Organohalogen compounds - Britannica.com

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chemical compound

Alkyl halides

Structure and physical properties

Alkyl halides (RX, where R is an alkyl group and X = F, Cl, Br, or I) are classified as primary, secondary, or tertiary according to the degree of substitution at the carbon to which the halogen is attached. In a primary alkyl halide, the carbon that bears the halogen is directly bonded to one other carbon, in a secondary alkyl halide to two, and in a tertiary alkyl halide to three.

The methods used to prepare alkyl halides and the reactions that alkyl halides undergo frequently depend on whether the alkyl halide is primary, secondary, or tertiary.

A halogen substituent draws the electrons in the C-X bond toward itself, giving the carbon a partial positive charge (+) and the halogen a partial negative charge (-). The presence of the resulting polar covalent bond makes most alkyl halides polar compounds. Because the bond dipole (the measure of the separation of charge) of a C-X bond is the product of a charge term (largest for fluorine and smallest for iodine) and a distance term (smallest for fluorine and largest for iodine), the molecular dipole moments of alkyl halides do not vary much from one halogen to another (see Table 37).

The most important reactions of organohalogen compounds involve breaking the carbon-halogen bond by processes in which the halogen retains both of the electrons from the original bond and is lost as a negatively charged ion (X^-). Consistent with the order of carbon-halogen bond strengths given in Table 37, which shows that the bond to fluorine is the strongest and the bond to iodine the weakest of the carbon-halogen bonds, fluorides are normally observed to be the least reactive of the alkyl halides and iodides the most reactive.

The last column in Table 37 indicates that the boiling points of ethyl halides increase as the atomic number of the halogen increases. With increasing atomic number the halogen becomes more polarizable, meaning that the electric field associated with the atom is more easily distorted by the presence of nearby electric fields. Fluorine is the least polarizable of the halogens and iodine the most polarizable. An increased polarizability is associated with stronger intermolecular attractive forces of the dipole-induced dipole and induced dipole-induced dipole types (see chemical bonding) and therefore with an increased boiling point.

Multiple halogen substitution tends to increase the boiling point: CH₃Cl boils at -24º C, CH₂Cl₂ at 40º C, CHCl₃ at 61º C; and CCl₄ at 77º C. Multiple fluorine substitution is an exception, however: CH₃CH₂F boils at -32º C, CH₃CHF₂ at -25º C, CH₃CF₃ at -47º C, and CF₃CF₃ at -78º C. By reducing the molecular polarizability, multiple fluorine substitution weakens the strength of induced dipole-induced dipole attractive forces between molecules. In the liquid state these weakened intermolecular forces are reflected in unusually low boiling points, and in the solid state they are responsible for the novel properties of fluorocarbon polymers.

The densities of alkyl halides are related to intermolecular attractive forces and tend to parallel boiling points, alkyl fluorides being the least dense and alkyl iodides the most dense. In general, alkyl fluorides and chlorides are less dense than water, and bromides and iodides are more dense than water. Alkyl halides are not soluble in water.

Natural occurrence

Estimates place the amount of chloromethane (CH₃Cl) that results from natural biological processes at more than five million tons (five billion kilograms) per year. Most of this is produced in the oceans by marine algae and kelp, but terrestrial organisms--especially fungi--also make a contribution. Smaller quantities (less than 250,000 tons per year) enter the atmosphere as a result of volcanic emissions, forest fires, and human activity. Ocean-living organisms are a source of bromomethane (CH₃Br) as well as large quantities of iodomethane (CH₃I). More than 50 organohalogen compounds, including CHBr₃, CHBrClI, BrCH₂CH₂I, CH₂I₂, Br₂CHCH=O, I₂CHCO₂H, and (Cl₃C)₂C=O, have been identified as being present in the Hawaiian red seaweed Asparagopsis taxiformis. Virtually every marine plant that has been assayed has been found to produce organohalogen compounds, many of which have quite complicated structures.

Several naturally occurring halogen-containing substances have pharmaceutical applications. An example is the antibiotic chloramphenicol produced by Streptomyces venezuelae.

Fluorine-containing natural products are relatively rare, the most prominent examples being -fluoro fatty acids. (The prefix indicates that the substitution occurs at the end of a chain.) Fluoroacetic acid, FCH₂CO₂H, occurs in the South African plant Dichapetalum cymosum and is quite toxic. A related Dichapetalum species contains 16-fluorohexadecanoic acid, FCH₂(CH₂)₁₄CO₂H, which is also poisonous when ingested because of its subsequent metabolic conversion to fluoroacetic acid.

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