How to Prepare Carboxylic Acids: Oxidation, Nitriles and Grignard + CO2
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
There are three main ways to make a carboxylic acid (-COOH). One, oxidise a primary alcohol or an alkylbenzene using a strong oxidiser like acidic KMnO4 or K2Cr2O7. Two, hydrolyse a nitrile (R-CN) with acid or base to get R-COOH. Three, react a Grignard reagent (R-MgX) with CO2 (dry ice) and then add water. Memory hook: "Oxidise it, hydrolyse it, or feed it CO2."
Three NEET routes to R-COOH: oxidise a primary alcohol (same carbon count), hydrolyse a nitrile, or add CO2 to a Grignard reagent (both add one carbon).
Your doubts, answered
How do I turn a primary alcohol into a carboxylic acid?
Use a STRONG oxidiser: acidic KMnO4 (KMnO4 + H2SO4 or KMnO4/KOH then H3O+), acidified K2Cr2O7, or CrO3-H2SO4 (Jones reagent). A primary alcohol (R-CH2OH) is oxidised all the way to R-COOH. Only primary alcohols and aldehydes give acids. Secondary alcohols give ketones (which stop there), and tertiary alcohols do not oxidise normally.
Why does Cu at 573K NOT give a carboxylic acid?
Cu at 573 K only DEHYDROGENATES a primary alcohol to an ALDEHYDE and stops there. It is a mild method, not a strong oxidiser. To reach the -COOH stage you need a strong oxidiser. NEET puts Cu/573K and CrO3/(CH3CO)2O (Collins-type) as traps because both stop at the aldehyde, not the acid.
How does nitrile hydrolysis make a carboxylic acid?
A nitrile R-C≡N is heated with dilute acid (H3O+) or with a base (NaOH) followed by acidification. Water adds across the C≡N and it becomes R-COOH, releasing NH3 (as NH4+ salt in acid). The nitrile itself is usually made first from an alkyl halide + KCN. So the full chain is: R-X → R-CN → R-COOH.
What does a Grignard reagent give with CO2?
R-MgX attacks the carbon of CO2 (usually solid CO2, dry ice). This forms R-COO-MgX, and after adding water/dilute acid you get the carboxylic acid R-COOH. Key point: the -COOH carbon comes from CO2, so the acid has ONE MORE carbon than the original halide (R-X). This is how you step up a carbon chain.
Does the carboxylic acid have the same number of carbons as the starting material?
It depends on the method. Oxidation of an alcohol keeps the SAME number of carbons (the -CH2OH just becomes -COOH). But both nitrile hydrolysis and Grignard + CO2 ADD one carbon, because the extra carbon comes from -CN or from CO2. NEET loves testing this carbon-count difference.
Which alkylbenzene oxidation gives benzoic acid?
Any alkylbenzene with a benzylic hydrogen (like toluene, ethylbenzene) is oxidised by hot acidic KMnO4 or K2Cr2O7 to BENZOIC ACID (C6H5COOH). The whole side chain, no matter how long, is cut down to a single -COOH on the ring. Tert-butylbenzene has no benzylic H, so it does not oxidise this way.
⚠️ The NEET trap ✗ Choosing Cu at 573K or CrO3/(CH3CO)2O as a reagent that converts an alcohol to a carboxylic acid. ✓ Only STRONG oxidisers reach the acid stage: acidic KMnO4, acidic K2Cr2O7, and CrO3-H2SO4. Cu/573K and CrO3 with acetic anhydride are MILD and stop at the aldehyde. 🧠 Strong oxidiser = full journey to -COOH. Mild reagent = stops at -CHO. If it is 'mild', it is a trap for the acid question.
Real NEET questions
NEET 2023 Phase 2
Reagents which can be used to convert alcohols to carboxylic acids are: (A) CrO3-H2SO4 (B) K2Cr2O7 + H2SO4 (C) KMnO4 + KOH / H3O+ (D) Cu, 573 K (E) CrO3, (CH3CO)2O. Choose the most appropriate answer.
A · (A), (B) and (C) only ✓
B · (A), (B) and (E) only
C · (B), (C) and (D) only
D · (B), (D) and (E) only
Solution: Primary alcohols go all the way to -COOH only with STRONG oxidisers: CrO3-H2SO4 (Jones), acidic K2Cr2O7, and alkaline KMnO4 with acidic work-up — so A, B, C work. Cu at 573K only dehydrogenates to the aldehyde, and CrO3 with acetic anhydride is a mild reagent that also stops at the aldehyde. So D and E fail. Answer: A, B and C only.
NEET 2023 Phase 1
Cyclohexanone [A] reacts with HCN to give [B], which on treatment with conc. H2SO4/heat gives [C]. C is:
A · cyclohexanol (ring with -OH)
B · cyclohexane carbaldehyde (ring with -CHO)
C · 1-hydroxycyclohexane-1-carbonitrile (ring with -OH and -CN)
D · cyclohexane carboxylic acid (ring with -COOH) ✓
Solution: Cyclohexanone adds HCN to form the cyanohydrin (ring bearing -OH and -CN) as [B]. Heating this nitrile with conc. H2SO4 HYDROLYSES the -CN group to -COOH. So [C] is cyclohexane carboxylic acid. This is nitrile hydrolysis making an acid. Answer: D.
NEET 2023 Phase 2
Identify the product: cyclobutyl chloride → (i) KCN → (ii) H2O/HCl, Δ → (iii) Br2/red P → (iv) H2O → Product.
A · Cyclobutane bearing -Cl and -Br
B · Cyclobutane bearing two -Br
C · Cyclobutane bearing -COBr
D · Cyclobutane bearing -COOH and an alpha -Br (alpha-bromocyclobutanecarboxylic acid) ✓
Solution: Step (i): KCN swaps -Cl for -CN (nitrile formed). Step (ii): acidic hydrolysis turns -CN into -COOH, giving cyclobutanecarboxylic acid. Steps (iii)-(iv): Br2/red P is the Hell-Volhard-Zelinsky reaction, which puts a Br on the alpha carbon. Product carries both -COOH and an alpha -Br. Answer: D.
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the three main methods to prepare carboxylic acids?
(1) Oxidation of primary alcohols or alkylbenzenes with strong oxidisers (acidic KMnO4 / K2Cr2O7). (2) Hydrolysis of nitriles (R-CN + H3O+ or NaOH then acid → R-COOH). (3) Grignard reagent + CO2 followed by water (R-MgX → R-COOH).
Which methods add an extra carbon atom?
Nitrile hydrolysis and Grignard + CO2 both add one carbon (from -CN or from CO2). Oxidation of an alcohol keeps the carbon number the same.
Can NaBH4 or Cu/573K make a carboxylic acid from an alcohol?
No. NaBH4 is a reducing agent, not an oxidiser. Cu at 573 K only dehydrogenates a primary alcohol to an aldehyde. You need a strong oxidiser to reach -COOH.
Why use dry ice (solid CO2) in the Grignard method?
Solid CO2 is easy to handle and controls the reaction so the Grignard adds cleanly to one CO2 molecule, giving R-COO-MgX, which becomes R-COOH after adding water.
Does oxidation of toluene give benzoic acid or benzaldehyde?
Hot acidic KMnO4 or K2Cr2O7 oxidises toluene fully to benzoic acid. Milder controlled methods (like Etard reaction with CrO2Cl2) stop at benzaldehyde.
