1. 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)

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

3. 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.

4. 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.

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

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

7. 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:

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

9. 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.

10. 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.

11. 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

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

13. Optical Activity – Measuring on Polarimeter

14. Optical Activity – Measuring on Polarimeter

15. Optical Activity – Measuring on Polarimeter

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

17. 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

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

19. 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

20. 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
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 but greater than zero

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

22. 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%

23. 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%!

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

25. Other Chirality In Organic Chemistry

26. Other Chirality In Organic Chemistry

27. Other Chirality In Organic Chemistry