The alkyl halide functional group, also called haloalkanes, is one of the most useful and common functional groups. This article talks about the structure of alkyl halides and some of their reactions and properties.
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What is an Alkyl Halide Functional Group?
As its name suggests, an alkyl halide is a hydrocarbon compound in which halogens take the place of one or more Hydrogen atoms. This includes Fluorine, Chlorine, Bromine, and Iodine. The generic structure is R-X, where R is an alkyl group and X is a halogen.
Quick Facts about the Alkyl Halide Functional Group
- Structure: An Alkyl Group with one or more halides attached to it
- Generic formula: R-X
- Atomic weight: Depends on alkyl group and halide
- Bond Polarity:
- Bonds to Fluorine and Chlorine are more polar than to Bromine and Iodine
- Reactions involving haloalkanes often involve replacing the halide with a different functional group
- The halogen in an alkyl halide can react with a lighter halogen to form a new alkyl halide with the smaller atom.
- For example, 2-iodopropane can react with Chlorine to form 2-chloropropane.
Naming haloalkanes is fairly simple. The first part is a prefix that indicates which halogen is in the compound. The second part is the name of the alkane. The halogen prefixes are Fluoro for Fluorine, Chloro for Chlorine, Bromo for Bromine, and Iodo for Iodine.
When there are different halogens on a compound, their prefixes both go before the alkane’s name in alphabetical order. Additionally, each halide should receive a number based on its location on the carbon chain.
Alkyl Halide Reactions
General Reaction Trends
Except for Fluorine, haloalkanes readily undergo SN1 and SN2 substitution reactions to replace the halide group with a different functional group. Fluoroalkanes are generally too stable to participate in the reactions that other haloalkanes do.
Formation of Alkyl Halides
The addition of HX or X2, where X is a halogen, to an alkene, yields a haloalkane. Following Markovnikov’s rule, the halogen will add to the more substituted side of the alkene.
Alkanes can participate in free radical halogenation, in which a hydrogen atom is replaced by a halide. This process happens when alkane radicals, which are present in low concentrations, interact with Halide molecules as shown below.
Often, a different functional group will replace the halide group on a haloalkane. This is the case with the Williamson Ether Synthesis, in which an alkoxide anion replaces the halide to form an ether.
When an alkyl halide encounters a carboxylate ion, the resulting reaction forms an ester.
Additionally, nitriles, alkynes, thiols, sulfides, and azides react similarly, replacing the halide with the respective functional group. For primary and secondary halide groups, these reactions are SN2. In tertiary halide groups, such reactions are SN1. In both cases, the anion of the substituting functional group acts as the nucleophile, and the carbon bonded to the halide acts as the electrophile.
Using a base, alkyl halides can be eliminated to form an alkene. The base pulls off a proton from a carbon next to the carbon bonded to the halide. The electrons from the former H-C bond then come down to form a double bond and kick off the halide.
Uses of Alkyl Halides
Haloalkanes have many applications, in laboratory settings, industry, and medicine. Due to the harm they often cause to the environment, governments have phased out many uses of haloalkanes. Halothane, a molecule that has Bromine, Chlorine, and Fluorine, is a common anesthetic, though how it works exactly is unknown.
Historically, chlorofluorocarbons, or CFCs, were used as refrigerants. However, when CFCs escaped into Earth’s Ozone layer, they depleted it. Dichloro-diphenyl-trichloroethane, DDT, is an insecticide first synthesized in the 1940s. Because of its negative impact on human and animal health, as well as on the environment, the EPA banned the use of DDT in 1972. Similarly, the EPA banned the pesticide Chlordane in 1988 because of its harmful effects on humans, animals, and the environment.
Dichloromethane, or DCM, is a common solvent in organic laboratories.
Plastics such as PVC are alkyl halide polymers. PVC stands for polyvinyl chloride and is a common material for plastic piping. Teflon, also called polytetrafluoroethylene, is an inert plastic that often coats non-stick cooking pans.