These are molecules that contain carbon and hydrogen only and include alkanes, alkenes, alkynes and aromatic compounds.

Reference: McMurry Ch 2, George et al Ch 1.1

• "Saturated" hydrocarbons – no multiple bonds
• General formula C_nH_2n+2 for acyclic alkanes
• Outer shell of a carbon atom has one s and three p orbitals

 Consider methane, CH₄

All C-H bonds are the same length (154 pm), strength (412 kJ mol^-1) and chemically equivalent.

Hybridisation gives four identical sp³ orbitals, which form a tetrahedral shape to minimise repulsion between the orbitals.

The name of an organic molecule consists of several parts:

The stem indicates the number of carbon atoms in the longest chain containing the functional group. The ending indicates the nature of the functional group present. Substituents are indicated by prefixes and a number is used to locate any substituent or functional group unambiguously in a molecule.

• Find and name the longest carbon chain ® stem
• Identify substituents
• Number longest carbon chain to give lowest numbering for substituents
• Allocate a number to every substituent
• List substituents in alphabetical order
• Identical side chains are indicated by using prefixes: di (2), tri (3), tetra (4) etc

"Straight chain" alkanes form a homologous series that differ by a CH₂ group. They have the general formula C_nH_2n+2

 e.g.

In these cases there is only one possible formula, however with C₄H₁₀ there are two possibilities:

• Molecules with the same formula but a different bond connectivity are called constitutional isomers (sometimes structural isomers)
• Constitutional isomers have different physical properties
• Molecules containing heteroatoms may have quite different functional groups, and hence chemistry. For example a compound with formula C₄H₈O may be a ketone, aldehyde, alcohol or ether.

As the number of carbon atoms increases the number of constitutional isomers increases rapidly.

C₅H₁₂ has 3 constitutional isomers

C₆H₁₄ has 5 constitutional isomers

C₁₀H₂₂ has 75 constitutional isomers

C₂₀H₄₂ has 366,319 constitutional isomers

Alkanes can be drawn in a number of different ways that differ by rotation around a C-C s bond. These are conformational isomers or conformers. Conformers normally can not be isolated from one another as interconversion of one conformer to another is rapid. Conformational isomers are part of a class of isomers called stereoisomers, which possess the same bond connectivity but differ in the position of the atoms in space.

Example: Ethane

• Cyclic, saturated alkanes; general formula C_nH_2n
• Named with cyclo

• Rings do not allow free rotation around the single bonds and thus obtain stereoisomers when two substituents are attached.

• Interconversion of the stereoisomers requires breaking of bonds. This does not occur at room temperature so the two forms may be separated from one another.

• These isomers are called diastereoisomers or diastereomers.

• Natural gas 90% methane, 10% ethane + others
• Petroleum refining higher alkanes

• Strong, non-polar C-C and C-H bonds mean alkanes are basically inert
• Good solvents
• Combustion in air to give CO₂, H₂O and heat

C₈H₁₈ + 12½ O₂ ® 8 CO₂ + 9 H₂O D H = -5512 kJ mol^-1

Questions on Alkanes

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