1. Types of Organic Compounds
Vast majority of over 20 million known compounds are based on C: organic compounds.
Generally contain C and H + other elements

2. Isomerism Isomers have identical composition but different structures
Two forms of isomerism
Constitutional (or structural)
Stereoisomerism
Constitutional
Same empirical formula but different atom-to-atom connections
Same atom-to-atom connections but different arrangement in space.

3. Constitutional Isomers
Look below at the structures of butane and 2-methylpropane.
Butane
2-methylpropane
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4. Constitutional Isomers
Even though both compounds have the formula C4H10, their boiling points and other properties differ.
Because their structures are different, they are different substances.
isomers have the same molecular formula but different molecular structures.
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5. Constitutional Isomers
Butane and 2-methylpropane represent a category of isomers called constitutional isomers, or structural isomers.
Constitutional isomers are compounds that have the same molecular formula but the atoms are joined together differently.
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6. Constitutional Isomers
Constitutional isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivities.
In general, the more highly branched the hydrocarbon structure is, the lower the boiling point of the isomer will be compared with less branched isomers.
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7. Which of these molecules will have a lower boiling point and why?
Butane
2-methylpropane
2-methylpropane will have a lower boiling point because it has a more highly branched structure.
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8. Structural Isomers

9. Hydrocarbons & Structural Isomerism
Isomers of C5H12?
C5H12 has 3 structural isomers.
Note names of isomers

10. Stereoisomers What are two types of stereoisomers? Stereoisomers
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11. Stereoisomers
Because molecules are three-dimensional structures, molecules with the same molecular formula and with atoms joined in exactly the same order may still be isomers.
Stereoisomers are molecules in which the atoms are joined in the same order but the positions of the atoms in space are different.
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12. Two types of stereoisomers are cis-trans isomers and enantiomers.
Two arrangements are possible for the methyl groups and hydrogen atoms with respect to the rigid double bond in 2-butene.
Cis configuration
Trans configuration
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13. Stereoisomers
Cis-Trans Isomers
Cis-trans isomers, also known as geometric isomers, have atoms joined in the same order but the spatial orientation of the groups differs.
The most common example of cis-trans isomerism occurs in molecules with double bonds.
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14. Stereoisomers
Cis-Trans Isomers
In the cis configuration, similar groups are on the same side of the double bond.
Cis configuration
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15. Stereoisomers
But, when similar groups extend from opposite sides of the double bond, the isomer is in the trans configuration.
Trans configuration
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16. Cis-trans isomers have different physical and chemical properties.
Stereoisomers
Cis-Trans Isomers
Cis-trans isomers have different physical and chemical properties.
You should be able to identify cis-trans isomers of alkenes when each carbon of the double bond has one substituent and one hydrogen.
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17. Stereoisomers: Optical
Optical isomers are molecules with non-superimposable mirror images.
Such molecules are called CHIRAL
Pairs of chiral molecules are enantiomers.
Chiral molecules in solution can rotate the plane of plane polarized light.

18. Chiral Compounds and Polarized Light

19. Stereoisomers
Enantiomers
A carbon with four different atoms or groups attached is an asymmetric carbon.
In the compound shown here, H, F, Cl, and Br atoms are attached to a single carbon atom, so the carbon is an asymmetric carbon.
CHFClBr
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20. Stereoisomers
Enantiomers
The relationship between the two molecules is similar to the relationship between right and left hands.
Sometimes the terms right-handed and left-handed are used to describe compounds with an asymmetric carbon.
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21. Stereoisomers
Enantiomers
By contrast, a pair of hands is distinguishable even though the hands have identical parts.
When you try to stack your hands on top of one another, the thumb of one hand lines up with the little finger of the other hand.
No matter how you turn your hands, you can’t get them to look exactly alike.
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22. Stereoisomers
Enantiomers
Pairs of molecules that are mirror images and not superimposable are called enantiomers, or optical isomers.
These molecules are examples of enantiomers.
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23. Stereoisomers Isomers
Chirality generally occurs when a C atom has 4 different groups attached.
Lactic acid

24. Stereoisomers Isomers
Lactic acid isomers are non-super-imposable

25. Chirality: Handedness in Nature
These molecules are non-superimposable mirror images.

26. Chirality: Handedness in Nature
These amino acids are non-superimposable mirror images.

27. Stereoisomers in Nature
Right- and left-handed seashells
The DNA here is right-handed

28. Stereoisomers
Enantiomers
Unlike other isomers, enantiomers have identical physical properties such as boiling points and densities.
Enantiomers do, however, behave differently when they interact with other molecules that have asymmetric carbons.
Many molecules in your body have asymmetric carbons, so each enantiomer can have a different effect on the body.
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29. Identifying Asymmetric Carbon Atoms
Sample Problem 22.4
Identifying Asymmetric Carbon Atoms
Which compound has an asymmetric carbon? b.
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30. Analyze Identify the relevant concepts.
Sample Problem 22.4
Analyze Identify the relevant concepts.
An asymmetric carbon has four different substituents attached. 1
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31. Solve Apply the concepts to this problem.
Sample Problem 22.4
Solve Apply the concepts to this problem.
Draw the structure in a way that makes it easier to compare the four different groups attached to the central carbon.
a. b. 2
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32. Solve Apply the concepts to this problem.
Sample Problem 22.4
Solve Apply the concepts to this problem.
Compare the groups. If all four groups are unique, the central carbon is asymmetric. If any two are the same, the central carbon is not asymmetric.
a. b.
Compound (b) is asymmetric. 2
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33. Key Concepts
Constitutional isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivities.
Two types of stereoisomers are cis-trans isomers and enantiomers.
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34. Glossary Terms
isomers: compounds that have the same molecular formula but different molecular structures
constitutional isomers: compounds that have the same molecular formula, but whose atoms are bonded in a different order
stereoisomers: molecules that have atoms in the same order, but which differ in the arrangement of the atoms in space
cis-trans isomers: compounds that have atoms in the same order, but differ in the orientation of groups around a double bond
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35. Glossary Terms
cis configuration: the configuration in which substituent groups are on the same side of a double bond
trans configuration: the configuration in which substituent groups are on the opposite sides of a double bond
asymmetric carbon: a carbon atom that has four different atoms or groups attached
enantiomers: molecules that differ from one another in the way that four different groups are arranged around a carbon atom
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