Does no optical rotation always implies optical inactivity? [duplicate]








**This question already has an answer here**:
 
 




[Are there chiral compounds that don't rotate plane-polarized light?](/questions/103048/are-there-chiral-compounds-that-dont-rotate-plane-polarized-light)

 (1 answer)
 

Closed last year.



This question popped up in my mind in reference to this question,
[Is a compound optically active if plane polarised light is deflected by an angle of n\*(2π) angles?](https://chemistry.stackexchange.com/q/166324/124759)


Suppose that I give a chemistry exam in future in which a question is asked like this,



> 
> A pure organic compound(not a racemic mixture) was analysed once and it was found that that there was absolutely zero rotation of PPL when passed through it(not even slight rotation as in cryptochirality). This means that it must be an optically active compound.
> 
> 
> 


Now there are two options whether the above statement is true or false. What will be the answer? Assume that the organic compound is pure ie it is not a mixture (not even a racemic mixture). Also assume that the enantiomers (if possible) are separable which means that there is nothing like amine inversion or any other similar mechanism through which enantiomers can interconvert into each other.


Though it might be quite intuitive that the answer should be false but think about the person who set this question. (It has quite often happened with me that even after having the correct logic, my answer is consider wrong by majority.)





> 
> Now there are two options whether the above statement is true or false. What will be the answer?
> 
> 
> 


I choose to *false*. We have two cases for no rotation of plane polarized light. One of the case is that the compound is optically inactive. And the other case is when the analyte is a [racemic mixture](https://en.m.wikipedia.org/wiki/Racemic_mixture).


Wikipedia states that,



> 
> A racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule.
> 
> 
> 


So the rotation of plane polarized light is a combined effect of nature of compound and nature of analyte. So no rotation *doesn't implies that the compound is optically active*.


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