Introduction to Organic Chemistry
The chemistry of carbon — the element of life
Organic chemistry is the scientific study of the structure, properties, composition, reactions, and synthesis of carbon-containing compounds. With over 10 million known organic compounds (and counting), it is one of the largest and most active branches of chemistry.
🌍 Organic Chemistry in the Real World
Medicine
Drug design, pharmacology, antibiotics, painkillers
Consumer Products
Soaps, dyes, perfumes, cosmetics, detergents
Agriculture
Pesticides, herbicides, fertilizers, growth hormones
Biochemistry
DNA, proteins, enzymes, carbohydrates, lipids
Materials
Plastics, rubber, fibers, adhesives, coatings
Energy
Petroleum, natural gas, biofuels, batteries
🔬 Organic vs. Inorganic Compounds
| Property | Organic Compounds | Inorganic Compounds |
|---|---|---|
| Contains | Always carbon (+ H, O, N, S, etc.) | May lack carbon |
| Bonding | Mostly covalent | Ionic or covalent |
| Melting Point | Generally low | Generally high |
| Solubility | Often in organic solvents | Often in water |
| Combustibility | Usually combustible | Usually non-combustible |
| Reaction Rate | Slow, need catalysts | Often fast, ionic |
| Isomerism | Very common | Rare |
📋 Key Foundational Concepts
- Tetravalency of Carbon: Carbon always forms exactly 4 bonds (single, double, or triple), allowing chains, branches, and rings.
- Catenation: Carbon atoms bond extensively with each other, forming chains of virtually unlimited length.
- Hybridization: sp³ (tetrahedral, 109.5°), sp² (trigonal planar, 120°), sp (linear, 180°) — determines molecular geometry.
- Functional Groups: Specific atom groupings (–OH, –COOH, –NH₂, etc.) that dictate chemical behavior.
- IUPAC Nomenclature: The systematic naming system ensuring every compound has a unique, unambiguous name.
📐 IUPAC Naming — Quick Guide
- Find the longest carbon chain — this is the parent chain (meth-, eth-, prop-, but-, pent-, hex-, etc.)
- Number the carbons — start from the end nearest a substituent or functional group
- Identify substituents/branches — name them with position numbers
- Add suffix — -ane (single bonds), -ene (double bond), -yne (triple bond), -ol (alcohol), etc.
- Alphabetize substituents — ignore prefixes like di-, tri- when alphabetizing
"Monkeys Eat Peanut Butter; People Hate Having Old Nuts Daily"
Meth-, Eth-, Prop-, But-, Pent-, Hex-, Hept-, Oct-, Non-, Dec-
📜 Brief History of Organic Chemistry
Before 1828, scientists believed organic compounds could only be produced by living organisms — a concept known as vitalism. Friedrich Wöhler shattered this belief by synthesizing urea (an organic compound) from ammonium cyanate (inorganic), proving that organic molecules follow the same chemical laws as all matter.
Since then, organic chemistry has grown explosively — from August Kekulé's discovery of the benzene ring structure (1865) to modern computational chemistry and total synthesis of complex natural products.
Chemical Bonding in Organics
The forces that hold organic molecules together
Bonding determines a molecule's shape, polarity, reactivity, and physical properties. In organic chemistry, the dominant bond type is covalent — atoms share electrons to achieve stable electron configurations.
🔗 Types of Covalent Bonds
| Bond Type | Composition | Rotation? | Bond Length | Bond Energy | Example |
|---|---|---|---|---|---|
| Single (σ) | 1 sigma bond | Yes (free) | 154 pm | 347 kJ/mol | C–C in ethane |
| Double (σ + π) | 1 sigma + 1 pi | No (restricted) | 134 pm | 614 kJ/mol | C=C in ethene |
| Triple (σ + 2π) | 1 sigma + 2 pi | No (restricted) | 120 pm | 839 kJ/mol | C≡C in ethyne |
⚛️ Hybridization in Detail
| Hybridization | Geometry | Bond Angle | Bonds | Unhybridized p | Example |
|---|---|---|---|---|---|
| sp³ | Tetrahedral | 109.5° | 4σ | 0 | CH₄ (methane) |
| sp² | Trigonal Planar | 120° | 3σ + 1π | 1 | C₂H₄ (ethene) |
| sp | Linear | 180° | 2σ + 2π | 2 | C₂H₂ (ethyne) |
📊 Electronegativity & Bond Polarity
Electronegativity (EN) measures an atom's ability to attract shared electrons. The difference (ΔEN) between bonded atoms determines polarity:
- ΔEN = 0: Nonpolar covalent (e.g., C–C, H–H)
- ΔEN = 0.1–0.4: Weakly polar (e.g., C–H)
- ΔEN = 0.5–1.7: Polar covalent (e.g., C–O, C–N, O–H)
- ΔEN > 1.7: Ionic (e.g., Na–Cl)
F (4.0) > O (3.5) > N (3.0) > Cl (3.0) > Br (2.8) > S (2.5) > C (2.5) > H (2.1)
↔️ Resonance Structures
When electrons can be delocalized across multiple bonds, we draw resonance structures — different Lewis structures that contribute to the actual electron distribution (the resonance hybrid).
- Resonance structures differ only in electron placement, NOT atom positions
- The actual molecule is a weighted average (hybrid) of all contributing structures
- More resonance stabilization = more stable molecule (e.g., benzene, carboxylate anion)
🤝 Intermolecular Forces (IMFs)
IMFs determine physical properties like boiling point, solubility, and viscosity:
| Force | Strength | Occurs In | Effect on BP |
|---|---|---|---|
| London Dispersion | Weakest | All molecules | ↑ with MW and surface area |
| Dipole-Dipole | Moderate | Polar molecules | Higher than nonpolar of same MW |
| Hydrogen Bonding | Strong | N–H, O–H, F–H | Significantly higher |
🧠 Deep Dive: Molecular Orbital Theory
Beyond hybridization, Molecular Orbital (MO) Theory explains bonding by combining atomic orbitals into molecular orbitals that span the entire molecule.
- Bonding MOs — lower energy, constructive interference, electron density between nuclei
- Antibonding MOs (σ*, π*) — higher energy, destructive interference, node between nuclei
- Bond Order = (bonding e⁻ − antibonding e⁻) / 2
MO theory is essential for understanding conjugated systems, aromaticity, and UV absorption in spectroscopy.
Alkanes — Saturated Hydrocarbons
The simplest organic compounds — only single bonds
Alkanes are hydrocarbons containing only C–C and C–H single bonds. They are called "saturated" because every carbon is bonded to the maximum number of hydrogen atoms possible. Alkanes are relatively unreactive, earning them the historical name "paraffins" (Latin: parum affinis = little affinity).
📐 General Formula
Cycloalkanes: CnH2n
📊 Homologous Series — First 10 Alkanes
| #C | Name | Formula | BP (°C) | State (25°C) |
|---|---|---|---|---|
| 1 | Methane | CH₄ | −162 | Gas |
| 2 | Ethane | C₂H₆ | −89 | Gas |
| 3 | Propane | C₃H₈ | −42 | Gas |
| 4 | Butane | C₄H₁₀ | −1 | Gas |
| 5 | Pentane | C₅H₁₂ | 36 | Liquid |
| 6 | Hexane | C₆H₁₄ | 69 | Liquid |
| 7 | Heptane | C₇H₁₆ | 98 | Liquid |
| 8 | Octane | C₈H₁₈ | 126 | Liquid |
| 9 | Nonane | C₉H₂₀ | 151 | Liquid |
| 10 | Decane | C₁₀H₂₂ | 174 | Liquid |
📝 IUPAC Nomenclature Rules
- Find the longest continuous carbon chain — this is the parent chain
- Number the carbons from the end nearest a branch point
- Identify substituents: methyl (–CH₃), ethyl (–C₂H₅), propyl (–C₃H₇), etc.
- Name alphabetically with position numbers (ignore di-, tri- when alphabetizing)
- Use multiplying prefixes: di-, tri-, tetra- for repeated substituents
CH₃–CH(CH₃)–CH(CH₃)–CH₃
Parent chain: butane (4 carbons). Two methyl groups at positions 2 and 3.
🔄 Conformational Analysis
Free rotation around C–C single bonds produces different conformations. For ethane:
- Staggered — H atoms as far apart as possible → most stable (lowest energy)
- Eclipsed — H atoms aligned → least stable (torsional strain ≈ 12 kJ/mol)
For butane, the most stable conformation is anti (methyl groups 180° apart), and the least stable is fully eclipsed (methyl groups 0° apart).
⚗️ Reactions of Alkanes
1. Combustion
Complete combustion produces CO₂ and H₂O. Incomplete combustion (limited O₂) produces CO or C (soot).
2. Free Radical Halogenation
Mechanism has three steps:
- Initiation: Cl₂ → 2Cl• (homolytic cleavage by UV)
- Propagation: Cl• + CH₄ → HCl + CH₃• then CH₃• + Cl₂ → CH₃Cl + Cl•
- Termination: Two radicals combine (Cl• + Cl•, CH₃• + Cl•, etc.)
3. Cracking
Large alkanes are thermally broken into smaller, more useful molecules (alkanes + alkenes). Industrial cracking is vital for producing gasoline from crude oil.
Alkenes — Unsaturated Hydrocarbons
Reactive molecules with C=C double bonds
Alkenes contain at least one carbon-carbon double bond (C=C), making them "unsaturated." The double bond consists of one σ bond and one π bond. The π bond is the site of reactivity — its electron density above and below the plane attracts electrophiles.
📐 General Formula
Each additional ring or double bond: subtract 2H (degree of unsaturation)
📊 Degree of Unsaturation (DoU)
Also called Index of Hydrogen Deficiency (IHD). Calculates the number of rings + double bonds:
where C = carbon, N = nitrogen, H = hydrogen, X = halogens
📝 Nomenclature & E/Z System
- Replace -ane with -ene; number C=C to give lowest locant
- cis/trans — for two identical groups: cis = same side, trans = opposite side
- E/Z system (more general, uses Cahn-Ingold-Prelog priority rules):
- Z (zusammen = together): higher-priority groups on SAME side
- E (entgegen = opposite): higher-priority groups on OPPOSITE sides
⚗️ Key Reactions of Alkenes
Alkenes primarily undergo addition reactions — the π bond breaks and two new σ bonds form:
| Reaction | Reagent | Product | Type |
|---|---|---|---|
| Hydrogenation | H₂ / Pd, Pt, or Ni | Alkane | Syn addition |
| Halogenation | Br₂ or Cl₂ | Vicinal dihalide | Anti addition |
| Hydrohalogenation | HBr, HCl, HI | Alkyl halide | Markovnikov |
| Hydration | H₂O / H⁺ | Alcohol | Markovnikov |
| Hydroboration-oxidation | BH₃ then H₂O₂/NaOH | Alcohol | Anti-Markovnikov, syn |
| Epoxidation | mCPBA (peracid) | Epoxide | Syn addition |
| Ozonolysis | O₃ then Zn/H₂O | Aldehydes/Ketones | Cleavage |
| Dihydroxylation | OsO₄ / KMnO₄ (cold, dilute) | Diol (1,2-diol) | Syn addition |
| Polymerization | Heat/pressure/catalyst | Polymer | Chain growth |
📏 Markovnikov's Rule
Why? The more substituted carbocation intermediate is more stable (hyperconjugation + inductive effects): 3° > 2° > 1° > methyl.
🧪 Test for Alkenes
Add bromine water (Br₂/H₂O) — the orange/brown color decolorizes if an alkene is present (addition across the double bond). Alkanes do not react.
🔄 Stability of Alkenes
More substituted alkenes are more stable (lower heat of hydrogenation):
Trans alkenes are generally more stable than cis due to reduced steric strain.
Alkynes — Triple Bond Compounds
Highly reactive molecules with C≡C triple bonds
Alkynes contain at least one carbon-carbon triple bond (C≡C), consisting of one σ bond and two π bonds. The carbons are sp hybridized with linear geometry (180° bond angle). The triple bond is the shortest and strongest C–C bond.
📐 General Formula
Degree of unsaturation = 2 (per triple bond)
📊 Comparison of C–C Bonds
| Property | Single (C–C) | Double (C=C) | Triple (C≡C) |
|---|---|---|---|
| Bond Length | 154 pm | 134 pm | 120 pm |
| Bond Energy | 347 kJ/mol | 614 kJ/mol | 839 kJ/mol |
| Hybridization | sp³ | sp² | sp |
| Geometry | Tetrahedral | Trigonal planar | Linear |
| Bond Angle | 109.5° | 120° | 180° |
📝 Classification & Nomenclature
- Terminal alkynes: H–C≡C–R (have an acidic terminal hydrogen, pKa ≈ 25)
- Internal alkynes: R–C≡C–R (no acidic hydrogen, generally more stable)
- Suffix: -yne (ethyne, propyne, but-1-yne, but-2-yne)
- Common name for ethyne: acetylene (used in welding torches)
⚗️ Key Reactions of Alkynes
| Reaction | Reagent | Product | Notes |
|---|---|---|---|
| Hydrogenation (full) | H₂ / Pd or Pt (excess) | Alkane | 2 equiv. H₂ added |
| Partial hydrogenation | H₂ / Lindlar's catalyst | cis-Alkene | Syn addition, stops at alkene |
| Dissolving metal reduction | Na / NH₃(l) | trans-Alkene | Anti addition |
| Halogenation | Br₂ (1 or 2 equiv.) | Dibromo- or tetrabromoalkane | Stepwise addition |
| Hydrohalogenation | HBr (1 or 2 equiv.) | Vinyl halide or geminal dihalide | Markovnikov |
| Hydration (Markovnikov) | H₂O / H₂SO₄ / HgSO₄ | Ketone (via enol) | Tautomerization |
| Hydroboration-oxidation | R₂BH then H₂O₂/NaOH | Aldehyde (from terminal) | Anti-Markovnikov |
| Acetylide formation | NaNH₂ (strong base) | Sodium acetylide (RC≡C⁻Na⁺) | Terminal alkynes only |
🔑 Acidity of Terminal Alkynes
Acidity order: sp C–H > sp² C–H > sp³ C–H
🔗 Alkylation of Acetylides
Sodium acetylides (RC≡C⁻) are powerful nucleophiles. They can attack primary alkyl halides via SN2 reaction to build longer carbon chains — a valuable synthetic strategy.
🧪 Keto-Enol Tautomerism
When a terminal alkyne undergoes acid-catalyzed hydration, the initial product is an enol (vinyl alcohol), which rapidly rearranges to the more stable keto form. This equilibrium is called tautomerism.
Aromatic Compounds
The remarkable stability of cyclic conjugated systems
Aromatic compounds are a special class of cyclic, planar, fully conjugated molecules that exhibit extraordinary thermodynamic stability. The parent compound is benzene (C₆H₆), whose structure puzzled chemists for decades until August Kekulé proposed the hexagonal ring in 1865.
🔑 Hückel's Rule for Aromaticity
- Cyclic — forms a continuous ring
- Planar — all atoms in the same plane
- Fully conjugated — every atom has a p orbital contributing to the π system
- (4n + 2) π electrons where n = 0, 1, 2, 3... → gives 2, 6, 10, 14... π electrons
📊 Aromatic vs. Antiaromatic vs. Nonaromatic
| Property | Aromatic | Antiaromatic | Nonaromatic |
|---|---|---|---|
| Cyclic? | Yes | Yes | May or may not |
| Planar? | Yes | Yes | No requirement |
| Conjugated? | Fully | Fully | Not fully |
| π Electrons | 4n + 2 | 4n | N/A |
| Stability | Extra stable | Extra unstable | Normal |
| Examples | Benzene (6π), Naphthalene (10π) | Cyclobutadiene (4π) | Cyclooctatetraene (8π, tub-shaped) |
💎 Structure of Benzene
Benzene is NOT three alternating single and double bonds (Kekulé structures). It is a resonance hybrid — all six C–C bonds are identical (139 pm, intermediate between single 154 pm and double 134 pm). The six π electrons are fully delocalized around the ring, giving benzene 36 kcal/mol of resonance stabilization energy.
⚗️ Electrophilic Aromatic Substitution (EAS)
The most important class of benzene reactions. An electrophile (E⁺) replaces one H on the ring:
| Reaction | Electrophile | Catalyst/Conditions | Product |
|---|---|---|---|
| Halogenation | X⁺ (Br⁺, Cl⁺) | FeX₃ or AlX₃ | Aryl halide |
| Nitration | NO₂⁺ | HNO₃ / H₂SO₄ | Nitroarene |
| Sulfonation | SO₃ | Fuming H₂SO₄ | Sulfonic acid |
| Friedel-Crafts Alkylation | R⁺ (carbocation) | RCl / AlCl₃ | Alkylbenzene |
| Friedel-Crafts Acylation | RCO⁺ (acylium) | RCOCl / AlCl₃ | Aryl ketone |
🎯 Directing Effects of Substituents
Existing substituents on benzene direct incoming electrophiles to specific positions:
| Type | Position | Ring Activity | Examples |
|---|---|---|---|
| Activating, ortho/para | o,p-directing | Activating (faster EAS) | –OH, –NH₂, –OR, –NHCOR, –R, –Ar |
| Weakly deactivating, ortho/para | o,p-directing | Deactivating | –F, –Cl, –Br, –I (halogens) |
| Deactivating, meta | m-directing | Deactivating (slower EAS) | –NO₂, –CN, –COOH, –COR, –SO₃H |
Electron-withdrawing groups (have multiple bonds to electronegative atoms) → meta directors.
🌐 Common Aromatic Compounds
Toluene
Methylbenzene — solvent, TNT precursor
Phenol
Hydroxybenzene — antiseptic, polymer precursor
Aniline
Aminobenzene — dyes, pharmaceuticals
Naphthalene
Fused bicyclic — mothballs (10 π e⁻, aromatic)
🧬 Heterocyclic Aromatics
Aromatic rings containing non-carbon atoms (N, O, S) are heterocycles. They are extremely important in biology and medicine:
- Pyridine (C₅H₅N) — 6-membered, nitrogen replaces one CH, 6π electrons
- Pyrrole (C₄H₅N) — 5-membered, nitrogen's lone pair is part of the π system, 6π electrons
- Furan (C₄H₄O) — 5-membered with oxygen, found in food chemistry
- Thiophene (C₄H₄S) — 5-membered with sulfur, used in pharmaceuticals
- Imidazole — found in histidine (amino acid) and histamine
- Purine & Pyrimidine — the bases of DNA and RNA (A, G, C, T, U)
Functional Groups
The reactive hearts of organic molecules
A functional group is a specific arrangement of atoms within a molecule that determines the compound's chemical reactivity, physical properties, and IUPAC name. The carbon skeleton provides the framework, but functional groups define behavior.
📊 Comprehensive Functional Group Table
| Functional Group | General Structure | IUPAC Suffix | Example | Found In |
|---|---|---|---|---|
| Alkyl Halide | R–X (X = F, Cl, Br, I) | halo- (prefix) | CH₃Cl (chloromethane) | Solvents, refrigerants |
| Alcohol | R–OH | -ol | CH₃OH (methanol) | Beverages, fuels, solvents |
| Phenol | Ar–OH | phenol | C₆H₅OH | Antiseptics, resins |
| Ether | R–O–R' | -oxy- / ether | CH₃OCH₃ (dimethyl ether) | Solvents, anesthetics |
| Aldehyde | R–CHO | -al | CH₃CHO (ethanal) | Flavors, preservatives |
| Ketone | R–CO–R' | -one | CH₃COCH₃ (propanone) | Solvents (acetone) |
| Carboxylic Acid | R–COOH | -oic acid | CH₃COOH (ethanoic acid) | Vinegar, fatty acids |
| Ester | R–COO–R' | -oate | CH₃COOCH₃ | Fragrances, fats |
| Amine | R–NH₂ / R₂NH / R₃N | -amine | CH₃NH₂ (methanamine) | Amino acids, dyes |
| Amide | R–CONH₂ | -amide | CH₃CONH₂ (ethanamide) | Proteins (peptide bond) |
| Nitrile | R–C≡N | -nitrile | CH₃CN (ethanenitrile) | Solvents, polymers |
| Thiol | R–SH | -thiol | CH₃SH (methanethiol) | Garlic, natural gas odorant |
📏 Priority Order for Naming (Highest → Lowest)
When multiple functional groups are present, the highest-priority group gets the suffix; others become prefixes.
🧪 Alcohol Classification & Properties
| Type | Structure | Oxidation Product | Example |
|---|---|---|---|
| 1° (Primary) | R–CH₂–OH | Aldehyde → Carboxylic Acid | Ethanol (CH₃CH₂OH) |
| 2° (Secondary) | R₂–CH–OH | Ketone | Propan-2-ol (isopropanol) |
| 3° (Tertiary) | R₃–C–OH | No oxidation (resistant) | 2-methylpropan-2-ol |
🔄 Interconversion of Functional Groups
Many reactions in organic chemistry involve converting one functional group to another. Key transformations include:
- Alkene → Alcohol (hydration or hydroboration-oxidation)
- Alcohol → Alkyl Halide (reaction with HX or SOCl₂)
- Alcohol → Aldehyde/Ketone (oxidation with PCC or Jones reagent)
- Aldehyde → Carboxylic Acid (oxidation with KMnO₄ or CrO₃)
- Carboxylic Acid + Alcohol → Ester (Fischer esterification, acid catalyst)
- Ester + NaOH → Carboxylate + Alcohol (saponification)
- Carboxylic Acid + Amine → Amide (with heat or coupling reagent)
- Nitrile + H₂O → Amide → Carboxylic Acid (hydrolysis)
🧠 Carbonyl Chemistry Overview
The carbonyl group (C=O) is the most important functional group in organic chemistry. Its polarity (Cᵟ⁺=Oᵟ⁻) makes the carbon electrophilic and the oxygen nucleophilic:
- Nucleophilic addition — nucleophiles attack the C=O carbon (aldehydes & ketones)
- Nucleophilic acyl substitution — nucleophiles replace leaving groups (esters, amides, acid chlorides)
- α-Carbon chemistry — protons adjacent to C=O are acidic (enolization, aldol reactions)
Major Types of Organic Reactions
The fundamental transformations that build and break molecules
All organic reactions can be classified into a few fundamental categories based on what happens to the bonds and atoms during the transformation.
1️⃣ Addition Reactions
Two reactants combine to form a single product. The π bond breaks, and two new σ bonds form. Common with alkenes, alkynes, and carbonyls.
Electrophilic Addition
Electrophile attacks π bond (HBr + alkene)
Nucleophilic Addition
Nucleophile attacks C=O (NaBH₄ + ketone)
Radical Addition
Free radicals add (HBr + ROOR + alkene)
2️⃣ Elimination Reactions
A small molecule (H₂O, HX, etc.) is removed from the substrate, creating a new π bond (double or triple).
| Type | Mechanism | Base Strength | Substrate | Product |
|---|---|---|---|---|
| E1 | 2-step, carbocation | Weak base | 3° and 2° RX | More substituted alkene (Zaitsev) |
| E2 | 1-step, concerted | Strong base | All RX | Zaitsev (usually) or Hofmann |
| E1cb | 2-step, carbanion | Strong base | Poor LG | Alkene |
3️⃣ Substitution Reactions
| Feature | SN1 | SN2 |
|---|---|---|
| Mechanism | 2-step (carbocation intermediate) | 1-step (concerted, backside attack) |
| Rate Law | Rate = k[substrate] | Rate = k[substrate][nucleophile] |
| Substrate | 3° > 2° (stabilized carbocation) | Methyl > 1° > 2° (steric access) |
| Nucleophile | Weak nucleophile OK | Strong nucleophile required |
| Solvent | Polar protic (stabilizes carbocation) | Polar aprotic (doesn't solvate Nu⁻) |
| Stereochemistry | Racemization (flat carbocation) | Inversion of configuration (Walden) |
| Rearrangements? | Yes (hydride/methyl shifts) | No |
4️⃣ Oxidation-Reduction (Redox)
In organic chemistry, oxidation-reduction is tracked by changes in carbon's oxidation state:
- Oxidation = loss of C–H bonds or gain of C–O/C–X bonds (adding oxygen, removing hydrogen)
- Reduction = gain of C–H bonds or loss of C–O bonds (adding hydrogen, removing oxygen)
Common oxidizing agents: KMnO₄, CrO₃, PCC, Jones reagent, NaOCl
Common reducing agents: NaBH₄ (mild), LiAlH₄ (strong), H₂/Pd (catalytic)
5️⃣ Condensation Reactions
Two molecules combine with the loss of a small molecule (usually water):
Other condensation examples: amide formation, acetal formation, aldol condensation.
6️⃣ Rearrangement Reactions
Atoms within a molecule reorganize to form a structural isomer. Common rearrangements include:
- 1,2-Hydride shift: H⁻ migrates to adjacent carbocation (stabilization)
- 1,2-Methyl shift: CH₃⁻ migrates to adjacent carbocation
- Beckmann rearrangement: Oxime → Amide
- Pinacol rearrangement: 1,2-diol → ketone
Reaction Mechanisms
Step-by-step electron flow — the heart of organic chemistry
A reaction mechanism describes the detailed, step-by-step sequence of bond-breaking and bond-forming events that transform reactants into products. Understanding mechanisms allows you to predict products, explain selectivity, and design new reactions.
🔑 Core Concepts
| Term | Definition | Example |
|---|---|---|
| Nucleophile | Electron-rich; donates electron pair | OH⁻, CN⁻, NH₃, H₂O, Br⁻ |
| Electrophile | Electron-poor; accepts electron pair | H⁺, R⁺ (carbocation), BF₃, C=O |
| Leaving Group | Departs with bonding electrons | X⁻, H₂O, OTs⁻, N₂ |
| Intermediate | Species formed and consumed during rxn | Carbocations, carbanions, radicals |
| Transition State | Highest energy point (not isolable) | ‡ notation on energy diagram |
| Activation Energy (Ea) | Energy barrier to reach TS | ΔG‡ determines reaction rate |
↗️ Curved Arrow Notation
Curved arrows show the movement of electron pairs (not atoms!):
- Full curved arrow (⟶): Movement of 2 electrons (heterolytic)
- Half (fishhook) arrow (⟶): Movement of 1 electron (homolytic / radical)
- Arrows always point FROM the electron source TO the electron sink
- Nucleophile → Electrophile (arrow from Nu to E⁺)
1. Never exceed an octet for 2nd-row elements (C, N, O, F)
2. Arrows flow from electron-rich to electron-poor
3. Formal charges must be tracked in each step
⚡ Carbocation Stability
Carbocations are key intermediates in SN1, E1, and electrophilic addition reactions:
Special: Benzylic ≈ Allylic > 3° (resonance-stabilized)
Stabilized by: hyperconjugation (σ-π overlap), inductive effects (electron-donating groups), and resonance (allylic, benzylic).
🔬 Detailed Mechanism: SN2
Result: Complete inversion of stereochemistry (Walden inversion) — like an umbrella turning inside out.
OH⁻ + CH₃–Br → CH₃–OH + Br⁻
🔬 Detailed Mechanism: SN1
Step 2: Nucleophile attacks the flat (sp²) carbocation from either face
Result: Racemization — mixture of both R and S products (often not exactly 50:50 due to ion pairing).
📊 SN1/SN2/E1/E2 Decision Guide
| Factor | Favors SN2 | Favors SN1 | Favors E2 | Favors E1 |
|---|---|---|---|---|
| Substrate | Methyl, 1° | 3° | 3° > 2° > 1° | 3° |
| Nucleophile | Strong Nu⁻ | Weak Nu | Strong base | Weak base |
| Solvent | Polar aprotic | Polar protic | Any | Polar protic |
| Temperature | Low–moderate | High | High | High |
⚡ Energy Diagrams
Reaction coordinate diagrams plot energy vs. reaction progress. Key features:
- Exergonic (ΔG < 0): Products lower in energy → spontaneous. Example: combustion.
- Endergonic (ΔG > 0): Products higher in energy → non-spontaneous.
- Activation energy (Ea): Height of the barrier from reactants to transition state
- Multi-step reactions: Multiple peaks (transition states) and valleys (intermediates)
- Rate-determining step: The step with the highest Ea (tallest peak)
🧠 Thermodynamic vs. Kinetic Control
Kinetic product — formed fastest (lower Ea, not necessarily most stable). Favored at low temperature and short reaction time.
Thermodynamic product — most stable product (lowest energy). Favored at high temperature and long reaction time (equilibrium conditions).
Classic example: 1,2- vs. 1,4-addition to conjugated dienes.
Stereochemistry
The 3D arrangement of atoms — where geometry meets reactivity
Stereochemistry studies how the three-dimensional arrangement of atoms in molecules affects their properties and reactions. Two molecules with the same molecular formula and connectivity but different spatial arrangements are called stereoisomers.
🗂️ Complete Classification of Isomers
├── Constitutional (Structural) Isomers — different connectivity
│ ├── Chain isomers (butane vs. isobutane)
│ ├── Position isomers (1-propanol vs. 2-propanol)
│ └── Functional group isomers (ethanol vs. dimethyl ether)
└── Stereoisomers — same connectivity, different spatial arrangement
├── Enantiomers — non-superimposable mirror images
└── Diastereomers — stereoisomers that are NOT mirror images
├── cis/trans (geometric) isomers
├── E/Z isomers
└── Compounds with multiple stereocenters
✋ Chirality & Chiral Centers
A molecule is chiral if it is not superimposable on its mirror image. The most common cause is a chiral center (stereocenter) — a carbon bonded to four different groups.
Example: a molecule with 3 stereocenters can have up to 2³ = 8 stereoisomers (4 pairs of enantiomers).
🔄 R/S Configuration (CIP Priority Rules)
- Assign priorities (1 → 4) to the four groups on the stereocenter by atomic number of the directly bonded atom (highest atomic # = priority 1)
- If tied, move outward along the chain until a point of difference is found
- Orient the molecule so priority 4 points AWAY from you (into the page)
- Trace a path from 1 → 2 → 3:
- Clockwise = R (rectus, Latin for right)
- Counterclockwise = S (sinister, Latin for left)
• Double bonds count as two single bonds to phantom atoms
• Triple bonds count as three single bonds to phantom atoms
• Isotopes: higher mass = higher priority (D > H, ¹⁴C > ¹²C)
🪞 Enantiomers — Properties
| Property | Enantiomers |
|---|---|
| Same melting point? | Yes |
| Same boiling point? | Yes |
| Same solubility? | Yes |
| Same density? | Yes |
| Same IR/NMR spectra? | Yes |
| Rotate plane-polarized light? | Yes, but in OPPOSITE directions |
| React differently with chiral reagents? | Yes — different rates! |
| Biological activity? | Often dramatically different |
🔬 Optical Activity
- Dextrorotatory (+, d): Rotates plane-polarized light clockwise
- Levorotatory (−, l): Rotates plane-polarized light counterclockwise
- Racemic mixture (±): Equal amounts of both enantiomers → net rotation = 0°
- Specific rotation: [α] = α / (l × c) where α = observed rotation, l = path length (dm), c = concentration (g/mL)
🧊 Meso Compounds
A meso compound has multiple stereocenters but an internal plane of symmetry, making it achiral overall. It is a special case where the stereoisomer count is less than 2ⁿ.
↔️ E/Z (Geometric) Isomerism
Restricted rotation around C=C double bonds or in rings creates geometric isomers:
- Z (zusammen): Higher-priority groups on the same side
- E (entgegen): Higher-priority groups on opposite sides
- Requirements: each doubly-bonded carbon must have two different substituents
🧠 Fischer Projections
Fischer projections are a 2D representation of 3D stereochemistry, widely used in carbohydrate and amino acid chemistry:
- Vertical lines = bonds going INTO the page (back)
- Horizontal lines = bonds coming OUT of the page (front)
- The intersection represents the chiral carbon
- Rule: you may rotate a Fischer projection 180° but NOT 90° (that inverts configuration)
- Swapping any two groups inverts the configuration (R↔S)
💊 Why Chirality Matters in Medicine
Since enzymes and receptors are chiral, they often distinguish between enantiomers:
- Ibuprofen: (S)-ibuprofen is the active painkiller; (R)-form is inactive
- Thalidomide: One enantiomer treats morning sickness; the other causes birth defects
- L-DOPA: Treats Parkinson's disease; D-DOPA is inactive
- Amino acids: All natural amino acids are L-configuration
- Sugars: Natural sugars are D-configuration
Modern pharmaceutical regulations often require testing of individual enantiomers, not just racemic mixtures.
Spectroscopy
Identifying organic compounds using electromagnetic radiation
Spectroscopy uses the interaction of electromagnetic radiation with matter to determine molecular structure. It is the primary tool for identifying unknown organic compounds and confirming synthetic products.
📡 The Four Major Techniques
IR Spectroscopy
Identifies functional groups by bond vibrations
¹H NMR
Reveals hydrogen environments & connectivity
¹³C NMR
Shows unique carbon environments
Mass Spectrometry
Determines molecular mass & fragmentation
🔴 Infrared (IR) Spectroscopy
IR light causes bonds to vibrate (stretch and bend). Different bonds absorb at characteristic frequencies (measured in wavenumbers, cm⁻¹).
| Bond | Wavenumber (cm⁻¹) | Appearance | Found In |
|---|---|---|---|
| O–H (alcohol) | 3200–3550 | Broad, strong | Alcohols, phenols |
| O–H (carboxylic acid) | 2500–3300 | Very broad | Carboxylic acids |
| N–H | 3300–3500 | Medium, 1 or 2 peaks | Amines, amides |
| C–H (sp³) | 2850–2960 | Strong | Alkanes, most organics |
| C–H (sp²) | 3020–3100 | Medium | Alkenes, aromatics |
| C–H (sp, ≡C–H) | 3300 | Strong, sharp | Terminal alkynes |
| C≡C | 2100–2260 | Weak–medium | Alkynes |
| C≡N | 2200–2260 | Medium | Nitriles |
| C=O | 1650–1750 | Strong, sharp | Aldehydes, ketones, esters, acids |
| C=C | 1620–1680 | Medium | Alkenes |
| C–O | 1000–1300 | Strong | Ethers, alcohols, esters |
🧲 ¹H NMR (Proton NMR)
NMR exploits the magnetic properties of atomic nuclei. Key information from a ¹H NMR spectrum:
- Number of signals: Number of unique hydrogen environments
- Chemical shift (δ, ppm): Position tells you the electronic environment
- Integration: Area under each peak → ratio of hydrogens
- Splitting pattern: n+1 rule — a proton with n neighboring non-equivalent protons splits into n+1 peaks
| Type of Proton | Chemical Shift (δ ppm) |
|---|---|
| R–CH₃ (alkyl) | 0.8–1.0 |
| R–CH₂–R (alkyl) | 1.2–1.4 |
| C=C–H (vinylic) | 4.5–6.5 |
| Ar–H (aromatic) | 6.5–8.5 |
| R–CHO (aldehyde) | 9.0–10.0 |
| R–OH (alcohol) | 1.0–5.0 (variable) |
| R–COOH (acid) | 10.0–12.0 |
💥 Mass Spectrometry (MS)
A mass spectrometer ionizes molecules and separates fragments by mass-to-charge ratio (m/z):
- Molecular ion peak (M⁺): Gives the molecular weight
- Base peak: Most abundant fragment (set to 100% intensity)
- M+1 peak: Due to ¹³C isotope — helps determine # of carbons
- Fragmentation pattern: Bonds break at specific locations revealing structure
−15 = loss of CH₃ (methyl)
−17 = loss of OH
−18 = loss of H₂O (alcohols)
−28 = loss of CO or C₂H₄
−29 = loss of CHO (aldehyde)
−31 = loss of OCH₃
−44 = loss of CO₂ (carboxylic acids)
−45 = loss of OC₂H₅ (ethyl ester)
Polymers
Giant molecules built from repeating monomer units
A polymer is a macromolecule composed of many repeating structural units called monomers, joined by covalent bonds. Polymers can be natural (proteins, DNA, cellulose) or synthetic (plastics, nylon, Teflon).
🏭 Types of Polymerization
| Feature | Addition (Chain-Growth) | Condensation (Step-Growth) |
|---|---|---|
| Mechanism | Chain reaction (initiation, propagation, termination) | Stepwise reaction between functional groups |
| Monomers | Unsaturated (C=C, typically) | Bifunctional (–OH + –COOH, etc.) |
| By-product? | No | Yes (H₂O, HCl, etc.) |
| MW builds | Rapidly (high MW early) | Gradually (step by step) |
| Examples | Polyethylene, PVC, polystyrene, Teflon | Nylon, polyester, Bakelite, proteins |
🧪 Common Addition Polymers
| Polymer | Monomer | Uses |
|---|---|---|
| Polyethylene (PE) | CH₂=CH₂ (ethene) | Plastic bags, bottles, packaging |
| Polypropylene (PP) | CH₂=CHCH₃ (propene) | Containers, automotive parts, textiles |
| Polystyrene (PS) | CH₂=CHC₆H₅ (styrene) | Packaging foam, insulation, cups |
| PVC | CH₂=CHCl (vinyl chloride) | Pipes, flooring, cables |
| Teflon (PTFE) | CF₂=CF₂ (tetrafluoroethene) | Non-stick coatings, seals |
| PMMA (Plexiglass) | Methyl methacrylate | Transparent sheets, lenses |
🧵 Common Condensation Polymers
| Polymer | Monomers | Bond Formed | Uses |
|---|---|---|---|
| Nylon 6,6 | Hexanediamine + adipic acid | Amide (–CONH–) | Textiles, rope, gears |
| Polyester (PET) | Ethylene glycol + terephthalic acid | Ester (–COO–) | Bottles, clothing, film |
| Kevlar | p-phenylenediamine + terephthaloyl chloride | Amide | Body armor, racing sails |
| Proteins | Amino acids | Peptide (amide) | Biological functions |
| DNA | Nucleotides | Phosphodiester | Genetic information |
🔗 Polymer Properties & Structure
- Thermoplastics — soften on heating, can be remolded (PE, PVC, nylon). No cross-links.
- Thermosets — harden permanently, cannot be remolded (Bakelite, epoxy). Extensive cross-links.
- Elastomers — flexible, return to shape (natural rubber, silicone). Light cross-links.
♻️ Polymer Sustainability & Recycling
Plastic pollution is a major environmental challenge. Key approaches to addressing it:
- Mechanical recycling: Melting and reshaping thermoplastics (most common)
- Chemical recycling: Breaking polymers back into monomers (depolymerization)
- Biodegradable polymers: PLA (polylactic acid) from corn starch, PHA from bacteria
- Resin identification codes: #1 PET, #2 HDPE, #3 PVC, #4 LDPE, #5 PP, #6 PS, #7 Other
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