Halogenated hydrocarbons are organic compounds consisting of C-C, C-H and C-X bonds where X is a halogen atom (F, Cl, Br, I). Carbon has a valence of four and thus requires four electrons or bonds to complete its octet in the neutral state. Hydrogen has a valence of one and thus requires a single electron or bond to complete its “duet” in the neutral state. Halogens have seven valence electrons and thus require a single electron or bond to complete its octet. Thus in halogenated hydrocarbons, carbon can form neutral bonding arrangements by forming single bonds with halogens, single bonds with hydrogen, and single, double or triple bonds with other carbons (or other atoms).
Halogenated hydrocarbons may be sub-classified based on the nature of the hydrocarbon fragment to which they are attached (alkane, alkene, alkyne, aromatic), and on the basis of the number of halogen atoms present (mono-, di- tri- tetra-, etc. halogenated compounds). The structures below show several examples of such sub-classification. In the simplest case, methane can be substituted with one, two, three or four halogens (chlorines, in the example below). These are examples of mono-, di-, tri- and tetra-halo substituted alkanes.
Figure.1． The sub-classification of Halogenated hydrocarbons
Alfa Chemistry provides a wide variety of halogenated hydrocarbons for different uses.
Halogenated hydrocarbons are widely used in synthetic chemistry. Under the appropriate conditions, they can undergo displacement, elimination, oxidation and other reactions which are described generally below:
Oxidation Reactions: Polyhalogenated compounds undergo chemical oxidation in the presence of thermal energy and oxygen. For example, the chloroform may undergo oxidation to yield phosgene. This is why chloroform and other polyhalogenated compounds usually contain small amounts of alcohol. Besides, halogenated compounds can also undergo oxidative metabolism catalyzed by human cytochromes.
Nucleophilic Displacement: Suitably halogenated hydrocarbons may undergo nucleophilic displacement reactions in the presence of nucleophiles and elimination reactions in the presence of bases. Halogens are relatively good leaving groups as electronegative atoms, so they can accommodate the negative charge resulting from heterolytic cleavage associated with displacement and elimination reactions. The larger the halogen atom is, the more "polarizable" (the greater the distance between the valence electrons and atomic nucleus) is and therefore the more likely it is to function as a leaving group. Furthermore, when bound to a carbon atom that can yield a stabilized carbocation (tertiary carbon, benzylic carbon or allylic carbon), a halogen can leave more easily.
Elimination Reactions: Halogenated hydrocarbons can also undergo elimination reactions in the existence of strong base to form alkenes. Halogenated hydrocarbon with halogens that are good leaving groups (such as I-) linked to carbon atoms are more likely to undergo elimination reaction. Based on the similarities in mechanism and structural requirements for reaction, and the fact that many nucleophiles are also good bases, it is not surprising that nucleophilic displacement and elimination reactions often compete with each other under certain conditions.