Until 1828, every known organic compound (citric acid, lactic acid, urea, soap, alcohol from grapes) had been isolated from a living thing. The accepted explanation was the Vital Force Theory — a mysterious "vis vitalis" in living organisms was needed to make organics.
In 1828, Friedrich Wöhler (1800–1882, German) heated ammonium cyanate NH₄OCN — an inorganic salt — and got urea (NH₂)₂C=O, the same urea that had been isolated from mammalian urine. One inorganic-to-organic experiment, no living tissue: vital force disproved.
Modern definition: organic = carbon-containing compounds, with a small set of conventional exceptions classed as inorganic: CO, CO₂, H₂CO₃ (carbonic acid), MCO₃ (carbonates), MHCO₃ (bicarbonates), MCN (cyanides), MOCN (cyanates).
Five distinguishing features vs inorganics: built from C/H/O/N/S/P/X; covalent bonds (not ionic); low melting/boiling points; density < 1 usually; low water solubility, good organic-solvent solubility (like-dissolves-like).
Chemical: organics are combustible, react slowly with side products and modest yields; inorganic ionic reactions are fast and clean.
Electronegativity (χ) = an atom's pull on the bond's electrons. Pauling scale (memorise the order): F 4.0 > O 3.5 > N, Cl 3.0 > Br 2.8 > C, S 2.5 > H 2.1.
A bond is nonpolar if Δχ ≈ 0 (C–C, C–H), polar covalent if Δχ ~ 0.5–1.9 (C–O, C–Cl, O–H), and essentially ionic if Δχ > ~1.9 (Na–Cl).
Polar bonds do NOT mean polar molecule. If geometry is symmetric, dipoles cancel.
| Molecule | Geometry | Polar molecule? |
|---|---|---|
| CCl₄ | Tetrahedral symmetric | No (dipoles cancel) |
| CHCl₃ | Tetrahedral, 3 Cl + 1 H | Yes |
| CO₂ | Linear O=C=O | No |
| H₂O | Bent 104.5° | Yes |
Bond parameters — memorise the row: C–C 154 pm / 347 kJ mol⁻¹ · C=C 134 pm / 614 · C≡C 120 pm / 839. Shorter = stronger. A triple bond is NOT three times as strong as a single — the σ component is the same, the two π's add less.
Atomic C is 2s²2p² → only 2 unpaired electrons. Yet CH₄ has 4 equivalent bonds at 109.5°. Resolution = orbital hybridization (Pauling, 1931): atomic orbitals of similar energy on the same atom recombine into a new equivalent hybrid set with different shape, energy and direction. Total orbital count is conserved.
VSEPR tells you the geometry: "the best arrangement of electron domains is the one that minimises repulsion".
| State | Mix | Shape | Angle | σ | π | Examples |
|---|---|---|---|---|---|---|
| sp³ | 1s + 3p → 4 | Tetrahedral | 109.5° | 4 | 0 | CH₄, all alkanes, R–OH carbon, R–NH₂ carbon |
| sp² | 1s + 2p → 3 + 1p leftover | Trigonal planar | 120° | 3 | 1 | C=C alkene, C=O carbonyl, benzene |
| sp | 1s + 1p → 2 + 2p leftover | Linear | 180° | 2 | 2 | C≡C alkyne, HCN, CO₂ |
Rotation rule: σ bond overlap is end-to-end, cylindrically symmetric → rotates freely. π bond overlap is sideways above/below the plane → rotation would tear it. Therefore cis/trans isomers of alkenes exist; this is what Ch 3 builds on.
Be fluent moving between all four. Exam will give one, ask for another.
Memorise every row. Recognising the FG predicts reactivity for the rest of the syllabus.
| Family | Functional group | General formula | Example | Suffix |
|---|---|---|---|---|
| Alkane | none (only single bonds) | CnH2n+2 | CH₃CH₃ ethane | -ane |
| Alkene | C=C | CnH2n | CH₂=CH₂ ethene | -ene |
| Alkyne | C≡C | CnH2n-2 | HC≡CH ethyne | -yne |
| Aromatic | benzene ring | C6+nH6+2n | C₆H₆ benzene | (ring) |
| Alcohol | –OH (sp³ C) | R–OH | CH₃CH₂OH | -ol |
| Phenol | –OH (Ar) | Ar–OH | C₆H₅OH | (ring+-ol) |
| Ether | R–O–R′ | R–O–R′ | CH₃OCH₃ | oxy-/ether |
| Aldehyde | –CHO | R–CHO | CH₃CHO | -al |
| Ketone | >C=O | R–CO–R′ | CH₃COCH₃ | -one |
| Carboxylic acid | –COOH | R–COOH | CH₃COOH | -oic acid |
| Ester | –COO–R′ | R–COO–R′ | CH₃COOCH₂CH₃ | -oate |
| Amine | –NH₂ (1°) | R–NH₂ | CH₃NH₂ | -amine |
| Amide | –CONH₂ | R–CONH₂ | CH₃CONH₂ | -amide |
Homologous series: same FG + same general formula, each next member adds CH₂. e.g. CH₃OH → C₂H₅OH → C₃H₇OH. Reactivity stays the same; physical properties change gradually.
Isomers: same molecular formula, different arrangement of atoms. In Ch 1 we meet structural / constitutional isomers only (different connectivity). Stereoisomers (same connectivity, different 3-D arrangement — cis/trans, R/S) are introduced in Ch 3 + Ch 8.
| Type | What changes | Worked example |
|---|---|---|
| Chain (skeletal) | Carbon skeleton | C₄H₁₀ → n-butane vs isobutane |
| Positional | FG position on same skeleton | C₃H₈O → 1-propanol vs 2-propanol |
| Functional-group | Different FG entirely | C₂H₆O → ethanol vs dimethyl ether |
Slide-page practice answers: C₄H₁₀ → 2 isomers, C₅H₁₂ → 3 (n-, iso-, neopentane), C₂H₆O → 2 (ethanol, DME).