Hydrocarbons, functional groups and homologous series
Hydrocarbons contain only carbon and hydrogen: alkanes, alkenes, alkynes. A functional group decides a compound's properties; a homologous series differs step by step by -CH₂-.
Grouping organic compounds by their carbon skeleton
Open chain (acyclic) compounds have carbon atoms in a straight or branched chain, for example butane. Closed chain (cyclic) compounds have carbon atoms joined in a ring. A ring made only of carbon atoms is homocyclic: it may be alicyclic (cyclohexane) or aromatic (benzene). A ring that also has another atom, such as oxygen or nitrogen, is heterocyclic (furan, pyridine).
Three families of hydrocarbons
| Family | General formula | Carbon-carbon bond | Name ends in | First members |
|---|---|---|---|---|
| Alkanes (saturated) | CₙH₂ₙ₊₂ | Single bonds only | -ane | Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀ |
| Alkenes (unsaturated) | CₙH₂ₙ | One double bond, C=C | -ene | Ethene C₂H₄, propene C₃H₆, butene C₄H₈ |
| Alkynes (unsaturated) | CₙH₂ₙ₋₂ | One triple bond, C≡C | -yne | Ethyne C₂H₂, propyne C₃H₄, butyne C₄H₆ |
Common functional groups
| Class | Functional group | Examples |
|---|---|---|
| Alcohol | -OH (hydroxyl) | CH₃OH methanol; C₂H₅OH ethanol |
| Aldehyde | -CHO | HCHO methanal; CH₃CHO ethanal |
| Ketone | -CO- (C=O joined to two carbon atoms) | CH₃COCH₃ propanone (acetone) |
| Carboxylic acid | -COOH (carboxyl) | HCOOH methanoic acid; CH₃COOH ethanoic acid |
| Ester | -COOR | CH₃COOC₂H₅ ethyl ethanoate |
| Ether | -O- | CH₃OCH₃ methoxymethane |
| Halide (haloalkane) | -X (X = Cl, Br or I) | CH₃Cl chloromethane |
Homologous series: definition and characteristics
- A homologous series is a group of organic compounds with the same functional group, in which each member differs from the next by a -CH₂- group.
- All members have the same general formula (alkanes CₙH₂ₙ₊₂; alcohols CₙH₂ₙ₊₁OH).
- Each member differs from the next by -CH₂-, that is, by 14 u in molecular mass.
- All members have the same functional group, so they show similar chemical properties.
- Physical properties change gradually (gradation): boiling point and melting point rise as the molecular mass rises.
- All members can be prepared by similar general methods.
Check the -CH₂- difference
Find the molecular masses of methane, ethane and propane, and the difference between them. (C = 12 u, H = 1 u)
- Methane, CH₄ = 12 + (4 × 1) = 16 u
- Ethane, C₂H₆ = (2 × 12) + (6 × 1) = 24 + 6 = 30 u
- Propane, C₃H₈ = (3 × 12) + (8 × 1) = 36 + 8 = 44 u
- Differences: 30 − 16 = 14 u and 44 − 30 = 14 u
- 14 u is the mass of one CH₂ group: 12 + (2 × 1) = 14 u
Each member is 14 u heavier than the one before it: the mass of one -CH₂- group.
The gas in an LPG cylinder (propane and butane) and the CNG in buses (methane) are alkanes. Ethyne (acetylene) burns with a very hot flame for welding; calcium carbide gives off ethyne, and using it to ripen mangoes is banned in India.
1 marks: “Write the general formula of alkynes.” CₙH₂ₙ₋₂, for example ethyne, C₂H₂.
2 marks: “What is a functional group? Name the functional group in ethanoic acid.” An atom or group of atoms that decides the chemical properties of an organic compound. Ethanoic acid has the carboxyl group, -COOH.
4 marks: “Write the characteristics of a homologous series.” Same general formula; successive members differ by -CH₂- (14 u); same functional group and similar chemical properties; gradual change in physical properties; similar methods of preparation.
Ane, ene, yne: one, two, three bonds between two carbons. More bonds, fewer hydrogens: 2n + 2, 2n, 2n − 2.