Organic Chemistry · Carboxylic Acids and Nitriles
Reactions of Carboxylic Acids: An Overview
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In 30 seconds
Carboxylic acids sit at a busy intersection of organic reactivity. The group R–COOH has two reactive centers: the acidic O–H bond and the electrophilic carbonyl carbon (δ+ because of the C=O and C–O bonds). Their reactions fall into five families:
- Acid–base reactions — deprotonation of the O–H to carboxylate salts.
- Nucleophilic acyl substitution Nucleophile adds to carbonyl carbon; leaving group departs Full entry → — replacing –OH with Cl, OR′, or NR′₂ to make acid halides, esters, and amides.
- Reduction — LiAlH₄ takes the acid to a primary alcohol.
- Decarboxylation Loss of CO₂ from a carboxylic acid Full entry → — loss of CO₂, key for β-keto acids and malonic acids.
- α-Substitution — halogenation at the carbon next to the carbonyl (Hell–Volhard–Zelinsky).
This topic is a map: each family is treated in depth elsewhere (Chapter 21 for acyl substitution); the goal here is to see how the acid's structure controls what it can do.
Why this matters
- Chapter 21 depends on this map. Every acyl derivative — Acid chloride R–COCl: most reactive acyl derivative Full entry →, anhydride, ester, amide — is made from the acid and converts back to it.
- Biology runs on acyl groups. Acetyl-CoA, esters in fats, amides in proteins — all made and broken by this substitution logic.
- Solubility and isolation: Deprotonation makes acids water-soluble (carboxylate salts); reprotonation returns the neutral acid — the pH switch behind extraction.
- Drugs: Many medications are given as salts (sodium ibuprofen) for solubility; ester prodrugs release the active acid in the body.
- Lab safety: Acid chloride preparations use corrosive reagents (SOCl₂, PCl₃) that release HCl — fume hood, gloves, goggles, slow addition; never add water to them.
The college version
Core Concepts
Acid–base reactions: the carboxylate salts
Carboxylic acids have pKₐ near 4–5, so hydroxide, carbonate, and amines deprotonate them:
R–COOH + OH- → R–COO- + H2O
The carboxylate ion is stabilized by resonance (the negative charge is shared between the two oxygens). Carboxylate salts are ionic, water-soluble solids; adding strong acid returns the neutral, often water-insoluble acid. This reversible switch is the basis of extraction and purification. Aromatic acids (pKₐ ≈ 4.2 for benzoic acid) behave the same way.
Nucleophilic acyl substitution: replacing –OH
The carboxyl carbon is electrophilic, but –OH is a poor leaving group — hydroxide is a strong base and does not leave easily. So the acid reacts sluggishly and must first be activated by converting –OH into a better leaving group:
- With SOCl₂ (thionyl chloride): R–COOH → R–COCl (acid chloride). The byproducts are SO₂ and HCl gas.
- With an alcohol + acid catalyst: R–COOH + R′–OH ⇌ R–COOR′ + H₂O (Esterification Acid + alcohol ⇌ ester + water Full entry →, an equilibrium).
- With amines: acid + amine first gives an ammonium salt; heating dehydrates it to an amide R–CONH₂.
The general picture — nucleophile adds to the carbonyl, then the leaving group departs — is nucleophilic acyl substitution, the organizing reaction of Chapter 21.
Reduction: acids to primary alcohols
Lithium aluminum hydride (LiAlH₄) reduces carboxylic acids cleanly to primary alcohols:
R–COOH LiAlH4, then H2O⟶ R–CH2OH
LiAlH₄ is a powerful hydride donor. Sodium borohydride (NaBH₄), by contrast, is too weak to reduce the acid's carbonyl, so reagent choice matters: NaBH₄ reduces aldehydes and ketones but not acids.
Decarboxylation
Heating a simple acid does little, but when a carbonyl sits two carbons away (β-keto acid) or a second –COOH exists (malonic acid), CO₂ is lost readily:
R–CO–CH2–COOH Δ⟶ R–CO–CH3 + CO2
The cyclic transition state that expels CO₂ is stabilized by the adjacent carbonyl, which is why simple acids do not decarboxylate under mild heat. This reaction powers biosynthetic pathways and the malonic ester synthesis.
α-Halogenation: the Hell–Volhard–Zelinsky reaction
With Br₂ and a catalytic amount of PBr₃ (or P), a carboxylic acid is halogenated at the α carbon (next to the carbonyl):
R–CH2–COOH Br2, PBr3⟶ R–CHBr–COOH
The reaction proceeds through the acid bromide and is a rare example of selective α-bromination of an acid; it is the gateway to α-amino acid synthesis (Chapter 26).
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Carboxylic acid reactivity | Aldehyde/ketone reactivity | Acids need activation before substitution; carbonyls add nucleophiles directly |
| LiAlH₄ reduction of acids | NaBH₄ reduction of acids | LiAlH₄ works (→ alcohol); NaBH₄ does not reduce acids |
| Esterification (acid + alcohol) | Saponification (ester + base) | Esterification makes esters (equilibrium); saponification hydrolyzes them to salts |
| Decarboxylation of β-keto acids | Decarboxylation of simple acids | Only acids with a β-carbonyl lose CO₂ easily |
| α-Bromination (HVZ) | Free-radical benzylic bromination | HVZ uses Br₂/PBr₃ on the α carbon; NBS targets allylic/benzylic C–H |
| Acid chloride formation | Esterification | SOCl₂ replaces –OH with Cl (irreversible); esterification is a catalyzed equilibrium |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A carboxylic acid is like a toolbox with two handles: one is the acidic hydrogen that can be swapped for a metal (making a salt, like vinegar fizzing with baking soda), and the other is the carbon end that can trade its –OH group for other parts. The –OH is a reluctant leaver — like a bolt that needs a special wrench (activation) before it comes out. Once you know which wrench fits, you can build esters, amides, and all the other derivatives.
Worked example
Example 1: pH of a carboxylic acid solution (acid–base chemistry with calculation)
Calculate the pH of a 0.100 M aqueous solution of acetic acid (Kₐ = 1.8 × 10⁻⁵).
Step 1 — Write the equilibrium expression.
Ka = [H+][CH3COO-][CH3COOH]
Let x = [H+] = [CH3COO-]. For a weak acid with small Kₐ, [CH3COOH] ≈ 0.100 - x ≈ 0.100, so:
Ka ≈ x20.100
Step 2 — Substitute and solve.
x2 = Ka × 0.100 = (1.8 × 10-5)(0.100) = 1.8 × 10-6
x = 1.8 × 10-6 = 1.34 × 10-3 M
Step 3 — Convert to pH.
pH = -log[H+] = -log(1.34 × 10-3) = 2.87
Dimensional check: Kₐ (mol/L) × concentration (mol/L) gives (mol/L)², whose square root is mol/L — the answer is a concentration, and pH is unitless. The 5% rule holds (1.3% dissociation), so the approximation was valid.
Example 2: Reading the reaction map — a three-step conversion
Show the reagents and conditions to convert butanoic acid to butanamide, then to butanenitrile, and state what each step does to the functional group.
Step 1 — Activate the acid, then react with ammonia. SOCl₂ converts butanoic acid to butanoyl chloride (replaces –OH with Cl). Adding ammonia gives butanamide:
CH3CH2CH2COOH SOCl2⟶ CH3CH2CH2COCl NH3⟶ CH3CH2CH2CONH2
Step 2 — Dehydrate the amide. P₂O₅ (or SOCl₂) removes water, giving butanenitrile (topic 7):
CH3CH2CH2CONH2 P2O5⟶ CH3CH2CH2CN
The carbon count never changes: the map transforms functional groups, not skeletons.
Example 3: Choosing a reducing agent
Convert pentanoic acid to pentan-1-ol. Which reagent — NaBH₄ or LiAlH₄ — is appropriate, and why?
LiAlH₄. The acid's carbonyl resists hydride addition, so only the powerful donor LiAlH₄ can reduce it; NaBH₄ leaves the acid untouched (it is reserved for aldehydes and ketones). Aqueous workup liberates the alcohol:
CH3CH2CH2CH2COOH LiAlH4, then H2O⟶ CH3CH2CH2CH2CH2OH
Key takeaways
- Two reactive centers: the acidic O–H (deprotonation) and the electrophilic carbonyl carbon (substitution); –OH is a poor leaving group, so acids need activation (SOCl₂, acid catalysis).
- Acid → alcohol: LiAlH₄ reduces R–COOH to R–CH₂OH; NaBH₄ does not.
- Acid → salt: R–COOH + OH⁻ → R–COO⁻; the pH switch drives extraction.
- Decarboxylation needs a β-carbonyl (β-keto acids, malonic acids): R–CO–CH₂–COOH → R–CO–CH₃ + CO₂.
- α-Bromination: Br₂/PBr₃ (Hell–Volhard–Zelinsky) puts Br on the α carbon.
- Esterification is an equilibrium; removing water drives it forward.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name the two reactive centers of a carboxylic acid and one reaction for each.
Show answer
The acidic O–H (deprotonation to the carboxylate salt) and the electrophilic carbonyl carbon (nucleophilic acyl substitution, reduction, etc.).
Why must a carboxylic acid be "activated" before nucleophilic acyl substitution?
Show answer
Hydroxide is a poor leaving group; converting –OH to a better one (e.g., Cl via SOCl₂) allows substitution to proceed.
Which reagent reduces R–COOH to R–CH₂OH, and which common one cannot?
Show answer
LiAlH₄ reduces the acid to the primary alcohol; NaBH₄ does not reduce carboxylic acids.
Write the equilibrium for esterification of ethanoic acid with ethanol, and name the ester.
Show answer
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O; the ester is ethyl ethanoate.
What structural feature lets an acid decarboxylate easily, and what are the two products?
Show answer
A carbonyl two carbons away (β-keto acid) or a second –COOH (malonic acid); products are a ketone (or acid) plus CO₂.
Calculate the pH of a 0.100 M acetic acid solution (Kₐ = 1.8 × 10⁻⁵).
Show answer
x = √(1.8×10⁻⁵ × 0.100) = 1.34×10⁻³ M; pH = 2.87.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Carboxylate salt
- R–COO⁻: deprotonated acid, charge shared over two oxygens
- Nucleophilic acyl substitution
- Nucleophile adds to carbonyl carbon; leaving group departs
- Activation
- Converting –OH into a better leaving group
- Acid chloride
- R–COCl: most reactive acyl derivative
- Esterification
- Acid + alcohol ⇌ ester + water
- Decarboxylation
- Loss of CO₂ from a carboxylic acid
- α-Carbon
- Carbon directly attached to the carbonyl carbon
- Hell–Volhard–Zelinsky reaction
- α-Bromination of acids with Br₂/PBr₃
Sources & references
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