How the MCAT tests organic reactions
Organic reactivity falls in AAMC content category 5D — the structure, function, and reactivity of biologically relevant molecules. The MCAT rarely asks for a synthesis; it asks you to read a mechanism, predict a product, or explain why one pathway wins. Thinking in terms of electron flow (nucleophile → electrophile) is the master skill.
Find the electron-rich site (nucleophile) and the electron-poor site (electrophile). Curved arrows always start at electrons (a lone pair or bond) and point to where they go.
Nucleophiles, electrophiles, and leaving groups
These three roles define every polar reaction. A nucleophile donates an electron pair; an electrophile accepts one; a leaving group departs with the bonding electrons.
- Good nucleophiles are electron-rich: negative charge, lone pairs, or π bonds (e.g., OH⁻, CN⁻, NH₃).
- Electrophiles are electron-poor: partial-positive carbons, especially carbonyl carbons and carbons bearing a leaving group.
- Good leaving groups are weak bases that are stable after leaving — halides (I⁻ > Br⁻ > Cl⁻), water, tosylate. OH⁻ and NH₂⁻ are poor leaving groups.
Nucleophilic substitution: SN1 vs SN2
In substitution, a nucleophile replaces a leaving group. The two pathways differ in timing and stereochemistry.
| Feature | SN2 | SN1 |
|---|---|---|
| Steps | One (concerted) | Two (carbocation intermediate) |
| Rate law | Second order: k[substrate][Nu] | First order: k[substrate] |
| Best substrate | Methyl / primary | Tertiary (stable carbocation) |
| Nucleophile | Strong | Weak (often solvent) |
| Stereochemistry | Inversion | Racemization |
Steric access drives SN2; carbocation stability drives SN1.
Elimination: E1 vs E2
Elimination removes two groups to form a π bond (an alkene). It competes with substitution and is favored by heat and strong, bulky bases.
- E2: one concerted step, second order, needs a strong base; the H and leaving group depart anti-periplanar. Follows Zaitsev (more substituted alkene) unless the base is bulky.
- E1: two steps through a carbocation, first order, favored by weak bases and good ionizing solvents — parallels SN1.
- Deciding factor: strong bulky base and heat → elimination; small strong nucleophile on a primary substrate → substitution.
Carbonyl chemistry
The carbonyl (C=O) is the most important functional group in organic and biochemistry. Its carbon is electrophilic, so nucleophiles add across the double bond.
Carboxylic-acid derivatives react by addition–elimination, and their reactivity follows leaving-group ability: acyl chloride > anhydride > ester ≈ carboxylic acid > amide. A more reactive derivative can be converted into a less reactive one, but not the reverse.
Oxidation and reduction
In organic chemistry, oxidation increases bonds to oxygen (or other heteroatoms) and reduction increases bonds to hydrogen. The alcohol → aldehyde → carboxylic acid ladder is the classic sequence.
- Oxidizing agents (PCC stops at the aldehyde; strong agents like chromic acid go to the carboxylic acid).
- Reducing agents (LiAlH₄ is strong and reduces esters/acids to alcohols; NaBH₄ is milder, reducing aldehydes and ketones).
- Secondary alcohols oxidize to ketones, which resist further oxidation.
How to study it — and the traps to avoid
Organic reactions are learned by drawing electron flow, not by rote lists. Practice curved-arrow pushing until it is automatic.
- For every reaction, label the nucleophile, electrophile, and leaving group first.
- Decide SN1/SN2/E1/E2 from substrate class, nucleophile/base strength, and conditions.
- Remember SN2 inverts configuration; SN1 racemizes.
- Track oxidation state by counting bonds to O versus H.
Do not treat a strong base as a good nucleophile automatically. A strong, bulky base (like tert-butoxide) mostly does E2 elimination rather than SN2 substitution because it is too hindered to attack carbon directly.