How do you prepare alkyl halides?

Alkyl halides are alkanes with one or more halogen substituents. They are a subset of the broader class of halocarbons, however this distinction is rarely emphasized. Alkyl halides are commonly employed in industry. They are utilized in flame retardants, extinguishers, refrigerants, propellants, solvents, and medicines. Many halocarbons have been demonstrated to be severe pollutants and poisons as a result of their widespread commercial use. Chlorofluorocarbons, for example, have been linked to ozone depletion. Methyl bromide is a divisive fumigant. Methyl iodide, a found naturally chemical, does not exhibit ozone-depleting qualities and has been recognized as a non-ozone layer depleter by the US Environmental Protection Agency. The compounds with the generic formula "RX" where R is an alkyl or substituted alkyl group & X is a halogen are known as alkyl halides or alkyl halides.

For ages, alkyl halides have been identified. Chloroethane was first synthesized in the 15th century. The systematic synthesis of such chemicals began in the nineteenth century, paralleling the advancement of organic chemistry and the knowledge of alkane structure. Methods for the selective production of C-halogen bonds have been devised. The addition of halogens to alkenes, hydrohalogenation of alkenes, and conversion of alcohols to alkyl halides were all very flexible processes. Because the halide may be further substituted by various functional groups, alkyl halideswere inexpensively available for application in industrial chemistry due to the reliability and ease of implementation of these procedures.

While the majority of alkyl halides are manufactured by humans, non-artificial alkyl halides do exist on Earth, mostly through enzyme-mediated synthesis by bacteria, fungus, and notably marine macroalgae. Over 1600 halogenated organics have been discovered, with bromoalkanes being the most frequent. Brominated organics in biology include methyl bromide, which is created physiologically, as well as non-alkane aromatics and unsaturated. Halogenated alkanes are less common in terrestrial plants, although they do exist, such as fluoroacetate, which is generated as a poison by at least 40 recognized plant species. There are also specific dehalogenase enzymes in bacteria that remove halogens from haloalkanes.

Alkyl halides are colorless, odorless, and hydrophobic, similar to their parent alkanes. The melting and boiling temperatures of bromo-, chloro-, and iodoalkanes are higher than those of similar alkanes, with the atomic weight and number of halides increasing with increasing atomic weight and number of halides. This phenomenon is caused by an increase in the intensity of the intermolecular forces—from London dispersion to dipole-dipole interaction as polarizability increases. Tetraiodomethane is therefore a solid, while tetrachloromethane is a liquid. Due to fluorine's reduced polarizability, several fluoroalkanes defy this tendency and have lower melting and boiling temperatures than their nonfluorinated cousins. Alkyl halidesare less flammable than alkanes because they have fewer C–H bonds, and some are employed in fire extinguishers. Because of their enhanced polarity, alkyl halidesare superior solvents than equivalent alkanes. The reactivity of alkyl halides comprising halogens other than fluorine is better than that of the parent alkanes; this reactivity is the cause of most disagreements. Many are alkylating agents, the most active being main alkyl halides as well as those containing heavier halogens. The photolability of the C–Cl bond causes CFCs to deplete the ozone layer.

Production of alkyl halides

From alkanes

When hydrogen halides are added to alkenes, they either obey Markovnikov's rule or exhibit the Kharash effect. Markovnikov addition reactions are all electrophilic addition reactions of alkenes that obey the Markovnikov rule. (In a nutshell, "Hydrogen is introduced to the carbon with the greatest hydrogens, while halide is put to the carbon with the fewest hydrogens.")

From alkenes and alkynes

An alkene interacts with a dry hydrogen halide (HX) such as hydrogen chloride (HCl) or hydrogen bromide (HBr) to generate a mono-haloalkane during hydrohalogenation. The alkene's double bond is replaced by two new bonds, one with the halogen and one with the hydrohalic acid's hydrogen atom. According to Markovnikov's rule, hydrogen is linked to the carbon with the most hydrogen.

In a halogen addition reaction, alkenes combine with halogens (X2) to generate haloalkanes with two nearby halogen atoms. Tetrahalo compounds are formed when alkynes react similarly. Because the reagent X2 is colored and the result is normally colorless, this is sometimes referred to as "decolorizing" the halogen.

From alcohols

Haloalkanes can be formed from alcohol. Tertiary choloroalkane is formed when tertiary alkahol combines with hydrochloric acid. An activator, such as zinc chloride, is required for primary and secondary alcohol. The Lucas test takes use of this response.

Thionyl chloride (SOCl2) is used in the "Darzens halogenation" to convert these less reactive alcohols to chlorides. Phosphorus pentachloride (PCl5) and phosphorus trichloride (PCl3) work in the same way.

Alcohols can also be transformed to bromoalkanes with the use of hydrobromic acid or phosphorus tribromide (PBr3). For the transformation of phosphorus and bromine, a catalytic quantity of PBr3 may be utilized; PBr3 is generated in situ.

Check One Example

Question: The best reagent for converting ethanol to chloroethane is -

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