Alkenes

Alkene is an unsaturated hydrocarbon that contains one or more carbon-carbon double bonds. The compounds are said to be unsaturated because they do not have the maximum number of atoms each carbon is able to accommodate. The molecular formula of this group is C_nH_2n (n is the number of carbon atoms). Alkenes have less hydrogen atoms than alkanes. Ethene (or ethylene as it is sometimes called) is the simplest alkene. Alkenes are often referred to as olefins, an old term derived from the fact that chlorine adds to gaseous ethylene to form an oily product.

Naming alkenes, we replace the suffix “-ane” of the corresponding alkane by “-ene”.

Physical properties of alkenes are similar to those of alkanes with the same carbon skeletons. The low-molecular-weight alkenes of importance in the petrochemical industry are gases. Most common alkenes found in the chemical laboratory are pungent-smelling liquids. Like alkanes, alkenes are relatively nonpolar compounds which are insoluble in water.

The alkenes are generally more reactive than the alkanes. The carbon-carbon double bond is an unsaturated linkage to which many chemical reagents can add. Chlorination and hydrogenation are two of the addition reactions that are typical of alkenes. The reaction of ethene with bromine is a simple example of an addition reaction. An example of hydrogenation is the reaction of propylene with hydrogen. Alkenes also undergo combustion reactions. For example, propene burns in a supply of air to give carbon dioxide and water. If there is not enough oxygen available, propene will burn with a yellow flame and produce soot (carbon) and carbon monoxide (CO).

An alkene that contains two double bonds is called a diene. If the two double bonds are separated by only one single bond, such as in 1,3-butadiene, the double bonds are said to be in conjugation.

Alkenes may be oxidized to ketones or carboxylic acids by an oxidizing agent. If the carbon in the double bond is attached to two alkyl groups, a ketone is the product. If the carbon has only one alkyl group, a carboxylic acid is the product.

Ethene (ethylene) and propene (propylene) are present in small quantities in natural gas and petroleum. The chemical industry prepares enormous quantities of these compounds by heating ethane and propane. Ethylene and propylene are used to manufacture plastics and a number of organic chemicals, including ethanol (alcohol), acetic acid, and ethylene glycol (antifreeze).

Alkynes

Alkynes are unsaturated hydrocarbons which contain one or more carbon-carbon triple bonds. The molecular formula of this group is CnH2n-2 (n is the number of carbon atoms). Alkynes have less hydrogen atoms than alkanes and alkenes. Alkynes are more unsaturated than alkenes. The simplest alkyne is ethyne (or acetylene (C2H2) as it is sometimes called) Alkynes are often referred to as acetylenes, a non IUPAC name (International Union of Pure and Applied Chemistry). They show greater tendency to polymerize or oligomerize than alkenes do. The resulting polymers, called polyacetylenes (which do not contain alkyne units) are conjugated and can exhibit semiconducting properties. Naming unbranched alkynes, we replace the suffix "-ane" of alkanes by "-yne".

The physical properties of alkynes are similar to those of alkenes and alkanes. These unsaturated compounds undergo additions similar to alkenes. Hydrogenation of alkynes produces alkanes and alkenes.

Acetylene, the dominant alkyne, is used as a fuel and a precursor to other compounds, e.g., acrylates. Hundreds of millions of kilograms are produced annually by partial oxidation of natural gas. Acetylene is widely used in the oxy-acetylene welding torch because it burns in oxygen with a hot flame. Ethyne (trivial name acetylene) burns in air with a luminous flame and produces a lot of soot. If a good supply of oxygen is available, ethyne burns with a colourless flame. Propyne, also industrially useful, is also prepared by thermal cracking of hydrocarbons. Most other industrially useful alkyne derivatives are prepared from acetylene, e.g. via condensation with formaldehyde.

Specialty alkynes are prepared by dehydrohalogenation of vicinal alkyl dihalides or vinyl halides. Via the Fritsch-Buttenberg-Wiechell rearrangement, alkynes are prepared from vinyl bromides. Alkynes can be prepared from aldehydes using the Corey-Fuchs reaction and from aldehydes or ketones by the Seyferth–Gilbert homologation. In the alkyne zipper reaction, alkynes are generated from other alkynes by treatment with a strong base.

Halogenation of alkynes gives the vinyl dihalides or alkyl tetrahalides. The hydroboration of alkynes gives vinylic boranes which oxidize to the corresponding aldehyde or ketone. Hydrohalogenation gives the corresponding vinyl halides or alkyl dihalides, again depending on the number of HX added.