Haloalkane and Haloarene are obtained by the replacement of a hydrogen atom of an alkane and arene respectively by a halogen atom (F , Cl , Br , I ). Haloalkane contain halogen atom attached to the sp3 hybridised carbon atom of an alkyl group whereas haloarenes contain halogen atom attached to sp2 hybridised carbon atoms of an aryl group.
Nomenclature of Haloalkanes
CH3 — Cl
CH3 — CH2—Br
CH3 — CH2— CH2— F
CH3 — CH2— CH2—CH2—Cl
CH3 —CCl2 —CH3
Nomenclature of Haloarenes (Aryl Halides)
Nature of C—X Bond
Due to electronegativity difference between carbon and halogen , shared pair of electron lie closer to halogen .
Thus , Carbon bears a small positive charge and halogen bears a small negative charge . Hence , C—X bond is a polar covalent bond .
As size of halogen atom increases , carbon halogen bond length increases and bond enthalpy decreases from C–F to C–I .
The above reaction is not applicable for preparation of aryl halide because carbon- oxygen bond in phenol has a partial double bond character and is difficult to break being stronger than a single bond.
b)By the action of Phosphorous Halide
Phosphorous halide react with alcohol to form haloalkane or alkyl halide in excellent yields (80% or above).
Similarily , bromoalkane and iodoalkane are prepared by the action of phosphorous tribromide and phosphorous tri-iodide respectively since PBr3 and PI3 are not very stable compound , these are generally prepared insitu by the action of red phosphorous on bromine and iodine respectively .
Chloroalkanes are conveniently prepared by refluxing alcohols with thionyl chloride in presence of pyridine . Thionyl chloride method is preferred to hydrogen chloride or phosphorous pentachloride for the preparation of chloroalkanes since both the by product SO2 and HCl in this reaction being gases escape leaving behind the chloroalkane in almost pure state.
vic-Dibromides can be prepared by addition of Br2 in CCl4 to alkenes
CH2 = CH2 + Br2 BrCH2—CH2Br (1,2-Dibromoethane)
When Br2 / CCl4 is added to alkene , the reddish brown colour of bromine is discharged due to formation of colourless vic-dibromide. Therefore , this reaction is used as test for detection of unsaturation in an organic molecule.
3) From Silver Salt of Fatty Acids
The decomposition of the silver salt of a carboxylic acid with Br2 in refluxing CCl4 to form an alkyl or aryl bromide with one carbon less than the original acid is called Hunsdiecker Reaction .
C6H5COOAg + Br2 C6H5–Br (Bromobenzene) + CO2 + AgBr
4) By Halide Exchange
Iodoalkanes can be easily prepared from the corresponding chloro- or bromoalkanes by heating with sodium iodide in acetone or methanol .
CH3-CH2 –Br + NaI CH3-CH2 –I (Iodoethane) + NaBr
This reaction is called Finkelstein Reaction . This reaction is very useful for preparing such alkyl iodides which cannot be prepared by direct addition of HI to alkenes.
Alkyl fluorides are more conveniently prepared indirectly by heating suitable chloro- or bromoalkanes with inorganic fluorides such as AgF , Hg2F2 etc . This is called Swarts Reaction .
CH3Br + AgF → CH3F+ AgBr
2CH3-CH2 –Cl + Hg2F2 → 2CH3-CH2 –F + Hg2Cl2
Physical Properties of Haloalkanes
Physical State , Colour , Odour
Alkyl Halides are colourless when pure , but iodides develop colour when exposed to light. Methyl chloride , methyl bromide , ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher chloro , bromo , iodo compounds are either liquids or solids. Many volatile halogen compounds have sweet smell.
Melting Points and Boiling Points
Due to greater polarity as well as higher molecular mass as compared to parent hydrocarbon , intermolecular force are stronger in halogen derivatives. Thus , boiling point of chlorides , bromides and iodides are comparitively higher than those of hydrocarbon of comparable molecular mass.
For the same alkyl group , boiling point of haloalkane decreases in order : RI > RBr > RCl > RF . This is because with increase in size and mass of halogen , the magnitude of vander waal force of attraction decreases.
For the same halogen atom , the boiling point increases with increase in size of alkyl group .
For isomeric alkyl halides , the boiling point decreases with branching because with branching the surface area of the alkyl halide decreases and hence magnitude of vanderwaal force of attraction decreases.
The boiling point of chloro , bromo and iodo compounds increase as the number of halogen atom increases.
Alkyl fluorides and chlorides are generally lighter than water whereas alkyl bromides and iodides and polyhalides are heavier.
Among the alkyl halide , methyl iodide has highest density.
As size of alkyl group increase , density of alkyl halides decreases.
Haloalkane are very slightly soluble in water. If a haloalkane is to dissolve in water , energy is required to overcome the forces of attraction already existing between haloalkane molecules and to break the hydrogen bond already existing between water molecules.
However , these are quite soluble in organic solvents of low polarity because the new force of attraction set up between haloalkane and solvent molecules are of same strength as force of attraction being broken
Since the strength of C-X bond decreases in order : C–F > C–Cl > C–Br > C–I
Thus , stability of haloalkanes having same alkyl group decreases in order :
Reaction in which a stronger nucleophile displaces a weaker nucleophile are called nucleophilic substitution reaction and the atom or group (halide ion in present case ) which departs with its bonding pair of electrons is called the leaving group . Better the leaving group , more facile is the nucleophilic substitution reaction .
Amongst the halide ions , the other in which the leaving groups departs follows the sequence :
I– > Br– > Cl– > F –
Substitution of Halogen by Hydroxyl Group (Formation of Alcohols)
Haloalkanes on treatment with boiling aqueous alkalies or silver oxide in boiling water undergo hydrolysis to form alcohols .
When the aqueous ethanolic solution of a haloalkane is heated with silver cyanide, alkyl isocyanide or alkyl isonitrile or alkyl carbylamine is produced as the chief product along with a small amount of the corresponding alkyl cyanide.
R—X + Ag—CN R—NC+AgX
CH3I + Ag—CN CH3NC + AgI
Substitution of Halogen by Nitrile Group
Haloalkanes react with sodium or potassium nitrile to form alkyl nitriles.
The reaction between tert-butyl bromide and OH– ion to yield tert-butyl alcohol and Br– ion follows first order kinetics, i.e., the rate of reaction depends upon concentration of tert-butyl bromide only and is independent of concentration of OH- ions.
The reaction occurs in two steps .
In first step ; tert-butyl bromide undergoes ionization to produce tert-butyl carbocation and a bromide ion . The step is slow and hence is rate determining step of reaction .
In the second step , carbocation being a reactive chemical species is immediately attacked by nucleophile . This step is fast and does not affect the rate.
Carbocations are intermediates.
Both backside and frontside attack of the nucleophile occurs through which the backside attack predominates.
Low concentration of nucleophiles generally favours them .
Polar protic solvents of high dielectric constants favours SN1 reaction .
The order of reactivity follows the sequence : 3º > 2º >1º > methyl halide.
Electronic factors affect the reaction rates greatly.
In Grignard reagent , carbon – halogen bond is covalent but highly polar due to large electronegativity difference between carbon and magnesium . The magnesium – halogen bond is ionic .
Due to high polarity , Grignard reagent are a potential source of carbanions and hence are very reactive. Thus, It is necessary to protect grignard reagent from moisture otherwise these will react with water to form hydrocarbons.
It involves the interaction of two molecules of an alkyl halide (preferably bromide or iodide ) with metallic sodium in presence of dry ether to form symmetrical alkanes containing double the number of carbon atom present in the alkyl halide.
If the halogen is present on any carbon atom within the chain , the alkyl halide can undergo dehydrohalogenation in two or more different directions . In all such cases , the more highly substituted alkene is the major product of dehydrohalogenation . This generalisation is known as Saytzeff’s rule and this mode of elimination is called Saytzeff elimination.
Preparation of Haloarenes
By Direct Halogenation
Chloroarenes and Bromoarenes can be prepared by direct chlorination or bromination of aromatic hydrocarbons.
This reaction is usually carried out in absence of sunlight and in presence of lewis acid such as anhydrous ferrous or aluminium halide as catalyst.
If toluene is used instead of benzene , a mixture of o-chlorotoluene (minor) and p-chlorotoluene(major) is obtained since CH3 group is o,p – directing .
By Side Chain Halogenation
When Cl2 is passed through boiling toluene in presence of sunlight and absence of halogen carrier , phenylchloromethane is formed.
From Diazonium Salt by Sandmeyer Reaction
Formation of Diazonium Salt is as follows:
The conversion of benzenediazonium chloride to chlorobenzene , bromobenzene and benzonitrile on treatment with CuCl/HCl , CuBr/HBr or CuCN/KCN respectively is called Sandmeyer reaction
However , iodoarenes are prepared by simply warming the diazonium salt solution with aqueous KI solution.
Balz- Schiemann Reaction
Fluoroarenes are prepared by heating the corresponding diazonium tetrafluoroborates which in turn are obtained by diazotisation of suitable aromatic primary amine with aqueous NaNO2 in presence of fluoroboric acid (HBF4) at 273-278K . This reaction is called Balz – Schiemann reaction.
By Gattermann Reaction
This is a modification of Sandmeyer reaction in which benzenediazonium chloride is treated with copper powder and a halogen acid (instead of cuprous halide dissolved in the corresponding halogen acid) to form aryl halides.
Physical Properties of Haloarenes
Melting Points and Boiling Points
The boiling point of haloarenes are nearly same as those of haloalkanes with the same number of carbon atoms.
For the same aryl group , the melting point and boiling point increases as size of halogen atom increases.
For the same halogen atom , the melting and boiling point increases as the size of aryl group increaes.
The boiling point of isomeric dihalobenzene are nearly same but their melting point are quite different. The melting point of p- isomer is always higher than o- and m- isomer since p-isomer is more symmetrical and hence its molecules pack close in the crystal lattice.
All aryl halides are heavier than water.
Chemical Reaction of Haloarenes
Nucleophilic Substitution Reaction
Why Haloarenes are extremely less reactive than haloalkanes toward nucleophilic substitution reaction ?
Resonance Effect : In haloarenes lone pair of electron on halogen atom are delocalized on the benzene ring . As a result , C—Cl bond acquires partial double bond character whereas in case of alkyl halides carbon is attached to chlorine by a pure single bond. Thus , C—X bond in aryl halides is stronger than alkyl halides and cannot be easily broken.
Difference in hybridization of carbon atom in C—X : In haloarenes halogen is attached to sp2 hybridized carbon while in haloalkanes , halogen is attached to sp3 hybridized carbon. C—Cl bond in haloarene should be shorter (due to small size of sp2 hybridized orbital) and hence stronger than haloalkane. Thus , C—Cl bond in haloarene is stronger than haloalkane.
Instability of phenyl cation : In haloarene , phenyl cation formed as a result of self ionization is not stabilized by resonance. Hence aryl halides does not easily undergo nucleophilic substitution reaction .
Replacement by Hydroxyl Group
Due to low reactivity , nucleophilic substitution reaction in aryl halides occur under drastic condition. Chlorobenzene when treated with aqueous solution of NaOH at 623 K under 300atm pressure, gives sodium phenoxide which upon acidification gives phenol . This reaction is called Dow’s process .
Further , presence of electron withdrawing group at o and p position activates the halogen towards nucleophilic displacement. Moreover , greater the no of such groups more reactive is the haloarene.
Electrophilic Substitution Reaction
Haloarenes undergo the usual electrophilic substitution reaction of the benzene ring such as halogenation , nitration , sulphonation and electrophilic substitution reaction . Halogen atom is slightly deactivating and o,p-directing .
Friedel Craft Reaction
Reaction with Metal
Wurtz Fittig Reaction
Haloarenes when treated with an ethereal solution of alkyl halide in presence of sodium , form alkyl derivative of benzene. This is called wurtz fitting reaction.
When only haloarenes are treated with sodium , diaryls are produced. This reaction is called Fittig reaction .
Reaction with Magnesium
Bromo and iodoarenes form grignard reagent when their ethereal solution is treated with magnesium turnings.
Compounds containing more than one halogen atom are called polyhalogeno compound.
Dichloromethane (Methylene chloride )
It is widely used as solvent for paint remover , as a propellant in aerosols and as a process solvent in manufacture of drugs.
Exposure to some level of methylene chloride in air causes slightly impaired hearing and vision.
High level of methylene chloride in air causes dizziness , nausea , tingling and numbness in fingers and toes.
Physical Properties : It is heavy colourless mobile liquid . It has sticky sweet smell nd burning taste. It is sparingly soluble in water but much readily dissolves in alcohol and ether. It is very good solvent for organic compound.
Chemical Properties :
When Chloroform is exposed to sunlight and air , it decomposes into phosgene .
2CHCl3 + [O] → 2COCl2 (phosgene) + 2HCl
Chloroform may be nitrated on heating with concentrated nitric acid to give chloropicrin (tear gas) .