Organic Chemistry I

4.1
Solutions included in the section.
4.2
Draw all conformers for 3-methylpentane by viewing along C2-C3 bond, and order them from the most stable to
least stable.
4.3
Determine
which
is
the
more
stable
isomer,
cis
-1-ethyl-2-methylcyclohexane
or
trans
-1-ethyl-2-methylcyclohexane?
The
trans
-isomer has the most stable conformer with both substituents are at equatorial positions, therefore
trans
-isomer is the more stable one.
136 | Answers to Practice Questions Chapter 4

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Answers to Practice Questions Chapter 4 | 137

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CHAPTER 5 STEREOCHEMISTRY
The term stereoisomer has been introduced in
Chapter 4,
in the topic of cis/trans isomer of disubstituted cycloalkane.
Stereoisomers are interesting phenomenon that exist in nature, also have great impact on the properties of organic
molecules. In this chapter, we will have in depth discussions on stereoisomers and stereochemistry.
Chapter 5 Stereochemistry | 139

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5.1 Summary of Isomers
In stereochemistry topic, we will learn more different types of isomers. To clarify the concepts, it is a good idea
to have a summary about the isomers in organic chemistry. There are two major types of isomers,
constitutional
isomers
and
stereoisomers
. We have had detailed discussions on constitutional isomers in
Chapter 2
, and will focus
on stereoisomers in this chapter. Stereoisomers are molecules with same bonding, but groups are in different spatial
arrangement. At beginning of this chapter, we will learn more about geometric isomers in alkenes and the E/Z naming
system. Then we will move on to a brand new category of stereoisomers, the isomers with chirality center. The
flowchart here (
Fig. 5.1a
) show the correlation and difference between different types of isomers (pay attention to the
definitions included) and provides useful guideline for the learning.
Fig. 5.1a Summarization of Isomers
5.1 Summary of Isomers | 141

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5.2 Geometric Isomers and E/Z Naming System