MCAT Organic Chemistry Reactions, IR, and NMR Reference Sheet

Key takeaways

• Organic chemistry is 15% of Chem/Phys and 5% of Bio/Biochem, which works out to roughly 12 questions on a 230-question exam.
• The exam rewards recognizing functional-group reactivity and reading spectra. It does not ask you to plan a multi-step synthesis.
• There is no reagent sheet and no calculator on test day, so the short list below is what you carry in.
• Two orderings decide most questions: carbonyl electrophilicity and leaving-group ability, and both come from the same electron-density logic.
• Spectroscopy items are answerable from tabulated data plus reasoning. You are never asked to draw a structure.

If you want one page of MCAT organic chemistry reactions to review the night before a full-length, this is it. Below are the mechanisms the AAMC reuses, the IR bands and proton NMR shifts that turn up as passage data, and the two reactivity orderings that settle most questions before you reach the answer choices.

It is much shorter than your organic chemistry course was, and that is the point. The MCAT tests first-semester organic chemistry, and it tests it the way a physician uses it: as the chemical logic underneath a lab workflow or an enzyme mechanism.

How the MCAT Actually Tests Organic Chemistry

Organic chemistry appears in two of the four scored sections.

Discipline percentages are from the AAMC Chemical and Physical Foundations of Biological Systems section overview and the AAMC Biological and Biochemical Foundations of Living Systems section overview, verified August 28, 2026. AAMC notes these percentages are approximated to the nearest 5% and vary between test forms.

So about 12 questions total. What matters more than the count is what those 12 questions ask. Three patterns cover almost all of them:

1. Recognize the functional group and predict what it does. Which carbon is electrophilic, what attacks it, and what leaves. A question that names a reagent and a substrate is asking you to place both in a mechanism family, not to recall a named reaction.
2. Read spectra as constraints. IR and ¹H NMR show up as passage data you interpret, not as standalone recall. Mass spectrometry appears mostly as a molecular ion.
3. Compare, do not calculate. Rank two acids, decide whether one base can deprotonate another, assign R or S, or say which of two carbonyls reacts faster.

What the MCAT does not do is hand you a target molecule and ask for a synthesis. Retrosynthesis, protecting-group strategy beyond acetals, aromatic substitution mechanisms, and pericyclic reactions are second-semester topics that the AAMC content outlines leave out. If you are pushing arrows through four-step sequences, you are studying for the wrong exam.

MCAT Organic Chemistry Reactions Sheet

No reagent list is provided on test day and no calculator is permitted. These are the reactions the AAMC reuses, grouped by the mechanism that drives them.

Substitution and Elimination

Oxidation and Reduction

Nucleophilic Addition to the Carbonyl

Enol, Enolate, and Alpha-Carbon Chemistry

Carboxylic Acid Derivatives (Nucleophilic Acyl Substitution)

Reactivity order of the derivatives — memorize this ranking, since it decides which interconversions are possible:

acid chloride greater than anhydride greater than ester ≈ carboxylic acid greater than amide greater than carboxylate

A more reactive derivative converts to a less reactive one directly. Going the other direction requires an activating step. The ranking tracks leaving-group ability and how strongly the attached heteroatom donates electron density back into the carbonyl.

Functional Group Reactivity: The Two Orderings That Decide Everything

Most organic questions on the MCAT are comparisons, and almost every comparison reduces to one of two rankings.

Which Carbonyls Are Most Electrophilic

A carbonyl carbon is electrophilic because oxygen pulls electron density off it. Anything that gives that density back makes the carbon less positive and less reactive. Two effects do the giving:

• Resonance donation from the attached heteroatom. A lone pair on the atom bonded to the carbonyl carbon delocalizes into the C=O. Nitrogen donates strongly, oxygen moderately, chlorine barely at all, since its lone pairs sit in a 3p orbital that overlaps poorly with carbon's 2p. This is the dominant term, and it is why an amide is the least reactive derivative and an acid chloride the most.
• Donation from alkyl groups. Alkyl substituents push a little density toward the carbonyl carbon and get in the way sterically. A ketone has two, an aldehyde has one, so aldehydes are more electrophilic than ketones.

Together they give the ordering in the acyl substitution table above: acid chloride, anhydride, ester ≈ carboxylic acid, amide, carboxylate. That series ranks the derivatives for nucleophilic acyl substitution, where the nucleophile adds and a leaving group departs. Aldehydes and ketones sit outside it because they have no leaving group and cannot substitute at all. For pure addition, the order is acid chloride, anhydride, aldehyde, ketone, ester, amide. Same logic, both times.

A carboxylate is not electrophilic at all: its charge is delocalized over both oxygens. That is exactly why saponification is irreversible.

Leaving-Group Ability

A good leaving group is a weak base, meaning it holds a negative charge comfortably. The proxy is the pKa of its conjugate acid, and lower is better.

Set that next to the acyl reactivity series and you will see they are the same list. Chloride leaves easily, so acid chlorides are reactive. Alkoxide leaves reluctantly, so esters are sluggish. An amide nitrogen is essentially never expelled as an anion, which is why peptide bonds survive in water for years. One honest exception: fluoride looks respectable by pKa (HF is about 3.2) but leaves poorly, because C–F is the strongest single bond carbon makes. The proxy is a heuristic, not a law.

This also explains why hydroxide never departs a carboxylic acid directly. To esterify an acid you protonate the OH first, turning a terrible leaving group into water. Every acid-catalyzed carbonyl mechanism is running some version of that trick.

The pKa Comparisons the Exam Reuses

You are almost never asked for an absolute value. You are asked whether one species can deprotonate another, and the reaction proceeds if the acid being deprotonated is stronger than the conjugate acid that forms.

The chloroacetic series shows the inductive effect adding up with each halogen. The basicity ordering, alkylamine then ammonia then aniline then amide, is one of the AAMC's favorite comparisons because it lets a passage ask about a protein side chain without naming a mechanism.

Treat the amide entry as a range rather than a figure. Published values for the conjugate acid of acetamide run from about −8 for the nitrogen-protonated form to roughly 0 for the oxygen-protonated form, and textbooks quote one or the other without always saying which. The qualitative conclusion is what gets tested and it is not in doubt: an amide nitrogen is a far weaker base than an amine nitrogen, because its lone pair is delocalized into the carbonyl. The dichloroacetic acid value is similarly approximate.

MCAT IR Absorption Reference

The exam almost never asks you to interpret a full IR spectrum. It hands you one or two bands and asks what they rule in or out. Intensity and shape matter as much as position.

Two patterns explain nearly every carbonyl question. Conjugation lowers the C=O frequency by roughly 20–40 cm⁻¹, because delocalization gives the bond partial single-bond character — an α,β-unsaturated ketone sits near 1680 rather than 1715. Ring strain raises it, which is why a cyclopentanone absorbs near 1745 and a cyclobutanone near 1780.

The carbonyl region is drawn on its own scale because the exam separates those bands by as little as 15 wavenumbers.

MCAT ¹H NMR Chemical Shift Reference

Chemical shift is set by shielding: electron density near a proton shields it and moves the signal upfield toward 0 ppm, while nearby electronegative atoms or π systems deshield it and move it downfield.

Three rules turn a shift table into answers:

1. Integration counts protons. The area under a signal is proportional to the number of hydrogens producing it, giving you a ratio, not an absolute count.
2. Multiplicity counts neighbors. The n + 1 rule: a signal split into six lines has five protons on adjacent carbons. Forgetting the plus one is the most common NMR error on the exam.
3. Exchangeable protons do not split their neighbors and vanish on a D₂O shake. O–H and N–H protons exchange too fast to couple, which is why a carboxylic acid proton appears as a broad singlet rather than a multiplet.

Worked Example: From a Formula, One IR Band, and a Splitting Pattern to a Structure

This is the shape an MCAT spectroscopy item takes: a molecular formula, one or two IR bands, a short NMR table, four candidate structures. Here is the order to attack it in.

The data. An unknown liquid has molecular formula C₄H₈O. Its IR shows a strong band at 1715 cm⁻¹, nothing between 3100 and 3600 cm⁻¹, and no bands between 2700 and 2900 cm⁻¹ beyond ordinary alkyl C–H. Its ¹H NMR shows δ 2.45 (2H, quartet), δ 2.13 (3H, singlet), δ 1.05 (3H, triplet).

Step 1. Count degrees of unsaturation. For C₄H₈O that is (2 × 4 + 2 − 8) / 2 = 1. Oxygen does not enter the formula. One degree means exactly one ring or one π bond and nothing more.

Step 2. Spend the degree using the IR. A strong band at 1715 cm⁻¹ is in the ketone window. A clean 3100–3600 region rules out an alcohol (3200–3600, broad), and the absence of a very broad 2500–3300 envelope rules out a carboxylic acid. No 2720/2820 pair rules out an aldehyde. So the single degree of unsaturation is a ketone carbonyl, not a ring and not an alkene.

Step 3. Check the integration closes. The 2:3:3 ratio sums to 8 protons, matching C₄H₈O exactly. Every hydrogen is accounted for, so there is no hidden exchangeable proton, which independently confirms there is no O–H.

Step 4. Read the splitting for connectivity. A 2H quartet needs three neighbors and a 3H triplet needs two, so those two are a CH₃CH₂ group coupled to each other. Place them by shift: the CH₂ at δ 2.45 sits in the 2.0–2.5 window for a proton α to a carbonyl, not at 3.3–4.0 where a CH₂ on oxygen would land. So the ethyl is bonded to the carbonyl carbon, not to an oxygen. The 3H singlet at δ 2.13 has no neighbors at all and sits in the same α window, so it is a methyl bonded straight to C=O.

Step 5. Assemble and verify. A carbonyl flanked by ethyl and methyl is CH₃COCH₂CH₃, butan-2-one. Four carbons, eight hydrogens, one oxygen. It closes.

Step 6. Kill the alternatives out loud.

No drawing, no arrow pushing. Degrees of unsaturation set the budget, IR spends it, integration confirms the formula, splitting supplies the connectivity. Run those four moves in that order and the timing takes care of itself.

What Spectroscopy Questions Actually Require

Worth being clear about the limits, because students overprepare this in one direction and underprepare it in another.

Every MCAT spectroscopy question is answerable from tabulated data and reasoning. You are never asked to draw a structure, propose a mechanism from a spectrum, or interpret ¹³C multiplicity, coupling constants, or a full fingerprint region. The passage hands you the bands and shifts it wants used.

What you do need is the ability to use a value as an exclusion. A very broad 2500–3300 absorption next to a band near 1710 does not merely suggest a carboxylic acid, it eliminates ketones, esters, and alcohols in one step. IR eliminates, NMR identifies, and the molecular formula keeps both honest. The tables above are ranges on purpose: real spectra shift with solvent, concentration, and hydrogen bonding, and the AAMC writes items so the answer sits comfortably inside a range. If you are deciding between two choices on a 5 cm⁻¹ difference, you have misread the question.

How to Drill This

Reading a reference sheet is not the same as recalling it under a 95-second clock. Once you can reproduce the carbonyl ordering and the handful of diagnostic IR bands from memory, put them under exam conditions with our free MCAT organic chemistry practice questions, which run 25 items in the AAMC interface with full worked solutions. The rest of that section is covered by our MCAT general chemistry practice questions and MCAT physics practice questions. For the wider picture, see how hard the MCAT is or our roundup of free MCAT resources.

Frequently Asked Questions

Which MCAT organic chemistry reactions do I actually need to know?

The ones in the sheet above and essentially nothing beyond them: SN1, SN2, E1, E2, alcohol oxidation and carbonyl reduction, Grignard addition, imine and enamine formation, acetal protection, keto-enol tautomerization, aldol addition and condensation, decarboxylation, and the acyl substitutions. The list is short because the AAMC specifies first-semester organic chemistry only.

What are the reactions to know if I only have a few days?

Triage to the carbonyl. Nucleophilic addition and nucleophilic acyl substitution, plus the reactivity ordering that governs both, account for more MCAT organic questions than any other topic, and they carry straight into biochemistry through aldolase, transamination, and thioester chemistry. Spectroscopy comes second because it pays back study time faster than anything else. Substitution and elimination come third.

Do I need to memorize IR wavenumbers exactly?

No. Learn a few anchors and two rules. Anchors: alcohol O–H broad at 3200–3600, carboxylic acid O–H very broad at 2500–3300, ketone C=O near 1715, ester near 1740, amide near 1650. Rules: conjugation lowers a C=O by roughly 20–40 cm⁻¹, ring strain raises it. You can reason to the rest.

Is second-semester organic chemistry tested?

No. Aromatic substitution mechanisms, pericyclic reactions, named multi-step syntheses, and retrosynthetic analysis fall outside the AAMC content outlines. If your course covered them, set them aside for this exam.

Do I have to draw or identify structures?

The exam does show structures, but much of the reasoning is verbal: comparing pKa values, choosing a mechanism family, ranking CIP priorities, reading tabulated spectra. You are never asked to produce a drawing.

MCAT Reference Sheets

The rest of the high-yield memorization set:

MCAT amino acid chart · MCAT equation sheet · MCAT hormone chart

Then test what stuck with our free MCAT practice questions — 25 questions per subject in the AAMC test interface, with full worked solutions.

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