[rleung]

Hey,

I am a little confused about the nomenclature for cycloalkanes.  It says in my book that if there are more than one alkyl groups, then you should start with the one that is alphabetically first.  When you do this, would I be correct in assuming:

1) you do not count the prefixes such as "di-, tri-, tetra-, etc."
2) you DO count the prefixes in established names such as sec-butyl and tert-butyl
3) you do NOT countthe prefix "cyclo-" for cycloalkyls

Also, do the above rules apply to listing the structures in alphabetical order too when you write the name of the molecule?

Sorry for these simple questions, but my book does not do a very good job of explaining nomenclature.

Ryan

[arnyk]

List the substiuent groups in alphabetical order when naming.  Use the base name always, bromo, chloro, ethyl...

Although you write them in alphabetical order, you will still number them to give the lowest for substituents.

[Winga]

1) you do not count the prefixes such as "di-, tri-, tetra-, etc."
2) you DO count the prefixes in established names such as sec-butyl and tert-butyl
3) you do NOT countthe prefix "cyclo-" for cycloalkyls

1. the prefixes, di, tri, tetra...are only used to describe how many substitutents (same kind) attached to the main carbon chain.

2. sec-butyl, tert-butyl, isopropyl...they are really the names of the whole substitutents, so the prefixes are counted.

3. I am not sure...

Also, do the above rules apply to listing the structures in alphabetical order too when you write the name of the molecule?

Yes.

[rleung]

Thanks.

I am still wondering one thing: I know that you number the C's to giv the lowest number of substituents, but what if there are two places you can begin counting from that will give you the same numbers for the same substituents?  In that case, the book says to start counting from the C that has the substituent that is alphabetically first.  In that case, do you ONLY use the base name to determine the alphabetical order, disregarding prefixes such as cyclo-?

Here is an example of where I am confused:

In this structure, the book answer names it as trans-1-t-butyl-3-ethylcyclohexane, but that is not what I get.  I started counting anti-clockwise starting with the ethyl substituent since that comes before the t-butyl substituent alphabetically.  Therefore, I get trans-1-ethyl-3-t-butylhexane.  

Thanks.

Ryan

[arnyk]

That is not a straight chain hydrocarbon.  You cannot count it that way, your base chain is the cyclic structure.

[movies]

2) you DO count the prefixes in established names such as sec-butyl and tert-butyl

These prefixes are usually not counted.  I think that will resolve the problem in your example.

If you look up t-butanol in the Aldrich catalog it's listed under 'b' not 't.'

[Winga]

But why "iso" is counted but not sec, tert.

How about "cyclo"?

Cyclopentyl & pentyl (just a n-pentyl), which one has a higher priority?

[movies]

The "iso" comes from a different set of naming conventions, I believe.  It also tends to refer to isomeric structures which probably weren't structurally confirmed at the time that they were named (e.g., isobutyl).  Even in those cases, I'm not certain that the prefix is included.  The iso nomenclature is becoming more and more rare.  Isopropanol is the most common use of this, but I think most chemical suppliers list it as 2-propanol now days.

[AWK]

But why "iso" is counted but not sec, tert.

Iso is not counted because it is not used in IUPAC nomenclature, but iso is used in common system of nomenclature

[Winga]

e.g.
IUPAC:
3-methyl-4-(1-methylehtyl)heptane

Non-IUPAC:
4-isopropyl-3-methylheptane

If we use isopropyl instead of 1-methylethyl, then iso will be counted. Is it true?

[AWK]

IUPAC uses 1-methylethyl or  propan-2-yl for radicals.
Isopropyl is traditional name (IUPAC - trivial)