Organic Chemistry
Organic chemistry is the study of carbon compounds — historically those derived from living organisms, today an immense field embracing petroleum, polymers, pharmaceuticals and biomolecules. Carbon's unique tetravalency and ability to catenate (form long chains and rings) underlie millions of known organic substances.
A specific arrangement of atoms within a molecule that determines its characteristic chemical reactivity. Examples: −OH (alcohol), −COOH (carboxylic acid), C=C (alkene), C≡C (alkyne).
Hydrocarbons
The simplest organic compounds contain only C and H. They divide into:
| Class | General formula | Example | Key reaction |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | CH₄, C₂H₆ | Free-radical substitution |
| Alkenes | CₙH₂ₙ | C₂H₄ | Electrophilic addition |
| Alkynes | CₙH₂ₙ₋₂ | C₂H₂ | Electrophilic/nucleophilic addition |
| Cycloalkanes | CₙH₂ₙ | Cyclohexane | Substitution, ring strain |
| Aromatic | Benzene rings | C₆H₆ | Electrophilic substitution |
Alkanes are unreactive ("paraffins" — little affinity). Alkenes and alkynes contain π-bonds and react readily with electrophiles. Benzene's six π-electrons are delocalised (aromaticity, Hückel's 4n+2 rule), giving exceptional stability.
Functional groups
| Group | Class | Example |
|---|---|---|
| −OH | Alcohol | CH₃CH₂OH (ethanol) |
| −O− | Ether | CH₃OCH₃ |
| C=O | Aldehyde / Ketone | CH₃CHO / CH₃COCH₃ |
| −COOH | Carboxylic acid | CH₃COOH |
| −COO− | Ester | CH₃COOC₂H₅ |
| −NH₂ | Amine | CH₃NH₂ |
| −CONH₂ | Amide | CH₃CONH₂ |
| −X (F, Cl, Br, I) | Haloalkane | CH₃Cl |
| −CN | Nitrile | CH₃CN |
| −NO₂ | Nitro | CH₃NO₂ |
The IUPAC name of an organic compound consists of a parent chain (longest C chain with the principal functional group), substituent prefixes, and a suffix for the principal group.
Reaction mechanisms
Organic reactions fall into a small set of mechanistic types:
- Substitution — one group replaces another. Examples: SN1 (carbocation intermediate) and SN2 (concerted, inverts stereochemistry).
- Elimination — two groups leave, forming a π-bond. E1 and E2 mechanisms parallel SN1/SN2.
- Addition — π-bond opens; two new σ-bonds form. Markovnikov's rule guides HX addition to alkenes.
- Rearrangement — atoms reorganise to a more stable structure.
- Radical — one-electron processes (chlorination of alkanes, combustion, polymerisation).
- Markovnikov's rule: when HX adds to an unsymmetrical alkene, H goes to the carbon with more hydrogens; X to the more-substituted carbon (forming the more stable carbocation).
- Saytzeff's rule (eliminations): the more-substituted alkene is the major product.
- Carbocation stability: tertiary > secondary > primary > methyl (hyperconjugation + induction).
- Aromatic rings undergo electrophilic substitution (nitration, halogenation, Friedel–Crafts) rather than addition.
Stereochemistry
Three-dimensional aspects of organic molecules:
- Constitutional (structural) isomers differ in connectivity (e.g., butane vs isobutane).
- Stereoisomers share connectivity but differ in 3-D arrangement. They subdivide into:
- Enantiomers — non-superimposable mirror images (chiral pairs). They rotate plane-polarised light in opposite directions.
- Diastereomers — stereoisomers that are not mirror images (cis/trans, multiple chiral centres).
- A carbon with four different substituents is a chiral centre. Designations R/S (Cahn–Ingold–Prelog) describe configuration.
Most biomolecules are chiral and biology uses only one enantiomer: L-amino acids in proteins, D-sugars in nucleic acids. The thalidomide tragedy of the 1960s — one enantiomer cured morning sickness; the other caused birth defects — drove modern emphasis on enantiopure pharmaceuticals.
Polymers
Polymers are macromolecules built from repeating monomer units. Two main types of synthesis:
- Addition (chain-growth) polymerisation: monomers with C=C bonds (e.g., ethylene → polyethylene, vinyl chloride → PVC, styrene → polystyrene).
- Condensation (step-growth) polymerisation: monomers join with loss of a small molecule, usually water (e.g., nylons, polyesters, Bakelite).
Natural polymers — cellulose, proteins, nucleic acids, rubber — illustrate the same principles. The Nobel-winning discovery of Ziegler–Natta catalysts (1953) enabled stereo-regular polymers like high-density polyethylene and isotactic polypropylene.
Biomolecules
Four classes dominate biochemistry:
- Carbohydrates — Cₙ(H₂O)ₙ; monosaccharides (glucose, fructose), disaccharides (sucrose, lactose), polysaccharides (starch, cellulose, glycogen).
- Lipids — fatty acids, triglycerides, phospholipids, steroids. Hydrophobic, energy-dense (9 kcal/g).
- Proteins — polymers of 20 standard L-amino acids linked by peptide bonds; folded structure determines function (enzymes, transport, structural).
- Nucleic acids — DNA and RNA; nucleotide building blocks of sugar + phosphate + base; encode and translate genetic information.
Spectroscopy quick reference
Organic chemists routinely identify structures using:
- IR (infrared) — functional groups (O–H ~3300, C=O ~1700, C–H ~2900 cm⁻¹).
- NMR (nuclear magnetic resonance) — atomic environment of ¹H and ¹³C.
- Mass spectrometry — molecular weight and fragmentation pattern.
- UV-visible — conjugated π-systems.
Together these tools allow elucidation of complex natural products, drug candidates, and reaction intermediates that would have taken months of degradation chemistry a century ago.