2,2,4-Trimethylpentane (iso-octane)

Description

2,2,4-Trimethylpentane is a branched alkane also known as iso-octane. It is one of the isomers of octane and is famously used as the standard reference fuel for octane ratings.

• Molecular formula: C8H18
• Molar mass: 114.23 g/mol
• IUPAC name: 2,2,4-trimethylpentane
• Common name: iso-octane
• Key role: defines the octane rating of 100 on the standard scale

Structure and nomenclature

• The main chain is a pentane (five carbons).
• Substituents:
  • Two methyl groups at carbon 2
  • One methyl group at carbon 4
• A condensed structural representation is:
  • CH3-C(CH3)2-CH2-CH(CH3)-CH3

This arrangement yields eight carbons in total (hence “octane” in its common name) and a highly branched structure.

Physical properties (typical)

• State at room temperature: liquid
• Boiling point: around 99 °C
• Density: roughly 0.69 g/mL at 25 °C
• Solubility: essentially insoluble in water; soluble in nonpolar solvents
• Safety: flammable; handle with appropriate precautions (avoid ignition sources, use in well-ventilated areas)

Octane rating and use

• Octane rating: defined as 100 for iso-octane under standard test conditions
• Practical significance: used as the high-octane reference fuel against which other gasoline components are measured. Gasoline blends are rated by comparing their tendency to knock to the iso-octane standard and to n-heptane (which is rated 0).

Related notes

• Iso-octane is one of several isomers of octane. Its high resistance to knocking is what gives it the 100 octane rating.
• In refining and fuel formulation, a mix of various isomers is used to achieve a target octane value and combustion characteristics.

Takeaway

2,2,4-Trimethylpentane (iso-octane) is the quintessential high-octane reference compound for gasoline quality, with the octane number of 100 and a branched C8H18 structure that maximizes resistance to knocking.