Organic Chemistry: Functional Groups and Reactions

MCAT organic chemistry rewards pattern recognition, not brute memorization. If you can identify a functional group, judge relative reactivity, and predict how nucleophiles and electrophiles interact, you can reason your way through most Chem/Phys passages. Master the frameworks below and the reactions become predictable rather than mysterious.

Core Idea

• Functional groups define reactivity. The carbon skeleton is mostly inert; the group attached to it (hydroxyl, carbonyl, amine) determines how a molecule behaves and reacts.
• Electron flow drives every reaction. Nucleophiles (electron-rich) attack electrophiles (electron-poor). Nearly every mechanism is just electrons moving from high density to low density.
• Structure dictates properties. Polarity, hydrogen bonding, and steric bulk explain boiling points, solubility, acidity, and which reaction pathway dominates.

Functional Groups and Relative Reactivity

Learn the groups in order of oxidation state and reactivity. Alkanes (C–C, C–H only) are the least reactive. Alkenes/alkynes add across their pi bonds. Alcohols (–OH) hydrogen bond, are weakly acidic, and can be oxidized. Moving up in oxidation state: aldehydes (terminal C=O) and ketones (internal C=O) are electrophilic at the carbonyl carbon; aldehydes are more reactive than ketones because they are less sterically hindered and less electron-rich.

At the highest common oxidation state sit the carboxylic acid derivatives. Reactivity toward nucleophilic acyl substitution decreases in the order: acid chlorides > anhydrides > esters > amides, tracking how good the leaving group is and how much the substituent donates electrons into the carbonyl. Carboxylic acids (–COOH) are the most acidic common group (pKa ~4-5) because the conjugate base is resonance-stabilized. Amines are the most common organic bases and act as nucleophiles.

Nomenclature Basics

Name the longest chain containing the principal functional group, number to give that group the lowest locant, and use suffixes: -ane (alkane), -ene (alkene), -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid). Priority for the principal group generally follows: carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine.

Isomerism and Stereochemistry

• Structural (constitutional) isomers share a molecular formula but differ in connectivity.
• Stereoisomers share connectivity but differ in spatial arrangement. Enantiomers are non-superimposable mirror images (differ at all chiral centers); diastereomers differ at some but not all centers and are not mirror images. Cis/trans (geometric) isomers are diastereomers arising from restricted rotation about a double bond or ring.
• Chirality requires a carbon with four different groups. R/S is assigned by ranking substituents by atomic number (Cahn-Ingold-Prelog), orienting the lowest priority away, and reading 1→2→3: clockwise is R, counterclockwise is S. Enantiomers rotate plane-polarized light in equal but opposite directions; a racemic mixture is optically inactive.

Common Reaction Types

• SN2: one concerted step, backbone inversion of configuration, favored by strong nucleophiles and unhindered (methyl/primary) substrates; rate depends on both substrate and nucleophile.
• SN1: two steps through a carbocation, favored by tertiary substrates and polar protic solvents; produces racemization.
• Elimination (E1/E2): forms alkenes; strong bulky bases and heat favor elimination over substitution (Zaitsev gives the more substituted alkene).
• Addition: nucleophiles or electrophiles add across pi bonds (Markovnikov addition puts H on the carbon with more H's).
• Oxidation/reduction: oxidation adds bonds to O (or removes H); primary alcohol → aldehyde → carboxylic acid, secondary alcohol → ketone. Reduction reverses this.
• Esterification: carboxylic acid + alcohol under acid catalysis yields an ester plus water (a reversible condensation).

Lab Techniques (Separation and Spectroscopy)

• Extraction separates by relative solubility between two immiscible solvents; acid-base extraction moves compounds between layers by protonation state.
• Distillation separates by boiling point; fractional distillation resolves close boiling points.
• Chromatography separates by differential affinity for a stationary vs. mobile phase (polarity in TLC/column; size, charge, or affinity in others).
• IR spectroscopy identifies functional groups: broad O–H ~3300, sharp C=O ~1700 cm⁻¹.
• NMR reveals the carbon-hydrogen framework: chemical shift shows environment, integration counts protons, and splitting (n+1 rule) shows neighboring hydrogens.

High-Yield Exam Patterns

• Reactivity ranking questions are constant: aldehyde > ketone for nucleophilic addition; acid chloride > anhydride > ester > amide for acyl substitution.
• Stereochemistry twists: expect SN2 = inversion, SN1 = racemization; know that enantiomers share physical properties but diastereomers do not.
• Spectroscopy clues: a broad IR peak near 3300 signals an O–H or N–H; a strong peak near 1700 signals a carbonyl.
• Solvent and substrate hints tell you the mechanism: tertiary + protic solvent → SN1/E1; primary + strong nucleophile → SN2.
• Acidity comparisons hinge on resonance and electronegativity: carboxylic acids beat alcohols because the carboxylate is resonance-stabilized.
• Extraction logic: converting an amine to its salt (add acid) pulls it into the aqueous layer.

Common Traps to Avoid

• Confusing enantiomers with diastereomers — mirror images at all centers are enantiomers; partial matches are diastereomers.
• Assuming more substituted always means more reactive — steric bulk slows SN2 and carbonyl addition.
• Forgetting that SN1 racemizes while SN2 inverts, and mislabeling the stereochemical outcome.
• Mixing up oxidation and reduction — gaining bonds to oxygen (or losing H) is oxidation.
• Treating IR and NMR interchangeably — IR identifies functional groups, NMR maps the hydrogen environment and connectivity.