Re: Are connecticivty and optical rotation enough information to uniquely

The answer is no; The very simple example of cis- and trans-2-butene provides two substances 
with zero optical rotation, and identical connectivity. Optical isomerism is not the only type of 
ambiguity for a system with fixed connectivity. There is also geometric isomerism. This can 
happen with C=C double bonds, C=N double bonds, or saturated ring systems like cis- and trans-
decalin.

Even with optical isomers, things can get rather complicated when there is more than one chiral 
carbon atom in the structure. You are probably aware of enantiomers and diastereomers. For 
example, with aldohexose sugars D-glucose is just one of 8 pairs of sugars with identical 
connectivity. One of the other fifteen is the mirror image of D-glucose (L-glucose, known as its 
enantiomer), but there are seven other mirror image pairs of sugars with the same connectivity 
and significantly different physical and chemical properties (each known as a diastereomer of D-
glucose). There are four chiral carbon atoms, and each sugar can be described in terms of the 
chirality of each carbon atom. One way of doing it, not the usual way, would be to call the chrality 
of each atom along the chain: D-glucose is "right-right-left-right", and L-glucose is "left-left-
right-left". Clearly there are other possibilities like "left-left-right-right".

Now, as it happens, each of the 8 pairs in this case has quite a different magnitude of optical 
rotation. But there is no reason why it must be so. It would be a co-incidence, but there is no good 
reason why in another similar series of compounds, two diastereomers should not turn out to have 
the same optical rotation.

But there is another situation where there is a **guarantee** that two diastereomers have the same 
optical rotation. The formula for tataric acid can be written:

(HOOC)(HO)HC--CH(OH)(COOH)

It has three forms, according to the arrangement of the three groups on each carbon atom. "right-
right" and "left-left" are a pair of optically active materials which are mirror images of each other. 
But if you make models, you will see that "right-left" is mirror-symmetric, and has no optical 
rotation. The three forms are referred to as D-tartaric acid, L-tartaric acid, and meso-tartaric acid.

Now, think about the acid

(HOOC)(HO)HC--HCOH--HOCH--CH(OH)(COOH)

You will find (using models) that both "right-left-right-left" and "right-right-left-left" are 
different mirror-symmetric meso- forms of this acid.

To summarize: connectivity and optical rotation are not sufficient to uniquely specify a molecular 
structure because of 

(1) geometric isomerism involving C=C (and C=N) double bonds, or 
(2) possible coincidence of similar rotation for diastereomers
(3) possibility of more than one zero rotation meso- form for a given potentially chiral 
connectivity.