Alkyl Halides

Alkyl Halides Chain Reaction Mechanism Uses of Halogenated Compounds

Alkyl Halides

Imagine that a pair of crystallizing dishes are placed on an overhead projector as shown in the figure below. An alkene is added to the dish in the upper-left corner of the projector and an alkane is added to the dish in the upper-right corner. A few drops of bromine dissolved in chloroform (CHCl3) are then added to each of the crystallizing dishes.

The characteristic red-orange color of bromine disappears the instant this reagent is added to the alkene in the upper-left corner as the Br2 molecules add across the C=C double bond in the alkene.

The other crystallizing dish picks up the characteristic color of a dilute solution of bromine because this reagent doesn't react with alkanes under normal conditions.

If the crystallizing dish in the upper-right corner is moved into the center of the projector, however, the color of the bromine slowly disappears. This can be explained by noting that alkanes react with halogens at high temperatures or in the presence of light to form alkyl halides.

The light source in an overhead projector is intense enough to initiate this reaction, although the reaction is still significantly slower than the addition of Br2 to an alkene.

The reaction between an alkane and one of the halogens (F2, Cl2, Br2, or I2) can be understood by turning to a simpler example.

CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)

This reaction has the following characteristic properties.

These facts are consistent with a chain-reaction mechanism that involves three processes: chain initiation, chain propagation, and chain termination.

return to top

Chain Reaction Mechanism

Chain Initiation

A Cl2 molecule can dissociate into a pair of chlorine atoms by absorbing energy in the form of either ultraviolet light or heat.

Cl2 2 Cl · Ho = 243.4 kJ/molrxn

The chlorine atom produced in this reaction is an example of a free radical -- an atom or molecule that contains one or more unpaired electrons.

Chain Propagation

Free radicals, such as the Cl· atom, are extremely reactive. When a chlorine atom collides with a methane molecule, it can abstract a hydrogen atom to form HCl and a CH3· radical.

CH4 + Cl· CH3· + HCl Ho = -16 kJ/molrxn

If the CH3· radical then collides with a Cl2 molecule, it can remove a chlorine atom to form CH3Cl and a new Cl· radical.

CH3· + Cl2 CH3Cl + Cl· Ho = -87 kJ/molrxn

Because a Cl· atom is generated in the second reaction for every Cl· atom consumed in the first, this reaction continues in a chain-like fashion until the radicals involved in these chain-propagation steps are destroyed.

Chain Termination

If a pair of the radicals that keep the chain reaction going collide, they combine in a chain-terminating step. Chain termination can occur in three ways.

2 Cl · Cl2 Ho = -243.4 kJ/molrxn
CH3· + Cl · CH3Cl Ho = -330 kJ/molrxn
2 CH3· CH3CH3 Ho = -350 kJ/molrxn

Because the concentration of the radicals is relatively small, these chain-termination reactions are relatively infrequent.

This chain-reaction mechanism for free-radical reactions explains the observations listed for the reaction between CH4 and Cl2.

Cl2 2 Cl· Ho = 243.4 kJ/molrxn
CH4 + Cl· CH3· + HCl
CH3· + Cl2 CH3Cl + Cl·
CH3Cl + Cl· CH2Cl· + HCl
CH2Cl· + Cl2 CH2Cl2 + Cl·, and so on
2 CH3· CH3CH3

Free-radical halogenation of alkanes provides us with another example of the role of atom-transfer reactions in organic chemistry. The net effect of this reaction is to oxidize a carbon atom by removing a hydrogen from this atom.

CH4 + Cl2 CH3Cl + HCl
-4 -2

The reaction, however, doesn't involve the gain or loss of electrons. It occurs by the transfer of a hydrogen atom in one direction and a chlorine atom in the other.

return to top

Uses of Halogenated Compounds

The chlorinated derivatives of methane have been known for so long that they are frequently referred to by the common names shown in the figure below.

Methyl chloride Methylene chloride Chloroform Carbon tetrachloride
BP = - 24.2șC BP = 40șC BP = 61.7șC BP = 76.5șC

The first member of this series is a gas at room temperature; the other three are liquids. These chlorinated hydrocarbons make excellent solvents for the kind of nonpolar solutes that would dissolve in hydrocarbons. They have several advantages over hydrocarbons; they are less volatile and significantly less flammable.

Chloroform (CHCl3) and carbon tetrachloride (CCl4) react with hydrogen fluoride to form a mixture of chlorofluorocarbons, such as CHCl2F, CHClF2, CCl3F, CCl2F2, and CClF3, which are sold under trade names such as Freon and Genetron. The freons are inert gases with high densities, low boiling points, low toxicities, and no odor. As a result, they once found extensive use as propellants in antiperspirants and hair sprays. Controversy over the role of chlorofluorocarbons in the depletion of the Earth's ozone layer led the Environmental Protection Agency to ban the use of CCl2F2 and CCl3F in aerosols in 1978. CCl2F2, CCl3F and CHFCl2 are still used as refrigerants in the air-conditioning industry, however.

The structures of a variety of more complex halogenated compounds are shown in the figure below. Halothane is an anesthetic and thyroxine is a thyroid hormone. DDT is an insecticide that has been shown to accumulate in the fatty tissue of birds and is therefore only used as a last resort. Chlordane is a potent insecticide that is still used to control termites.

return to top

Organic Chemistry: Functional Groups

Functional Groups     |   Alkyl Halides  |   Alcohols and Ethers     |    Aldehydes and Ketones     |    The Carbonyl Group    |    Amines, Alkaloids, and Amides     |     Grignard Reagents

Research in the 1990's: The Chemistry of Garlic

Periodic Table    |    Glossary    |     Cool Applets

Gen Chem Topic Review      |    General Chemistry Help Homepage     |     Search: The general chemistry web site.