The specific rotation of (R)-(+)-glyceraldehyde is +8.7. If the observed specific rotation of a mixture of (R)-glyceraldehyde and (S)-glyceraldehyde is +2.2, what percent of glyceraldehyde is present as the R enantiomer?
1 Answer
The mixture contains
Explanation:
The rotations of the two enantiomers cancel each other, so the rotation of the mixture will be that of the excess enantiomer.
The mixture has a positive sign of rotation, so the
The formula for enantiomeric excess is
#color(blue)(|bar(ul(color(white)(a/a) ee = "observed specific rotation"/"maximum specific rotation" × 100 %color(white)(a/a)|)))" "#
We can calculate the percent of each enantiomer as described in this Socratic question.
If we have a mixture of (+) and (-) isomers and (+) is in excess,
#color(blue)(|bar(ul(color(white)(a/a) % ("+") = (ee)/2 +50 %color(white)(a/a)|)))" "#
We have a 25.3 % enantiomeric excess of (+).
∴
So, the mixture contains 63 % (