Since the 1913 edition of Webster's dictionary, the nomenclature of hydrocarbons has become more specific in regards to structure. Butane is now specifically a string of four linked carbon atoms, like so:

  H H H H
  | | | | 
H-C-C-C-C-H
  | | | |
  H H H H

The other form of C4H10 is no longer considered a butane, but instead it is identified as 2-methyl-propane, as that is its longest string of carbon atoms:

  H H H
  | | | 
H-C-C-C-H
  | | |
  H | H
  H-C-H
    |
    H

Butane is the n = 4 case in the series of alkane single-bond chain hydrocarbons, each made up of n carbon atoms and 2n+2 hydrogen atoms. The two forms of butane are n-butane and 2-methyl-propane. An unofficial name for the second form of butane is isobutane, because it is the one and only isomer of the chemical.

n-butane boils at -0.5° C (31.1° F); isobutane boils at -11.7° C (10.9° F).

Bu"tane (?), n. [L. butyrum butter. See Butter.] Chem.

An inflammable gaseous hydrocarbon, C4H10, of the marsh gas, or paraffin, series.

 

© Webster 1913.