The MCAT rarely asks organic chemistry questions the way a college exam does. Instead of "draw the product of this synthesis," you're more likely to see a question that asks you to reason about *why* something happens. These 20 questions are written in that spirit — conceptual, reasoning-based, and focused on the ideas that repeatedly show up on the real exam.
1. What is the hybridization of each carbon atom in ethene (CH2=CH2)?
sp2. Each carbon is bonded to three groups (two H atoms and one carbon), giving trigonal planar geometry and leaving one unhybridized p orbital on each carbon to form the π bond.
2. In an SN2 reaction at a stereocenter, what happens to the configuration of the product?
It inverts. The nucleophile attacks from the side opposite the leaving group, flipping the spatial arrangement like an umbrella turning inside out — this is called Walden inversion.
3. Which combination of factors most favors an SN1 mechanism over SN2?
A tertiary substrate, a stable carbocation intermediate, a weak nucleophile, and a polar protic solvent. The stability of the carbocation is usually the deciding factor.
4. A bulky, strong base is used with a tertiary alkyl halide. Which product distribution is favored?
The Hofmann (less substituted) alkene is favored. A bulky base has difficulty removing a hindered, more substituted β-hydrogen, so it removes a more accessible, less hindered one instead.
5. Why does trifluoroacetic acid (CF3COOH) have a lower pKa than acetic acid (CH3COOH)?
The three electronegative fluorine atoms withdraw electron density inductively, stabilizing the conjugate base's negative charge and making the O-H bond easier to break.
6. A compound shows a strong, broad IR absorption around 3300 cm⁻¹. What functional group is likely present?
An O-H group, as in an alcohol or carboxylic acid. The broadness comes from hydrogen bonding, which is why this band is wide rather than sharp.
7. In ¹H NMR, a proton has 3 neighboring, non-equivalent hydrogens. How many peaks will its signal split into?
4 peaks (a quartet), following the n+1 rule, where n is the number of adjacent, non-equivalent protons.
8. What determines whether a stereocenter is labeled R or S?
CIP (Cahn-Ingold-Prelog) priority rules rank the four substituents by atomic number; viewing the molecule with the lowest-priority group pointing away, a clockwise 1→2→3 sequence is R, counterclockwise is S.
9. HBr adds to propene without peroxides. Where does the bromine end up, and why?
On the more substituted carbon (Markovnikov addition), because the reaction proceeds through the more stable secondary carbocation intermediate.
10. HBr adds to propene in the presence of peroxides. How does the product differ?
The addition becomes anti-Markovnikov: bromine ends up on the less substituted carbon. Peroxides trigger a radical mechanism instead of a carbocation mechanism, changing which intermediate is favored.
11. What oxidation state change occurs when a primary alcohol is fully oxidized to a carboxylic acid?
Carbon goes from an alcohol (one C-O bond) through an aldehyde intermediate to a carboxylic acid (two C-O bonds, one being a C=O), an increase in oxidation state at that carbon of +4 overall.
12. What is a typical reagent for reducing a ketone to a secondary alcohol?
NaBH4 (sodium borohydride) or LiAlH4 (lithium aluminum hydride). Both deliver a hydride to the carbonyl carbon; NaBH4 is milder and more selective.
13. What is the key structural difference between enantiomers and diastereomers?
Enantiomers are non-superimposable mirror images that differ at every stereocenter. Diastereomers are stereoisomers that differ at some, but not all, stereocenters.
14. What makes a meso compound optically inactive despite having stereocenters?
A meso compound has an internal plane of symmetry, so the optical rotation contributed by one half of the molecule is exactly canceled by the mirror-image half.
15. What are the criteria for a ring system to be considered aromatic?
It must be cyclic, planar, fully conjugated, and contain 4n+2 π electrons (Hückel's rule) — for example, benzene has 6 π electrons, satisfying n = 1.
16. Why is a simple amine a stronger base than an amide?
In an amide, the nitrogen's lone pair is delocalized into the adjacent carbonyl through resonance, making it far less available to accept a proton than the lone pair on an amine nitrogen.
17. What is keto-enol tautomerism?
The equilibrium between a ketone (or aldehyde) form and its enol form, interconverted by moving a proton and shifting a double bond — they're constitutional isomers in rapid equilibrium, not resonance structures.
18. Rank acid chlorides, esters, and amides by reactivity toward nucleophilic acyl substitution.
Acid chlorides > esters > amides. This tracks with how good a leaving group each carbonyl compound has and how strongly the substituent donates electron density into the carbonyl.
19. In a Newman projection of butane, which conformation is lowest in energy?
The anti conformation, with the two methyl groups 180° apart, minimizes steric strain between the largest substituents compared to gauche or eclipsed arrangements.
20. Why can 1,2-addition and 1,4-addition give different major products with a conjugated diene?
1,2-addition is typically the kinetic product (forms faster, favored at low temperature), while 1,4-addition is typically the thermodynamic product (more stable, favored at higher temperature or longer reaction time) — the reaction proceeds through a resonance-stabilized allylic carbocation that can be attacked at either position.