Organic Chemistry · Carboxylic Acids and Nitriles
Preparing Carboxylic Acids
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In 30 seconds
A Carboxylic acid R–COOH: carbonyl carbon bonded to –OH Full entry → has the structure R–COOH: a carbonyl carbon bonded to a hydroxyl group. This topic is a toolbox of reactions that create the –COOH group. Five families matter most:
- Oxidation of primary alcohols and aldehydes — the carbon climbs oxidation states, oxygen already present.
- Oxidation of alkyl side chains on aromatic rings — a toluene-like methyl group is oxidized all the way to –COOH.
- Hydrolysis of nitriles — the C≡N bond is hydrated to the acid plus ammonia.
- Carboxylation of Grignard reagents — a carbanion equivalent attacks CO₂, adding one carbon.
- Oxidative cleavage Breaking a C=C or C≡C bond with oxidant Full entry → of alkenes and alkynes — strong oxidants sever the π bond, leaving –COOH groups.
The route you choose depends on the starting material and where the –COOH must end up. Chain-extension methods (nitriles, Grignards) add one carbon; oxidation methods keep the skeleton unchanged.
Why this matters
- Pharmaceuticals: Aspirin, ibuprofen, and penicillin are carboxylic acids or derivatives; syntheses often build the acid group last.
- Polymers: Adipic acid (nylon-66) and terephthalic acid (PET) come from oxidation routes.
- Food chemistry: Benzoic acid is a preservative; acetic acid is vinegar; citric acid is the tang in citrus.
- Exam strategy: "Propose a synthesis" problems test this material most.
- Lab safety: Oxidants (KMnO₄, CrO₃) are hazardous — fume hood, gloves, goggles, slow addition, and no strong oxidants with flammable solvents.
The college version
Core Concepts
Oxidation of primary alcohols and aldehydes
A Primary alcohol R–CH₂OH: –OH on a carbon with one carbon neighbor Full entry → is oxidized first to an aldehyde, then to the carboxylic acid:
R–CH2OH KMnO4 or CrO3⟶\text{excess oxidant} R–COOH
The carbon of –CH₂OH sits near oxidation state −1; in –COOH about +3: a four-electron oxidation. Warm aqueous KMnO₄, Na₂Cr₂O₇/H₂SO₄, and Jones reagent (CrO₃/H₂SO₄) all work. The aldehyde intermediate is easily over-oxidized and cannot be isolated — aldehydes need milder reagents (Chapter 19). Example: butan-1-ol → butanoic acid.
Oxidation of alkyl side chains on aromatic rings
An alkyl group on a benzene ring, such as the methyl of toluene (C₆H₅–CH₃), is oxidized to –COOH by hot KMnO₄ or Na₂Cr₂O₇:
C6H5–CH3 KMnO4, H2O⟶\text{heat} C6H5–COOH
Only the Benzylic carbon Carbon directly attached to a benzene ring Full entry → (directly attached to the ring) is attacked, because the ring stabilizes the intermediate there. Longer chains behave the same way: propylbenzene gives benzoic acid, not phenylpropanoic acid — the whole side chain degrades to a single –COOH.
Hydrolysis of nitriles
A Nitrile R–C≡N: carbon triple-bonded to nitrogen Full entry → (R–C≡N) hydrolyzes to a carboxylic acid under acidic or basic conditions:
R–C≡N + 2 H2O + H+ → R–COOH + NH4+
The reaction passes through an amide intermediate (R–CONH₂). Because nitriles are made by S_N2 displacement of alkyl halides with cyanide ion (Chapter 10), the sequence R–X → R–CN → R–COOH is a classic one-carbon chain extension: 1-bromopropane gives butanenitrile, which hydrolyzes to butanoic acid. Under basic conditions the product is the Carboxylate salt R–COO⁻: deprotonated acid, anionic form Full entry → R–COO⁻, needing acid workup for the free acid.
Carboxylation of Grignard reagents
Grignard reagents (R–MgX) act as carbanion equivalents — powerful carbon nucleophiles. Attack on carbon dioxide gives a carboxylate salt; acid workup gives the acid:
R–MgX + O=C=O → R–COO-MgX+ H3O+⟶ R–COOH
This route also extends the chain by one carbon and shines for tertiary and aromatic acids, where oxidation fails because no C–H exists at the carbon to be oxidized. Example: bromobenzene → phenylmagnesium bromide + CO₂ → benzoic acid.
Oxidative cleavage of alkenes and alkynes
Hot, concentrated KMnO₄ cleaves carbon–carbon π bonds. A terminal alkene R–CH=CH₂ gives R–COOH (the =CH₂ end becomes CO₂); an internal alkene R–CH=CH–R′ gives two acids. Ozonolysis with oxidative workup (H₂O₂) does the same more controllably; alkynes cleave to two carboxylic acids. Useful when the double bond already exists, such as in locating double bonds in natural products.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Oxidation of an alcohol to an acid | Oxidation of an aldehyde to an acid | Same strong oxidants; the aldehyde is an intermediate, usually not isolated |
| Benzylic side-chain oxidation | Oxidation of the aromatic ring itself | The ring resists KMnO₄/CrO₃; only benzylic C–H is oxidized |
| Nitrile hydrolysis (acidic) | Nitrile reduction | Hydrolysis gives R–COOH + NH₄⁺; LiAlH₄ gives R–CH₂NH₂ |
| Grignard + CO₂ | Grignard + aldehyde | CO₂ gives an acid; aldehydes give secondary alcohols |
| Basic hydrolysis product | Acidic hydrolysis product | Base gives the carboxylate salt R–COO⁻; acid workup gives free R–COOH |
| Theoretical yield | Actual yield | Stoichiometry gives the maximum; side reactions and losses lower the real number |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Making a carboxylic acid is like upgrading a tool: sometimes you sand wood until smooth (oxidizing an alcohol), sometimes you add a handle (Grignard + CO₂ adds a carbon), sometimes you break a plank in two and polish the ends (oxidative cleavage). Pick the right upgrade for the wood you have.
Worked example
Example 1: One-carbon extension by the nitrile route (with stoichiometry)
Propose a synthesis of butanoic acid from 1-bromopropane and calculate the theoretical mass of butanoic acid from 12.3 g of 1-bromopropane.
Step 1 — Plan. Displace bromide with cyanide (S_N2), then hydrolyze:
CH3CH2CH2Br + NaCN → CH3CH2CH2CN + NaBr
CH3CH2CH2CN + 2 H2O + H+ → CH3CH2CH2COOH + NH4+
Step 2 — Convert mass to moles (1-bromopropane C₃H₇Br, molar mass 123.0 g/mol):
n = mM = 12.3 g123.0 g mol-1 = 0.100 mol
Step 3 — Apply the 1:1 mole ratio to butanoic acid (C₄H₈O₂, molar mass 88.11 g/mol):
m = n × M = 0.100 mol × 88.11 g mol-1 = 8.81 g
Dimensional check: g ÷ (g mol⁻¹) = mol; mol × (g mol⁻¹) = g. The theoretical maximum is 8.81 g; real yields are lower.
Example 2: Grignard carboxylation (with stoichiometry)
How many grams of benzoic acid can be prepared from 15.7 g of bromobenzene via the Grignard route, assuming quantitative conversion?
Step 1 — Form the Grignard reagent and carboxylate it:
C6H5Br + Mg → C6H5MgBr
C6H5MgBr + CO2 → C6H5COO-MgBr+ H3O+⟶ C6H5COOH
Step 2 — Convert mass to moles (bromobenzene C₆H₅Br, molar mass 157.0 g/mol):
n = 15.7 g157.0 g mol-1 = 0.100 mol
Step 3 — Scale to product. The 1:1 ratio gives 0.100 mol benzoic acid (C₇H₆O₂, molar mass 122.12 g/mol):
m = 0.100 mol × 122.12 g mol-1 = 12.2 g
12.2 g of benzoic acid is the theoretical mass. In practice, Grignard reactions are moisture-sensitive and run under anhydrous conditions.
Example 3: Choosing between oxidation and chain extension
Show how to prepare 2-methylpropanoic acid from 2-bromopropane, and explain why alcohol oxidation would be a poor choice.
2-Bromopropane is a secondary alkyl halide with no C–H to oxidize, so oxidation is impossible. The Grignard route works: 2-bromopropane + Mg gives (CH₃)₂CH–MgBr, which reacts with CO₂ to give, after acid workup, 2-methylpropanoic acid, (CH₃)₂CH–COOH.
Key takeaways
- Oxidation ladder: primary alcohol → aldehyde → carboxylic acid; hot KMnO₄/CrO₃ goes all the way.
- Benzylic oxidation: any alkyl side chain on a benzene ring becomes –COOH; only the benzylic carbon is attacked.
- Nitrile hydrolysis: R–CN + 2 H₂O + H⁺ → R–COOH + NH₄⁺; nitriles come from S_N2 with CN⁻ (one-carbon extension).
- Grignard + CO₂: R–MgX + CO₂ → R–COO⁻MgX⁺ → (H₃O⁺) → R–COOH; one-carbon extension; works for tertiary and aromatic acids.
- Oxidative cleavage: R–CH=CH₂ → R–COOH (+ CO₂); internal alkenes give two acids.
- Chain growth: oxidation preserves the carbon count; nitrile and Grignard routes add exactly one carbon. Basic hydrolysis gives the carboxylate salt; acid workup gives the free acid.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What oxidation states does the carbon of a primary alcohol pass through en route to –COOH?
Show answer
Roughly −1 (R–CH₂OH) → +1 (R–CHO) → +3 (R–COOH): a four-electron oxidation.
Toluene is treated with hot KMnO₄. What product forms, and why is the ring itself untouched?
Show answer
Benzoic acid, C₆H₅–COOH. The ring resists oxidation; the benzylic C–H is attacked and the side chain degrades to a single –COOH.
Write the balanced transformation for acidic hydrolysis of butanenitrile, and name both products.
Show answer
CH₃CH₂CH₂CN + 2 H₂O + H⁺ → CH₃CH₂CH₂COOH + NH₄⁺: butanoic acid and ammonium ion.
Why do the nitrile and Grignard routes each add exactly one carbon to the chain?
Show answer
CN⁻ supplies it in the nitrile route, CO₂ in the Grignard route; the skeleton is untouched, so the chain grows by one.
12.3 g of 1-bromopropane → butanoic acid by the nitrile route. Theoretical mass of product?
Show answer
0.100 mol of 1-bromopropane → 0.100 mol butanoic acid; 0.100 × 88.11 = 8.81 g.
Which route makes benzoic acid from bromobenzene, and what electrophile does the Grignard reagent R–MgX: carbon–magnesium bond acting as a carbanion Full entry → attack?
Show answer
The Grignard route: C₆H₅MgBr attacks CO₂, giving a benzoate salt, protonated to benzoic acid.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Carboxylic acid
- R–COOH: carbonyl carbon bonded to –OH
- Primary alcohol
- R–CH₂OH: –OH on a carbon with one carbon neighbor
- Benzylic carbon
- Carbon directly attached to a benzene ring
- Nitrile
- R–C≡N: carbon triple-bonded to nitrogen
- Grignard reagent
- R–MgX: carbon–magnesium bond acting as a carbanion
- Carboxylate salt
- R–COO⁻: deprotonated acid, anionic form
- Oxidative cleavage
- Breaking a C=C or C≡C bond with oxidant
Sources & references
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