[Axmann8]

Why is 2-methyl-1-propanol

H3C -- CH -- C   -- OH
            |     H2
            |
          CH3

the same as isobutanol, but not isopropanol?

If the longest straight carbon chain is 3, why isn't it propanol?

[curiouscat]

The logic in the IUPAC versus conventional naming systems differs.

Only IUPAC uses longest chains. Traditional names go by the total number of C atoms in backbone.

e.g.

[fledarmus]

You have to look at the information that was available to the people who first discovered these compounds. An early researcher could use reactivity to determine that his compound was an alcohol, and elemental analysis combined with physical properties to determine an empirical formula of C₄H₁₀O. For an early researcher, then, this compound is some form of butanol. Long chains of links and guesses and some very, very careful measurements were required to separate and identify the different forms of butanol, but they were pretty easy to separate from the different forms of propanol, or the different forms of pentanol. So to an early researcher, the most important distinction among the low molecular weight alcohols was the total number of carbons in the molecule.

Now we do organic chemistry more in the context of structural interactions and structural information, our database of compounds is enormous, and our instrumental methods give much better structural information. It is more important now that our chemical names be unique, systematic, and rely on a few easily remembered building blocks.