Constitutional Isomers Wrapping Up: Subdivision of Isomers Isomers (different compounds with same molecular formula) Constitutional Isomers (isomers whose atoms have a different connectivity) Stereoisomers (isomers that have the same connectivity but differ in spatial arrangement of their atoms) Enantiomers (stereoisomers that are nonsuperimposable mirror images of each other) Diastereomers (stereoisomers that are NOT mirror images of each other)

Determining the relationship between molecules If the molecular formula is different they are not isomers Example: benzene vs. cyclohexane

Determining the relationship between molecules If the molecular formula is identical, but they are connected differently they are structural isomers Example: cis-1,2 vs. cis-1,3-dimethylcyclohexane Example: diethyl ether vs. 1-butanol © 2014 by John Wiley & Sons, Inc. All rights reserved.

Determining the relationship between molecules If the connectivity is identical, but they differ in the 3-D arrangement of atoms in space they are steroisomers.

Determining the relationship between molecules If the molecules are not superimposable and mirror images, they are enantiomers:

Determining the relationship between molecules If the molecules are not superimposable nor are they mirror images, they are diastereomers:

Determining the relationship between molecules In molecules with multiple stereocenters, this may be difficult to visualize. Simply, if every R, S center flips between the molecules you are comparing, they are enantiomers:

Determining the relationship between molecules If at least one center remains the same and at least one other flips, they are diastereomers!

Physical Properties of Stereoisomers Enantiomers have identical physical properties (e.g. melting point, boiling point, refractive index, solubility etc.) Compound bp (oC) mp (oC) (R)-2-Butanol 99.5 (S)-2-Butanol (+)-(R,R)-Tartaric Acid 168 – 170 (–)-(S,S)-Tartaric Acid (+/–)-Tartaric Acid 210 – 212 © 2014 by John Wiley & Sons, Inc. All rights reserved.

Physical Properties of Stereoisomers Enantiomers also: Have the same chemical properties (except reaction/interactions with chiral substances) Show different behavior only when they interact with other chiral substances (enzymes) Rotate plane-polarized light in equally in opposite directions - this property of enantiomers is called optical activity © 2014 by John Wiley & Sons, Inc. All rights reserved.

Optical Activity The property possessed by chiral substances of rotating the plane of polarization of plane-polarized light The electric field (like the magnetic field) of light is oscillating in all possible planes When this light passes through a polarizer (Polaroid lens), we get plane-polarized light (oscillating in only one plane) Polaroid lens

Optical Activity – Measuring on Polarimeter Polarimeter – instrument to measure optical activity a = observed optical rotation

Optical Activity – Measuring on Polarimeter

Optical Activity – Measuring on Polarimeter

Optical Activity – Measuring on Polarimeter

Optical Activity – Calculating Specific Rotation observed rotation temperature ℓ wavelength of light (e.g. D-line of Na lamp, l = 589.6 nm) length of cell in dm (1 dm = 10 cm) concentration of sample solution in g/mL

Optical Activity – Calculating Specific Rotation The value of a depends on the particular experiment (since there are different concentrations with each run) But specific rotation [aD] should be the same regardless of the concentration

Optical Activity – Calculating Specific Rotation Two enantiomers should have the same value of specific rotation, but the signs (+/-) are opposite

Optical Activity – Calculating Specific Rotation An equimolar mixture of two enantiomers is called a racemic mixture (or racemate or racemic form) A racemic mixture causes no net rotation of plane-polarized light rotation equal & opposite rotation by the enantiomer

Racemic Forms and Enantiomeric Excess A sample of an optically active substance that consists of a single enantiomer is said to be enantiomerically pure or to have an enantiomeric excess of 100% An enantiomerically pure sample of (S)-(+)-2-butanol shows a specific rotation of +13.52 A sample of (S)-(+)-2-butanol that contains less than an equimolar amount of (R)-(–)-2-butanol will show a specific rotation that is less than 13.52 but greater than zero

Calculating Enantiomeric Excess or Optical Purity Also known as optical purity It can also be caluclated from optical rotation data

Calculating Enantiomeric Excess or Optical Purity A mixture of the 2-butanol enantiomers showed a specific rotation of +6.76o The specific rotation of pure (S)-2-butanol is +13.5o The enantiomeric excess of the (S)-(+)-2-butanol is 50%

Calculating Enantiomeric Excess or Optical Purity It is important to know what this means. Since any R impurity will ‘cancel’ the rotation of an equal amount of S: A sample with an ee of 50% S is actually 50% pure S and 50% racemic R/S. The total S enantiomer in the sample is actually 75%!

Physical Properties of Diastereomers Stereoisomers that are not enantiomers Unlike enantiomers, diastereomers usually have substantially different chemical and physical properties

Other Chirality In Organic Chemistry

Other Chirality In Organic Chemistry

Other Chirality In Organic Chemistry