Essential Organic Chemistry Functional Groups Cheatsheet

Introduction: The Building Blocks of Organic Chemistry

Functional groups are specific atoms or arrangements of atoms within organic molecules that determine their chemical behavior and reactivity. Understanding these groups is fundamental to organic chemistry as they dictate how compounds interact, react, and function in biological systems and industrial processes. This cheatsheet provides a comprehensive reference for identifying, naming, and understanding the reactivity patterns of major functional groups in organic chemistry.

Core Concepts of Functional Groups

Definition and Importance

• Functional groups: Specific arrangements of atoms that give molecules their characteristic chemical properties
• Homologous series: Compounds with the same functional group but differing carbon chain lengths
• Reaction sites: Areas of molecules where chemical reactions typically occur
• Predictive power: Functional groups allow chemists to predict chemical behavior and design synthesis pathways

Functional Group Notation

• Structural formula: Shows all atoms and bonds (e.g., CH₃CH₂OH)
• Condensed structural formula: Simplifies by grouping (e.g., C₂H₅OH)
• Line-angle representation: Carbon atoms at vertices, hydrogens implied (zigzag lines)
• R group notation: R represents variable alkyl group (e.g., R-OH for alcohols)

Hydrocarbon Functional Groups

Alkanes (C-C Single Bonds)

Naming: -ane suffix (methane, ethane, propane)
Physical properties: Low reactivity, nonpolar, low boiling points
Common reactions:

• Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O
• Free radical halogenation: CH₄ + Cl₂ → CH₃Cl + HCl

Alkenes (C=C Double Bonds)

Naming: -ene suffix (ethene, propene)
Physical properties: More reactive than alkanes, slightly polar
Common reactions:

• Addition of H₂, X₂, HX, H₂O
• Oxidation (e.g., KMnO₄ gives diols)
• Polymerization

Alkynes (C≡C Triple Bonds)

Naming: -yne suffix (ethyne, propyne)
Physical properties: Terminal alkynes slightly acidic (pKa ≈ 25)
Common reactions:

• Addition reactions (similar to alkenes but can add twice)
• Terminal alkyne deprotonation with strong bases

Aromatic Compounds (Benzene Rings)

Structure	General Formula	Example	Properties	Key Reactions
	C₆H₆	Benzene	Planar, resonance-stabilized	Electrophilic substitution

Naming: Based on substituents (toluene, phenol, etc.)
Physical properties: Unusually stable, planar structure
Common reactions:

• Electrophilic aromatic substitution (not addition)
• Friedel-Crafts alkylation and acylation
• Nitration, sulfonation, halogenation

Oxygen-Containing Functional Groups

Alcohols (R-OH)

Classification: Primary (1°), secondary (2°), tertiary (3°)
Physical properties: Hydrogen bonding → higher BP than similar MW hydrocarbons
Common reactions:

• Oxidation (1° → aldehyde → carboxylic acid; 2° → ketone)
• Dehydration to alkenes
• Conversion to alkyl halides (with HX, PX₃, SOCl₂)
• Esterification with carboxylic acids

Ethers (R-O-R’)

Naming: Alkyl alkyl ether or alkoxy prefix
Physical properties: Lower BP than alcohols, good solvents
Common reactions:

• Relatively unreactive (why they’re good solvents)
• Cleavage with strong acids (HBr, HI)

Aldehydes (R-CHO)

Naming: -al suffix or -carbaldehyde
Physical properties: Polar, often pleasant smelling
Common reactions:

• Oxidation to carboxylic acids
• Reduction to 1° alcohols
• Nucleophilic addition (CN⁻, RMgX, RLi, alcohols, etc.)
• Aldol condensation

Ketones (R-CO-R’)

Naming: -one suffix
Physical properties: Polar, often used as solvents
Common reactions:

• Reduction to 2° alcohols
• Nucleophilic addition (similar to aldehydes but less reactive)
• Alpha-halogenation
• Enolate formation and aldol reactions

Carboxylic Acids (R-COOH)

Naming: -oic acid suffix
Physical properties: Acidic (pKa ≈ 4-5), strong hydrogen bonding
Common reactions:

• Esterification with alcohols
• Conversion to acid chlorides, anhydrides
• Reduction to aldehydes or alcohols
• Decarboxylation

Esters (R-COO-R’)

Naming: Alkyl alkanoate
Physical properties: Often pleasant fragrance, lower BP than acids
Common reactions:

• Hydrolysis (acidic or basic conditions)
• Reduction to alcohols
• Transesterification
• Ammonolysis to amides

Nitrogen-Containing Functional Groups

Amines (R-NH₂, R₂NH, R₃N)

Classification: Primary (1°), secondary (2°), tertiary (3°)
Physical properties: Basic (accepts H⁺), lower BP than alcohols
Common reactions:

• Alkylation
• Acylation to form amides
• Reaction with nitrous acid (1° → diazonium salts)
• Hofmann elimination

Amides (R-CO-NH₂, R-CO-NHR’, R-CO-NR’₂)

Classification: Primary, secondary, tertiary (based on N substitution)
Physical properties: Strong hydrogen bonding, resonance-stabilized
Common reactions:

• Hydrolysis to carboxylic acids and amines
• Reduction to amines
• Dehydration to nitriles

Nitriles (R-C≡N)

Naming: -nitrile suffix or alkyl cyanide
Physical properties: Polar, moderate BP
Common reactions:

• Hydrolysis to carboxylic acids
• Reduction to amines
• Addition of organometallics

Nitro Compounds (R-NO₂)

Naming: Nitro- prefix
Physical properties: Strongly electron-withdrawing
Common reactions:

• Reduction to amines
• Activates aromatic rings for nucleophilic substitution
• Deactivates for electrophilic substitution (ortho/para director)

Halogen-Containing Functional Groups

Alkyl Halides (R-X)

Classification: Primary, secondary, tertiary (affects reaction mechanism)
Physical properties: Increasing BP with halogen size (F < Cl < Br < I)
Common reactions:

• Nucleophilic substitution (SN1, SN2)
• Elimination (E1, E2)
• Formation of Grignard reagents (RMgX)
• Reduction

Sulfur-Containing Functional Groups

Thiols (R-SH)

Naming: -thiol suffix
Physical properties: Strong, unpleasant odor, lower BP than alcohols
Common reactions:

• Oxidation to disulfides
• Formation of thioethers
• Metal complexation

Sulfides/Thioethers (R-S-R’)

Naming: Alkyl alkyl sulfide
Physical properties: Less polar than ethers
Common reactions:

• Oxidation to sulfoxides and sulfones
• Alkylation to sulfonium salts

Phosphorus-Containing Functional Groups

Phosphines (R₃P)

Naming: Trialkylphosphine
Physical properties: Nucleophilic, often air-sensitive
Common reactions:

• Oxidation to phosphine oxides
• Metal complexation (ligands)
• Mitsunobu reaction

Common Functional Group Transformations

Interconversion Map

Alkenes ⟶ Alkyl halides ⟶ Grignard reagents
   ↓          ↓  ↑              ↓
Alcohols ⟶ Ethers             Alcohols
   ↓                            ↓
Aldehydes/Ketones  ⟵⟶  Acetals/Ketals
   ↓
Carboxylic acids ⟶ Acid chlorides ⟶ Esters, Amides
   ↓
CO₂ (Decarboxylation)

Key Transformations

Spectroscopic Identification of Functional Groups

IR Spectroscopy Characteristic Absorptions

¹H NMR Chemical Shifts

¹³C NMR Chemical Shifts

Reactivity Patterns & Mechanisms

Electrophilic vs. Nucleophilic Sites

• Electrophilic sites: Electron-deficient centers (C=O carbon, C⁺, etc.)
• Nucleophilic sites: Electron-rich centers (lone pairs, π-bonds)

Common Reaction Mechanisms

1. Nucleophilic Substitution:

   • SN2: Backside attack, inversion, 2nd order kinetics
   • SN1: Carbocation intermediate, 1st order kinetics

2. Elimination:

   • E2: Concerted, requires anti-periplanar arrangement
   • E1: Carbocation intermediate, then proton loss

3. Addition:

   • Electrophilic addition to alkenes (Markovnikov)
   • Nucleophilic addition to carbonyls

4. Substitution on Aromatics:

   • Electrophilic aromatic substitution
   • Nucleophilic aromatic substitution (with EWGs)

5. Radical Reactions:

   • Chain processes (initiation, propagation, termination)

Activating/Deactivating Groups in Aromatic Substitution

Solving Common Organic Chemistry Problems

Identifying Functional Groups

1. Look for characteristic atoms (O, N, halogens, S)
2. Identify specific arrangements (C=O, OH, NH₂, etc.)
3. Classify by primary structure (alcohol, amine, etc.)

Predicting Reactivity

1. Identify functional groups present
2. Determine electron-rich and electron-poor sites
3. Consider reagent types (nucleophiles, electrophiles, etc.)
4. Apply reaction patterns for specific functional groups

Naming Compounds with Multiple Functional Groups

1. Identify highest priority group (COOH > CHO > C=O > OH > NH₂ > C=C)
2. Use that group for suffix
3. List other groups as prefixes
4. Number chain to give priority group lowest possible number

Resources for Further Learning

Recommended Books

• “Organic Chemistry” by Jonathan Clayden, Nick Greeves, and Stuart Warren
• “Organic Chemistry as a Second Language” by David Klein
• “Pushing Electrons” by Daniel Weeks

Online Resources

• Master Organic Chemistry (masterorganicchemistry.com)
• Khan Academy Organic Chemistry
• ChemTube3D for interactive mechanisms

Mobile Apps

• ChemDoodle
• Functional Groups in Organic Chemistry
• ChemSpider

Practice Problem Sources

• Journal of Chemical Education
• ACS Exams
• Previous organic chemistry course problem sets

This cheatsheet provides a foundation for understanding functional groups in organic chemistry. Remember that success in organic chemistry comes from practice, understanding reaction mechanisms, and developing pattern recognition skills rather than pure memorization.