Alkyl Halides or Haloalkanes
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Isomerism in haloalkanes is of two types: Haloalkanes containing four or more carbon atoms exhibit chain isomerism in which the isomers differ in the chain of carbon atoms. For example, C4H9Br has three chain isomers, such as:
Haloalkanes with three or more carbon atoms exhibit position isomerism in which the isomers differ in the position of halogen atom. For example, C3 H7I has two position isomers:
In the laboratory, Haloalkanes are generally prepared either from hydrocarbons or alcohols. Haloalkanes can be prepared from alkanes and alkenes when the halogen atom replaces the hydrogen atom. In the presence of ultra-violet light halogens (Cl2, Br2, I2) react with alkanes to form haloalkanes. In this method, polysubstituted halides are obtained that are difficult to separate. When pure components are needed, they can be obtained by fractional distillation.
(Carbon tetra chloride)
In case of higher alkanes, a mixture of different isomeric products are formed even when monosubstitution is carried out. Note: With rare exceptions, halogenation of alkanes is not suitable for laboratory preparation of haloalkanes. This is mainly because mixtures of different isomers are formed which are difficult to separate. However, chlorination of alkanes is useful on an industrial scale, particularly when haloalkanes are used as solvents. The mixture of isomers is as suitable as a pure compound and is much cheaper. Haloalkanes can be prepared by the addition of halogen acids (HBr, HCl or HI).
In case of addition to symmetrical alkenes (HC=CH), the carbon atoms joined by the double bond are equivalent and, therefore, only one addition product is formed. For example,
In the addition of halogen acids to unsymmetrical alkenes, Markownikoff's rule is followed in which 'the negative part of the attacking reagent attaches itself to the carbon atom carrying lesser number of hydrogen atoms while the positive part goes to the carbon atom with more number of hydrogen atoms'. However, it has been observed that when HBr is added to an unsymmetrical double bond in the presence of organic peroxides such as benzoyl peroxide (C6H5 CO-O-O-COC6 H5), the reaction takes place against Markownikoff's rule. In the anti Markownikoff's rule or peroxide effect or Kharasch effect the negative part of the attacking reagent (Br) will go to the carbon atom carrying more hydrogen atoms while 'H' atom will go to the other carbon atom containing lesser number of hydrogen atoms.
cnti Markownikoff's rule or peroxide effect applies to the addition of HBr only and not to the addition of HI or HCl. c When alkenes are heated with Br2, or Cl2 at a high temperature of about 800 K, the hydrogen atom of allylic carbon is substituted with halogen atom forming allyl halides.
Reactions in which halogenation occurs at the allylic position of an alkene are called allylic halogenation reactions. cllylic bromination may also be carried out by treating alkene with N-bromo succinimide (NBS) in the presence of light. Bromination occurs at the methyl group forming a good yield of 3-bromoprop-1-ene (allyl bromide).
cllylic chlorination can also be carried out by treating alkene with sulphuryl chloride (SO2Cl2) at 475 K, in the presence of light and traces of peroxide.
The most widely used method for the preparation of haloalkanes. Here, the hydroxyl group (-OH) of the alcohol is replaced by the halogen atom (X).
clcohols can be converted into haloalkanes by treatment with halogen acids (HX). However, the rate of the reaction depends on both the nature of alcohol and the halogen acid. For example: Primary and secondary alcohols form chloroalkanes when hydrochloric acid gas is passed through alcohol in the presence of anhydrous zinc chloride (Groove's process).
ZnCl2, is a Lewis acid and it readily coordinates with the oxygen atom of the alcohols. cs a result, the C-O bond weakens and breaks to form carbocation, which reacts with chloride ion to form chloroalkanes. Thus, anhydrous ZnCl2 helps in the cleavage of the C-O bond. Tertiary alcohols, are very reactive and therefore, they react readily with concentrated HCl even in the absence of the chloride.
Similarly, bromo alkanes are obtained by heating an alcohol with hydrobromic acid (48%). HBr can also be generated in situ (during the reaction) by the action of concentrated H2SO4 on KBr.
Iodoalkanes are obtained by heating alcohols with constant boiling hydroiodic acid (57%) generated in situ by the action of phosphoric acid on potassium iodide.
The secondary and tertiary bromides and iodides unlike alkyl halides, cannot be prepared from the respective
alcohols. This is because the secondary and tertiary alcohols undergo dehydration on heating with concentrated H2SO4 to form alkenes. The preparation of fluoroalkanes is not practical by this method. The order of reactivity of halogen acids on alcohols is in accordance with the bond dissociation energies of H-X bonds: HI HBr > HCl Reactivity of alcohols towards this reaction is: tertiary > secondary > primary The cleavage of C-O bond becomes easy and reactivity increases when the polarity of C-OH increases with the number of electron releasing groups on the -carbon atom of the alcohol. Haloalkanes can be prepared by the action of phosphorus halides on alcohols. Chloro alkanes can be prepared by the action of phosphorus pentachloride (PCl5 ) or phosphorus trichloride (PCl3) on alcohols.
Likewise, Bromoalkanes and iodoalkanes are prepared by the action of phosphorus tribromide (PBr3) and phosphorus tri-iodide (PI3) respectively on alcohols. cs PBr3 and PI3, are not very stable compounds, they are prepared in situ by the action of red phosphorus on Br2, or I2, as:
Chloro alkanes can be also be prepared from alcohols by refluxing alcohols with thionyl chloride in the presence of pyridine.
This method is preferred than other methods because both the products of the reaction (SO2 and HCl) are gases and can easily escape leaving behind pure alkyl halide.
When chloro or bromoalkanes are heated with a concentrated solution of sodium iodide in acetone, iodoalkanes are obtained. This method is useful for preparing iodoalkanes.
Sodium bromide gets precipitated from the solution and can be removed by filtration. This reaction is known as Finkelstein reaction. Fluoro alkanes are prepared by treating alkyl chlorides with salts such as mercurous fluoride (Hg2F2).
When the silver salts of the carboxylic acids dissolved in CCl4 are decomposed by bromine bromoalkanes are formed. This reaction is called Borodine Hundsdiecker reaction.
The yield of halide is primary > secondary > tertiary. Chloroalkanes can also be obtained by this method by using Cl2 instead of Br2, but, the yield of chloroalkanes is very poor. Iodoalkanes cannot be obtained by this reaction.