• Because of the special properties of the carbon atom:

1. Carbon can join to other carbon atoms and to atoms of other elements (H, O, N, halogens, S …) by covalent bonds.
2. It can form single, double and triple bonds (C–C, C=C, C≡C; also C=O, C≡N).
3. Carbon atoms can link together in many different arrangements: straight chains, branched chains, and rings (homocyclic rings contain only carbon; heterocyclic rings contain carbon plus another atom such as N, O or S).
4. the same molecular formula can correspond to several different structures, multiplying the number of compounds.

• Because there are so many compounds, chemists classify them into families (groups), each with its own functional group and naming rules; we study the important ones in this chapter.

• Ethanol CH₃CH₂OH — b.p. 78.5 °C, m.p. −117 °C; contains an –OH group; reacts with sodium giving hydrogen.
• Dimethyl ether CH₃–O–CH₃ — b.p. −24 °C (a gas at room temperature); no –OH, does not react with sodium.
• (The much higher boiling point of ethanol is due to hydrogen bonds between its molecules.)

• Structural formulas look flat, but real molecules are three-dimensional (for example the four bonds of carbon in methane point toward the corners of a tetrahedron, H–C–H angle 109.5°).
• Molecular models (coloured plastic balls and sticks) are used to picture the true shape.

• An alkyl radical is the group obtained by removing one hydrogen atom from an alkane.
• It cannot exist alone.
• General formula CₙH₂ₙ₊₁–, symbol R.
• The name replaces -ane by -yl: methane → methyl (CH₃–), ethane → ethyl (C₂H₅–), propane → propyl (C₃H₇–), butane → butyl (C₄H₉–).
• Examples of compounds: CH₃Cl methyl chloride, C₂H₅Br ethyl bromide, C₃H₇I propyl iodide, C₄H₉Cl butyl chloride.

• The strong C–C and C–H σ bonds make alkanes relatively inert; they react only under special conditions.
• 1. Combustion. All alkanes burn in excess air giving CO₂ and H₂O and a lot of heat (highly exothermic) — hence their use as fuels.

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + heat

• (With limited oxygen, carbon monoxide or soot forms.)
• 2. Substitution by halogens. In ultraviolet light or at about 400 °C, halogens replace hydrogen atoms one by one (substitution). With methane and chlorine, the products depend on the ratio:
• CH₄ + Cl₂ —(UV)→ CH₃Cl + HCl (chloromethane, methyl chloride)

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane)

• CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (trichloromethane, chloroform)
• CHCl₃ + Cl₂ → CCl₄ + HCl (tetrachloromethane, carbon tetrachloride)
• With ethane the first product is chloroethane CH₃CH₂Cl, then dichloroethanes (1,1- and 1,2-) etc.
• Uses of halogenated alkanes:
• Chloroform was used as an anaesthetic, but it is dangerous (dose is hard to control and it can cause death); halothane, CF₃–CHBrCl (2-bromo-2-chloro-1,1,1-trifluoroethane), is the safer modern anaesthetic.
• 1,1,1-Trichloroethane is used in dry cleaning.
• Freons (chlorofluorocarbons, e.g. CF₂Cl₂ dichlorodifluoromethane) were used in refrigerators and air conditioners, as aerosol propellants and cleaning agents, because they are cheap, easily liquefied, non-poisonous and non-corrosive. But they destroy the ozone layer that protects us from harmful ultraviolet radiation, so an international agreement bans them.
• 3. Thermal (catalytic) cracking. Heating heavy, long-chain petroleum fractions at high temperature and pressure with a catalyst breaks them into smaller molecules, giving (a) short-chain alkanes used as petrol (gasoline) and (b) short-chain alkenes (ethene, propene) used for making polymers and other chemicals. For example:
• C₁₂H₂₆ → C₈H₁₈ + C₄H₈ (one possible cracking; the products here: octane + butene)

C₈H₁₈ → C₄H₁₀ + C₄H₈ (octane → butane + butene)