Organic Chemistry · Polar Covalent Bonds; Acids and Bases

Organic Acids and Organic Bases

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pKa values (acetic acid 4.76, phenol ~10, ethanol ~16, anilinium 4.6, methylammonium 10.6, chloroacetic acid 2.87, glycine 2.3/9.6) are standard textbook reference values; verify against current primary sources before formal citation.

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
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In 30 seconds

An is a carbon-containing compound that can donate a proton, and an is a carbon-containing compound that can accept a proton — but the two classes are far more diverse than the simple definitions suggest. The most important organic acids are the carboxylic acids (general formula RCOOH, pKa ≈ 4–5), phenols (pKa ≈ 10), and alcohols (pKa ≈ 16–18). The most important organic bases are the amines (general formula R₃N, whose conjugate acids have pKa ≈ 10–11). What makes organic acids and bases interesting is that their strength is tunable: structural features such as electronegativity, resonance, hybridization, and inductive effects change pKa values by many orders of magnitude. Understanding why acetic acid is about 10¹¹ times more acidic than ethanol — both contain an O–H bond — is the payoff of this topic. This chapter section also introduces acidic and basic sites within the same molecule, such as amino acids, which carry both a and an amine group.

Why this matters

Drug chemistry: most drugs are weak acids or weak bases, and their ionization state (set by pKa and pH) determines whether they dissolve in water, cross membranes, or bind to receptors. Aspirin (acid, pKa 3.5), ibuprofen (acid, pKa 4.9), and many antidepressants (amines) are classic examples.
Enzyme and metabolic biochemistry: the amino acid side chains — carboxylates, ammonium groups, imidazole of histidine, phenol of tyrosine — do the acid–base work inside enzyme active sites.
Synthesis: protonating an alcohol converts it into a good leaving group (ROH₂⁺); deprotonating an alkyne or amine generates a powerful nucleophile. Acid–base chemistry is the first step of countless mechanisms.
Everyday science: why acetic acid in vinegar is a weak acid while HCl is strong, why amine salts (like ammonium chloride) are water-soluble solids, and why soap works better in basic conditions.

The college version

Core Concepts

The three major families of organic acids

Family	Example structure	Typical pKa	Why the conjugate base is stabilized
Carboxylic acid	CH₃COOH (acetic acid)	4.76	Resonance: the carboxylate ion CH₃COO^- delocalizes the negative charge over two equivalent oxygens
Phenol	C₆H₅OH	~10	Resonance into the ring: the phenoxide ion delocalizes charge onto ortho/para ring carbons
Alcohol	CH₃CH₂OH (ethanol)	~16	No resonance; the alkoxide ion RO^- carries the full negative charge on one oxygen

The pattern to internalize: the more the spreads out its negative charge, the more stable it is, and the more acidic the parent acid. Acetic acid's conjugate base shares charge between two oxygens (resonance), so acetic acid is far more acidic than ethanol, whose conjugate base localizes charge on a single oxygen.

What makes a base strong: the conjugate acid test

Basicity is measured by the pKa of the conjugate acid. The higher the pKa of BH⁺, the stronger the base B. Amines have conjugate acids with pKa ≈ 10–11 (CH₃NH₃⁺, pKa 10.6), so alkylamines are strong organic bases. Structural changes that stabilize the conjugate acid (or destabilize the lone pair) raise basicity; changes that delocalize the lone pair lower it. Aniline (C₆H₅NH₂, conjugate acid pKa 4.6) is a much weaker base than cyclohexylamine (conjugate acid pKa 10.6) because the nitrogen lone pair is delocalized into the aromatic ring.

Factors that tune acid and base strength

Electronegativity: across a row, more electronegative atoms hold negative charge better — HF > H₂O > NH₃ > CH₄ in acidity.
Resonance: stabilizes conjugate bases (carboxylate, phenoxide) and destabilizes lone pairs (aniline, amides) — the single largest effect in organic acids.
Inductive effects: electron-withdrawing groups (Cl, F, NO₂) near the acidic site stabilize the conjugate base and raise acidity. Chloroacetic acid (pKa 2.87) is stronger than acetic acid (4.76); trifluoroacetic acid (pKa 0.5) is nearly a strong acid.
Hybridization: the more in the orbital holding the lone pair or the C–H bond, the more electronegative the atom and the more acidic the proton. Acetylene (sp, pKa 25) is far more acidic than ethylene (sp², pKa 44), which is more acidic than ethane (sp³, pKa 50).
Solvation: in water, small conjugate bases are stabilized by hydrogen bonding with solvent; this is why F^- is not as basic as size trends predict in water.

Amino acids: acids and bases in one molecule

Glycine, H₂N-CH₂-COOH, contains both a basic amine and an acidic carboxylic acid. In neutral water it exists as a , ⁺H₃N-CH₂-COO^-, with the proton transferred from the carboxyl group to the amine. The carboxyl group donates its proton (pKa ≈ 2.3) and the ammonium group holds it (pKa ≈ 9.6), so amino acids are buffered over a wide pH range — the reason proteins and peptides resist pH change in the body.

Common Confusions

Do Not Confuse	With	Difference
Carboxylic acids and alcohols	Both have O–H bonds	Carboxylates benefit from two-oxygen resonance (pKa 4–5); alkoxides do not (pKa 16–18). ~10¹¹ difference.
Basicity of the amine	pKa of the amine	Amines are bases; their strength is quoted as the pKa of the conjugate acid (e.g., CH₃NH₃⁺, 10.6). Higher BH⁺ pKa = stronger base.
Aniline basicity	Alkylamine basicity	Aniline's lone pair is ring-delocalized (BH⁺ pKa 4.6); alkylamines' lone pairs are localized (BH⁺ pKa 10–11).
Inductive vs resonance effects	Both affect acidity	Inductive effects act through sigma bonds and fall off with distance; resonance acts through π systems and is usually much larger.
Alkyne C–H acidity	Alkane C–H acidity	sp-hybridized carbon is more electronegative: acetylene pKa 25 vs ethane pKa ~50.
"Organic acid" = carboxylic acid only	Other organic acids	Phenols, alcohols, thiols, and terminal alkynes are also organic acids, just weaker ones.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Some molecules want to give away a hydrogen atom (acids), and some want to grab one (bases). Organic acids are like people with an extra umbrella they're happy to lend — but only if letting go makes them feel more stable, like spreading the "crowd" of electrons over more space. Carboxylic acids spread the crowd over two oxygens, so they lend easily; alcohols can't spread it, so they hold on tight. Amines are the borrowers: they love to grab a hydrogen because it makes them positively charged and stable.

Worked example

Example 1: Why is acetic acid so much stronger than ethanol?

Problem: Rank CH₃COOH (pKa 4.76), C₆H₅OH (pKa 10), and CH₃CH₂OH (pKa 16) in order of acidity and explain the gaps.

Plan and formula: acidity follows conjugate-base stability, which follows the number of resonance forms and the electronegativity of the atom carrying the charge. Count the resonance stabilization of each conjugate base:

Acetate, CH₃COO^-: the negative charge is shared equally between two oxygen atoms (two equivalent resonance forms). Strongest stabilization → most acidic.
Phenoxide, C₆H₅O^-: charge delocalizes onto the oxygen and onto ortho/para ring carbons. Intermediate stabilization → intermediate acidity.
Ethoxide, CH₃CH₂O^-: no resonance; the charge sits entirely on one oxygen. Weakest stabilization → least acidic.

Answer: CH₃COOH > C₆H₅OH > CH₃CH₂OH. Quantitatively, acetic acid is 10^{16 - 4.76} ≈ 10¹¹ times more acidic than ethanol — an enormous difference driven entirely by resonance.

Example 2: Which is the stronger base — aniline or cyclohexylamine?

Problem: Compare the basicity of aniline, C₆H₅NH₂, and cyclohexylamine, C₆H₁₁NH₂. Conjugate acid pKas: C₆H₅NH₃⁺ = 4.6; C₆H₁₁NH₃⁺ = 10.6.

Plan and formula: base strength is proportional to conjugate-acid stability, i.e., the pKa of BH⁺. Higher pK_a(BH⁺) = stronger base. Compare the two values:

$p K a (C 6 H 11 NH 3 +) = 10.6 > p K a (C 6 H 5 NH 3 +) = 4.6$

Explanation: in aniline, the nitrogen lone pair is delocalized into the aromatic ring (resonance with the π system), so it is less available for protonation; the conjugate acid C₆H₅NH₃⁺ is correspondingly less stabilized. In cyclohexylamine the lone pair is localized on nitrogen, fully available, and the ammonium ion is well stabilized by solvation. Answer: cyclohexylamine is the stronger base by about 10⁶ — aniline is roughly 10⁶ times weaker than a typical alkylamine.

Example 3: Which amine is a stronger base — ammonia or methylamine?

Problem: Compare NH₃ (pK_a(NH₄⁺) = 9.24) with CH₃NH₂ (pK_a(CH₃NH₃⁺) = 10.6).

Plan and formula: again compare conjugate-acid pKas:

$p K a (CH 3 NH 3 +) = 10.6 > p K a (NH 4 +) = 9.24$

Explanation: the methyl group is an electron-donating group (inductive effect), which stabilizes the positive charge on the protonated amine CH₃NH₃⁺. More stabilized conjugate acid → stronger base. Answer: methylamine is the stronger base — roughly 10^1.4 ≈ 25 times stronger than ammonia. This is why alkylamines, not ammonia, are usually the bases of choice in organic synthesis.

Key takeaways

Carboxylic acids: pKa ≈ 4–5 (resonance-stabilized carboxylate). Phenols: pKa ≈ 10 (ring resonance). Alcohols: pKa ≈ 16–18 (no resonance).
Basicity is judged by the conjugate acid's pKa: higher pK_a(BH⁺) = stronger base. Alkylamine conjugate acids pKa ≈ 10–11; anilinium pKa ≈ 4.6.
Resonance stabilizes conjugate bases → more acidic parent; resonance on the base itself → less basic (aniline, amides, pyrrole).
Inductive electron withdrawal raises acidity: ClCH₂COOH (2.87) > CH₃COOH (4.76); CF₃COOH ≈ 0.5.
Hybridization: more s-character, more acidic — sp alkyne (25) > sp² alkene (44) > sp³ alkane (50).
Amino acids are zwitterions in neutral water: ⁺H₃N-CH₂-COO^-. The carboxyl pKa ≈ 2.3, ammonium pKa ≈ 9.6.
Protonated alcohols/ethers (pK_a ≈ -2 to −3) are excellent leaving groups — acid catalysis converts ROH into ROH₂⁺.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

Rank the acidity of CH₃COOH, CH₃CH₂OH, and C₆H₅OH and give the structural reason for the ranking.
Show answer
CH₃COOH (4.76) > C₆H₅OH (10) > CH₃CH₂OH (16). Acetate has two-oxygen resonance; phenoxide has ring resonance; ethoxide has no resonance.
Why does substituting chlorine for hydrogen on the methyl group of acetic acid (giving chloroacetic acid, pKa 2.87) make the acid stronger?
Show answer
Chlorine is electron-withdrawing (inductive effect). It pulls electron density away from the carboxylate ion, stabilizing the negative charge, which raises acidity. Two chlorines (dichloroacetic acid) are stronger still, and three (trichloroacetic acid) approach strong-acid behavior.
Which is the stronger base: pyridine (conjugate acid pKa 5.2) or triethylamine (conjugate acid pKa 10.7)? By roughly how much?
Show answer
Triethylamine is stronger. ΔpK_a = 10.7 - 5.2 = 5.5, so triethylamine is about 10^5.5 ≈ 3 × 10⁵ times stronger. Alkyl groups donate electron density that stabilizes the conjugate acid.
Draw (describe) the zwitterion form of glycine and state which group holds the positive charge and which holds the negative charge at neutral pH.
Show answer
⁺H₃N-CH₂-COO^-: the protonated amino group (-NH₃⁺) carries the positive charge and the carboxylate (-COO^-) carries the negative charge.
Why is HC ≡ CH (pKa 25) so much more acidic than CH₃CH₃ (pKa ~50)?
Show answer
Hybridization: the sp carbon of acetylene is more electronegative than the sp³ carbon of ethane because the sp orbital has more s-character (50% vs 25%), so the C–H bond is more polar and the proton is more readily lost.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Organic acid: A carbon-containing proton donor (RCOOH, ArOH, ROH, RC≡CH, RNH–)
Organic base: A carbon-containing proton acceptor, typically an amine R₃N
Carboxylic acid: RCOOH; the archetypal organic acid, pKa ≈ 4–5
Conjugate base: What remains after an acid loses a proton
Zwitterion: A neutral molecule with both + and − charges
Inductive effect: Electron withdrawal/push through sigma bonds by nearby atoms
s-character: Fraction of s orbital in a hybrid (sp > sp² > sp³)
Amine salt: R₃NH⁺ X^-, formed by protonating an amine

Sources & references

openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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In 30 seconds

Why this matters

The college version

Core Concepts

The three major families of organic acids

What makes a base strong: the conjugate acid test

Factors that tune acid and base strength

Amino acids: acids and bases in one molecule

Common Confusions

The same idea, in plain words

Worked example

Example 1: Why is acetic acid so much stronger than ethanol?

Example 2: Which is the stronger base — aniline or cyclohexylamine?

Example 3: Which amine is a stronger base — ammonia or methylamine?

Key takeaways

Check yourself

Keep learning

Key vocabulary

Related lessons

Sources & references