Properties of Chiral Molecules: Optical Activity

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Optical Activity/ Optical Rotation

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Presentation transcript:

Properties of Chiral Molecules: Optical Activity 1

Optical Activity 2

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. 2

Light: electromagnetic radiation 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. 4

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 5

Polarization of light “White Light” 6

Polarization of light Nicol prism 6

Polarization of light Nicol prism 6

Rotation of plane-polarized light 7

Rotation of plane-polarized light 7

Rotation of plane-polarized light 7

Rotation of plane-polarized light 7

Rotation of plane-polarized light 7

Measuring the rotation of a chiral molecule: Polarimeter 7

Measuring the rotation of a chiral molecule: Polarimeter Clockwise (+); d Counterclockwise (-); l Different from absolute (R,S) configuration 7

Specific rotation observed rotation (a) depends on the number of molecules encountered and is proportional to: path length (l), and concentration (c) 8

therefore, define specific rotation [a] as: observed rotation (a) depends on the number of molecules encountered and is proportional to: path length (l), and concentration (c) therefore, define specific rotation [a] as: 100 a cl concentration = g/100 mL length in decimeters [a] = 8

a racemic mixture is optically inactive (a = 0) a 50:50 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 be either an achiral substance or a racemic mixture 9

enantiomeric excess = % one enantiomer – % other enantiomer Optical purity an optically pure substance consists exclusively of a single enantiomer enantiomeric excess = % one enantiomer – % other enantiomer % optical purity = enantiomeric excess 10

observed specific rotation optical purity = specific rotation of the pure enantiomer = + 5.44o / +13.60o x 100 =40% d- 40% d- in excess 70% d- and 30% l- 60% Racemic