The percent of each enantiomer in the given solution has to be calculated. Concept introduction: The pair of Enantiomers has different configurations. Pair of enantiomers has different optical activity, which means if one will show levorotatory then the other will show dextrorotatory. The direction and magnitude of rotation of plane-polarized light by an optically compound is measured by using polarimeter . This rotation of polarized light is specific for a compound at specified temperature and wavelength, so it is called as Specific rotation of that compound. The specific rotation of enantiomers has equal magnitudes and opposite signs. Specific rotation of compound is calculated by, [ α ] λ T = α l ( d m ) × c ( g / 100 m L ) [ α ] λ T − s p e c i f i c r o t a t i o n ( a t T a n d λ ) α − o b s e r v e d r o t a t i o n c − c o n c e n t r a t i o n o f t h e s a m p l e l − l e n g t h o f t h e s a m p l e t u b e If the mixture contain unequal amount of enantiomers of a compound, then the amount of excess of enantiomer is called enantiomeric excess or optical purity. It is calculated by, enantiomeric excess = observed specific rotation specific rotation of the pure enantiomer ×100%

Question

8. A pure sample of the S enantiomer of a compound has specific rotation, [a], of +100°. A solution containing 0.20 g/mL of a mixture of the two enantiomers of the compound rotates plane polarized light by -16° in a 1 dm polarimeter. What is the enantiomeric excess (% ee) of the mixture? A) 32% S B) 80% S C) 80% R D) 16% R E) 16% S 9. How many chiral centers are there in the following molecule? (The naturally occuring stereoisomer with this structure is cortisol.) A) 5 B) 6 C) 7 D) 8 HO HO .OH E) 9

Answer 1

Question

Chapter 4, Problem 91P

Interpretation Introduction

Interpretation:

The percent of each enantiomer in the given solution has to be calculated.

Concept introduction:

The pair of Enantiomers has different configurations.

Pair of enantiomers has different optical activity, which means if one will show levorotatory then the other will show dextrorotatory.

The direction and magnitude of rotation of plane-polarized light by an optically compound is measured by using polarimeter.

This rotation of polarized light is specific for a compound at specified temperature and wavelength, so it is called as Specific rotation of that compound.

The specific rotation of enantiomers has equal magnitudes and opposite signs.

Specific rotation of compound is calculated by,

[α]λT=αl ( dm)×c (g/100mL) [α]λT−specific rotation (at T and λ)α−observed rotationc−concentration of the samplel−length of the sample tube

If the mixture contain unequal amount of enantiomers of a compound, then the amount of excess of enantiomer is called enantiomeric excess or optical purity. It is calculated by,

enantiomeric excess =observed specific rotationspecific rotation of the pure enantiomer×100%

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