Is regioisomer a synonym of stereoisomer?

Answer 1

I don't think so... A stereoisomer is definitely an isomer which is related to another molecule by the orientation of its atoms in space rather than its connectivity. In other words, if you have a chiral molecule, with a stereocenter, that molecule can exist as multiple stereoisomers.

I couldn't find much on regioisomer, but "regioselectivity" generally refers to the preference of one direction of http://www.answers.com/ making or breaking over all other possible directions. In other words, when adding a molecule, such as HX, where X is a halide, across a double bond, the regioselectivity refers to the preference for adding the H to one side of the bond and the X to the other.

See the Web Links to the left of this answer for some more information. From what I can gather regioisomer is rather more synonymous with structural/constitutional isomer, where the atoms differ in their connectivity. E.g. ethanol and dimethylether, which both have the molecular formula C2H6O but have different arrangements of their atoms (CH3-CH2-OH and CH3-O-CH3 respectively). I can't find a clear definition of regioisomer anywhere, which makes me wonder if there's more to it, but the online dictionaries and encyclopedias I've looked at all refer me to structural isomers when I search for it. Also, my textbook lists various 'regiospecific' and 'regioselective' reactions, where the product is one of a number of possible STRUCTURAL isomers. This contrasts to the stereospecific and stereoselective reactions given, where the product is one of a number of potential stereoisomers, i.e. molecules that have the SAME atom connectivity but different 3D arrangements in space. So I'd use regioiomer as another term for structural/constitutional isomer, not stereoisomer.