Organochlorine chemistry

Two representations of chloroform.

Organochlorine chemistry is concerned with the properties of organochlorine compounds, or organochlorides, organic compounds containing at least one covalently bonded atom of chlorine. The chloroalkane class (alkanes with one or more hydrogens substituted by chlorine) includes common examples. The wide structural variety and divergent chemical properties of organochlorides lead to a broad range of names, applications, and properties. Organochlorine compounds have wide use in many applications, though some are of profound environmental concern, with TCDD being one of the most notorious.^[1]

Physical and chemical properties

Many organochlorine compounds have been isolated from natural sources ranging from bacteria to humans.[2]^[3] Chlorinated organic compounds are found in nearly every class of biomolecules and natural products including alkaloids, terpenes, amino acids, flavonoids, steroids, and fatty acids.^[2][4] Dioxins, which are of particular concern to human and environmental health, are produced in the high temperature environment of forest fires and have been found in the preserved ashes of lightning-ignited fires that predate synthetic dioxins.^[5] In addition, a variety of simple chlorinated hydrocarbons including dichloromethane, chloroform, and carbon tetrachloride have been isolated from marine algae.^[6] A majority of the chloromethane in the environment is produced naturally by biological decomposition, forest fires, and volcanoes.^[7]

The natural organochloride epibatidine, an alkaloid isolated from tree frogs, has potent analgesic effects and has stimulated research into new pain medication. However, because of its unacceptable therapeutic index, it is no longer a subject of research for potential therapeutic uses.^[8] The frogs obtain epibatidine through their diet which is then sequestered into their skin. Likely dietary sources are beetles, ants, mites, and flies.^[9]

Preparation

From chlorine

The haloform reaction, using chlorine and sodium hydroxide, is also able to generate alkyl halides from methyl ketones, and related compounds. Chloroform was formerly produced thus.

Chlorine adds to the multiple bonds on alkenes and alkynes as well, giving di- or tetra-chloro compounds.

Reaction with hydrogen chloride

Alkenes react with hydrogen chloride (HCl) to give alkyl chlorides. For example, the industrial production of chloroethane proceeds by the reaction of ethylene with HCl:

H₂C=CH₂ + HCl → CH₃CH₂Cl

In oxychlorination, hydrogen chloride instead of the more expensive chlorine is used for the same purpose:

CH₂=CH₂ + 2 HCl + 1⁄2 O₂ → ClCH₂CH₂Cl + H₂O.

Secondary and tertiary alcohols react with hydrogen chloride to give the corresponding chlorides. In the laboratory, the related reaction involving zinc chloride in concentrated hydrochloric acid:

${\displaystyle {\ce {{R-OH}+HCl->[{\ce {ZnCl2}}][\Delta ]{\overset {alkyl\ halide}{R-Cl}}+H2O}}}$

Called the

Other chlorinating agents

Alkyl chlorides are most easily prepared by treating alcohols with thionyl chloride (SOCl₂) or phosphorus pentachloride (PCl₅), but also commonly with sulfuryl chloride (SO₂Cl₂) and phosphorus trichloride (PCl₃):

ROH + SOCl₂ → RCl + SO₂ + HCl

3 ROH + PCl₃ → 3 RCl + H₃PO₃

ROH + PCl₅ → RCl + POCl₃ + HCl

In the laboratory, thionyl chloride is especially convenient, because the byproducts are gaseous. Alternatively, the Appel reaction can be used:

Reactions

Alkyl chlorides are versatile building blocks in organic chemistry. While alkyl bromides and iodides are more reactive, alkyl chlorides tend to be less expensive and more readily available. Alkyl chlorides readily undergo attack by nucleophiles.

Heating alkyl halides with

Most low molecular weight chlorinated hydrocarbons such as chloroform, dichloromethane, dichloroethene, and trichloroethane are useful solvents. These solvents tend to be relatively non-polar; they are therefore immiscible with water and effective in cleaning applications such as degreasing and dry cleaning. Several billion kilograms of chlorinated methanes are produced annually, mainly by chlorination of methane:

CH₄ + x Cl₂ → CH_4−xCl_x + x HCl

The most important is dichloromethane, which is mainly used as a solvent. Chloromethane is a precursor to

The two main groups of organochlorine

• The DDT like compounds work on the peripheral nervous system. At the axon's sodium channel, they prevent gate closure after activation and membrane depolarization. Sodium ions leak through the nerve membrane and create a destabilizing negative "afterpotential" with hyperexcitability of the nerve. This leakage causes repeated discharges in the neuron either spontaneously or after a single stimulus.^[10]: 255
• Chlorinated cyclodienes include
  gamma-aminobutyric acid (GABA) chloride ionophore complex, which inhibits chloride flow into the nerve.^[10]
  ^: 257
• Other examples include
  hydrophobic, depending on their molecular structure.^[11]

Polychlorinated biphenyls (PCBs) were once commonly used electrical insulators and heat transfer agents. Their use has generally been phased out due to health concerns. PCBs were replaced by polybrominated diphenyl ethers (PBDEs), which bring similar toxicity and bioaccumulation concerns.

Toxicity

Some types of organochlorides have significant toxicity to plants or animals, including humans. Dioxins, produced when organic matter is burned in the presence of chlorine, are

Rachel Carson brought the issue of DDT pesticide toxicity to public awareness with her 1962 book Silent Spring. While many countries have phased out the use of some types of organochlorides such as the US ban on DDT, persistent DDT, PCBs, and other organochloride residues continue to be found in humans and mammals across the planet many years after production and use have been limited. In Arctic areas, particularly high levels are found in marine mammals. These chemicals concentrate in mammals, and are even found in human breast milk. In some species of marine mammals, particularly those that produce milk with a high fat content, males typically have far higher levels, as females reduce their concentration by transfer to their offspring through lactation.^[16]

See also

• Organic halide

References