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7.4 Properties of Chiral Molecules: Optical Activity. Optical Activity. A substance is optically active if it rotates the plane of polarized light. In order for a substance to exhibit optical activity, it must be chiral and one enantiomer must be present in excess of the other. Light.

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7.4 Properties of Chiral Molecules: Optical Activity

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7.4Properties of Chiral Molecules:Optical Activity


Optical Activity

A substance is optically active if it rotates the plane of polarized light.

In order for a substance to exhibit opticalactivity, it must be chiral and one enantiomer must be present in excess of the other.


Light

has wave properties

periodic increase and decrease in amplitude of wave


Light

optical activity is usually measured using light having a wavelength of 589 nm

this is the wavelength of the yellow light from a sodium lamp and is called the D line of sodium


Polarized light

ordinary (nonpolarized) light consists of many beams vibrating in different planes

plane-polarized light consists of only those beams that vibrate in the same plane


Polarization of light


Polarization of light

Nicol prism


Polarization of light

Nicol prism


Rotation of plane-polarized light


Rotation of plane-polarized light


Rotation of plane-polarized light


Rotation of plane-polarized light

a


Specific rotation

observed rotation (a) depends on the number of molecules encountered and is proportional to:path length (l), andconcentration (c)


100 a

cl

Specific rotation

observed rotation (a) depends on the number of molecules encountered and is proportional to:path length (l), andconcentration (c)

therefore, define specific rotation [a] as:

concentration = g/100 mL

length in decimeters

[a] =


Racemic mixture

a mixture containing equal quantities of enantiomers is called a racemic mixture

a racemic mixture is optically inactive(a = 0)

a sample that is optically inactive can beeither an achiral substance or a racemicmixture


Optical purity

an optically pure substance consists exclusively of a single enantiomer

enantiomeric excess = % one enantiomer – % other enantiomer

% optical purity = enantiomeric excess


7.5AbsoluteandRelative Configuration


Configuration

Relative configuration compares the arrangement of atoms in space of one compound with those of another.

Absolute configuration is the precise arrangement of atoms in space.


Configuration

Relative configuration compares the arrangement of atoms in space of one compound with those of another.until the 1950s, all configurations were relative

Absolute configuration is the precise arrangement of atoms in space.we can now determine the absolute configuration of almost any compound


CH3CHCH

CH3CHCH2CH3

CH2

OH

OH

Relative configuration

Pd

No bonds are made or broken at the stereogenic centerin this experiment. Therefore, when (+)-3-buten-2-ol and (+)-2-butanol have the same sign of rotation, the arrangement of atoms in space is analogous. The twohave the same relative configuration.

[a] + 33.2°

[a] + 13.5°


H

HO

H

HO

OH

OH

H

H

Two possibilities

H2, Pd

But in the absence of additional information, we can't tell which structure corresponds to(+)-3-buten-2-ol, and which one to (–)-3-buten-2-ol.

H2, Pd


H

HO

H

HO

OH

OH

H

H

Two possibilities

H2, Pd

Nor can we tell which structure corresponds to(+)-2-butanol, and which one to (–)-2-butanol.

H2, Pd


H

HO

H

HO

OH

OH

H

H

Absolute configurations

H2, Pd

[a] +33.2°

[a] +13.5°

H2, Pd

[a] –13.5°

[a] –33.2°


CH3CH2CHCH2OH

CH3CH2CHCH2Br

CH3

CH3

Relative configuration

HBr

Not all compounds that have the same relativeconfiguration have the same sign of rotation. No bondsare made or broken at the stereogenic center in thereaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation changes.

[a] -5.8°

[a] + 4.0°


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