halogenated hydrocarbons

(organic chemistry) One of a group of halogen derivatives of organic hydrogen- and carbon-containing compounds; the group includes monohalogen compounds (alkyl or aryl halides) and polyhalogen compounds that contain the same or different halogen atoms.

An aliphatic or aromatic hydrocarbon in which one or more hydrogen atoms are substituted by halogen. See also Halogen elements; Halogenation.

Alkyl halides are compounds in which one hydrogen of an alkane has been replaced by halogen [fluorine (F), chlorine (Cl), bromine (Br), or iodine (I)], for example, bromoethane (ethyl bromide; CH₃CH₂Br). Many alkyl halides have been prepared; the chlorides and bromides are most useful and most common. Alkyl halides are important starting materials for the preparation of many other functionally substituted compounds. A general reaction for chlorides, bromides, and iodides is nucleophilic substitution, in which an ion or molecule with an available electron pair (a nucleophile) displaces a halide ion. See also Quaternary ammonium salts.

Compounds with halogen bonded directly to a benzene or other aromatic ring are called aryl halides. Halogen is introduced by electrophilic substitution, with a Lewis acid catalyst such as FeCl₃ or FeBr₃ to enhance the positive character of the halogen.

In fluorocarbons, every hydrogen atom is replaced by fluorine. Fluorocarbons can be named simply by using the prefix perfluoro- with the parent name. Because of the small atomic radius and high electronegativity of fluorine, these compounds are chemically inert and have properties quite unlike those of other halogenated organic compounds. See also Fluorocarbon.

Hydrofluorocarbons contain combinations of fluorine and hydrogen to satisfy the valency requirement of carbon. Hydrofluorocarbons are also known as HFCs. The development of specific molecules for particular applications, previously satisfied by chlorofluorocarbons, has been international in scope. Because they contain hydrogen, hydrofluorocarbons are more likely to be degraded in the lower regions of the atmosphere. Since they do not contain chlorine, these compounds do not contribute to ozone depletion.

Perfluorocarbons contain only carbon and fluorine. They are named by using the prefix perfluoro- along with the name of the equivalent hydrocarbon. Perfluorocarbons are chemically very inert and also have excellent thermal stability. This inertness and the resulting long atmospheric lifetime is reflected in higher global warming potentials compared to hydrofluorocarbons, hydrochlorofluorocarbons, and chlorofluorocarbons. This is due, partly, to the high strength of the C-F bond. The electronegativity of fluorine shields the carbon backbone from chemical attack. Under normal conditions, perfluorocarbons are unaffected by strong acids or bases and by oxidizing or reducing agents. See also Electronegativity.

A number of compounds with two or more halogen atoms are of special importance. Methane (CH₄) can be substituted with as many as four halogen atoms to give compounds such as CH₂Cl₂, CHI₃, and CF₃Br. Several polyhalomethane, -ethane, and -ethylene derivatives have major uses, and they are industrial chemicals produced in large quantities.

Chlorination of methane leads to mixtures of mono-, di-, tri-, and tetrachloro products. The relative amounts can be controlled by adjusting the ratio of starting materials. Methyl chloride is manufactured by this chlorination process and also by reaction of methanol and HCl. Methylene chloride, CH₂Cl₂, is the major product from methane, and is utilized primarily as a cleaning solvent or as a blowing agent for plastic foam. Methylene chloride is more volatile and much less toxic than CHCl₃ or CCl₄.

Chlorofluorocarbons are methane and ethane derivatives with all hydrogen atoms replaced by combinations of chlorine and fluorine. Chlorofluorocarbons are known collectively as CFCs. The first of these compounds, dichlorofluoromethane (CCl₂F₂), was introduced as a nontoxic, nonflammable working fluid in refrigeration equipment to replace ammonia and sulfur dioxide. Other compounds were developed to meet the requirements of specific uses such as air-conditioning equipment in buildings and vehicles, and propellants for aerosols.

Hydrochlorofluorocarbons contain combinations of hydrogen, chlorine, fluorine, and carbon to satisfy the valency requirement of carbon. Also known as HCFs, the hydrochlorofluorocarbons have been developed as interim substitutes for chlorofluorocarbons. Since they have at least one hydrogen atom in the molecule, they are more likely to be degraded in the troposphere by reaction with hydroxyl (OH) radicals. Thus, the potential that hydrochlorofluorocarbons have to deplete ozone by migration to the stratosphere is reduced.

General anesthetic agents such as halo-thane and isoflurane, which work with a minimum of nausea and mucous membrane irritation. They produce drowsiness for several hours after the primary effects wear off.