A) \[16.7%\] and \[83.3%\]
B) \[83.3%\] and \[16.7%\]
C) \[66.6%\] and \[33.4%\]
D) \[33.4%\] and \[66.6%\]
Correct Answer: A
Solution :
| [a] 1. To find the percentage of optical purity (optically active enantiomer), divide the observed specific rotation by that of the pure enantiomer and multiply the quotient by 100%. |
| The optical purities are: |
| Chloride \[=\frac{30{}^\circ }{40{}^\circ }\times\] (100%) = 75% |
| Alcohol \[=\frac{-6.0{}^\circ }{-12.0{}^\circ }\times\] (100%) = 50% |
| 2. The percentage of inversion is calculated by dividing the percentage of enantiomer of alcohol by that of reacting chloride and multiplying the quotient by 100%. The percentage of racemisation is the difference between this percentage and 100% |
| Percentage inversion \[=\frac{50%}{70%}\times\] 100%=66.6% |
| Percentage recemisation = 100% - 66.6% = 33.4% |
| 3. Inversion involves only back-side attack, 'while recemisation result from equal back-side and front- side attacks. The percentage of back-side attack is the sum of the inversion and one-half of the percentage of recemisation. The percentage of front- side attack is the remaining half of the percentage of racemisation. |
| Percentage of back-side attack is: |
| 66.6% +\[\frac{1}{2}\] (33.4%) = 83.3% |
| Percentage of front-side attack |
| \[=\frac{1}{2}\] (33.4%) = 16.7% |
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