1. Structural, Geometric, Enantiomer
Isomers
Structural, Geometric, Enantiomer

2. Isomers
Compounds that have the same molecular formula but difference structure and different properties
3 types:
Structural isomers
Geometric isomers
Enantiomers

4. Structural Isomer
same KIND and AMOUNT of atoms (molecular formula) but the atoms have different connectivity
Structural isomers usually have different physical and chemical properties

5. Structural Isomer

6. Structural Isomer

7. Structural Isomer

8. Structural Isomer

9. Structural Isomer
How many different structures can you make from C5H12?

10. Geometric Isomers
SAME molecular formula and the SAME connectivity but different spatial arrangements
Due to inflexibility of a bond, usually either:
double bonds (cis and trans formation)
bonds that can’t rotate (i.e. in a ring formation)

11. Geometric Isomers

12. Geometric Isomer Why is example 1 not a pair of geometric isomers?
Hint: Look at example 2 & 3 and compare to example 1. What is the minimum requirement for a pair of molecules to be considered geometric isomers?

13. Geometric Isomer
Answer: Each carbon of the double bond must have two different substituent groups
So if example 1 is not a pair of geometric isomers, what are they classified as?

14. Geometric Isomer
Another type of geometric isomer is found in ring structures and doesn’t involve double bonds
Due to the inflexible nature of the bonds that make up the ring
Note: The ring lies on a flat plane (e.g. your paper) and the OH groups are perpendicular to the plane (e.g. up and down)

15. Enantiomer Molecules that are mirror images of each other
Note: left and right hands are a pair of enantiomers (mirror images not identical)
What other body parts that are enantiomers?

16. Enantiomer Activity: Build these molecule pairs
Compare them to determine whether they are enantiomers

17. Enantiomer

18. Enantiomer
How are the lower pairs of molecules different from the upper pairs?
What condition do you think is necessary for a pair of molecules to be enantiomers? X

19. Enantiomer
Requirement: Enantiomers can only occur when each of the four groups attached to the central carbon atom are all different.
The central carbon is known as a chiral carbon and the molecule is chiral.

20. Chiral Molecules
Asymmetric carbon: carbon with 4 different groups bonded to it
No plane of symmetry
Mirror image is non-superimposable

21. Achiral Molecule
lacks chiral properties
has a plane of symmetry

22. Practice: Identify the chiral carbons
Double bond to carbon = achiral

23. Application: Amino acids
Amino acids are building blocks of proteins
Some amino acids can exist as enantiomers because of their chiralty
Switching an enantiomer in a biological system can have detrimental effects

24. Application: Enzymes Enzymes are always chiral.
Their binding sites are in a specific orientation that fits only one form of an enantiomer.
Binding sites won’t fit if the wrong enantiomer is present.

25. Racemic mixture
A mixture that contains equal quantities of both enantiomers
Enantiomers can interconvert in vivo

26. Racemic mixture Story of thalidomide (1960’s)
A racemic drug given to pregnant women to combat morning sickness
One of the enantiomers caused birth defects (teratogen) and death

27. Racemic mixture What’s in Advil (ibuprofen)?
Production results in a racemic mixture
One of the enantiomers is effective as an anti-inflammatory
Takes about 30 minutes for the inactive enantiomer to be converted
The World of Chemistry, 4 ed Thomson Brooks/Cole. Joesten, Castellion, & Hogg.

28. Steps to identifying isomers
Do the molecules have the same chemical formula?
If the formula is different, they are NOT isomers but completely different molecules
Are the molecules identical?
If they are, they are NOT isomers.
Are all the atoms connected in the same way to other atoms?
If not, they are STRUCTURAL isomers
Look for an inflexible bond (double bond or ring) for geometric isomers
Look for mirror images for enantiomers

29. Summary Isomer Same Different Requirements Structural Geometric
Enantiomer

30. Summary Isomer Same Different Requirements Structural
Molecular formula
Connectivity
Geometric
Enantiomer

31. Summary Isomer Same Different Requirements Structural
Molecular formula
Connectivity
Geometric
Molecular formula & connectivity
Spatial arrangement
Double bond
Different substituent groups on each carbon
Enantiomer

32. Summary Isomer Same Different Requirements Structural
Molecular formula
Connectivity
Geometric
Molecular formula & connectivity
Spatial arrangement
Double bond
Different substituent groups on each carbon
Enantiomer
Chiral molecules
Mirror image

33. Practice: Identify the chiral carbon

34. Practice: Identify the chiral carbon* * * * * *

35. Practice: Identify the chiral carbons
structural isomers
structural isomers
geometric isomers

36. Practice: Identify the chiral carbons*
structural isomers * * * * * * * * *
structural isomers
geometric isomers * * * *