Orgo 1 foundation · U.S. undergraduate · en-US
Organic Chemistry Acid–Base & pKa: How to Pick the Proton, Conjugate Base & What Reacts First (Exam Guide)
Acid–base chemistry is the hidden engine behind substitution, elimination, and carbonyl reactions. Exams do not always label a problem “pKa” — they show two reagents and ask which proton is removed or which site is deprotonated. This guide gives one exam workflow: most acidic H → conjugate base → what reacts first. It bridges general chemistry pKa to orgo 1 mechanisms without re-teaching SN1 vs SN2; use the linked hubs when you know the proton story.
Quick answer — organic chemistry acid–base & pKa
On organic chemistry exams, find the most acidic proton by comparing pKa values for the functional groups present (lower pKa = more acidic). Deprotonation gives the conjugate base that actually reacts — often an alkoxide, carboxylate, enolate, or amide anion. Stronger bases (higher pKa of conjugate acid) can remove protons that weaker bases cannot. After you identify the conjugate base, decide nucleophile vs base: bulky or strong base at high temperature favors elimination; good nucleophile in polar aprotic solvent favors SN2. Carbonyl α hydrogens (pKa ~16–25) matter for enolate and aldol logic in orgo 2.
Why acid–base shows up everywhere in orgo
Substitution and elimination start with a base removing a proton or attacking a carbon. Carbonyl chemistry uses acids and bases to protonate oxygens, form enolates, and drive acyl substitution. If you skip pKa reasoning, you guess mechanisms from reagent names alone.
General chemistry taught you Ka, pKa, and conjugate pairs. Organic chemistry asks you to apply those ideas on molecules with several different proton environments in one structure.
- Orgo 1: alkyl halides + base/nucleophile, alcohols, terminal alkynes, carboxylic acids in acid–base prep.
- Orgo 2: α hydrogens on aldehydes/ketones, ester enolates, amide NH acidity vs carbonyl reactivity.
- MCAT: protonation state of amino acid side chains and simple acid–base equilibria in passages.
pKa trends you should recognize (not memorize every number)
Patterns beat flashcard lists. Carboxylic acids and sulfonic acids are the most acidic common organic acids; alcohols and water are middling; alkanes are very weak acids. Alkyne terminal C–H is unusually acidic for an sp carbon. α hydrogens next to carbonyls are much more acidic than typical alkane C–H because the conjugate base is resonance-stabilized.
When two protons compete, compare their functional group class first, then resonance (phenol vs alcohol), then hybridization (sp vs sp³), then electronegativity (allylic/benzylic stabilization).
From conjugate base to mechanism family
The conjugate base you draw is the species that attacks or eliminates. tert-Butoxide as base → think E2 with bulky base. Methoxide in protic solvent on a tertiary halide → think SN1/E1 competition. Enolate from a ketone → carbonyl chemistry, not SN2 on the halide.
Link forward: after acid–base setup, use the SN1/SN2 and E1/E2 hubs for pathway choice and the carbonyls hub when the nucleophile is an enolate or alkoxide adding to C=O.
Exam workflow: most acidic H → conjugate base → what reacts first
Run every acid–base step through this order before you draw curved arrows for the full mechanism.
List every acidic proton environment
Mark O–H, N–H, terminal alkyne C–H, carboxylic acid O–H, α-to-carbonyl C–H, and ordinary alkyl C–H. Ignore vinylic/aromatic C–H for deprotonation unless the problem states strong base and specific conditions.
Compare pKa — pick the most acidic proton
Lower pKa wins. Use the table below for typical ranges. If two sites are close, check whether the problem gives a weak base (only the strongest acid deprotonates) or a strong base like LDA or NaNH₂.
Draw the conjugate base
Show the bond that broke and the charge on the atom that held the proton (O⁻, N⁻, carbanion on enolate, acetylide anion). Resonance structures explain why enolates and carboxylates are stable.
Ask: nucleophile, base, or both?
A species can be both. Bulky base → elimination bias. Strong, unhindered nucleophile in polar aprotic solvent → SN2 bias. Enolates act as nucleophiles at α carbon toward electrophiles (including carbonyls).
Connect to the mechanism hub for the reaction family
Substitution/elimination: SN1/SN2/E1/E2 hub. Carbonyl addition or acyl substitution: carbonyls or carboxylic acid derivatives hub. Do not mix pathways — the acid–base step tells you which anion starts the mechanism.
Organic chemistry pKa comparison table (typical undergraduate ranges)
Approximate values for exam reasoning; your textbook may list slightly different numbers. Lower pKa = stronger acid = easier to deprotonate.
| Functional group / proton | Approx. pKa | Notes |
|---|---|---|
| Alkane C–H | ~50 | Very weak acid; needs very strong base (e.g. NaNH₂ for terminal alkyne only — not alkane) |
| Alkene C–H (allylic/benzylic) | ~43–25 | Allylic/benzylic more acidic than ordinary alkyl; resonance stabilizes conjugate base |
| Terminal alkyne C–H | ~25 | Deprotonated by NaNH₂, BuLi — acetylide anion |
| α-H (aldehyde/ketone) | ~16–20 | Enolate formation; LDA, NaOEt, etc. |
| Ester / nitrile α-H | ~25–30 | Harder to enolize than aldehydes/ketones |
| Alcohol O–H | ~16–18 | Alkoxides from ROH + NaH or Na metal |
| Water | ~15.7 | Reference point; hydroxide as base |
| Phenol O–H | ~10 | Resonance-stabilized phenoxide |
| Carboxylic acid O–H | ~4–5 | Carboxylate conjugate base; weaker bases do not fully deprotonate |
| Sulfonic acid O–H | ~-2 to 0 | Very strong organic acid; rare on intro exams |
Acid–base trap answers on orgo exams
Deprotonating the wrong oxygen or carbon on a multi-functional molecule
Rank all acidic sites with pKa before drawing arrows. Carboxylic acid beats alcohol; α-H beats alkyl C–H only when base is strong enough.
Calling every strong base an SN2 nucleophile
NH₂⁻, RO⁻, and OH⁻ can eliminate. Tertiary substrates with heat often favor E1/E2 regardless of your intended substitution.
Forgetting resonance-stabilized conjugate bases
Enolates and carboxylates are delocalized — the charge drawing matters for where attack occurs next.
Using pKa numbers without units or context
Exams test relative acidity (which proton first), not reciting 18.2 vs 19.2 from memory unless a table is provided.
Acid–base ideas that unlock mechanism questions
Master these links before midterms; each points to a mechanism hub for the next step.
Skill: Equilibrium
Weaker base deprotonates only the stronger acid
If pKa(acid) < pKa(conjugate acid of base), deprotonation is favorable. A problem with NaOH and a carboxylic acid goes to carboxylate, not alkoxide from an alcohol on the same molecule if the acid is far more acidic.
Skill: Orgo 1
Alkoxide and hydroxide as bases in substitution
RO⁻ and OH⁻ can be nucleophiles and bases. Substrate and solvent decide SN2 vs E2 — not the label on the flashcard.
Skill: Elimination
Bulky base → Hofmann / E2 on hindered systems
t-BuOK deprotonates less hindered β protons when Zaitsev product is sterically blocked.
Skill: C–C bond formation
Terminal alkyne + NaNH₂ → acetylide nucleophile
After deprotonation, acetylide attacks primary alkyl halides (SN2) or carbonyls — know pKa ~25 justifies the anion.
Skill: Orgo 2 / carbonyls
α deprotonation → enolate chemistry
The conjugate base is resonance-stabilized; enolates add to carbonyls or participate in aldol-type logic.
Skill: Mechanism setup
Protonation of carbonyl oxygen (acid catalysis)
Acid increases electrophilicity at carbonyl carbon — different step than enolate formation, but still acid–base.
One-week acid–base & pKa study plan
Use the one-week midterm study guide if your exam is sooner and acid–base is only one unit among several.
Days 1–2 — pKa ranking
Compare functional groups
- Memorize the order: carboxylic acid > phenol > alcohol/water > terminal alkyne > α-carbonyl > alkane.
- Ten problems: circle the most acidic H on each structure without mechanisms.
Days 3–4 — Conjugate bases
Draw the anion that forms
- For each structure, show deprotonation product with correct charge and one resonance form where relevant.
- Link five problems to SN1/SN2/E1/E2 hub practice prompts.
Days 5–7 — Mixed acid–base + mechanisms
Full exam stems
- Mixed set: acid–base setup then predict major product (use predict-major-product blog workflow).
- Pair with mechanism arrow-pushing study guide for curved-arrow grading format.
Acid–base exam checklist
- I can rank O–H, N–H, terminal alkyne C–H, α-carbonyl C–H, and alkyl C–H by typical pKa.
- I draw the conjugate base with correct charge and key resonance.
- I decide whether the reacting species behaves mainly as base or nucleophile under the stated conditions.
- I connect deprotonation to the correct mechanism hub (substitution, elimination, or carbonyl).
- I do not start curved arrows until the acid–base step is identified.
Mechanism hubs after acid–base setup
- SN1 vs SN2 — SN1 mechanism vs SN2
- E1 vs E2 — E1 vs E2
- Carbonyl chemistry — aldol mechanism
- Grignard mechanism — grignard mechanism
Organic chemistry pKa & acid–base FAQ
- What is pKa in organic chemistry?
- pKa measures acid strength: lower pKa means a stronger acid and a proton that is easier to remove. On exams you use pKa to decide which proton deprotonates first when a base is present, and to predict whether an acid–base equilibrium favors products or reactants.
- Which proton is most acidic in organic chemistry?
- Compare functional groups: carboxylic acid protons are most acidic among common organic acids, then phenol, then alcohols and water, then terminal alkynes and α hydrogens next to carbonyls, then ordinary alkyl C–H. Resonance and hybridization can shift ranking within a class — always list all acidic sites on the molecule before choosing.
- How does acid–base relate to SN1 and SN2?
- Bases and nucleophiles often remove protons or attack carbon after deprotonation. The conjugate base’s strength and bulk help decide substitution vs elimination. Acid–base does not replace the SN1/SN2 decision tree — it sets up which anion starts the mechanism.
- What is the difference between a nucleophile and a base?
- A base removes a proton (forms a new bond to H). A nucleophile donates electrons to an electrophilic atom (often carbon). Many species do both — RO⁻ can deprotonate or attack an alkyl halide. Exam conditions (substrate, solvent, temperature) determine which pathway dominates.
- Why are α hydrogens on ketones more acidic?
- Deprotonation gives an enolate resonance-stabilized between oxygen and carbon. That stabilization lowers the effective pKa compared with alkane C–H, enabling enolate formation with bases like LDA or alkoxides in orgo 2 carbonyl chemistry.
- Do I need to memorize every pKa value for orgo?
- Memorize relative order and typical ranges (carboxylic acid ~4–5, alcohol ~16–18, terminal alkyne ~25, α-ketone ~16–20, alkane ~50). Exams usually test which site reacts first, not exact decimals unless a table is provided.
AAMC & undergraduate orgo references (external)
Open textbooks and reference sites for the same topics covered above. We link out to established science education publishers — not affiliate pages.
- Organic Chemistry (OpenStax) — OpenStax
- Organic Chemistry LibreTexts — LibreTexts
- MIT OpenCourseWare — Organic Chemistry I — MIT OCW
- PubChem — NIH NLM
Continue on this site
Practice mechanisms after you pick the right proton
Once you know which proton leaves and which base attacks, predict products and order curved arrows in OrgoMech — Reaction Predictor and Daily Drill use the same predict-then-mechanism workflow as your exams.