route: benzyl alcohol → benzyl chloride ($\ce{PCl5}$/$\ce{SOCl2}$) → benzyl cyanide ($\ce{KCN}$) → phenylethanoic acid (acidic hydrolysis). Direct oxidation cannot work — it would give benzoic acid, one carbon short.

(iii) 3-Nitrobromobenzene → 3-Nitrobenzoic acid

Reagent: $\ce{Mg}$/dry ether, then $\ce{CO2}$, then $\ce{H3O+}$ (Grignard carboxylation — the textbook's route)

Steps:

1. The bromine atom ($\ce{-Br}$) is not oxidised; the carboxyl group is introduced through a Grignard reagent.
2. Treat 3-nitrobromobenzene with magnesium in dry ether to form the arylmagnesium bromide.
3. Pass in $\ce{CO2}$ — the Grignard adds to it to give the magnesium carboxylate; acidification then releases 3-nitrobenzoic acid.

Reaction (Grignard carboxylation):

(iv) 4-Methylacetophenone → Benzene-1,4-dicarboxylic acid

Reagent: Hot alkaline $\ce{KMnO4}$ (strong oxidation)

Steps:

1. The methyl group ($\ce{-CH3}$) on the benzene ring is oxidised to $\ce{-COOH}$.
2. The acetyl group ($\ce{-COCH3}$) is also oxidised to $\ce{-COOH}$.
3. Both side chains become carboxyl groups at positions 1 and 4.

Reaction:

(Product: terephthalic acid)

(v) Cyclohexene → Hexane-1,6-dioic acid …

Here are the common mistakes students make on these specific oxidation transformations, along with the conceptual corrections and exam-smart strategies to avoid them.

(i) Butan-1-ol → Butanoic acid

Common Mistake: Using $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7$ in neutral/alkaline medium and stopping at the aldehyde stage, or using a weak oxidising agent like PCC.

Why it’s wrong:

• Butan-1-ol is a primary alcohol.
• To get the carboxylic acid, you need a strong oxidising agent in acidic medium.
• In neutral/alkaline $\text{KMnO}_4$, the reaction often stops at the aldehyde or gives a mixture.

Correct Approach:

Use acidified $\text{K}_2\text{Cr}_2\text{O}_7$ or acidified $\text{KMnO}_4$ (warm).

Avoid by:

• Always check the medium (acidic vs. neutral/alkaline).
• Remember: Primary alcohol → Aldehyde → Carboxylic acid requires strong oxidant + acid.

(ii) Benzyl alcohol → Phenylethanoic acid

Common Mistake: Thinking direct oxidation of benzyl alcohol gives phenylethanoic acid.

Why it’s wrong:

• Benzyl alcohol is $\text{C}_6\text{H}_5\text{CH}_2\text{OH}$.
• Direct oxidation (e.g., with $\text{KMnO}_4$) gives benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$), not phenylethanoic acid ($\text{C}_6\text{H}_5\text{CH}_2\text{COOH}$).
• The carbon chain length changes — you need to increase the carbon chain first.

Correct Approach:

This is a two-step process:

1. Convert benzyl alcohol to benzyl chloride (using $\text{PCl}_5$ or $\text{SOCl}_2$).
2. Do a nucleophilic substitution with $\text{KCN}$ to get benzyl cyanide, then hydrolyse (acidic) to get phenylethanoic acid.

Avoid by:

• Recognising that direct oxidation of a side chain always shortens it to $\text{COOH}$ at the benzylic carbon.
• To retain or lengthen the chain, use cyanide + hydrolysis.

(iii) 3-Nitrobromobenzene → 3-Nitrobenzoic acid

Common Mistake: Trying to oxidise the bromine atom directly.

Why it’s wrong:

• Bromine is a halogen, not an alkyl group. It cannot be oxidised to $\text{COOH}$.
• You need to replace the bromine with a group that can be oxidised.

Correct Approach:

1. Convert bromobenzene to a Grignard reagent (using Mg in dry ether).
2. React with $\text{CO}_2$ (dry ice) to form the carboxylate salt.
3. Acidify to get 3-nitrobenzoic acid.

Avoid by:

• Remember: Halogens are not oxidisable to acids. Use Grignard or cyanide substitution.
• Check the functional group: if it’s a halogen, think nucleophilic substitution first.

(iv) 4-Methylacetophenone → Benzene-1,4-dicarboxylic acid

Common Mistake: Oxidising only the methyl group or only the acetyl group, or using a weak oxidant.

Why it’s wrong:

• You need to oxidise both the methyl ($\text{CH}_3$) and the acetyl ($\text{COCH}_3$) groups to $\text{COOH}$.
• Weak oxidants (e.g., $\text{SeO}_2$) only affect the methyl group.

Correct Approach:

Use strong oxidising conditions — alkaline $\text{KMnO}_4$ followed by acidification.

Avoid by:

• Recognise that both alkyl and acyl groups attached to benzene can be oxidised to $\text{COOH}$ under strong conditions.
• Use alkaline $\text{KMnO}_4$ (not acidic) to avoid side reactions.

(v) Cyclohexene → Hexane-1,6-dioic acid

Common Mistake: Using $\text{KMnO}_4$ in cold/neutral conditions (which gives a diol) or using ozonolysis (which gives dialdehyde).

Why it’s wrong:

• Cold $\text{KMnO}_4$ gives cis-1,2-cyclohexanediol (not ring opening).
• Ozonolysis gives hexanedial (dialdehyde), not the diacid.

Correct Approach:

Use hot, concentrated $\text{KMnO}_4$ (or $\text{K}_2\text{Cr}_2\text{O}_7$ in acid) to cleave the double bond and oxidise both ends to $\text{COOH}$.

Avoid by:

• Remember: Hot, strong oxidants cleave alkenes to carboxylic acids.
• Cold/neutral conditions give diols — not what’s asked.

(vi) Butanal → Butanoic acid …