STEREO ISOMERISM OPTICAL ACTIVITY

OPTICAL ACTIVITY BASIC

Dear STUDENTS

Now moving on to the next topic of isomerism that is Geometrical Isomerism.
here we should check three basic properties i.e. 
1).Restricted rotation
2).Groups attached to carbon should be different
3).All the groups concerned should be same plane.
System displaying cylindrical symmetry like triply bonded tetravalent

or trivalent atoms that does not have any stereoisomer .                                                                                                      

The most important term that we come across in isomerism is the Enantiomers.
Each enantiomer of a pair has the same physical and chemical properties in
achiral enviornment with the important exceptions of their interactions with
1)Plane polarized light (optical activity)
2).Chiral Reagents. 
Here i would like to clearify the three very important terms

which are commonly used in framing  questions.

1).Asymmetric cente
2)Chiral centre
3).Stereogenic centre

All the above are alternative terms that are in current use for carbon atoms
which have four different groups Chirality is the property of the whole molecule
and cannot be localized in a particular atom or centre.
Thus the more widely used term "chiral center "like asymmetric center  is
strictly in correct and should be replaced by stereogenic center
or simply stereocenter i.e.center giving rise to stereoisomers.    
                
The ultimate criterion for chirality is the nonsupeimposability of a structure and its mirror image.
Question: Name a compound which has a mirror image however, but the mirror image on rotation 
around an appropriate axis is superimposable, such structure are homomeric and not enantiomeric.

Question: Trans -cyclooctene is chiral without a stereocenter but what about 1,2,3,4-tetramethylcyclooctatetraene?

Now before i sign off let me answer the last post's questions
Diethyl malonate enol content is around 1% which makes it almost no tautomerism.
2-pyridone is more stable due intermolecular hydrogen bonding.