[lip]

I am bit confused about definition of optical isomers. As per my knowledge optical isomer is a compound which show optical activity, but how can we  differentiate enantiomers and diastereomers with this. If enantimer have more than one chiral center should we still call it an optical isomer (because it still may have optical activity). what about enantiomer with more than one chiral center which has no net rotation of plane polarized light?

[Corribus]

I'm not sure what the question is here.  As far as I am aware, every chiral molecule, by virtue of its symmetry and therefore its interaction with light, will be "optically active" (an admittedly ambiguous term that persists because of historical usage) to some degree.  Sometimes however the amount of interaction is too small to be measured.

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

Do also note that possessing a chiral center does not necessarily make a molecule chiral. On the flip side, not every chiral molecule has a chiral carbon center.  The most famous example is BINAP.

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

Anyway, this is a fairly broad topic and I suggest you do some reading at an introductory textbook level so as to identify more specifically where your knowledge difficulty is.

[lip]

Thank for you answer. Actually my main question is what are optical isomers?
Do we consider both enantiomers and configurational  diastereomers(not cis/trans) as optical isomers?
Are there  any enantiomers with no optical activity(if it has more than one chiral center and optical rotation by one center cancel out the effect of other)?

[lip]

I think you answered my main question (optical isomers).But some text book define optical isomer as a synonym to an enantiomer. I feel  confused about this usage. As per my knowledge enantiomers with one chiral center can be optical isomer, but what happen if enantiomer has more than one chiral center and no optical effect(no overall rotation of plane polarized light). so is above definition hold true for all the instants ?

[Dan]

But some text book define optical isomer as a synonym to an enantiomer. I feel  confused about this usage.

It is an obsolete term usually used as a synonym for enantiomer. I was surprised to see that IUPAC consider it a synonym for the broader term "stereoisomer" (link).

what happen if enantiomer has more than one chiral center and no optical effect(no overall rotation of plane polarized light).

Then it's not an enantiomer. Two enantiomers are related as non-superimposable mirror images. A compound that is achiral despite having more than one stereogenic centre is called a meso compound. It cannot be one of a pair of enantiomers because it is superimposable on its mirror image.

You might find it useful to think about enantiomer and diastereomer in comparative terms - i.e. two compounds can be related as enantiomers or related as diastereoisomers.

[lip]

Thanks.