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I is the first of a two-part study of the compounds containing
a double bond. It addresses the structure of alkenes, the degrees
of unsaturation, nomenclature, the physical properties of these
compounds, the nature of the carbocation, and alkene preparations.
A study of the structure of alkenes leads to a study of degrees
of unsaturation, the way in which those degrees may be determined,
and the role played by elements other than carbon and hydrogen.
Degrees of unsaturation are important in analysis throughout the
Nomenclature of alkenes is addressed using the functional group
as the important characteristic. However, the limitations of cis
and trans notations are quickly realized and the Z (zusammen) and
E (entgegen) terms are introduced, explained, and utilized. The
Cahn-Ingold-Prelog rules used to determine the order of groups in
stereochemistry are reviewed and used here to determine Z and E
The physical properties of alkenes are considered and compared to
alkanes. The impact of the double bond, as well as cis and trans
orientations on physical properties is addressed.
The first preparation, dehydration of alcohols, is used to introduce
the concept of the carbocation. This reactive structure is studied
in depth, and the role it plays in how a reaction occurs is presented
clearly and vividly using a series of animations. The manner in
which structural geometry shifts in the course of a reaction is
readily demonstrated using these animations, a feature virtually
impossible to achieve in a textbook. The halide shift and alkyl
shift are also clearly described. With these most appropriate visuals,
it becomes easy to understand the order of stabilities of the carbocations
produced and why the Saytzeff rule works as it does.
The preparations of alkenes addressed are dehydration of alcohols,
dehydrohalogenation of alkyl halides, dehalogenation of vicinal
dihalides, the Corey-House, and reduction of alkynes. The carbocation
mechanism for applicable preparations is discussed and animated
where helpful so the student can be thoroughly familiar with the
application of this important mechanism. The versatile and useful
Corey-House reaction with its Gilman reagent is described at length
and the few limitations noted.
Students frequently become confused when trying to learn why hydrogenation
of alkynes results in cis with hydrogen and palladium coated barium
sulfate, as opposed to trans addition. However, when the student
sees the animated reaction, it becomes immediately clear why the
cis structure must predominate. It is techniques such as this that
makes the Chemistry Professor so successful in teaching.
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