Chapter 15.7, Problem 15.18P

Interpretation Introduction

(a)

Interpretation:

The structure of ( R )-alanine or enantiomer of ( S )-alanine needs to be drawn.

Concept introduction:

The R and S notational system is used for the organic compounds with a chiral center. As alanine has a chiral center, this classification can be used.

In this classification, first thing is to assign priorities for each bonded group surrounding the stereocenter. The priorities are assigned by considering the atomic weight of the atom, which is directly connected to the chiral center, in each bonded group. When 2 atomic groups are having the same first atom, the atomic number of the second atom should be considered.

Then the lowest priority group should be located in a backward direction, out of the plane of the page. Usually this lowest priority group is denoted with a dashed line.

Then the remaining 1, 2, 3 priorities should be directed from lowest to highest. A counterclockwise direction is S configuration. A clockwise direction is R.

Interpretation Introduction

(b)

Interpretation:

The melting point of (R)-alanine needs to be determined.

Concept introduction:

Enantiomers are a part of stereoisomers. In chemistry, enantiomers are two stereoisomers that are mirror images of each other. As they are mirror images, they are non-superimposable which means not identical.

Interpretation Introduction

(c)

Interpretation:

The specific rotation of (R)-alanine needs to be determined.

Concept introduction:

Depending on the rotating side of the plane of polarized light a classification exits among optical isomers. If a molecule rotates the plane of polarized light to the right [clockwise, or (+)], it is classified as dextrorotatory (d) compound. In contrast, if a molecule rotates the plane of polarized light to the left [anti-clockwise, or (-)], it is classified as levorotatory (l) compound.