Haloalkane


The haloalkanes are a group of chemical compounds
derived from alkanes 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,
consequently, are known under many chemical and commercial names. They are used as flame retardants, fire extinguishants,
refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce,
many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have
been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes which contain chlorine, bromine,
and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory
may have activity as greenhouse 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, see Halomethane. 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. Haloalkanes have been known for centuries. Chloroethane was produced synthetically 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 of alcohols
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 most haloalkanes are human-produced,
non-artificial-source haloalkanes do occur on Earth, mostly through enzyme-mediated synthesis
by bacteria, fungi, and especially sea macroalgae. More than 1600 halogenated organics have been
identified, with bromoalkanes being the most common haloalkanes. Brominated organics in biology range from
biologically produced methyl bromide to non-alkane aromatics and unsaturates. Halogenated alkanes in land plants are more
rare, but do occur, as for example the fluoroacetate produced as a toxin by at least 40 species
of known plants. Specific dehalogenase enzymes in bacteria
which remove halogens from haloalkanes, are also known. Classes of haloalkanes
From the structural perspective, haloalkanes can be classified according to the connectivity
of the carbon atom to which the halogen is attached. In primary haloalkanes, the carbon that carries
the halogen atom is only attached to one other alkyl group. An example is chloroethane (CH
3CH 2Cl). In secondary haloalkanes, the carbon that
carries the halogen atom has two C–C bonds. In tertiary haloalkanes, the carbon that carries
the halogen atom has three C–C bonds. Haloalkanes can also be classified according
to the type of halogen. Haloalkanes containing carbon bonded to fluorine,
chlorine, bromine, and iodine results in organofluorine, organochlorine, organobromine and organoiodine
compounds, respectively. Compounds containing more than one kind of
halogen are also possible. Several classes of widely used haloalkanes
are classified in this way chlorofluorocarbons, hydrochlorofluorocarbons and hydrofluorocarbons. These abbreviations are particularly common
in discussions of the environmental impact of haloalkanes. Properties
Haloalkanes generally resemble the parent alkanes in being colorless, relatively odorless,
and hydrophobic. Their boiling points are higher than the corresponding
alkanes and scale with the atomic weight and number of halides. This is due to the increased strength of the
intermolecular forces—from London dispersion to dipole-dipole interaction because of the
increased polarity. Thus carbon tetraiodide (CI
4) is a solid whereas carbon tetrafluoride (CF
4) is a gas. As they contain fewer C–H bonds, halocarbons
are less flammable than alkanes, and some are used in fire extinguishers. Haloalkanes are better solvents 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 are alkylating agents, with primary haloalkanes
and those containing heavier halogens being the most active. The ozone-depleting abilities of the CFCs
arises from the photolability of the C–Cl bond. Occurrence
Haloalkanes are of wide interest because they are widespread and have diverse beneficial
and detrimental impacts. The oceans are estimated to release 1-2 million
tons of bromomethane annually. A large number of pharmaceuticals contain
halogens, especially fluorine. An estimated one fifth of pharmaceuticals
contain fluorine, including several of the most widely used drugs. Examples include 5-fluorouracil, fluoxetine,
paroxetine, ciprofloxacin, mefloquine, and fluconazole. The beneficial effects arise because the C-F
bond is relatively unreactive. Fluorine-substituted ethers are volatile anesthetics,
including the commercial products methoxyflurane, enflurane, isoflurane, sevoflurane and desflurane. Fluorocarbon anesthetics reduce the hazard
of flammability with diethyl ether and cyclopropane. Perfluorinated alkanes are used as blood substitutes. Chlorinated or fluorinated alkenes undergo
polymerization. Important halogenated polymers include polyvinyl
chloride, and polytetrafluoroethene. The production of these materials releases
substantial amounts of wastes. Nomenclature
IUPAC The formal naming of haloalkanes should follow
IUPAC nomenclature, which put the halogen as a prefix to the alkane. For example, ethane with bromine becomes bromoethane,
methane with four chlorine groups becomes tetrachloromethane. However, many of these compounds have already
an established trivial name, which is endorsed by the IUPAC nomenclature, for example chloroform
and methylene chloride. For unambiguity, this article follows the
systematic naming scheme throughout. Production
Haloalkanes can be produced from virtually all organic precursors. From the perspective of industry, the most
important ones are alkanes and alkenes. From alkanes Alkanes react with halogens by free 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. The reactive 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 compounds mono- or multihalogenated at various positions. It is possible to predict the results of a
halogenation reaction based on bond dissociation energies and the relative stabilities of the
radical intermediates. Another factor to consider is the probability
of reaction at each carbon atom, from a statistical point of view. Due to the different dipole moments of the
product mixture, it may be possible to separate them by distillation. From alkenes and alkynes
In hydrohalogenation, an alkene reacts with a dry hydrogen halide like hydrogen chloride
or hydrogen bromide 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 in this reaction,
the halogen is more likely to become attached to the more substituted carbon. This is an electrophilic addition reaction. Water must be absent otherwise there will
be a side product of a halohydrin. The reaction is necessarily to be carried
out in a dry inert solvent such as CCl 4 or directly in the gaseous phase. The reaction of alkynes are similar, with
the product being a geminal dihalide; once again, Markovnikov’s rule is followed. Alkenes also react with halogens to form haloalkanes
with two neighboring halogen atoms in a halogen 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. From alcohols
Alcohol undergoes nucleophilic substitution reaction by halogen acid to give Haloalkanes.Tertiary
alkanol reacts with hydrochloric acid directly to produce tertiary chloroalkane, but if primary
or secondary alkanol is used, an activator such as zinc chloride is needed. This reaction is exploited in the Lucas test. The most popular conversion is effected by
reacting the alcohol with thionyl chloride (SOCl
2) in the “Darzen’s Process,” 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 convert the hydroxyl group to the
chloride. Alcohols may likewise be converted to bromoalkanes
using hydrobromic acid or phosphorus tribromide. A catalytic amount of PBr
3 may be used for the transformation using phosphorus and bromine; PBr
3 is formed in situ. Iodoalkanes may similarly be prepared using
red phosphorus and iodine. 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 acids
Two methods for the synthesis of haloalkanes from carboxylic acids are the Hunsdiecker
reaction and the Kochi reaction. Biosynthesis
Many chloro and bromolkanes are formed naturally. The principal pathways involve the enzymes
chloroperoxidase and bromoperoxidase. Reactions
Haloalkanes are reactive towards nucleophiles. They are polar molecules: the carbon to which
the halogen is attached is slightly electropositive where the halogen is slightly electronegative. This results in an electron deficient carbon
which, inevitably, attracts nucleophiles. Substitution
Substitution reactions involve the replacement of the halogen with another molecule—thus
leaving saturated hydrocarbons, as well as the halogenated product. Haloalkanes behave as the R+ synthon, and
readily react 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–
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 a covalent bond between the two. Thus C–X is broken by heterolytic fission
resulting in a halide ion, X–. As can be seen, the OH is now attached to
the alkyl group, creating an alcohol.. Reaction with ammonia give primary amines. Chloro- and bromoalkanes are readily substituted
by iodide in the Finkelstein reaction. The iodoalkanes produced easily undergo further
reaction. Sodium iodide is used thus as a catalyst. Haloalkanes react with ionic nucleophiles;
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 using lithium aluminium hydride. Azoalkanes may be reduced to primary amines
by the Staudinger reduction or lithium aluminium hydride. Amines may also be prepared from alkyl halides
in amine alkylation, the Gabriel synthesis and Delepine reaction, by undergoing nucleophilic
substitution with potassium phthalimide or hexamine respectively, followed by hydrolysis. In the presence of a base, haloalkanes alkylate
alcohols, amines, and thiols to obtain ethers, N-substituted amines, and thioethers respectively. They are substituted by Grignard reagents
to give magnesium salts and an extended alkyl compound. Mechanism
Where the rate-determining step of a nucleophilic substitution reaction is unimolecular, it
is known as an SN1 reaction. In this case, the slowest is the heterolysis
of a carbon-halogen bond to give a carbocation and the halide anion. The nucleophile attacks the carbocation to
give the product. SN1 reactions are associated with the racemization
of the compound, as the trigonal planar carbocation may be attacked from either face. They are favored mechanism for tertiary haloalkanes,
due to the stabilization of the positive charge on the carbocation by three electron-donating
alkyl groups. They are also preferred where the substituents
are sterically bulky, hindering the SN2 mechanism. Elimination Rather than creating a molecule with the halogen
substituted with something else, one can completely eliminate both the halogen and a nearby hydrogen,
thus forming an alkene by dehydrohalogenation. For example, with bromoethane and sodium hydroxide
in ethanol, the hydroxide ion HO- abstracts a hydrogen atom. Bromide ion is then lost, resulting in ethylene,
H2O and NaBr. Thus, haloalkanes can be converted to alkenes. Similarly, dihaloalkanes can be converted
to 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 alkylmagnesium
compounds: Grignard reagents. Haloalkanes also react with lithium metal
to give organolithium compounds. Both Grignard reagents and organolithium compounds
behave as the R- synthon. Alkali metals such as sodium and lithium are
able to cause haloalkanes to couple in the Wurtz reaction, giving symmetrical alkanes. Haloalkanes, especially iodoalkanes, also
undergo oxidative addition reactions to give organometallic compounds. Applications
Haloalkanes are widely used as synthon equivalents to alkyl cation in organic synthesis. They can also participate in a wide variety
of other organic reactions. Short chain haloalkanes such as dichloromethane,
trichloromethane and tetrachloromethane are commonly used as hydrophobic solvents in chemistry. They were formerly very common in industry;
however, their use has been greatly curtailed due to their toxicity and harmful environmental
effects. Chlorofluorocarbons were used almost universally
as refrigerants and propellants due to their relatively low toxicity and high heat of vaporization. Starting in the 1980s, as their contribution
to ozone depletion became known, their use was increasingly restricted, and they have
now largely been replaced by HFCs. See also
Halogenation Halomethane
Halogenoarene References

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