Organic Chemistry I

R
and
S
3-bromo-3-methylhexane were obtained with the bromination.
Figure 9.5a Bromination reaction of (±)-3-methylhexane
This can be explained by the stereochemistry of the propagation steps in the mechanism. The carbon radical generated
in step 1 is in trigonal planar shape as mentioned earlier. When the radical reacts with bromine in step 2, the reaction
can occur at either side of the plane. Because both sides are identical, the probability of the reaction by either side is
the same, therefore equal amount of the
R
– and
S
– enantiomer are obtained as a racemic mixture.
Figure 9.5b Propagation steps
The stereochemistry of the radical substitution is similar to that of S
N
1 reaction, because both carbon radical and
carbocation are in trigonal planar shape.
312 | 9.5 Stereochemistry for Halogenation of Alkanes

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Examples
Show the bromination product(s) with stereoisomers when applied.
Solutions:
The racemic mixture is obtained.
Exercises 9.3
Show all the mono-chlorination products of butane with any stereoisomers when applied.
Answers to Practice Questions Chapter 9
9.5 Stereochemistry for Halogenation of Alkanes | 313

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9.6 Synthesis of Target Molecules: Introduction of
Retrosynthetic Analysis
We have learned three major types of reactions so far, nucleophilic substitution, elimination and halogenation of alkane
(radical substitution), now we will see how to put the knowledge of these reactions together for application, that is to
design synthesis route for a target (desired) compound from available starting materials.
Building larger, complex organic molecules from smaller, simple molecules is the goal of organic synthesis. Organic
synthesis have great importance for many reasons, from testing the newly developed reaction mechanism or method,
to replicate the molecules of living nature, and to produce new molecules that have potential applications in energy,
material or medicinal fields.
It usually take multiple steps, from several to 20 or more, to synthesize a desired compounds, and therefore it would
be challenging to visualize from the start all the steps necessary. The common strategy to design a synthesis is to
work
backward,
that is instead of looking at the starting material and deciding how to do the first step, we look at
the product and decide how to do the last step. This process is called