3. After reading the following sentences, answer the questions below The .amino

Question

3. After reading the following sentences, answer the questions below The .amino acid phenylalanine has an asymmetric carbon atom. Thus there is a pair of enantiomers, D'phenylalanine and L-phenylalanine, that can not be superimposed each other. These enantiomers have same physical and chemical properties except for a direction of optical rotation, and are called optical isomers. A 1:1 mixture of optical isomers is called racemate and its solution does not show optical rotation. Dipeptides shown by two plane structures A and B were chemically synthesized using a racemate of phenylalanine COOH COOH H-C-NH2 LR.R) CS.52 CH2 CH2 CS. R) D-Phenylalanine L-Phenylalanine H2N- CH-C-N-CH CH2 CH2 CH2 HN-CH CH2 Plane structure A Plane structure B 1) How many stereoisomers with the plane structure A are there? 2) How many stereoisomers with the plane structure B are there? 3) Among the linear dipeptides answered in 1), how many of them do not show optical rotation in chi atena after complete acid hydrolysis? 4) Among the cyclic dipeptides answered in 2), how many of them do not show optical rotation after complete acid hydrolysis?

Explanation / Answer

1. No of stereoisomers For unsymmetrical substituted molecules = 2n, is the no of chiral centers.  So, Total 4 stereoisomers(RR, RS, SS and SR) are possible since it has 2 chiral centers.  

2. It also has 4 stereoisomers(RR, SS, SR and RS) because it has two chiral carbons.

3. Total 2 stereoisomers(SS, RR) are optically inactive.

4. Two isomers( RR, SS) won't show optical activity.

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