Ask A Scientist© Chemistry Archive

name         Jerry D.
status       educator
age          40s

Question -   I am teaching a class in Organic Chemistry to High School
juniors. We are starting with nomenclature and a question came up that I
could not answer without guessing. I would like to make sure that I was
correct and if there was a rule for it.
The question was about which way to number a complex alkane.
Would the compound be called 3,3,5-trimethyl-5-ethyloctane or
3,5,5-trimethyl-3-methyloctane.
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My guess was that the larger the substituent, the greater the precedence
for its naming and therefore get numbered from that end.

The Handbook of Chemistry and Physics devotes some 60 pages of very fine
print to the definitive nomenclature of organic compounds, which can only be
loved by the most obsessive compulsive organic chemist so, even as a
professional chemist, I am somewhat at a loss as to, "Who cares!".  None the
less, the rule is:

"The longest chain is numbered from one end to the other by Arabic numerals,
the direction being so chosen as to give the lowest numbers possible to the
side chains.  When series of locants containing the same number of terms are
compared term by term, that series is "lowest" which contains the lowest
number on the occasion of the first difference. This principle is applied
irrespective of the nature of the substituents."

Is that clear?

The answer is probably 5-ethyl- 3,3,5- trimethyl octane because the numbered
substituents in all cases is 16; however, ethyl comes before methyl in the
alphabet. That is in section 2.3 vi (b) (i).

See what I mean!!

Vince Calder
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On reflection, I think maybe I was overly curt in my previous response to
this question, and I apologize to Jerry D. The point I want to make is that
there are many very interesting topics to discuss in organic chemistry. The
subject is fascinating and quite likely it is the discussion of what organic
chemists make and do that turns many students on to chemistry. It seems to
me that trying to decide the proper name of an obscure hydrocarbon, that
quite possibly even many organic chemist would have to look up to make sure
it is correct, is not a topic that is likely to excite students about
chemistry.

Vince Calder
=========================================================
James,

You have a problem that wwill not be easily solved. Silver nitrate is freely
soluble in water. Alas, the stain left by silver nitrate in your clothes 
is not
silver nitrate -- it is silver metal. There is no common cleaning agent that
will remove the silver if it has soaked into the fibers. Of course, it will
dissolve in nitric acid. However, I think you can appreciate the folly of that
approach. I think you will have to live with the stain and be more careful in
the future to not get silver nitrate on your skin or clothes.

Regards,
ProfHoff
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Dear Jerry,
I floated your question in front of a colleague of mine
who is an organic chemist (I'm not one) just as a sanity check.

First of all, 3,3,5-trimethyl-5-ethyloctane is not a proper
name because "ethyl" comes before "methyl" alphabetically
(the tri is unimportant). But 5-ethyl-3,3,5-trimethyloctane is
okay.

However, neither of us can figure out what you mean by
3,5,5-trimethyl-3-methyloctane . Was there a typo in your
original message?

Maybe you'd like to make a GIF or JPEG of the molecule you're
interested in (download ISIS/Draw, it's free), post in on
the web again, and send another post to NEWTON with the URL?

best, prof. topper
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