[BrainScience]

So the question is starting with an R stereogenic reactant and gives both the S and R stereo products. It says that if the reaction was SN2 alone, the product would have an observed optical rotation of +100 but the actual rotation that is observed is +20.

a) What percentage of the product is R and what percentage of the product is S?

b) what percentage of the reactant underwent SN2?

Any help is greatly appreciated!

[sjb]

So the question is starting with an R stereogenic reactant and gives both the S and R stereo products. It says that if the reaction was SN2 alone, the product would have an observed optical rotation of +100 but the actual rotation that is observed is +20.

a) What percentage of the product is R and what percentage of the product is S?

b) what percentage of the reactant underwent SN2?

Any help is greatly appreciated!

It would depend to an extent on the actual reaction. Pure SN2 reactions can give R or S products from pure R starting material. What is the reaction you are referring to? How does optical rotation relate to concentration?

[fledarmus]

I believe this is just a theoretical question - you shouldn't need to know the actual reaction. Real reactions may not be as easy to calculate.

Start with three relationships:

1) The rotation of a pure R isomer of the compound will be the negative of the rotation of a pure S isomer of the same compound.

2) A racemic (1:1) mixture of isomers will show no optical rotation; the rotation of the R isomers cancels out the rotation of the S isomers.

3) S_N2 reactions give complete inversion of configuration; S_N1 reactions give racemic mixtures of products.

From these relationships and the relationship between concentration and optical rotation, you should be able to answer the question.