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Name: Jerry D.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2000

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.

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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