Q.Give names of the reagents to bring about the following transformations:

Concept: Oxidation Reactions — controlled oxidation of alcohols, alkenes, and nitriles to aldehydes or ketones using specific reagents.

Reasoning steps:

1. Primary alcohol → aldehyde: Use a mild oxidising agent that stops at the aldehyde stage, avoiding over-oxidation to carboxylic acid.
2. Secondary alcohol → ketone: Use a standard oxidising agent; ketones resist further oxidation.
3. Methylarene → benzaldehyde: Use chromyl chloride (Etard reaction) to oxidise the methyl group directly to aldehyde.
4. Nitrile → aldehyde: Use DIBAL-H (partial reduction) followed by hydrolysis.
5. Allyl alcohol → α,β-unsaturated aldehyde: Use a mild oxidant like PCC that does not attack the double bond. …

Oxidation reactions in organic chemistry are all about controlling the degree of oxidation. For alcohols, a primary alcohol can be oxidized to an aldehyde or further to a carboxylic acid. The trick is to choose a reagent that stops at the aldehyde stage — typically a mild oxidant like PCC (pyridinium chlorochromate) or a controlled reduction of a nitrile. For alkenes, ozonolysis cleaves the double bond to give carbonyl compounds. Let's go through each transformation.

1. Hexan-1-ol to hexanal

   Hexan-1-ol is a primary alcohol. To get the aldehyde without over-oxidation, use PCC in dichloromethane. PCC is a mild, anhydrous reagent that stops at the aldehyde. Strong oxidants like K₂Cr₂O₇/H₂SO₄ would give hexanoic acid.

   Reagent: PCC (pyridinium chlorochromate) in CH₂Cl₂.

2. Cyclohexanol to cyclohexanone

   Cyclohexanol is a secondary alcohol. Oxidation of a secondary alcohol gives a ketone, and there's no risk of further oxidation (ketones are resistant to mild oxidants). The textbook's reagent is anhydrous CrO₃ — without water there is no scope for over-oxidation. PCC or Na₂Cr₂O₇/H₂SO₄ also work for a secondary alcohol.

   Reagent: Anhydrous CrO₃ (also acceptable: PCC or Na₂Cr₂O₇/H₂SO₄).

3. p-Fluorotoluene to p-fluorobenzaldehyde

   This is a side-chain oxidation of a methyl group attached to an aromatic ring. Direct oxidation with KMnO₄ would give the carboxylic acid (p-fluorobenzoic acid). To stop at the aldehyde, use the Etard reaction: chromyl chloride (CrO₂Cl₂) reacts with the methyl group to form a chromium complex, which is then hydrolysed with H₃O⁺ to release the aldehyde.

   Reagent: CrO₂Cl₂ (Etard reagent) then H₃O⁺.

4. Ethanenitrile to ethanal

   A nitrile (R-C≡N) can be reduced to an aldehyde using DIBAL-H (diisobutylaluminium hydride) at low temperature, followed by hydrolysis. DIBAL-H adds hydride to the nitrile carbon, forming an imine intermediate that hydrolyzes to the aldehyde. LiAlH₄ would over-reduce to the primary amine.

   Reagent: DIBAL-H (in hexane or toluene, -78°C), then H₃O⁺.

5. Allyl alcohol to propenal …

Here is the clear solution method for the given transformations, focusing on the concept of controlled oxidation.

Method: Controlled Oxidation using Specific Oxidizing Agents

The core concept is that different oxidizing agents have different strengths. To stop an oxidation at the aldehyde stage (without over-oxidizing to a carboxylic acid), we must use a mild or specific reagent.

Steps for each transformation:

(i) Hexan-1-ol to hexanal

• Reagent: PCC (Pyridinium Chlorochromate) in anhydrous conditions (e.g., dry $CH_2Cl_2$).
• Why: PCC is a mild oxidizing agent. It oxidizes primary alcohols to aldehydes without further oxidizing them to carboxylic acids. Strong agents like $K_2Cr_2O_7/H_2SO_4$ would give hexanoic acid.

(ii) Cyclohexanol to cyclohexanone

• Reagent: Anhydrous $CrO_3$ (the textbook's choice; PCC or Jones reagent also work).
• Why: This is the oxidation of a secondary alcohol. Secondary alcohols oxidize to ketones (which cannot be further oxidized easily without breaking carbon-carbon bonds). Both mild (PCC) and strong (Jones) reagents work here, but PCC is often preferred for sensitive molecules.

(iii) p-Fluorotoluene to p-fluorobenzaldehyde

• Reagent: Chromyl chloride ($CrO_2Cl_2$) in $CS_2$ or $CCl_4$, followed by hydrolysis (aqueous workup). This is the Etard reaction.
• Why: Direct oxidation of a methyl group attached to a benzene ring (side-chain oxidation) usually gives benzoic acid. The Etard reaction specifically stops at the aldehyde stage by forming a stable complex with the benzylic carbon.

(iv) Ethanenitrile to ethanal

• Reagent: Diisobutylaluminium hydride (DIBAL-H) at low temperature ($-78^\circ C$), followed by hydrolysis.
• Why: Nitriles ($R-C \equiv N$) are reduced by DIBAL-H to an imine intermediate. Controlled hydrolysis of this imine yields the aldehyde. This is a reduction, not an oxidation, but it is the standard method to get an aldehyde from a nitrile.

(v) Allyl alcohol to propenal

• Reagent: PCC (Pyridinium Chlorochromate) in $CH_2Cl_2$.
• Why: Allyl alcohol is a primary allylic alcohol. PCC selectively oxidizes the $-OH$ group to an aldehyde without attacking the carbon-carbon double bond. Stronger oxidants would break the double bond.

(vi) But-2-ene to ethanal …

Here are the common mistakes students make when tackling these specific oxidation transformations, along with the precise conceptual corrections needed to avoid them.

General Mistake: Confusing "Strong" vs. "Mild" Oxidising Agents

The most frequent error is using a strong oxidising agent (like $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+$) for every reaction. Strong agents often over-oxidise, turning an aldehyde into a carboxylic acid or breaking carbon-carbon bonds.

• The Concept: Aldehydes ($\text{RCHO}$) are easier to oxidise than primary alcohols ($\text{RCH}_2\text{OH}$). To stop at the aldehyde, you must use a mild reagent that cannot further oxidise the product.
• How to Avoid: Memorise the "mild" list: PCC (Pyridinium Chlorochromate), Collins reagent, or Swern oxidation. For strong oxidation (to acid), use $\text{K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4$ or $\text{KMnO}_4$.

(i) Hexan-1-ol to hexanal

Common Mistake: Using $\text{K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4$ (acidified dichromate).

• Why it's wrong: This is a strong oxidising agent. It will convert the primary alcohol ($\text{CH}_3(\text{CH}_2)_4\text{CH}_2\text{OH}$) all the way to hexanoic acid ($\text{CH}_3(\text{CH}_2)_4\text{COOH}$), not stopping at the aldehyde.
• Correct Reagent: PCC (Pyridinium Chlorochromate) in anhydrous $\text{CH}_2\text{Cl}_2$.
• How to Avoid: Remember the rule: Primary alcohol $\xrightarrow{\text{PCC}}$ Aldehyde. PCC is the "stop sign" for oxidation.

(ii) Cyclohexanol to cyclohexanone

Common Mistake: Using PCC here when a cheaper, stronger reagent works perfectly.

• Why it's wrong: PCC is correct but expensive. Students often forget that secondary alcohols can be safely oxidised to ketones using strong agents because ketones are resistant to further oxidation (they have no $\text{H}$ on the carbonyl carbon to lose).
• Correct Reagent: Anhydrous $\text{CrO}_3$ — the textbook's answer. Acidified dichromate and PCC also oxidise a secondary alcohol cleanly.
• How to Avoid: Distinguish between primary and secondary alcohols. Secondary $\rightarrow$ Ketone is safe with strong oxidants. Primary $\rightarrow$ Aldehyde requires mild oxidants.

(iii) p-Fluorotoluene to p-fluorobenzaldehyde

Common Mistake: Trying to oxidise the methyl group ($\text{CH}_3$) directly with $\text{KMnO}_4$ or $\text{CrO}_3$.

• Why it's wrong: Strong oxidants on a methyl group attached to a benzene ring will oxidise it all the way to p-fluorobenzoic acid ($\text{COOH}$). You cannot stop at the aldehyde stage with these reagents.
• Correct Reagent: Chromyl chloride ($\text{CrO}_2\text{Cl}_2$) in the Etard reaction.
• How to Avoid: For toluene $\rightarrow$ benzaldehyde (or substituted variants), the specific reagent is Etard's reagent ($\text{CrO}_2\text{Cl}_2$). Do not use general oxidisers.

(iv) Ethanenitrile to ethanal

Common Mistake: Trying to oxidise the nitrile ($\text{CH}_3\text{CN}$) directly.

• Why it's wrong: Nitriles are not alcohols. You cannot "oxidise" a nitrile to an aldehyde. You must reduce it in a controlled way (DIBAL-H) and then hydrolyse the intermediate imine to the aldehyde.
• Correct Reagent: DIBAL-H (diisobutylaluminium hydride) at low temperature, then hydrolysis — the textbook's reagent. (Stephen reduction, $\text{SnCl}_2/\text{HCl}$, is an older alternative route.)
• How to Avoid: Recognise that this is a controlled reduction of a nitrile, not an oxidation. The reaction is: $\text{RCN} \xrightarrow[2.\ \text{H}_3\text{O}^+]{1.\ \text{DIBAL-H}} \text{RCHO}$.

(v) Allyl alcohol to propenal

Common Mistake: Using $\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+$ which might also attack the double bond.

• Why it's wrong: Strong oxidants like $\text{KMnO}_4$ or hot $\text{HNO}_3$ can cleave the carbon-carbon double bond ($\text{C=C}$) in allyl alcohol ($\text{CH}_2=\text{CH}-\text{CH}_2\text{OH}$), destroying the molecule. …