How to Use This Study Guide

Functional groups are the reactive centers of organic molecules. Knowing each group's structure, naming conventions, and reactivity patterns is the single most important skill in organic chemistry. This guide summarizes every major functional group in a concise, exam-ready format. Work through each section, then quiz yourself using the summary table at the end.

1. Hydrocarbons (C and H only)

  • Alkane (–CH2–, –CH3): Single bonds only. Relatively inert; undergoes radical halogenation.
  • Alkene (C=C): Double bond; undergoes electrophilic addition (HX, H2O, Br2), hydrogenation.
  • Alkyne (C≡C): Triple bond; undergoes addition and can form terminal alkyne anions (pKa ~25) for nucleophilic reactions.
  • Arene (benzene ring): Aromatic; undergoes electrophilic aromatic substitution (EAS).

2. Oxygen-Containing Groups

  • Alcohol (–OH): Hydrogen bonding; can act as nucleophile or undergo dehydration, oxidation, or substitution.
  • Ether (–O–): Relatively inert; cleaved only by strong acid (HI, HBr). Good solvents (THF, diethyl ether).
  • Aldehyde (–CHO): Electrophilic carbonyl; undergoes nucleophilic addition, oxidation to carboxylic acid.
  • Ketone (C=O, flanked by C): Similar to aldehyde but less reactive; undergoes nucleophilic addition, not easily oxidized.
  • Carboxylic Acid (–COOH): Acidic (pKa ~4–5); undergoes esterification, amide formation, reduction.
  • Ester (–COOR): Formed from acid + alcohol; hydrolyzed under acid or base; undergoes transesterification.
  • Amide (–CONH2/–CONHR): Stable to hydrolysis; forms hydrogen bonds; found in peptide bonds.
  • Anhydride (–CO–O–CO–): Reactive acylating agent; reacts with alcohols and amines.
  • Acyl chloride (–COCl): Highly reactive acylating agent; hydrolyzes rapidly.
  • Epoxide (three-membered ring with O): Highly reactive; opened by nucleophiles under acidic or basic conditions with regioselectivity.

3. Nitrogen-Containing Groups

  • Amine (–NH2, –NHR, –NR2): Basic (pKa of conjugate acid ~10); nucleophilic; reacts with acids, acyl chlorides, alkyl halides.
  • Amide (–CONH–): (Also listed above) Reduced basicity of N due to resonance.
  • Nitrile (–C≡N): Hydrolyzed to amide then carboxylic acid; reduced to amine.
  • Nitro (–NO2): Electron-withdrawing; can be reduced to amine (useful in synthesis).
  • Imine (C=N–): Formed from carbonyl + primary amine; hydrolyzed back under acidic aqueous conditions.

4. Sulfur and Halide Groups

  • Thiol (–SH): Analogous to alcohol but more acidic (pKa ~10–11) and better nucleophile; oxidized to disulfides.
  • Thioether/Sulfide (–S–): Oxidized to sulfoxide, then sulfone.
  • Alkyl halide (–X, X = F, Cl, Br, I): Undergoes SN1, SN2, E1, E2 depending on conditions. Leaving group ability: I > Br > Cl >> F.

Quick-Reference Summary Table

GroupKey FeatureKey Reaction Type
AlkeneC=CElectrophilic addition
Alcohol–OHOxidation, substitution
Aldehyde/KetoneC=ONucleophilic addition
Carboxylic Acid–COOHAcyl substitution
Ester/Amide–COOR / –CONH–Hydrolysis, transesterification
Amine–NH2Nucleophilic, basic
Alkyl Halide–XSN1, SN2, E1, E2
EpoxideCyclic ether (3-membered)Ring opening by nucleophile

Study Tips

  1. Draw, don't just read: Always sketch the functional group structure and the mechanism of its key reaction.
  2. Group by carbonyl: Acyl chlorides, anhydrides, esters, amides, and carboxylic acids all share the carbonyl core — learn them as a family with decreasing reactivity.
  3. Use pKa logic: Knowing relative pKa values tells you which species is the acid or base in a reaction.
  4. Connect to real molecules: Identify these groups in drugs, natural products, and biochemical molecules you already know.
  5. Practice arrow-pushing: Draw curved arrow mechanisms for each reaction type until it becomes automatic.

Conclusion

Mastering functional groups is the gateway to understanding all of organic chemistry. Once you can recognize a group on sight and recall its reactivity, every reaction mechanism becomes a predictable, logical process rather than rote memorization.