Alkynes introduces the student to the second type of unsaturation, the triple bond. The unit on alkynes addresses the structure around this functional group, nomenclature, physical properties, preparations, reactions, and analysis.
That there are two types of alkynes, internal and terminal, is an
important concept and one that is addressed throughout the unit
on alkynes. The differences in the way each reacts, the differences
in properties, and the ease with which terminal alkynes are identified
as opposed to the difficulty in identification of internal alkynes
using simple chemical tests are all considered. More detailed analysis
involving extensive procedures is addressed in that portion of this
unit considering reactions of alkynes. Similarities between
nomenclature and physical properties of alkynes and nomenclature
and physical properties of alkenes are noted. The naming of
enynes is addressed as is the nomenclature based on acetylene.
Preparations of alkynes includes hydration of calcium carbide and
the historical role this reaction played in mining. Other preparations
studied include dehydrohalogenation of dihalides, chain lengthening,
and other preparations using acetylides. The factors which must
be considered to reduce competition between elimination and substitutions
reactions, particularly in chain lengthening processes, is noted.
The dehalogenation of tetrahalides is discussed, particularly from
its use as a method of preserving the triple bond functional group.
Preparation of alcohols containing a triple bond is examined using
the reactions with carbonyls and the Grignard./p>
In the section dealing with reactions, hydrogenation is addressed
and includes the use of the poisoned catalyst, Lindlarís catalyst,
to produce cis alkenes. The radical anion encountered when
an active metal such as sodium is used with ammonia is discussed.
Other reactions studied include the classic halogenation and hydrohalogenation
in which Markovnikov and anti-Markovnikov additons are again seen,
oxidative cleavage, and formation of acetylides. Markovnikov hydration
using mercuric sulfate in sulfuric acid is discussed at length and
the formation of the very important equilibrium, keto-enol tautomerism,
is developed. An anti-Markovnikov hydration using disiaborane as
the reagent is introduced, and the student is given several opportunities
to compare the products formed when hydration in mercuric sulfate/sulfuric
acid is compared to those from disecondaryisoamyl borane.
The fun reaction, Diels-Alder, is introduced with very light treatment
given to the mechanism at this point. The student is shown a way
of dealing with determining the products formed without going into
the mechanism at length. Detailed instruction in the mechanism is
addressed in the unit concerning conjugated systems, and the use
of heat vs. light is also developed in that later topic.