1. 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
http://www.thalidomide.ca/the-canadian-tragedy/

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
http://www.brookscole.com/chemistry_d/templates/student_resources/0534389996_mcmurry/CHEM_A_WORK/chapter9.htm
The World of Chemistry, 4 ed. 2007. 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
http://2.bp.blogspot.com/_K91FA3B4cpM/SJ_NUmBWWGI/AAAAAAAAACY/UzL8H8Q4oag/s400/ist2_1744503_frustration.jpg

29. Summary
Isomer Same Different Requirements
Structural
Geometric
Enantiomer

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

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

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

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 * *
* *