Enantiomers & Diastereomers. PPT

Save (0)
Close

Recommended

Description

Course Code: BP401T

Course Title: Pharmaceutical Organic Chemistry III

Enantiomers & Diastereomers

 

Session Objectives

By the end of this session, students will be able to:

➢ Explain configuration of enantiomers

➢ Classify Enantiomers and Diastereomers

➢ Assign D and L system to isomers

➢ Define meso compounds

www.DuloMix.com 2

 

Optical activity
• A solution of optically active molecule (enantiomer) is placed in a sample tube,

plane-polarized light is passed through the tube and a rotation of the polarization
plane takes place

• The light then goes through a second polarizer called an analyser
• By rotating the analyser until the light passes through it, the new plane of

polarization can be found, and the extent of rotation that has taken place can be
measured

www.DuloMix.com

 

Optical activity

• A mixture of enantiomers with the same amount of each is called a racemic mixture
• Racemic mixtures are optically inactive (i.e. they cancel each other out) and are

denoted by (±)
• Note: the amount of rotation depends on sample concentration and sample path

length
• To obtain a meaningful optical rotation data, we have to choose standard conditions

www.DuloMix.com

 

Optical activity
• The specific rotation of a compound, designated as [α]D, is defined as the

observed rotation, when the sample path length l is 1 dm, the sample
concentration C is 1g/mL and light of 599.6 nm wavelength (the D line of a sodium
lamp, which is the yellow light emitted from common sodium lamps) is mostly
used

• As the specific rotation also depends on temperature, the temperature at which
the rotation is measured and denoted more precisely as

www.DuloMix.com

 

Enantiomers can be described as (+) or (-)

• We can use the fact that two enantiomers rotate plane-polarized light in opposite
directions to assign each a label that doesn’t depend on knowing its configuration

• We call the enantiomer that rotates plane-polarized light to the right (gives a
positive rotation) the (+)-enantiomer (or the dextrorotatory enantiomer)

• And the enantiomer that rotates plane-polarized light to the left (gives a negative
rotation) the (–)-enantiomer (or the laevorotatory enantiomer)

www.DuloMix.com

 

Enantiomers and Diastereomers

• In general molecule with n chiral centers, the maximum number of stereoisomers
possible is 2n

• That means for one chiral center its 2, for 2 its 4, for 3 its 8 and so forth
• Let us consider 2,3,4-trihydroxybutanal

www.DuloMix.com

 

Enantiomers and Diastereomers

• In general molecule with n chiral centers, the maximum number of stereoisomers
possible is 2n

• Let us consider 2,3,4-trihydroxybutanal
• It contains two chiral centers and 4 stereoisomers are possible

www.DuloMix.com

 

Enantiomers and Diastereomers
• Stereoisomers (a) and (b) are nonsuperposable mirror images and are, therefore, a

pair of enantiomers
• Stereoisomers (c) and (d) are also nonsuperposable mirror images and are a

second pair of enantiomers
• On naming, enantiomers (a) and (b) as (2R,3R)-erythrose and (2S,3S)-erythrose;

enantiomers (c) and (D) as (2R,3S)-threose and (2S,3R)-threose

www.DuloMix.com

 

Enantiomers and Diastereomers
• Both belongs to the class of carbohydrates and erythrose is present

in erythrocytes (red blood cells)
• What is the relationship between (a) and (c), (a) and (d), (b) and (c),

(b) and (d) ??
• Answer is diastereomers
• Diastereomers are the stereoisomers that are not mirror images
• Molecules with at least two chiral centers can have diastereomers

www.DuloMix.com

 

Meso compounds

• Molecules containing two or more chiral centers have special
symmetry properties that reduce the number of stereoisomers to
fewer than the maximum number predicted by the 2n rule

• For example, 2,3-dihydroxybutanedioic acid commonly called as
tartaric acid

www.DuloMix.com

 

Meso compounds

• In tartaric acid, carbons 2 and 3 are chiral centers, number of stereoisomers
possible is 4

• (a) and (b) are nonsuperposable mirror images, enantiomers
• (c) and (d) are superposable mirror images, meso compounds

www.DuloMix.com

 

Meso compounds
• Also (c) has plane of symmetry and is achiral
• A meso compound contains two or more chiral centers and is achiral
• To be a meso compound, molecule must also have chiral isomers
• Hence, tartaric acid has three stereosisomers, one pair of enantiomers and a meso

compound
• Enantiomers of tartaric acid will have same melting point, boiling point, solubility in

water and other common solvents, same value of pKa, and they undergo the same
acid-base reactions

www.DuloMix.com

 

Meso compounds

• But differ in optical activity
• Diastereomers have different physical and chemical properties, even in achiral

environments
• Meso tartaric acid has different physical properties from those of the enantiomers

and can be separated from them by methods such as crystallization

www.DuloMix.com

 

Physical properties of tartaric acid

www.DuloMix.com

 

Summary
• Enantiomer that rotates plane-polarized light to the right (gives a positive

rotation) the (+)-enantiomer (or the dextro-rotatory enantiomer)
• Enantiomer that rotates plane-polarized light to the left (gives a negative

rotation) the (–)-enantiomer (or the laevo-rotatory enantiomer)
• The direction in which light is rotated is not dependent on whether a stereogenic

centre is R or S
• Optical activity does not tell us the actual configuration of an enantiomer
• In general molecule with n chiral centers, the maximum number of stereoisomers

possible is 2n

• Diastereomers are the stereoisomers that are not mirror images
• Molecules with at least two chiral centers can have diastereomers
• A meso compound contains two or more chiral centers and is achiral

www.DuloMix.com 16

 

Disclaimer

All data and content provided in this presentation are
taken from the reference books, internet – websites
and links, for informational purposes only.

 

Thank You
www.DuloMix.com