Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Carbon Functional Groups and Organic Nomenclature
2.6 Unsaturated Hydrocarbons
In this section, compounds that contain at least two sp2 or sp carbon atoms will be examined. These compounds are also known as unsaturated hydrocarbon compounds and contain the alkene and alkyne functional groups. Examples are shown below.
One of the most popular alkenes is ethylene, which is produced naturally in the ripening of fruits. Ethylene is a sweet-smelling gas, which is an asphyxiant, anesthetic to animals, and phytotoxic to plants. Compounds that contain the alkene functionality are typically synthesized from petroleum products; fossil products, which were pointed out earlier, are mostly hydrocarbons. Saturated hydrocarbons under catalytic conditions and appropriate temperatures can be converted into alkenes.
Benzene is one compound of a very large category of compounds, which are described as aromatic compounds and will be studied in more details in Chapter 17. Benzene was isolated from compressed illuminating gas in 1825 by Michael Faraday of the Royal Institution. In 1834, Eilhardt Mitscherlich of the University of Berlin synthesized benzene by heating benzoic acid with calcium oxide, and he also showed that the molecular formula of benzene is C6H6. Today, benzene is obtained primarily from the distillation of petroleum, and it is used as the starting material for many useful chemicals in our society. Benzene is also a solvent that is sometimes used in chemical labs, both industrial and academic research labs.
Acetylene is the simplest alkyne. It has just one carbon—carbon triple bond, two carbons, and two hydrogens. It is easily made by heating lime and coke to produce calcium carbide, and the addition of water to calcium carbide yields acetylene as shown below in Reactions (2-1) and (2-2).
(2-1)
(2-2)
Acetylene is one of the most useful of the alkyne family of compounds. It is used in combination with pure oxygen to produce a very high temperature when it burns; as a result, it is used as the primary fuel for oxyacetylene welding torches in industry to cut and weld steel. When acetylene is burned in the presence of oxygen, a very hot blue flame is produced; temperatures as high as 3000 °C can be generated making it an ideal fuel for this purpose. Propyne, which is also known as methylacetylene, is sometimes used as a substitute for acetylene as fuel for welding torches and even has been tested as a possible rocket fuel. Another major use of alkynes is that they are often used as starting materials in the synthesis of polymers. Even though there is a triple bond in alkynes, it is a nonpolar bond. As a result, alkynes are nonpolar molecules and are immiscible in polar solvents, such as water. They have relatively low boiling points, but the boiling points of alkynes with high molecular weights can be relatively high.
Since the carbons of the alkyne functionality are sp hybridized, which means that they contain 50% s character, these carbons are relatively electronegative, compared to carbons that are sp2 (33% s character) and sp3 (25% s character). Remember that the s orbital is closer to the nucleus, compared to the more diffused p orbitals. As a result, hydrogens that are bonded to alkynes are relatively acidic, compared to alkenes and alkanes and can be removed by a strong enough base as shown in the Reaction (2-3).
(2-3)
As we will see in later chapters, the salts of alkynes are useful synthetic intermediate of larger useful compounds.