Why is 1,3-thiazole called thiazole and 1,2-thiazole called isothiazole?

Question

I'm really confused about heterocyclic nomenclature that the lecturer went through today. I'm also not sure when to use the word iso in front? Thank you so much, I really need the help :(

Answer 1

"Iso" is just another way of saying "isomer". Unfortunately, you will have to remember the structures of the parent heterocycles first. So, using your example, thiazole has a Sulfur in the 1-position and Nitrogen in the 3-position. Similarly, imidazole has a Nitrogen at the 1- and 3-positions, etc. Therefore, isothiazole is a structural isomer of thiazole - in this case, there is only one isomer possible, that being 1,2-thiazole...

Hope this helps!!!

Answer 2

thiazole and isothiazole are 'old-fashioned', non-systematic names for these two ring compounds.

Just as propanol and isopropanol were the two (old fashioned) names for the two ISOmers propan-1-ol and propan-2-ol.

Though you might still see them around, you should concentrate on the systematic names which tell you precisely what's happening.

So "1,3-thiazole" tells you ...

[1] "ole" = 5-membered ring

[2] "1 & thia" = a sulphur at position 1 (thion is greek for sulphur)

[3] "3 & az(a)" = a nitrogen in position 3 (french for nitrogen is still azote)

There are, though, several, important old-fashioned names that are still in permanent use, and you will have to learn them: pyrrole, pyridine, furan, thiophene ...