Alkynes

Overview:

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.

In the section dealing with reactions, hydrogenation is addressed and includes the use of the poisoned catalyst, Lindlar’s 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.