[Bjc51192]

I know the rule for using Iso in nomenclature is when it is an isomer, but are there any rules for Tert and sec, Im so confused since they look so similar.

Thanks

[Honclbrif]

http://en.wikipedia.org/wiki/Butyl

[fledarmus]

Yes, common names can be a real problem sometimes. That's one reason the IUPAC system was born. I assume you're thinking in terms of things like butyl and pentyl alcohols?

In broad terms, sec and tert refer to the number of substituents attached to the carbon to which your functional group is also attached. For example, in tert-butyl alcohol, the alcohol is attached to a carbon atom which is attached to three other carbon atoms (tertiary). Since there is only one possible way of hooking four carbons and an oxygen together such that you can have a tertiary carbon attached to the alcohol, it is sufficiently unique. The same applies to tert-amyl alcohol (amyl has five carbons, you just have to remember that.) In tert-butyl alcohol, the carbon attached to the -OH is also attached to three methyl groups - in tert-amyl alcohol, the carbon attached to the -OH is also attached to two methyl groups and an ethyl group.

In sec-butyl alcohol, the alcohol is attached to a carbon atom which is attached to two other carbon atoms (secondary). Since there is only one possible way of hooking four carbons and an oxygen together such that you can have a secondary carbon attached to the alcohol, this is also sufficiently unique. The -OH is attached to a carbon which is also attached to a hydrogen atom, a methyl group, and an ethyl group.

In general, iso means that the last methyl group on a straight chain has been moved forward one carbon, to leave two methyl groups on the penultimate carbon. For example, if you take n-butyl alcohol, CH3-CH2-CH2-CH2-OH, and think of the last methyl group being moved forward one carbon, you get isobutyl alcohol, (CH3)2-CH-CH2-OH. If you take n-amyl alcohol, CH3-CH2-CH2-CH2-CH2-OH and move the last methyl group forward one carbon, you get isoamyl alcohol, (CH3)2-CH-CH2-CH2-OH. If you take n-propanol, CH3-CH2-CH2-OH and move the last methyl group forward one carbon, you get isopropanol, (CH3)2-CH-OH.

These can also be combined - if you look back at the iso-amyl alcohol structure, there also exists a sec-isoamyl alcohol, which is (CH3)2-CH-CH(OH)-CH3.

Unfortunately, when you get behind five carbons, it rapidly turns into a morass of ambiguity, due to the exponentially increasing number of unique ways to hook the carbon atoms together.