Haloalkanes (also known as halogenoalkanes or alkyl halides ) are a group of chemical compounds derived from hydrocarbons containing one or more halogens . They are a subset of the general class of halocarbons , although the distinction is not often made. Haloalkanes are widely used commercially and as a result, are known under a number of chemical and commercial names. They are used as flame retardants , fire extinguishers , refrigerants , propellants , solvents , and pharmaceuticals., After widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxic substances.
For example, chlorofluorocarbons have been shown to lead to ozone depletion . Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine and iodine are a threat to the ozone layer , but in theory fluorinated volatile haloalkanes may have activity as greenhouse gases . methyl iodide, a naturally occurring substance, however, it does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone depleting compound. See halomethane for more details . Haloalkanes or alkyl halides are compounds with the general formula “RX”, where R is an alkyl or substituted alkyl group and X is a halogen (F, Cl, Br, I).
Haloalkanes have been known for centuries. Chloroethane was produced in the 15th century. The systematic synthesis of such compounds developed in the 19th century with the development of organic chemistry and an understanding of the structure of alkanes. Methods were developed for the selective formation of C-halogen bonds. Particularly versatile methods included the addition of halogens to alkanes, the hydrohalogenation of alkanes , and the conversion of alcohols to alkyl halides . These methods are so reliable and so easily implemented that haloalkanes become inexpensively available for use in industrial chemistry because halides can be replaced by other functional groups.
While most haloalkanes are human-produced, non-artificial-source haloalkanes are found on Earth, mostly through enzyme-mediated synthesis by bacteria, fungi, and especially marine macroalgae (seaweeds). Over 1600 halogenated organics have been identified, with bromoalkanes being the most common haloalkanes. Brominated organics in biology range from biologically produced methyl bromides to non-alkane aromatics and unsaturates (indoles, terpenes, acetogenins, and phenols). [1] [2] Halogenated alkanes are more rare in land plants, but are, for example, fluoroacetate produced as a toxin by at least 40 species of plants known . The specific dehalogenase enzyme in bacteria that produces haloalkanesremove halogens from
Classrooms
From a structural point of view, haloalkanes can be classified according to the valency of the carbon atom to which the halogen is attached. In primary (1°) haloalkanes, the carbon carrying the halogen atom is attached to only one other alkyl group. An example is chloroethane ( CH.)3Chieftain 2CL ). In secondary (2°) haloalkanes, the carbon carrying the halogen atom has two C-C bonds. In tertiary (3°) haloalkanes, the carbon carrying the halogen atom has three C-C bonds.
Haloalkanes can also be classified according to the type of halogen at group 7 that reacts to a specific halogenalkane. Fluorine , chlorine , bromine , and iodine bonded to carbon-containing haloalkanes result in organofluorine , organochlorine , organobromine, and organoiodine compounds , respectively. Compounds containing more than one type of halogen are also possible. There are several classes of widely used haloalkanes such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons .(HFC) is classified. These abbreviations are particularly common in discussions of the environmental impact of haloalkanes.
Virtue
Haloalkanes generally resemble parent alkanes by being colourless, relatively odorless and hydrophobic. Chloro-, bromo- and iodoalkanes have higher melting points and boiling points than similar alkanes, scaling with atomic weight and number of halides. This is due to the increased strength of intermolecular forces – from the London dispersion to the increased polarization due to the dipole–dipole interaction. Thus tetraiodomethane ( CI.)
4) is a solid while tetrachloromethane ( CCl)
4) is a liquid. However, many fluoroalkanes go against this trend and have lower melting points and boiling points than their non-fluorinated analogues due to the lower polarizability of fluorine. For example, methane ( CH.)
4) has a melting point of -182.5 °C while tetrafluoromethane ( CF.)
4) has a melting point of -183.6 °C.
Since they contain fewer C–H bonds, haloalkanes are less flammable than alkanes, and are used in some firefighting. Because of their increased polarity, haloalkanes are better solvents than related alkanes . Halogen-containing haloalkanes other than fluorine are more reactive than parent alkanes—this reactivity is the basis of most controversy. There are many alkylating agents , with the primary haloalkanes being the most active (fluoroalkanes do not act as alkylating agents under normal conditions). The ozone-depleting capabilities of CFCs arise from the photolability of the C–Cl bond.
Natural phenomena
It is estimated that 1-2 million tonnes of bromomethane are released annually from the oceans. [3]
Glossary
IUPAC
The formal nomenclature of haloalkanes must follow the IUPAC nomenclature, which keeps the halogen as a prefix to the alkene. For example, ethane with bromine becomes bromoethane, methane with four chlorine groups becomes tetrachloromethane. However, many of these compounds already have an established trivial name, which is supported by IUPAC nomenclature, for example chloroform (trichloromethane) and methylene chloride (dichloromethane). But nowadays, the IUPAC nomenclature is used. For the sake of unambiguity, this article follows a systematic naming scheme.
Production
Haloalkanes can be produced from almost all organic precursors. From the point of view of industry, the most important are alkanes and alkenes.
From alkanes
Alkanes react with halogens by free radical halogenation. In this reaction a hydrogen atom is removed from the alkene, then replaced by a halogen atom by reacting with a diatomic halogen molecule. The reactant intermediate in this reaction is a free radical and the reaction is called a radical chain reaction .
Free radical halogenation typically produces a mixture of mono- or multihalogenated compounds under various conditions. It is possible to predict the results of the halogenation reaction based on the bond dissociation energy and the relative stability of the radical intermediate. From a statistical point of view, the probability of a reaction at each carbon atom is another factor to consider.
Due to the different dipole moments of the product mixture, it may be possible to separate them by distillation.
Chloroalkanes can be formed on the reaction of an alkane with chlorine in the presence of sunlight. CH4 + Cl2→ CH3Cl+HCl Methane + Chlorine → Methyl chloride (or chloromethane) + Hydrogen chloride
And chloroalkanes can again be converted to dichloroalkanes by reacting chloroalkanes with chlorine. CH3Cl+Cl2→ CH2Cl2 + HCl chloromethane + chlorine → dichloromethane + hydrogen chloride.
CH2Cl2 + Cl2 → CHCl3 + HCl dichloromethane + chlorine → trichloromethane (or chloroform) + hydrogen chloride
CHCl3 + Cl2 →CCl4 + HCl chloroform (or trichloromethane) + chlorine → tetrachloromethane (or carbon tetrachloride + hydrogen tetrachloride)
From alkynes and alkynes
In hydrohalogenation, an alkene reacts with a dry hydrogen halide (HX) like hydrogen chloride ( HCl ) or hydrogen bromide ( HBr ) to form a mono haloalkane. The double bond of the alkene is replaced by two new bonds, one with the halogen and the other with the hydrogen atom of the hydrohalic acid. Markovnikov’s law states that hydrogen is attached to the carbon that has the highest number of hydrogens. This is an electrophilic addition reaction. Water must be absent otherwise there will be a side product of the halohydrin. The reaction must necessarily be carried out in a dry inert solvent such as CCl in
4or directly in the gaseous phase. The reaction of alkynes is similar, with the product being the geminal dihalide; Once again, Markovnikov’s law is followed.
In the halogen addition reaction, alkanes also react with halogens (X 2 ) to form haloalkanes with two neighboring halogen atoms . Alkynes react similarly, forming tetrahalo compounds. This is sometimes referred to as “decolorizing” the halogen, because reagent X 2 is colored and the product is usually colorless and odorless.
by wine
The alcohol undergoes a nucleophilic substitution reaction by the halogen acid to give the haloalkane. Tertiary alkanols react directly with hydrochloric acid to produce tertiary chloroalkanes (alkyl chlorides), but if primary or secondary alcohols are used, a catalyst such as zinc chloride is required. This reaction is exploited in the Lucas test.
The most popular conversion is affected by the reaction of alcohol with thionyl chloride ( SOCl .)2) in “Dargens halogenation”, which is one of the most convenient laboratory methods because the byproducts are gaseous. Both phosphorus pentachloride ( PCl.)5) and phosphorus trichloride ( PCl.)3) also converts the hydroxyl group to chloride.
Similarly alcohols can be converted into bromolkans using hydrobromic acid or phosphorus tribromide (PBr3) . PBr. a stimulating amount of Can be used for transformation by using phosphorus and bromine; PBR3is made in the situation .
Iodoalkanes can similarly be prepared using red phosphorus and iodine (the phosphorus equivalent of triiodide). The Appel reaction is also useful for preparing alkyl halides. The reagent is tetrahalomethane and triphenylphosphine; The co-products are haloform and triphenylphosphine oxide.
From carboxylic acid
The two methods of synthesis of haloalkanes from carboxylic acids are Hunsdicker reaction and Kochi reaction.
Biosynthesis
Many chloro and bromoalkanes occur naturally. The main pathways include the enzymes chloroperoxidase and bromoperoxidase.
By Raydon’s method
On heating an alcohol with a halogen in the presence of triphenyl phosphate, haloalkanes are produced.
Reactions
Haloalkanes are reactive towards nucleophiles. They are polar molecules: the carbon to which the halogen is attached is slightly electronegative where the halogen is slightly electronegative. This results in an electron deficient (electrophilic) carbon, which essentially attracts the nucleophile.
Replacement
Substitution reactions involve the replacement of a halogen with another molecule—thus giving up the saturated hydrocarbon, as well as the halogen product. Haloalkanes behave as R + synthons, and react readily with nucleophiles.
Hydrolysis, a reaction in which water breaks a bond, is a good example of the nucleophilic nature of haloalkanes. The polar bond attracts a hydroxide ion, OH – (NaOH (aq) is a common source of this ion). It is a nucleophile with OH – clearly negative charge, because it has extra electrons, it donates them to the carbon, resulting in a covalent bond between the two. Thus C-X breaks down by heterolytic fission resulting in a halide ion, X- . As can be seen, the OH is now attached to the alkyl group, forming an alcohol. (Hydrolysis of bromoethane, for example, gives ethanol). Reaction with ammonia gives primary amines.
Chloro- and bromoalkanes are readily replaced by iodides in the Finkelstein reaction. The produced iodoalkanes readily undergo further reaction. Thus sodium iodide is used as a catalyst.
haloalkanes react with ionic nucleophiles (eg cyanide, thiocyanate, azide); The halogen is replaced by the corresponding group. It is of great synthetic utility: chloroalkanes are often available cheaply. For example, after undergoing substitution reactions, cyanoalkanes can be hydrolyzed to carboxylic acids, or reduced to primary amines using lithium aluminum hydride. Azolkans can be reduced to primary amines by Staudinger reduction or by lithium aluminum hydride. Amines can also be prepared from alkyl halides in alkylation, the Gabriel synthesis and the Dlepine reaction, by hydrolysis followed by nucleophilic substitution with potassium phthalimide or hexamine, respectively.
In the presence of a base, haloalkanes alkylate ethers to obtain alcohols, amines, and thiols, N -substituted amines, and thioethers, respectively. They are replaced by Grignard reagents to give magnesium salts and an expanded alkyl compound.
Mechanism
Where the rate-determining step of a nucleophilic substitution reaction is one-sided, it is known as an SN1 reaction. In this case, the slowest (thus the rate-determining step) is the heterolysis of the carbon–halogen bond to give a carbocation and halide anion. The nucleophile (electron donor) attacks the carbocation to give the product.
SN1 reactions are associated with racemization of the compound, as the tri-planar carbocation can be attacked from any face. Because of the stabilization of the positive charge on the carbocation by the three electron-donating alkyl groups, they are the preferred mechanism for tertiary haloalkanes. They are also preferred where the substituents are sterically heavy, inhibiting the N2 mechanism.
To remove
Instead of forming a molecule with the halogen substituted with something else, one can completely eliminate both the halogen and the nearby hydrogen, thus forming an alkene by dehydrohalogenation. For example, in ethanol with bromoethane and sodium hydroxide (NaOH), the hydroxide ion HO – abstracts a hydrogen atom. The bromide ion is then destroyed, resulting in ethene, H 2 O and NaBr. Thus, haloalkanes can be converted into alkenes. Similarly, dihaloalkanes can be converted into alkynes.
In related reactions, 1,2-dibromocompounds are debrominated by zinc dust to give alkenes and geminal dihalides can react with strong bases to give carbenes.
Other
Haloalkanes undergo free-radical reactions with elemental magnesium to give the alkylmagnesium compound: Grignard reagent. Haloalkanes also react with lithium metal to give organolithium compounds. Both Grignard reagent and organolithium compound behave as R – synthon. Alkali metals such as sodium and lithium are able to pair in the Wurtz reaction to produce haloalkanes, giving symmetric alkanes. Haloalkanes, especially iodoalkanes, also undergo oxidative addition reactions to obtain organometallic compounds.
Application
Chlorinated or fluorinated alkenes undergo polymerization. Important halogenated polymers include polyvinyl chloride (PVC), and polytetrafluoroethane (PTFE, or Teflon). The production of these materials generates a considerable amount of waste.alkyl fluoridesAn estimated fifth drug contains fluorine, including many top medicines. Most of these compounds are alkyl fluorides. [4] Examples include 5-fluorouracil, flunitrazepam (Rohypnol), fluoxetine (Prozac), paroxetine (Paxil), ciprofloxacin (Cipro), mefloquine and fluconazole. Fluorine substituted ethers are volatile anesthetics, including commercial products methoxyflurane, enflurane, isoflurane, sevoflurane and desflurane.alkyl chlorideSome low molecular weight chlorinated hydrocarbons such as chloroform, dichloromethane, dichloroethane and trichloroethane are useful solvents. Several million tons of chlorinated methane are produced annually.
Chloromethane is the precursor of chlorosilane and silicon. Chlorodifluoromethane (CHClF 2 ) is used to make Teflon. [5]alkyl bromidesLarge-scale applications of alkyl bromides exploit their toxicity, which also limits their usefulness. Methyl bromide is also an effective fumigant, but its production and use are controversial.alkyl iodidesNo large-scale applications for alkyl iodides are known. Methyl iodide is a popular methylating agent in organic synthesis.chloroChlorofluorocarbons were almost universally used as refrigerants and propellants due to their relatively low toxicity and high heat of vaporization. In the 1980s, as their contribution to ozone depletion became known, their use became increasingly restricted, and they are now largely replaced by HFCs.
