Carboxylic Acids

Carboxylic acids are everywhere on the MCAT — fatty acid metabolism, amino acid chemistry, the citric acid cycle. The whole functional group flows from one idea: the carbonyl makes the carboxyl carbon electrophilic and the O–H acidic.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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Structure and Acidity of Carboxylic Acids

The Carboxyl Group

Must know

A carboxylic acid contains a carboxyl group ($\ce{-COOH}$): a carbonyl ($\ce{C=O}$) fused directly to a hydroxyl ($\ce{-OH}$) on the same, sp$^2$ (planar, ~120°) carbon. The carbonyl strongly withdraws electron density, so the $\ce{O-H}$ is far more polarized than in an alcohol — both groups act together, and that is the key to all carboxylic acid chemistry.

Why Carboxylic Acids Are Acidic

Must know

Losing the proton gives a carboxylate anion ($\ce{-COO-}$) whose negative charge is delocalized equally over both oxygens by resonance (both C–O bonds equivalent, bond order ≈ 1.5). That resonance stabilization of the conjugate base is why carboxylic acids (pKa ≈ 4–5) are vastly more acidic than alcohols (pKa ≈ 16–18), whose alkoxide charge is localized.

$\ce{RCOOH <=> RCOO- + H+}$

$\ce{R-C(=O)-O^{-} <-> R-C(-O^{-})=O}$

Factors That Tune pKa

Must know

Electron-withdrawing groups near the α-carbon (e.g., $\ce{-Cl}$, $\ce{-F}$, $\ce{-NO2}$) stabilize the carboxylate inductively → lower pKa (stronger acid). Electron-donating alkyl groups do the opposite → higher pKa. The effect is stronger with more EWGs and the closer they sit to the carboxyl (inductive effects fall off with distance).

Passage-level

Reference values: acetic ≈ 4.75, formic ≈ 3.75, chloroacetic ≈ 2.86, trichloroacetic ≈ 0.66, benzoic ≈ 4.2. Recognize the trend; don't memorize the numbers.

Quick check: Dichloroacetic acid has two chlorines on the alpha-carbon. Is it a stronger or weaker acid than chloroacetic acid (one chlorine)?

Answer: Stronger acid (lower pKa ≈ 1.48). Each additional electron-withdrawing chlorine further stabilizes the carboxylate anion inductively.

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Nomenclature

IUPAC Naming

Must know

Name from the longest chain including the carboxyl carbon, replacing alkane "-e" with "-oic acid" ($\ce{CH3COOH}$ = ethanoic acid; $\ce{CH3CH2COOH}$ = propanoic acid; $\ce{HOOC-CH2CH2-COOH}$ = butanedioic acid). When $\ce{-COOH}$ hangs off a ring it can't join, use "-carboxylic acid" (e.g., cyclohexanecarboxylic acid).

Common Names

Must know

The MCAT uses common names freely, especially for small acids and metabolic intermediates. Worth knowing cold: formic ($\ce{HCOOH}$), acetic ($\ce{CH3COOH}$), benzoic ($\ce{C6H5COOH}$), and the di-acids oxalic ($\ce{HOOCCOOH}$), malonic ($\ce{HOOCCH2COOH}$), succinic ($\ce{HOOC(CH2)2COOH}$), plus citric-acid-cycle members (fumaric, malic, citric). Propionic/butyric/valeric (3/4/5-carbon) are passage-level.

The Greek letters $\alpha$, $\beta$, $\gamma$ label C2, C3, C4 (carboxyl carbon = C1) — used in substituent positions and in decarboxylation.

Quick check: How many carbons are in "β-hydroxybutyric acid," and which carbon bears the hydroxyl?

Answer: 4 carbons ("butyr-" = 4). The $\beta$ position is C3 (carboxyl C1, $\alpha$ = C2, $\beta$ = C3). $\beta$-hydroxybutyrate is a ketone body — relevant to MCAT biochemistry.

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Physical Properties

Boiling Points and Hydrogen Bonding

Must know

Carboxylic acids have unusually high boiling points because they form cyclic hydrogen-bonded dimers — two molecules each donate an $\ce{O-H}$ to the other's carbonyl oxygen, effectively doubling the mass to vaporize. (Acetic acid bp 118 °C vs. propanol bp 97 °C, both MW 60.)

Solubility

Must know

Short-chain acids (≤ ~4 C) are miscible with water; solubility falls as the nonpolar chain lengthens — which is why long-chain fatty acids form micelles/bilayers rather than dissolving. The carboxylate anion (formed at physiological pH and above) is more water-soluble than the neutral acid, which is why soaps (fatty acid salts) dissolve.

Salt Formation and Acid–Base Extraction

Must know

Being acidic (pKa ≈ 4–5), carboxylic acids are deprotonated even by weak base like bicarbonate to give water-soluble carboxylate salts:

$\ce{RCOOH + NaHCO3 -> RCOO^- Na^+ + H2O + CO2 ^}$

This is the basis of acid–base extraction: aqueous $\ce{NaHCO3}$ pulls the carboxylic acid into the aqueous layer (as carboxylate) while neutral compounds — and even phenols (pKa ≈ 10, too weak for bicarbonate) — stay in the organic layer. Re-acidifying recovers the acid; the $\ce{CO2}$ effervescence also distinguishes carboxylic acids from phenols.

Quick check: Would you expect octanoic acid (8 carbons) to be more or less water-soluble than acetic acid? Why?

Answer: Much less soluble. The long nonpolar chain dominates over the polar carboxyl group, paralleling how long-chain fatty acids partition into membranes.

Spectroscopic Identification

Passage-level

• IR: very broad O–H, ~2500–3300 cm⁻¹ (broadest O–H you'll see) plus strong C=O ~1710 cm⁻¹ — the pair is essentially diagnostic.
• ¹H NMR: carboxyl proton extremely downfield, ~10–12 ppm (broad, exchangeable).

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Nucleophilic Acyl Substitution: The Master Mechanism

Must know

This mechanism underlies ester, amide, and anhydride formation — essentially all carboxyl reactions. The carboxyl carbon is electrophilic ($\delta+$).

1. Addition: a nucleophile attacks the carbonyl carbon, breaking the $\pi$ bond → tetrahedral intermediate (oxygen now negative).
2. Elimination: the intermediate collapses, a leaving group departs, re-forming the carbonyl.

$\text{Nu}^- + \ce{R-C(=O)-LG -> [R-C(Nu)(O^-)(LG)] -> R-C(=O)-Nu + LG^-}$

Reactivity order (set by leaving-group ability):

$\text{Acid chlorides} > \text{Anhydrides} > \text{Esters} \approx \text{Carboxylic acids} > \text{Amides}$

Intuition: $\ce{Cl-}$ is an excellent leaving group; the amide nitrogen is a terrible one. This order recurs across MCAT questions.

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Carboxyl Group Reactions

Fischer Esterification

Must know

Carboxylic acid + alcohol, acid-catalyzed ($\ce{H2SO4}$/$\ce{HCl}$), gives an ester + water:

$\ce{RCOOH + R'OH <=>[H+, \Delta] RCOOR' + H2O}$

It's an equilibrium — drive it toward ester (Le Chatelier) with excess alcohol or by removing water. Transesterification (swapping one alcohol for another in an ester) runs by the same mechanism. (Mechanism: protonate carbonyl → alcohol attacks → tetrahedral intermediate → lose water.)

Amide Formation

Must know

Acid + amine gives an amide ($\ce{RCONHR'}$), but direct mixing just makes an ammonium salt — you must heat to drive off water:

$\ce{RCOOH + H2NR' ->[\Delta] RCONHR' + H2O}$

In the lab a more reactive acyl derivative (acid chloride/anhydride) is used instead; in biology, ribosomes form amide bonds from activated amino acids (aminoacyl-tRNA). The amide (peptide) bond has partial double-bond character (N lone pair donates into the carbonyl), making it planar with restricted rotation — foundational for protein secondary structure.

Anhydride Formation

Must know

Carboxylic anhydrides ($\ce{RC(=O)-O-C(=O)R'}$) form by condensing two acids with loss of water; they are reactive acylating agents (above esters/acids, below acid chlorides) and react with alcohols → esters, amines → amides. Acetic anhydride is the common example.

$\ce{2 RCOOH ->[\Delta, P_2O_5] RC(=O)-O-C(=O)R + H2O}$

Quick check: Aspirin (acetylsalicylic acid) is made by reacting salicylic acid (which has a phenolic $\ce{-OH}$) with acetic anhydride. What type of reaction is this, and what functional group forms on the aspirin molecule?

Answer: Nucleophilic acyl substitution — the phenolic $\ce{-OH}$ attacks the anhydride, forming an ester linkage (other product: acetic acid). Aspirin is therefore an ester.

Conversion to Acid Chlorides

Must know

Because the free acid is low on the reactivity ladder, you first convert it up to the acid chloride using thionyl chloride ($\ce{SOCl2}$) ($\ce{PCl3}$/$\ce{PCl5}$ also work):

$\ce{RCOOH + SOCl2 -> RCOCl + SO2 ^ + HCl ^}$

Gaseous byproducts drive it to completion. The acid chloride then reacts readily with alcohols (→ esters) and amines (→ amides) — the standard way to funnel an unreactive acid into "downhill" acyl chemistry.

Lactones and Lactams

Must know

A lactone is a cyclic ester (intramolecular $\ce{-OH}$ + acid); a lactam is a cyclic amide (intramolecular amine + acid). Ring strain governs reactivity: 4-membered (β) rings are strained/reactive, 5- and 6-membered (γ, δ) are stable.

Optional

The strained β-lactam ring is the reactive core of penicillin/cephalosporin antibiotics.

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Reduction of Carboxylic Acids

Must know

The carboxyl carbon is at the highest common oxidation state (short of $\ce{CO2}$); reduction lowers it.

The high-yield distinction: $\ce{LiAlH4}$ (a strong hydride donor) reduces carboxylic acids all the way to a primary alcohol; $\ce{NaBH4}$ is too mild and does not touch carboxylic acids (or esters) — it reduces only aldehydes/ketones.

$\ce{RCOOH ->[1. LiAlH4, THF][2. H3O+] RCH2OH}$

Know the logic

Two hydride equivalents are delivered to the carboxyl carbon (acid → primary alcohol).

Biological connection: the acid → alcohol reduction parallels fatty acid biosynthesis steps, where NADPH plays the role of the hydride source instead of LAH.

Quick check: You want to selectively reduce a ketone in a molecule that also contains a carboxylic acid group. Which reagent would you choose?

Answer: $\ce{NaBH4}$ — it reduces the ketone to a secondary alcohol while leaving the carboxylic acid untouched. LiAlH4 would reduce both.

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Decarboxylation

Must know

Decarboxylation = loss of $\ce{CO2}$. It is easy only with the right structural feature, which makes it highly testable.

Beta-Keto Acids

Must know

β-keto acids (carbonyl at C3) decarboxylate readily because a six-membered cyclic transition state lets a proton transfer to the β-carbonyl oxygen as $\ce{CO2}$ leaves, giving a ketone (via enol) rather than an unstable carbanion. Malonic acid ($\ce{HOOCCH2COOH}$, a β-diacid) decarboxylates the same way on heating because the second $\ce{-COOH}$ mimics the β-carbonyl.

Passage-level

Examples: acetoacetic acid → acetone + $\ce{CO2}$ (the exhaled acetone of ketoacidosis); oxaloacetate → pyruvate + $\ce{CO2}$.

Biological Decarboxylations

Passage-level

Many metabolic steps lose $\ce{CO2}$ — pyruvate → acetyl-CoA (PDH), isocitrate → α-ketoglutarate and α-ketoglutarate → succinyl-CoA (TCA), and amino acids → biogenic amines (histidine → histamine; DOPA → dopamine).

Quick check: Would you expect malonic acid ($\ce{HOOCCH2COOH}$) to decarboxylate more easily or less easily than acetic acid ($\ce{CH3COOH}$)? Why?

Answer: More easily. With two carboxyls on the central carbon, when one leaves as $\ce{CO2}$ the other accepts the proton through the 6-membered cyclic transition state — the β-keto-acid mechanism. Acetic acid has no β-activating group and does not decarboxylate.

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Reactions at the Alpha Position (α-Substitution)

Must know

The α-carbon (C2) bears slightly acidic hydrogens (inductive withdrawal by the carbonyl), enabling α-substitution.

Hell–Volhard–Zelinsky (HVZ) Reaction

Must know

The Hell–Volhard–Zelinsky reaction is α-halogenation of a carboxylic acid with $\ce{Br2}$ (or $\ce{Cl2}$) and catalytic phosphorus, placing the halogen specifically at the α-carbon:

$\ce{RCH2COOH ->[Br2, P (cat.)] RCHBrCOOH + HBr}$

Optional

Phosphorus converts the acid to an acyl halide, which enolizes and halogenates at the α-position; hydrolysis gives the α-haloacid.

Why α-Substitution Matters

Know the logic

α-halocarboxylic acids are useful intermediates — the halide is a good leaving group, so $\text{S}_\text{N}2$ by nucleophiles ($\ce{-OH}$, $\ce{-NH2}$, $\ce{-CN}$) builds α-substituted acids, including α-amino acids (all protein amino acids are α-amino acids).

Quick check: In the HVZ reaction, why is phosphorus necessary? Can you simply mix $\ce{Br2}$ with a carboxylic acid and get α-bromination?

Answer: No. A carboxylic acid's enol content is too low for direct α-halogenation. Phosphorus converts it in situ to an acyl bromide, which enolizes readily (more electrophilic carbonyl) and brominates at the α-position; hydrolysis then gives the α-bromocarboxylic acid.

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Common Confusions & Tricks

1. NaBH4 vs. LiAlH4 for acids.
The #1 trap. NaBH4 does NOT reduce carboxylic acids (only aldehydes/ketones). LiAlH4 reduces everything: acids, esters, amides, ketones, aldehydes.

2. Ester vs. ether.
An ester has a carbonyl next to the oxygen ($\ce{-COO-}$); an ether is just $\ce{-O-}$. A lactone is a cyclic ester, not a cyclic ether.

3. Decarboxylation requires a β-activating group.
Not every acid decarboxylates on heating — you need a β-carbonyl (ketone or carboxyl at C3). "Readily loses CO₂ on heating" = look for the β-keto acid or malonic pattern.

4. Directionality of the acyl substitution product.
In Fischer esterification the carbonyl oxygen of the acid is retained; the acid's $\ce{-OH}$ leaves as part of water, and the alcohol oxygen becomes the bridging ester oxygen — the basis of isotope-labeling questions.

5. Acidity order trap.
Carboxylic acids (pKa ≈ 4–5) ≫ phenols (≈ 10) ≫ alcohols (≈ 16–18). Asked for "most acidic," the carboxylic acid wins decisively.

6. Lactam vs. lactone.
Lactam = cyclic amide (nitrogen in ring); lactone = cyclic ester (oxygen in ring). Mnemonic: lact-am contains N (amide).

7. Fischer esterification is reversible; acid chloride esterification is not.
Fischer needs excess alcohol or water removal; acid chloride + alcohol is irreversible ($\ce{HCl}$ leaves readily) and used for complete conversion.

8. Hell-Volhard-Zelinsky is specifically for carboxylic acids.
"Br₂ + P" → HVZ → α-bromocarboxylic acid. (α-Halogenation of ketones is a different acid/base enol mechanism.)

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Key Equations

Equation	Variables / When to Use
$\ce{RCOOH <=> RCOO- + H+}, \quad K_a = \frac{[\ce{RCOO-}][\ce{H+}]}{[\ce{RCOOH}]}$	Acid dissociation; larger $K_a$ = stronger acid = lower pKa.
$\text{pH} = \text{pK}_a + \log\dfrac{[\ce{RCOO-}]}{[\ce{RCOOH}]}$	Henderson-Hasselbalch; protonation-state / buffer reasoning at a given pH.
$\text{pK}_a = -\log K_a$	Converts $K_a$ to pKa. Higher pKa = weaker acid.
$\ce{RCOOH + R'OH <=>[H+, \Delta] RCOOR' + H2O}$	Fischer esterification; equilibrium, driven by excess alcohol or water removal.
$\ce{RCOOH ->[1.~LiAlH4,~THF][2.~H3O+] RCH2OH}$	Reduction to primary alcohol; LiAlH4, not NaBH4.
$\ce{RCH2COOH ->[Br2,~P] RCHBrCOOH + HBr}$	Hell-Volhard-Zelinsky α-bromination.
$\ce{\beta\text{-keto acid} ->[\Delta] \text{ketone} + CO2}$	Decarboxylation; needs β-keto/β-carboxyl group; 6-membered cyclic TS.
Reactivity: $\text{Acid chloride} > \text{Anhydride} > \text{Ester} \approx \text{Acid} > \text{Amide}$	Nucleophilic acyl substitution order; by leaving-group ability.