Cartões: Lecture 12 Part 2: Properties of Enantiomers and Diastereomers | Quizlet

Physical properties of diastereomers are

different

Physical properties of enantiomers are

the same except they rotate plane-polarized light equally but in opposite directions

How do polarized glasses work?

- they only let in the vertical light, not horizontal
- polarized glasses reduce glare from the road

What does the polarizing filter do?

- A polarizing filter only lets waves through that are vibrating in a certain direction.
- only let through light in the same path of the vertical strip
- light goes in waves, so the wave will either go through the paper or the wave will not match the filter

What is "unpolarized light"?

- A light wave that is vibrating in more than one plane
- no specific orientation of oscillation

Can achiral and meso compounds rotate plane-polarized light?

- NO, achiral and meso compounds can not rotate plane-polarized ("optically inactive")

*only chiral compounds can rotate plane-polarized light

Can chiral compounds rotate plane-polarized light?

- chiral compounds can rotate plane-polarized light

*achiral and meso compounds can NOT ("optically inactive")

What type of compounds can rotate plane-polarized light?

chiral compounds can rotate plane-polarized light

*achiral and meso can NOT ("optically inactive")

Are enantiomers chiral?

YES

A pair of enantiomers will rotate plane polarized light _______, but in _______ directions.

a pair of enantiomers will rotate plane polarized light equally, but in opposite directions

Dextrorotatory vs Levorotatory

1. Dextrorotatory (+) = "right"
2. Levorotatory (-) = "left"

What are the 2 names for enantiomers based on direction?
(remember: in a pair of enantiomers, each will rotate light equally, but in opposite directions)

1. Dextrorotatory (+) = "right"
2. Levorotatory (-) = "left"

In a pair of enantiomers, each will rotate light equally, but in opposite directions. Therefore, we call them dextrorotatory (+) for right, and levorotatory (-) for left.

1. Is there a relationship between R/S and +/-?

2. Can we predict +/- from the structure alone? What about R/S?

1. No, there is no relationship between R/S and +/-

2. Can't predict +/- from the structure alone (but can assign R vs S from paper)

What is a racemic mixture?

- a racemic mixture contains equal amounts of both enantiomers

- (+-) symbol

- an equal mixture of 2 enantiomers (50%/50%)

- physical properties of racemic mixtures are different enantiomers

Physical properties of racemic mixtures are ___ enantiomers?

physical properties of racemic mixtures are different enantiomers

- mp and bp different because have 2 molecules, 1 right handed and 1 left handed

What does specific rotation mean?

- observed rotation under standard conditions

In a racemic mixture of 2 enantiomers, what is the observed optical rotation?

0 degrees

50% (+) + 50% (-)

*equal amounts so their optical rotations cancel

In any other mixture of 2 enantiomers (not racemic), what does the observed rotation depend on?

- in any other mixture of two enantiomers, observed rotation depends on which enantiomer is in excess, and by how much

- i.e. 20%(+) + 80%(-) --> 20%(+) + 20%(-) + 60%(-) excess

Example: What is the enantiomer excess?
20%(+) + 80%(-) -->

20%(+) + 80%(-) --> 20%(+) + 20%(-) + 60%(-) excess

What is enantiomeric excess (ee or %ee)?

- the amount of one enantiomer that is "not cancelled" in the mixture

Ex. 20%(+) + 80%(-) --> 20%(+) + 20%(-) + 60%(-) excess

Ex. if 5% (+) + 95% (-) -> 90% ee of (-)

Example: What is enantiomer excess (ee or %ee)?
if 5% (+) + 95% (-) ?

Ex. if 5% (+) + 95% (-) -> 90% ee of (-)

Ex. The specific rotation of (-)-cholesterol is -31.5 degrees. For a mixture of 25% (-)-cholesterol and 75% (+)-cholesterol, what is the %ee?

25% (-) + 25% (+) + 50% (+) -> 50% ee of (+)

Ex. The specific rotation of (-)-cholesterol is -31.5 degrees. For a mixture of 25% (-)-cholesterol and 75% (+)-cholesterol, what is the %ee and what is the observed optical rotation of the mixture?

1. 25% (-) + 25% (+) + 50% (+) -> 50% ee of (+)

2. 15.8 degrees

50% ee (+) -> 50% of max rotation observed

(0.5)(+31.5 degrees) = 15.8 degrees

*only the "excess" portion rotates light (rest cancels out)