Q.Name the reagents used in the following reactions:

Concept: Reagent-specific transformations in organic chemistry — each reaction requires a particular oxidising, halogenating, reducing, or dehydrating agent.

Reasoning:

1. Primary alcohol → carboxylic acid: Requires a strong oxidising agent that does not stop at the aldehyde stage. Acidified $K_2Cr_2O_7$ (or $KMnO_4$) works.
2. Primary alcohol → aldehyde: Needs a milder oxidant that stops at the aldehyde; PCC (pyridinium chlorochromate) in anhydrous conditions is the standard choice.
3. Phenol → 2,4,6-tribromophenol: Bromine water ($Br_2/H_2O$) — the activating –OH group directs electrophilic substitution to all three ortho/para positions.
4. Benzyl alcohol → benzoic acid: Strong oxidation with alkaline $KMnO_4$ or acidified $K_2Cr_2O_7$ oxidises the side-chain completely to –COOH. …

Let’s go through each one. The logic here is simple: you need to know which reagent does what, and why. For oxidation, the strength of the oxidising agent and the reaction conditions decide whether you stop at an aldehyde or go all the way to a carboxylic acid. For the others, it’s about recognising characteristic reactions — like how phenol is so reactive that bromine water gives a tribromo product instantly, or how ketones are reduced to secondary alcohols.

1. Oxidation of a primary alcohol to carboxylic acid.

A primary alcohol ($\ce{R-CH2OH}$) can be oxidised in two stages: first to an aldehyde ($\ce{R-CHO}$), then to a carboxylic acid ($\ce{R-COOH}$). To get the acid directly, you need a strong oxidising agent that doesn’t stop at the aldehyde. The classic choices are acidified potassium dichromate ($\ce{K2Cr2O7/H2SO4}$) or acidified potassium permanganate ($\ce{KMnO4/H2SO4}$). Both are strong enough to push the oxidation all the way.

2. Oxidation of a primary alcohol to aldehyde.

Here you want to stop at the aldehyde stage. If you use a strong oxidising agent, it will over-oxidise. So you need a milder, specific reagent. The go-to is PCC (pyridinium chlorochromate, $\ce{C5H5NH+CrO3Cl-}$) in anhydrous conditions (e.g., in dichloromethane). It oxidises primary alcohols to aldehydes without touching them further. Another option is the Swern oxidation (DMSO + oxalyl chloride), but PCC is the standard for exams.

3. Bromination of phenol to 2,4,6-tribromophenol.

Phenol is highly activated towards electrophilic substitution because the –OH group donates electrons into the ring. Bromine water ($\ce{Br2/H2O}$) is a mild source of electrophilic bromine. The reaction is so fast that all three ortho and para positions get substituted instantly, giving a white precipitate of 2,4,6-tribromophenol. No catalyst is needed.

4. Benzyl alcohol to benzoic acid.

Benzyl alcohol ($\ce{C6H5CH2OH}$) is a primary alcohol attached to a benzene ring. To oxidise it to benzoic acid ($\ce{C6H5COOH}$), you need a strong oxidising agent. Acidified potassium permanganate ($\ce{KMnO4/H2SO4}$) works perfectly — it oxidises the –$\ce{CH2OH}$ group all the way to –$\ce{COOH}$. Note that the benzene ring is unaffected under these conditions. …

Here is the clear solution method for the given reactions, following a Concept-First approach.

Method: Reagent Mapping for Functional Group Transformations

This method relies on identifying the starting functional group and the target functional group, then selecting the specific reagent that achieves that transformation without over-oxidizing or under-reacting.

Steps for Each Reaction

(i) Oxidation of a primary alcohol to carboxylic acid.

• Concept: Primary alcohols ($RCH_2OH$) can be oxidized to carboxylic acids ($RCOOH$) using a strong oxidizing agent that does not stop at the aldehyde stage.
• Reagent: Acidified potassium dichromate ($K_2Cr_2O_7/H_2SO_4$) or Acidified potassium permanganate ($KMnO_4/H_2SO_4$).
• Why: The dichromate ion ($Cr_2O_7^{2-}$) in acidic medium provides a strong oxidizing environment that pushes the reaction past the aldehyde to the acid.

(ii) Oxidation of a primary alcohol to aldehyde.

• Concept: To stop at the aldehyde ($RCHO$), you must use a milder oxidizing agent that cannot further oxidize the aldehyde to the acid.
• Reagent: PCC (Pyridinium Chlorochromate) in anhydrous conditions (e.g., $CH_2Cl_2$).
• Why: PCC is a specific, mild oxidant that converts primary alcohols to aldehydes without over-oxidation. (Note: $K_2Cr_2O_7$ would give the acid here).

(iii) Bromination of phenol to 2,4,6-tribromophenol.

• Concept: Phenol ($C_6H_5OH$) is highly activated towards electrophilic substitution due to the -OH group. Bromine water is sufficient; no catalyst (like $FeBr_3$) is needed.
• Reagent: Bromine water ($Br_2/H_2O$).
• Why: The reaction is so fast that a dilute solution of bromine water instantly gives a white precipitate of 2,4,6-tribromophenol.

(iv) Benzyl alcohol to benzoic acid.

• Concept: Benzyl alcohol ($C_6H_5CH_2OH$) is a primary alcohol attached to a benzene ring. Strong oxidation cleaves the side chain to the carboxylic acid.
• Reagent: Alkaline potassium permanganate ($KMnO_4/KOH$) followed by acidification, or Acidified $K_2Cr_2O_7$.
• Why: The strong oxidizing agent ($KMnO_4$) oxidizes the $CH_2OH$ group directly to $COOH$.

(v) Dehydration of propan-2-ol to propene.

• Concept: Dehydration of an alcohol ($ROH$) to an alkene ($RCH=CH_2$) requires an acid catalyst and heat to remove a water molecule.
• Reagent: Concentrated sulphuric acid ($H_2SO_4$) at 443 K (170°C). …

Here are the common mistakes students make on this question, along with the concept-first reasoning to avoid each.

General Mistake: Confusing Reagent Names with Reaction Conditions

Students often write the chemical formula but forget the specific reagent name or medium (e.g., acid/base). In exams, the name must be exact.

(i) Oxidation of a primary alcohol to carboxylic acid.

Common Mistake: Writing only KMnO4 or K2Cr2O7 without specifying acidic medium.

• Why it’s wrong: In neutral/alkaline medium, KMnO4 can stop at aldehyde or give incomplete oxidation.
• Correct Reagent: Acidified potassium dichromate ($K_2Cr_2O_7/H_2SO_4$) or acidified potassium permanganate ($KMnO_4/H_2SO_4$).
• How to avoid: Remember: Carboxylic acid from primary alcohol needs strong oxidizing agent in acidic medium. Always write “acidified” or “$H_2SO_4$”.

(ii) Oxidation of a primary alcohol to aldehyde.

Common Mistake: Using the same strong oxidant as in (i) (e.g., acidified $K_2Cr_2O_7$).

• Why it’s wrong: Strong oxidants over-oxidize the aldehyde to carboxylic acid.
• Correct Reagent: PCC (Pyridinium chlorochromate) or Collins reagent ($CrO_3 \cdot 2Py$).
• How to avoid: Link aldehyde with mild, anhydrous oxidants. PCC is the exam favourite. Do not write $K_2Cr_2O_7$ here.

(iii) Bromination of phenol to 2,4,6-tribromophenol.

Common Mistake: Writing $Br_2$ alone, or $Br_2/FeBr_3$ (as in benzene bromination).

• Why it’s wrong: Phenol is highly activated — it does not need a Lewis acid catalyst. $Br_2$ alone in water works instantly.
• Correct Reagent: Bromine water ($Br_2/H_2O$).
• How to avoid: Remember: Phenol + Br2 water → white precipitate of tribromophenol. No catalyst needed.

(iv) Benzyl alcohol to benzoic acid.

Common Mistake: Writing $K_2Cr_2O_7/H_2SO_4$ (which works, but is not the specific reagent often expected).

• Why it’s wrong: While acidified dichromate works, the question expects a strong oxidizing agent that is commonly used for this specific conversion.
• Correct Reagent: Alkaline potassium permanganate ($KMnO_4/KOH$) followed by acidification, or acidified $KMnO_4$.
• How to avoid: For benzyl alcohol → benzoic acid, think of $KMnO_4$ (hot, conc.) as the standard. Write “alkaline $KMnO_4$” or “acidified $KMnO_4$”.

(v) Dehydration of propan-2-ol to propene.

Common Mistake: Writing only “heat” or “$H_2SO_4$” without specifying concentrated.

• Why it’s wrong: Dilute acid does not cause dehydration; it may just dissolve the alcohol.
• Correct Reagent: Concentrated sulphuric acid ($Conc. H_2SO_4$) at 170°C (or Alumina $Al_2O_3$ at 350°C).
• How to avoid: Always write “conc. $H_2SO_4$” and mention heat (170°C). For exam, “conc. $H_2SO_4$, heat” is sufficient.

(vi) Butan-2-one to butan-2-ol. …