Organic Chem Question

[taki]

When naming cycloalkanes under the IUPAC system, and numbering the main carbon chain (or in this case ring), where do you start numbering the carbons for reference? I know that you are supposed to number them according to according to alphabetical alkyl group. Like if you have a arene with a methyl and ethyl group you would start with the ethyl and go clockwise with the numbering. However, when it is a cycloalkane and the subsituents are both ethyl groups at two differnt spots, where you start the numbering?

 

Thanks for replies and sorry if this sounds confusing.

[ronthepon]

No, the numbering begins with the heaviest alkyl group. You'll start from ethyl, rather than methyl not because E comes before M. It's because ethyl has two carbon atoms.

 

In any case, in the situation you mentioned above, the numbering will be done in such a way so as to have the smallest numbers associated with the groups.

 

I'll come back later with an example, if you don't understand.

[taki]

No, the numbering begins with the heaviest alkyl group. You'll start from ethyl, rather than methyl not because E comes before M. It's because ethyl has two carbon atoms.

 

In any case, in the situation you mentioned above, the numbering will be done in such a way so as to have the smallest numbers associated with the groups.

 

I'll come back later with an example, if you don't understand.

 

 

Ok I undertand that you start writing the name with group with the most carbons, but I still don't understand how do you know where to start numbering if you have 2 of the exact same groups at two different spots. Like if you have two ethyl groups. As far as having the smallest numbers, for example in a ring you could have the groups at the 1,3 positions or the 3,1 positions depending on how you number the ring. In this case the substituents are the same.

 

Also, do you always number clockwise?

[Mercedes Benzene]

I'm still not sure what you're trying to ask, beyond what ronthepon has already said. I hope that this helps:

 

1.For a monosubstituted cycloalkane the ring supplies the root name and the substituent group is named as usual. A location number is unnecessary.

2.If the alkyl sustituent is large and/or complex, the ring may be named as a substituent group on an alkane.

3.If two different substituents are present on the ring, they are listed in alphabetical order, and the first cited substituent is assigned to carbon #1. The numbering of ring carbons then continues in a direction (clockwise or counter-clockwise) that affords the second substituent the lower possible location number.

4.If several substituents are present on the ring, they are listed in alphabetical order. Location numbers are assigned to the substituents so that one of them is at carbon #1 and the other locations have the lowest possible numbers, counting in either a clockwise or counter-clockwise direction.

5.The name is assembled, listing groups in alphabetical order and giving each group (if there are two or more) a location number. The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing.

 

google: IUPAC naming subsearch: organic compounds.

[taki]

For example:

 

1,1-dimethyl-3-(1,1,3-trimethylbutyl)cyclooctane

 

why do you start numbering the "dimethyl" group as one instead of the "butyl" group?

 

I am confused because from what I understand: "Number the longest chain beginning with the end nearest a branch."

 

Arent both these branches?

[ronthepon]

Hmm... You sure about that?

 

I'd normally see the '1,1,3-trimethylbutyl' group as being numbered as one.

 

Kinda like named as

 

1-(1,1,3-trimethylbutyl)-3,3-dimethylcyclooctane