Ask A Scientist© Chemistry Archive

 
   > >    name         Jerry
 > >    status       educator
 > >    age          40s

 > >    Question -   I started teaching a course in Organic Chemistry to
 > > advanced high
 > >  school students. I was go over the rules for nameing alkanes and
 > >  was wondering about one of the names. Why is the IUPAC name for
 > >  isopentane, 2-methylbutane. There is no ambiguity when calling it
 > >  methylbutane since there can only be one structure for it. Is the
 > >  2- really necessary and is it correct with it and without it?



Dear Mr. Deutsch;
The 2 seems to be required. The best reason I can
give is that if it is included, there is no doubt that
one is using IUPAC nomenclature.

Also, seems to me that what is "correct" depends on the context.
Any working chemist knows what isopentane means, and so
there will always be a meeting on the minds. But if one is
publishing in a journal that requires IUPAC notation for
everything, that 2 has to be there and it must be 2-isobutane.

In English, virtually every time a letter q appears it is
followed by a u. Why? No good reason. It doesn't help us
pronounce the word any differently than if it weren't there.
It's just a spelling convention. But if it weren't there,
we'd say is was "wrong." I view the 2 the same way. Things like
that don't happen often in IUPAC because it has a much smaller set of
rules and less history. But it still happens.

best regards,
prof. topper
dept of chemistry
the cooper union
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As far as I know, you don't absolutely need the number if the structure is
unambiguously specified by the name without it.  However, it would get kind
of hairy to make a rule to say exactly when you don't need a number.  For
example, along the same lines as your 2-methylbutane, what about
3-ethylpentane or 4-propylheptane?  Thej numbers here are also redundant,
because if the substituents were at any other positions, the root alkanes
would be different.

In reality, although naming rules are important to unsderstand, so that
organic chemists can communicate with each other, preocticing organic
chemists don't spend much of their time worrying about nomenclature.
Reactivity is the real heart of chemistry.


                Richard Barrans Jr., Ph.D.
                Chemical Separations Group
                Chemistry Division CHM/200
                Argonne National Laboratory
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