Saturated hydrocarbons: contain only single (C–C) bonds, e.g. alkanes (general formula C_nH_2n+2) such as ethane.

Unsaturated hydrocarbons: contain at least one double (C=C) or triple (C≡C) bond, e.g. alkenes (C_nH_2n) and alkynes (C_nH_2n−2) such as ethene and ethyne.

Organic chemistry is the branch of chemistry that deals with the study of hydrocarbons and their derivatives, i.e. the compounds of carbon (except oxides of carbon, carbonates, bicarbonates, carbides, etc.).

Condensed formula: CH₃–CH(OH)–CH₂–CH₃

    H   H   H   H
    │   │   │   │
H ─ C ─ C ─ C ─ C ─ H
    │   │   │   │
    H   O   H   H
        │
        H

Organic compounds are broadly classified into two main classes:

Organic compounds
├── Acyclic (open chain)
│   ├── Straight chain
│   └── Branched chain
└── Cyclic (closed chain)
    ├── Homocyclic (Carbocyclic)
    │   ├── Alicyclic
    │   └── Aromatic
    └── Heterocyclic

Ethylamine (C₂H₅NH₂) is a derivative of the hydrocarbon ethane (C₂H₆) in which one hydrogen atom is replaced by the amino group (−NH₂). It contains carbon and hydrogen bonded together, so it is an organic compound.

Carbocyclic (homocyclic) compounds are cyclic compounds whose ring is made up of carbon atoms only. They are of two types: alicyclic (e.g. cyclopropane, cyclohexane) and aromatic (e.g. benzene).

Condensed formula: HCOOC₂H₅ or HCOOCH₂CH₃

    O       H   H
    ║       │   │
H ─ C ─ O ─ C ─ C ─ H
            │   │
            H   H

Alicyclic compounds are carbocyclic compounds that contain a ring of carbon atoms but behave like aliphatic (open-chain) compounds. The ring may have single or multiple bonds, e.g. cyclopropane, cyclohexane and cyclohexene.

The phenomenon in which two or more compounds have the same molecular formula but different structural formulae (different arrangement of atoms) is called structural isomerism. Example: butane and 2-methylpropane, both C₄H₁₀.

A formula that shows all the atoms present in a molecule and all the covalent bonds between them (each bond drawn as a line) is called a displayed or full structural formula.

    H   H
    │   │
H ─ C ─ C ─ H
    │   │
    H   H

Butane (four carbon atoms in a straight chain, all single bonds).

All three contain the carbonyl group (C=O). In each, the carbonyl carbon is also bonded to another atom/group: −OH in carboxyl, −NH₂ in amide and −OR in ester.

  O
  ║
─ C ─ O ─ H

  O
  ║
─ C ─ N ─ H
        │
        H

  O
  ║
─ C ─ O ─ R

Condensed formula: CH₃COOCH₃

    H   O       H
    │   ║       │
H ─ C ─ C ─ O ─ C ─ H
    │           │
    H           H

Because all members of a homologous series contain the same functional group, and the chemical properties of an organic compound are mainly determined by its functional group.

C_nH_2n+2 (where n = 1, 2, 3 …), e.g. CH₄, C₂H₆, C₃H₈.

Both have the formula C₄H₁₀ but different structures, so their physical properties differ. n-Butane is a straight chain with a larger surface area and stronger van der Waals forces, hence a higher boiling point (about −0.5 °C). 2-Methylpropane is branched and more compact, with weaker forces and a lower boiling point (about −11.7 °C). Their chemical properties are similar.

CH₃ ─ CH₂ ─ CH₂ ─ CH₃

      CH₃
       │
CH₃ ─ CH ─ CH₃

Aromatic compounds are carbocyclic compounds that contain one or more benzene rings (a planar six-carbon ring with alternate single and double bonds), and usually have a characteristic smell. Examples: benzene (C₆H₆), toluene, phenol.

Pentane, C₅H₁₂, shows three isomers:

CH₃ ─ CH₂ ─ CH₂ ─ CH₂ ─ CH₃

CH₃ ─ CH ─ CH₂ ─ CH₃
      │
      CH₃

      CH₃
      │
CH₃ ─ C ─ CH₃
      │
      CH₃

No. Structural isomerism is also found in unsaturated hydrocarbons (e.g. butene: but-1-ene and but-2-ene) and in compounds with functional groups (e.g. ethanol CH₃CH₂OH and dimethyl ether CH₃OCH₃, both C₂H₆O).

Butene, C₄H₈:

CH₂ = CH ─ CH₂ ─ CH₃

CH₃ ─ CH = CH ─ CH₃

      CH₃
       │
CH₃ ─ C = CH₂

Phenols are compounds in which the hydroxyl group (−OH) is attached directly to a carbon atom of a benzene (aromatic) ring, e.g. phenol C₆H₅OH. In alcohols, the −OH group is attached to a saturated carbon atom of an alkyl group, e.g. ethanol. Phenols are weakly acidic, while alcohols are neutral.

CH₃ ─ CH₂ ─ OH

• Aldehyde (−CHO): carbonyl carbon is bonded to at least one hydrogen; it lies at the end of the chain.
• Ketone (>C=O): carbonyl carbon is bonded to two carbon atoms, within the chain.
• Carboxylic acid (−COOH): carbonyl carbon is also bonded to a hydroxyl (−OH) group.

    O
    ║
R ─ C ─ H

     O
     ║
R ─ C ─ R'

    O
    ║
R ─ C ─ O ─ H

An ester name has two words: the first is the alkyl group of the alcohol part, and the second is the alkanoate name from the acid part (replace “-oic acid” with “-oate”). Example: CH₃COOCH₃ = methyl ethanoate; HCOOC₂H₅ = ethyl methanoate.