Enantiomerism and Diasteriomerism

I am really confused between them. I know that an enantiomer is an isomer having optical activity, non super-imposability of image on original configuration, same molecular formula(Correct me if I am wrong). The thing that is causing me trouble is what diffrentiates this from diasteriomerism. It would be appreciated if you provide an example to clarify like a fischer projection




Your definition of enantiomer is correct. 


This simple further classification might be helpful. Let's take 2 molecules with the same molecular formula. These molecules are either constitutional (or structural) isomers or stereoisomers. If they have different structures (or connectivity patterns) and are separated by an energy barrier, then they are constitutional isomers. If they have the same structure or connectivity pattern, then they are stereoisomers. 


Among stereoisomers there are 3 classes that we can place a pair of molecules into. Take any two stereoisomers and compare them, they are either


1. the same (2 identical molecules)
2. mirror images of one another (enantiomers)


If they are not the same or enantiomers, then they are


3. different, and we call them diastereomers.


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One additional thing to add to @ron's answer is that *diastereomers* have different chemical properties .. they can be distinguished using conventional spectroscopic techniques, or even by measuring thermodynamic parameters (at least in principle .. sometimes the differences are very small and hard to resolve). They can also have different chemical reactivities. Apart from their different optical activities, *enantiomers* have identical chemical properties ... that is to say, you have to do an experiment that exploits their chiral natures to tell them apart.


Another point is that diasteromers always have at least two chiral centers ... a molecule with a single chiral center will have two enantiomers. A molecule having two chiral centers will have two enantiomers and two disasteromers. There is a nomenclature convention that can be used to index the various choices: each stereocenter is labeled "R" or "S" depending on whether the components are arranged clockwise (R) or counter-clockwise (S) around it, according to a conventional order of preference. In the case of a molecule with two stereo centers, the (R,R) and (S,S) stereoisomers are mirror images, and therefore are enantiomers, and the (R,S) and (S,R) stereoisomers are not mirror images, and therefore are *usually* diastereomers. As pointed out by @ron in the comments, if the molecule is highly symmetrical, the (S,R) and (R,S) forms may actually be indistinguishable, which would be a "meso" compound.


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