Question

Identify product when R- and (S)-2-butanol reacts with (R, R) tartaric acid in acidic medium.
(A) Racemic
(B) Diastereomers
(C) Meso
(D) Pure enantiomer

Answer 1

The R, R-tartaric acid reacts with R and S-butanol . The reaction proceeds with the removal of water molecules. The hydrogen atom of tartaric acid and the hydroxyl group of the butanol is removed as a water molecule to form new stereocenters.

Complete Solution :
The stereoisomers which do not form mirror images of each other and non –superimposable on one another with its stereocenters are called the diastereomers.
The (R, R) tartaric acid and the R and S- 2-butanol enantiomers react with each other.

1. When R-2-Butanol reacts with (R, R) –tartaric acid we get a product as shown below:
   
   The hydrogen atom of the tartaric acid and the hydroxyl group of R-butanol is removed as a water molecule. The resultant product has three stereocenters. All stereocenters have an R configuration.
2. When S-2-Butanol reacts with (R, R) –tartaric acid we get a product as shown below:
   
   The hydrogen atom of the tartaric acid and the hydroxyl group of R-butanol is removed as a water molecule. The resultant product has three stereocenters. Here two stereocenters (From the tartaric acid) have an R configuration while the other stereocenters formed by the removal of water have an S configuration.
   The product (1) and (2) have three stereocenters .Out of those two stereocenters are the same and they differ at only one stereocenter.The mirror image of the product (1) is not a mirror image of the product (2).
   Since products (1) and (2) are not mirror images of each other. Therefore, products (1) and (2) are diastereoisomers.
   So, the correct answer is “Option B”.

Note: Note that, diastereomers can be easily determined by looking at the designation or the configuration. The molecule which has the same or a designation is non-identical are diastereomers. For example, (R, R, R) and (R, S, R), etc.