[jena]

Hi,

I have two question and it is:

What is the optical rotation of a 1:4 mixture of enantimers given that the rotation value of pure mirror enantiomers would be 100 degrees?

Could someone help me break down the question so that I may understand it

and for the second question:

Which cyclohexane chair isomer is most stable?
My answer: (1R, 2S)-1,2-dimethylcyclohexane

is this correct :confused:



Thank you  

[movies]

For your first question think about what you material is really like; you have a 1:4 mixture of the two enantiomers, but you could also think of that as 2:3 mixture of racemic mixture to pure enantiomer (the one in excess).  How would this affect the observed rotation?

For your second question, the ring flip that the cyclohexane resides in will determine the stability.  You will have to draw out the correct one since it can't be defined by R/S nomenclature alone.  Also not that the enantiomer must have the same energy.

[dexangeles]

1. hint: a 1:1 (racemic) mixture is optically inactive

2. are their choices to chose from? I have this feeling that you have choices because the question alone doesn't make sense

[jena]

Yes this is a multiple choice question. the choices are

a. (1R, 2S)-1,2-dimethylcyclohexane
b. (1R, 2R)-1,2-dimethylcyclohexane
c. (1S, 3S)-1,3-dimehtylcyclohexane
d. a-c are all equally stable
e. not enough information is given to answer this question

If someone could please explain how you tell the difference in these I would appreciate that alone. The only thing that I really know about this R is rectus(Right) and S is sinister(Left) and those pertain to their direction.

Thank You

[dexangeles]

hint: chair conformation and equatorial is more stable than axial

[jena]

So if chair and equatorial conformation are more stable. Then the answer would be b. (1R, 2R)-1,2-dimethylcyclohexane.  
If this isn't the answer, then where may I have gone wrong in figuring it out.

Thank you