Thehaloalkanes (also known ashalogenoalkanes oralkyl halides) arealkanes containing one or morehalogen substituents of hydrogen atom.[1] They are a subset of the general class ofhalocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used asflame retardants,fire extinguishants,refrigerants,propellants,solvents, andpharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be seriouspollutants and toxins. For example, thechlorofluorocarbons have been shown to lead toozone depletion.Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine, and iodine are a threat to theozone layer, but fluorinated volatile haloalkanes in theory may have activity asgreenhouse gases.Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, seeHalomethane. Haloalkane or alkyl halides are the compounds which have 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 in step with the development of organic chemistry and the understanding of the structure of alkanes. Methods were developed for the selective formation of C-halogen bonds. Especially versatile methods included the addition of halogens to alkenes,hydrohalogenation of alkenes, and the conversion ofalcohols to alkyl halides. These methods are so reliable and so easily implemented that haloalkanes became cheaply available for use in industrial chemistry because the halide could be further replaced by other functional groups.
While many haloalkanes are human-produced, substantial amounts are biogenic.
From the structural perspective, haloalkanes can be classified according to the connectivity of the carbon atom to which the halogen is attached. In primary (1°) haloalkanes, thecarbon that carries the halogen atom is only attached to one other alkyl group. An example ischloroethane (CH
3CH
2Cl). In secondary (2°) haloalkanes, the carbon that carries the halogen atom has two C–C bonds. In tertiary (3°) haloalkanes, the carbon that carries the halogen atom has three C–C bonds.[citation needed]
Haloalkanes can also be classified according to the type of halogen on group 17 responding to a specific halogenoalkane. Haloalkanes containing carbon bonded tofluorine,chlorine,bromine, andiodine results inorganofluorine,organochlorine,organobromine andorganoiodine compounds, respectively. Compounds containing more than one kind of halogen are also possible. Several classes of widely used haloalkanes are classified in this waychlorofluorocarbons (CFCs),hydrochlorofluorocarbons (HCFCs) andhydrofluorocarbons (HFCs). These abbreviations are particularly common in discussions of the environmental impact of haloalkanes.[citation needed]
Haloalkanes generally resemble the parent alkanes in being colorless, relatively odorless, and hydrophobic. The melting and boiling points of chloro-, bromo-, and iodoalkanes are higher than the analogous alkanes, scaling with the atomic weight and number of halides. This effect is due to the increased strength of theintermolecular forces—fromLondon dispersion to dipole-dipole interaction because of the increased polarizability. Thustetraiodomethane (CI
4) is a solid whereastetrachloromethane (CCl
4) is a liquid. Many fluoroalkanes, however, go against this trend and have lower melting and boiling points than their nonfluorinated analogues due to the decreased polarizability of fluorine. For example,methane (CH
4) has a melting point of −182.5 °C whereastetrafluoromethane (CF
4) has a melting point of −183.6 °C.[citation needed]
As they contain fewer C–H bonds, haloalkanes are less flammable than alkanes, and some are used in fire extinguishers. Haloalkanes are bettersolvents than the corresponding alkanes because of their increased polarity. Haloalkanes containing halogens other than fluorine are more reactive than the parent alkanes—it is this reactivity that is the basis of most controversies. Many arealkylating agents, with primary haloalkanes and those containing heavier halogens being the most active (fluoroalkanes do not act as alkylating agents under normal conditions). The ozone-depleting abilities of the CFCs arises from thephotolability of the C–Cl bond.[citation needed]
An estimated 4,100,000,000 kg ofchloromethane are produced annually by natural sources.[2] The oceans are estimated to release 1 to 2 million tons ofbromomethane annually.[3]
The formal naming of haloalkanes should followIUPAC nomenclature, which put the halogen as aprefix to the alkane. For example,ethane withbromine becomesbromoethane,methane with fourchlorine groups becomestetrachloromethane. However, many of these compounds have already an established trivial name, which is endorsed by the IUPAC nomenclature, for examplechloroform (trichloromethane) and methylene chloride (dichloromethane). But nowadays, IUPAC nomenclature is used. To reduce confusion this article follows the systematic naming scheme throughout.[citation needed]
Haloalkanes can be produced from virtually all organic precursors. From the perspective of industry, the most important ones are alkanes and alkenes.
Alkanes react with halogens byfree radical halogenation. In this reaction a hydrogen atom is removed from the alkane, then replaced by a halogen atom by reaction with a diatomic halogen molecule. Free radical halogenation typically produces a mixture of compounds mono- or multihalogenated at various positions.[citation needed]
Inhydrohalogenation, analkene reacts with a dry hydrogen halide (HX)electrophile likehydrogen chloride (HCl) orhydrogen bromide (HBr) to form a mono-haloalkane. The double bond of the alkene is replaced by two new bonds, one with the halogen and one with the hydrogen atom of the hydrohalic acid.Markovnikov's rule states that under normal conditions, hydrogen is attached to the unsaturated carbon with the most hydrogen substituents. The rule is violated when neighboring functional groupspolarize the multiple bond, or in certain additions of hydrogen bromide (addition in the presence ofperoxides and theWohl-Ziegler reaction) which occur by a free-radical mechanism.[citation needed]
Alkenes also react with halogens (X2) to form haloalkanes with two neighboring halogen atoms in ahalogen addition reaction. Alkynes react similarly, forming the tetrahalo compounds. This is sometimes known as "decolorizing" the halogen, since the reagent X2 is colored and the product is usually colorless and odorless.[citation needed]
Alcohol can be converted to haloalkanes. Direct reaction with ahydrohalic acid rarely gives a pure product, instead generatingethers. However, some exceptions are known: ionic liquids suppress the formation or promote the cleavage of ethers,[4]hydrochloric acid converts tertiary alcohols to choloroalkanes, and primary andsecondary alcohols convert similarly in the presence of aLewis acid activator, such aszinc chloride. The latter is exploited in theLucas test.[citation needed]
In the laboratory, more active deoxygenating and halogenating agents combine with base to effect the conversion. In the "Darzens halogenation",thionyl chloride (SOCl
2) withpyridine converts less reactive alcohols to chlorides. Bothphosphorus pentachloride (PCl
5) andphosphorus trichloride (PCl
3) function similarly, and alcohols convert to bromoalkanes underhydrobromic acid orphosphorus tribromide (PBr3). The heavier halogens do not require preformed reagents: A catalytic amount ofPBr
3 may be used for the transformation using phosphorus and bromine;PBr
3 is formedin situ.[5] Iodoalkanes may similarly be prepared using redphosphorus andiodine (equivalent tophosphorus triiodide).[citation needed]
One family ofnamed reactions relies on thedeoxygenating effect oftriphenylphosphine. In theAppel reaction, the reagent is tetrahalomethane andtriphenylphosphine; the co-products arehaloform andtriphenylphosphine oxide. In theMitsunobu reaction, the reagents are anynucleophile, triphenylphosphine, and adiazodicarboxylate; the coproducts are triphenylphosphine oxide and ahydrazodiamide.[citation needed]
Two methods for the synthesis of haloalkanes fromcarboxylic acids areHunsdiecker reaction andKochi reaction.[citation needed]
Many chloro- and bromoalkanes are formed naturally. The principal pathways involve the enzymeschloroperoxidase andbromoperoxidase.[citation needed]
Primary aromaticamines yielddiazonium ions in a solution ofsodium nitrite. Upon heating this solution with copper(I) chloride, the diazonium group is replaced by -Cl. This is a comparatively easy method to make aryl halides as the gaseous product can be separated easily from aryl halide.[citation needed]
When an iodide is to be made, copper chloride is not needed. Addition ofpotassium iodide with gentle shaking produces the haloalkane.[citation needed]
Haloalkanes are reactive towardsnucleophiles. They arepolar molecules: the carbon to which the halogen is attached is slightlyelectropositive where the halogen is slightlyelectronegative. This results in anelectron deficient (electrophilic) carbon which, inevitably, attractsnucleophiles.[citation needed]
Substitution reactions involve the replacement of the halogen with another molecule—thus leavingsaturated hydrocarbons, as well as the halogenated product. Haloalkanes behave as the R+synthon, and readily react with nucleophiles.[citation needed]
Hydrolysis, a reaction in whichwater breaks a bond, is a good example of the nucleophilic nature of haloalkanes. The polar bond attracts ahydroxide ion, OH− (NaOH(aq) being a common source of this ion). This OH− is a nucleophile with a clearly negative charge, as it has excess electrons it donates them to the carbon, which results in acovalent bond between the two. Thus C–X is broken byheterolytic fission resulting in a halide ion, X−. As can be seen, the OH is now attached to the alkyl group, creating analcohol. (Hydrolysis of bromoethane, for example, yieldsethanol). Reactions with ammonia give primary amines.[citation needed]
Chloro- and bromoalkanes are readily substituted by iodide in theFinkelstein reaction. The iodoalkanes produced easily undergo further reaction.Sodium iodide is used as acatalyst.[citation needed]
Haloalkanes react with ionic nucleophiles (e.g.cyanide,thiocyanate,azide); the halogen is replaced by the respective group. This is of great synthetic utility: chloroalkanes are often inexpensively available. For example, after undergoing substitution reactions, cyanoalkanes may be hydrolyzed to carboxylic acids, or reduced to primary amines usinglithium aluminium hydride. Azoalkanes may be reduced to primary amines byStaudinger reduction orlithium aluminium hydride. Amines may also be prepared from alkyl halides inamine alkylation,Gabriel synthesis andDelepine reaction, by undergoing nucleophilic substitution withpotassium phthalimide orhexamine respectively, followed by hydrolysis.[citation needed]
In the presence of a base, haloalkanesalkylate alcohols, amines, and thiols to obtainethers,N-substituted amines, and thioethers respectively. They are substituted byGrignard reagent to give magnesium salts and an extended alkyl compound.[citation needed]
Indehydrohalogenation reactions, the halogen and an adjacent proton are removed from halocarbons, thus forming analkene. For example, withbromoethane and sodium hydroxide (NaOH) inethanol, the hydroxide ion HO− abstracts a hydrogen atom. ABromide ion is then lost, resulting inethene, H2O and NaBr. Thus, haloalkanes can be converted to alkenes. Similarly, dihaloalkanes can be converted toalkynes.[citation needed]
In related reactions, 1,2-dibromocompounds are debrominated byzinc dust to give alkenes andgeminal dihalides can react with strong bases to givecarbenes.[citation needed]
Haloalkanes undergo free-radical reactions with elemental magnesium to give alkyl-magnesium compound:Grignard reagent. Haloalkanes also react withlithium metal to giveorganolithium compounds. Both Grignard reagents and organolithium compounds behave as the R− synthon. Alkali metals such assodium andlithium are able to cause haloalkanes to couple inWurtz reaction, giving symmetrical alkanes. Haloalkanes, especially iodoalkanes, also undergooxidative addition reactions to giveorganometallic compounds.[citation needed]
Chlorinated or fluorinated alkenes undergo polymerization. Important halogenated polymers includepolyvinyl chloride (PVC), andpolytetrafluoroethene (PTFE, or teflon).[citation needed]
Nature produces massive amounts of chloromethane and bromomethane. Most concern focuses on anthropogenic sources, which are potential toxins, even carcinogens. Similarly, great interest has been shown in remediation of man made halocarbons such as those produced on large scale, such as dry cleaning fluids. Volatile halocarbons degrade photochemically because the carbon-halogen bond can be labile. Some microorganisms dehalogenate halocarbons. While this behavior is intriguing, the rates of remediation are generally very slow.[8]
Asalkylating agents, haloalkanes are potential carcinogens. The more reactive members of this large class of compounds generally pose greater risk, e.g.carbon tetrachloride.[9]
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