[rleung]

Hey,

I am confused about the way the book names this one cyclobutane.  The book's name is 2-ethyl-1,1-dimethylcyclobutane, but I get 1-ethyl-2,2-dimethylcyclobutane.  I started numbering the C's starting with the C with the ethyl substituent since that comes first alphabetically (I discount the "di-" prefix in the methyls attached to the other Carbon).  The book's answer seems to numbered the C attached to the methyl group as 1 instead.  The only reason why I think they would start with that is because of they counted the prefix in "dimethyl" when they alphabetized, but when they list the name, they list it alphabetically, discounting the "di-" in dimethyl.  Am I wrong or is the book wrong?

Ryan

[arnyk]

I believe they count it clockwise (which is what I usually do).  If you count clockwise starting with the ethyl group, you would not get the lowest number for substituents.

[movies]

Counting clockwise won't work.  What if the structure were drawn flipped over?  You would come up with a different name for the same compound.

I think it is numbered that way because of the priority number of the carbons in the ring.  Since the carbon bearing the two methyl groups has 4 carbons attached to it and the carbon bearing the ethyl group only has three, the quaternary carbon is assigned a higher priority.

[arnyk]

Oh yeah I just noticed the flippage factor.  But I never knew the about that priority rule.  So whichever carbon has the most carbons attached to it would take priority and be counted first?  And that carbon would be called the quaternary carbon (since it has 4 attached).  Is there a prefix system for that also?  

[movies]

There is no prefix symbol that I know of.  It's sometimes abbreviated as 4^o.

It's the same sort of priority designation you would use for determining R/S at a chiral center.

I might be totally wrong about the priority thing though.  It might just be numbered that way so that you give the most groups the lowest number (2 methyls vs. 1 ethyl).

Does anyone know for sure?

[AWK]

2-ethyl-1,1-dimethylcyclobutane
Rule:
Number the substituents so that the total of the numbers for all substituents is as small as possible

[arnyk]

Ahh, so that's fairly straight-forward.  Simple addition eh?  Thanks.

[jdurg]

I had always read that when dealing with substituents, the most electronegative substiuent is given the higher priority.  If the electronegativity is the same, then the substituent with the lowest molecular mass gets the priority.

[movies]

The system in question is the Cahn-Ingold-Prelog (CIP) system for assigning molecular chirality.  According to this system, the precedence is determined first by atomic number, then mass number.  There are some other criteria as well, but nothing that pertains to electronegativity.

There are two superb review papers:

Cahn, R. S.; Ingold, C.; Prelog, V. Angew. Chem. Int. Ed. 1966, 4, 385-415.
Prelog, V.; Helmchen, G. Angew. Chem. Int. Ed. Eng. 1982, 21, 567-583.

Links, for the lucky few that have online access (I had to go to the library!):

http://www3.interscience.wiley.com/cgi-bin/abstract/106574771/ABSTRACT
http://www3.interscience.wiley.com/cgi-bin/abstract/106583617/ABSTRACT