CHAPTER 3: Question 8

 

2D explanations

a) (R)-Phenylalanine is the enantiomer of (S)-phenylalanine. Enantiomers have identical physical properties except for the direction in which they rotate the plane of polarized light. Hence, (R)-phenylalanine will have the same melting point (272-274oC) as the (S)-enantiomer and will have a specific rotation [a]D20 = +34.

b) (RS)-Phenylalanine is the racemate of phenylalanine, ie it is composed of equal amounts of the (R)- and (S)-enantiomers of phenylalanine. Therefore, it will have a specific rotation of 0 since the rotations of the two enantiomers of the compound will cancel out. The melting point of the racemate cannot be predicted from the
information given. As discussed in the text, the intermolecular interactions between like enantiomers (ie both R or both S) are different to those between different enantiomers. Hence, the melting point of the racemate may be different to that of a pure enantiomer.

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