What is organic chemistry?
Organic chemistry is the study of compounds of carbon (except simple ones like CO₂, carbonates and cyanides). Long ago people thought these compounds came only from living things, which is why they are called organic. In 1828 Friedrich Wöhler made urea in a lab from a non-living salt, and that idea ended.
Carbon is special for two reasons. It makes 4 bonds, and it can join to other carbon atoms again and again to make long chains and rings. This joining is called catenation.
Tetravalence of carbon
Carbon (atomic number 6) has the configuration 1s² 2s² 2p². It has 4 valence electrons (outer electrons). It shares all 4 to reach 8 electrons. So carbon is tetravalent: it always forms 4 bonds.
The 4 bonds can be arranged as: four single bonds; two single + one double; one single + one triple; or two double bonds.
Hybridisation and shapes of carbon compounds
Hybridisation means mixing of orbitals to make new, equal orbitals. The number of π (pi) bonds on a carbon tells its hybridisation.
| Bonds on C | Hybrid | Shape | Angle | Example |
|---|---|---|---|---|
| 4 single | sp³ | tetrahedral | 109.5° | CH₄, ethane |
| 1 double | sp² | trigonal planar (flat) | 120° | ethene, HCHO |
| 1 triple or 2 double | sp | linear | 180° | ethyne, CO₂ |
How hybridisation changes bond length and strength
The more s-character an orbital has, the closer its electrons stay to the nucleus. sp has 50% s, sp² 33%, sp³ 25%. So an sp carbon holds electrons most tightly (it is the most electronegative), and its bonds are shortest. C–H length: sp³ 109 pm, sp² 108 pm, sp 106 pm.
π bonds and reactivity
A π bond forms by sideways overlap of p orbitals, above and below the bond line. Its electrons are exposed, so double and triple bonds are the reactive sites where many reactions start. The p orbitals must stay parallel, so atoms joined by a double bond cannot rotate freely.
Structural formulae: ways to draw a molecule
Complete (expanded) formula
Every atom and every bond is shown by a line. Single bond = one dash, double = two, triple = three.
Condensed formula
Bonds are hidden and identical groups are grouped: propan-1-ol is CH₃CH₂CH₂OH; hexane CH₃(CH₂)₄CH₃.
Bond-line (skeletal) formula
Only the carbon skeleton is drawn as a zigzag. Rules: every corner and every line end is a carbon; H atoms on carbon are not shown (count them so each C has 4 bonds); all other atoms (O, N, Cl…) are written. A ring is drawn as a polygon: cyclohexane is a hexagon.
Three-dimensional (wedge-dash) formula
Solid wedge (▲) = bond coming out of the page towards you. Dashed wedge = bond going behind the page. Normal line = bond in the plane of the paper. Ball-and-stick and space-filling models are 3D models too.
Classification of organic compounds
By structure
- Acyclic / open-chain (aliphatic): straight or branched chains. Examples: ethane, isobutane, acetaldehyde.
- Cyclic (closed chain):
- Alicyclic: ring of carbon atoms that behaves like an aliphatic compound (cyclopropane, cyclohexane).
- Aromatic: benzene and similar rings. Benzenoid (benzene, naphthalene) and non-benzenoid (tropolone).
- Heterocyclic: ring containing another atom such as O, N or S (furan, pyridine, thiophene).
- Homocyclic rings have only carbon.
By functional group
A functional group is the atom or group that decides the chemical properties: –OH (alcohol), –CHO (aldehyde), >C=O (ketone), –COOH (carboxylic acid), –NH₂ (amine), –X (haloalkane), C=C (alkene), C≡C (alkyne).
Homologous series
A family with the same functional group where next members differ by –CH₂– (14 u). Members share a general formula (alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH), have similar chemical properties and show a steady change in physical properties.
Try it
Clay-ball and toothpick model: 4 sticks = methane (tetrahedral), then join two sticks to show a double bond (flat). In the 3D, step 6: add CH₂ units and write the formula of each homologue before the screen shows it.
Key formulas and definitions
- Carbon: 4 valence electrons → 4 bonds (tetravalent)
- 0 π bonds → sp³, 109.5°; 1 π → sp², 120°; 2 π → sp, 180°
- % s-character: sp 50%, sp² 33.3%, sp³ 25%
- Alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂, alcohols CₙH₂ₙ₊₁OH
- Consecutive homologues differ by CH₂ = 14 u
Worked examples
1. State the hybridisation of each carbon in CH₂=CH–C≡N.
Line 1: C1 (CH₂=) has one double bond → 1 π → sp². Line 2: C2 (=CH–) has one double bond → sp². Line 3: C3 (–C≡N) has a triple bond → 2 π → sp. Answer: sp², sp², sp.
2. Write the condensed and bond-line formula of butan-2-ol.
Line 1: Chain of 4 carbons, –OH on carbon 2. Line 2: Condensed: CH₃CH(OH)CH₂CH₃. Line 3: Bond-line: a zigzag of 3 lines (4 ends/corners = 4 C), with OH written on the second carbon.
3. A bond-line drawing is a zigzag of 4 lines with a double bond between the 1st and 2nd carbons. Find its molecular formula.
Line 1: 4 lines in a chain → 5 carbon atoms. Line 2: One C=C, so it is an alkene: CₙH₂ₙ. Line 3: n = 5 → C₅H₁₀ (pent-1-ene).
4. Classify: (a) cyclohexane (b) pyridine (c) benzene (d) isobutane.
(a) Carbon-only ring, non-aromatic → alicyclic. (b) Ring with N → heterocyclic (aromatic). (c) Benzenoid aromatic. (d) Branched open chain → acyclic (aliphatic).
5. The 3rd member of the alcohol series is C₃H₇OH. Write the 5th member and its molar mass.
Line 1: Each next member adds CH₂. Line 2: 5th member: C₅H₁₁OH. Line 3: M = 5(12) + 12(1) + 16 = 88 g/mol.
6. How many σ and π bonds are in CH₂=CH–CH=CH₂ (buta-1,3-diene)?
Line 1: C–H bonds: 6, all σ. Line 2: C–C links: 3 (each has one σ) → 3 σ. Line 3: Two double bonds → 2 π. Answer: 9 σ and 2 π.
Common mistakes
- Forgetting the hidden hydrogens in a bond-line formula. Each corner carbon needs enough H to make 4 bonds.
- Calling benzene 'alicyclic'. Benzene is aromatic; alicyclic rings behave like chains.
- Thinking the carbon in CO₂ is sp². It has two double bonds (2 π), so it is sp and linear.
- Thinking members of a homologous series have the same physical properties. Chemical properties are similar; boiling point and density change steadily.