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

  H H H H H
  | | | | |
H-C-C-C-C-C-H
  | | | | |
  H H H H H
Other forms of C5H12 are not considered pentane, but are identified by naming the longest string of carbon atoms, then referencing the branches off of them, as in 2-methyl-butane, below:
  H H H H
  | | | |
H-C-C-C-C-H
  | | | |
  H | H H
  H-C-H
    |
    H

Pen"tane (?), n. [See Penta-.] Chem.

Any one of the three metameric hydrocarbons, C5H12, of the methane or paraffin series. They are colorless, volatile liquids, two of which occur in petroleum. So called because of the five carbon atoms in the molecule.

 

© Webster 1913.

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