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

  H H H H H H H
  | | | | | | |
H-C-C-C-C-C-C-C-H
  | | | | | | |
  H H H H H H H
Other forms of C7H16 are not considered heptane, but are identified by naming the longest string of carbon atoms, then referencing the branches off of them, as in 2-methyl-hexane, below:
  H H H H H H
  | | | | | |
H-C-C-C-C-C-C-H
  | | | | | |
  H | H H H H
  H-C-H
    |
    H

Hep"tane (?), n. [Gr. "epta` seven.] Chem.

Any one of several isometric hydrocarbons, C7H16, of the paraffin series (nine are possible, four are known); -- so called because the molecule has seven carbon atoms. Specifically, a colorless liquid, found as a constituent of petroleum, in the tar oil of cannel coal, etc.

 

© Webster 1913.

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