Iso is a prefix of names in organic chemistry used when it was difficult to determine the structure, the way atoms in a molecule were linked. The total number of atoms of the elements could be determined. Two substances with the same gross formula, yet distinct other properties were distinguished by this prefix.
For example, the compound butane (C4H10) has been known and described when a second substance of this formula was found. This was an isomer of butane. The term "isomer of butane" quickly led to the name isobutane. Today, the IUPAC name for this compound is "methylpropane"
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