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Haloalkanes & Haloarenes
Hydro carbon mainly is of two types:
1. Aliphatic Halogen compounds.
2. Aromatic Halogen compounds.
1. Aliphatic Halogen: These are obtain by replacement of one or more hydrogen atom of an
aliphatic hydrocarbon by an equal number of halogen atoms.
Mainly three types of Aliphatic Halogen
A. Haloalkanes  single bond. (𝑐𝑛 𝐻2𝑛+1 ) X
B. Haloalkenes  double bond. (𝑐𝑛 𝐻2𝑛−1 ) X
C. Haloalkynes  triple bond. (𝑐𝑛 𝐻2𝑛−3 ) X
Haloalkanes
Monohalolakanes
-
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The halogen derivates of alkanes are called haloalkanes. These are classified as fluoro, chloro, bromo or
iodo compounds- according to type of halogen present.
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Chloromethane (methyle chloride)
Dichloromethane (methylene chloride)
Trichloromethane (chloroform)
Tetrachloromethane (carbon tetrachloride)
CH3 − CH2 − Br 𝐁𝐫𝐨𝐦𝐨𝐞𝐭𝐡𝐚𝐧𝐞 (ethyl bromide)
(Primary)
𝐶𝐻3 − 𝐶𝐻2 − 𝐶𝐻2 − 𝐼 1-Iodopropane (n-propyl-iodide)
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(Secondary)
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(Tertiary)
2-chloro-2methyl propane
A. Haloalkenes or Alkenyle halides: {𝒏 = 𝟐, 𝟑 … . . }
𝐶𝐻2 = 𝐶𝐻 − 𝐶𝑙
𝐵𝑟 − 𝐶𝐻2 − 𝐶𝐻 = 𝐶𝐻2
Chloroethene(vinyl chloride)
3-Bromoprop-1-ene (Allyl bromicle)
4
3
2
1
𝐶𝐻3 − 𝐶𝐻 = 𝐶𝐻 − 𝐶𝐻2 𝐶𝑙
1-chlorobut -2 ene (Crotyl chloride)
1
2
3
𝐶6 𝐻5 − 𝐶𝐻 = 𝐶𝐻 − 𝐶𝐻2 𝐶𝑙
3-CHLORO-1-Phenyl prop-1-ene (cinnamyle chloride)
B. Haloalkynes or Alkynyl halides:{𝒏 = 𝟐, 𝟑, 𝟒 … . . }
3
2
1
3
𝐵𝑟 − 𝐶𝐻2 − 𝐶 ≡ 𝐶𝐻
2
1
𝐶𝐻3 − 𝐶 ≡ 𝐶 − 𝐵𝑟
3-Bromoprop-1-yne
1-Bromoprop-1-yne
𝐻 − 𝐶 ≡ 𝐶 − 𝐶𝑙
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Chloroethyne
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Nomenclature of haloalkanes.
A. General methods of preparation of haloakanes.
1. From Alkanes
Direct halogenations of alkanes in presence of sunlight, heat or a suitable catalyst yields
haloalkanes. The reactivity of alkanes follows the following order.
Tertiary alkanes > Secondary alkanes > Primary
ℎ𝜐
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Alkyl halide
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Alkanes
R-X + HX
Page
R-X + X2
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Direct halogenaton does not form pure monohalogen derivatives but a mixture of polyhalogen
derivatives is always obtained.
Ex .
This method is thus not used in laboratory because it is not easy to separate the individual members
from the mixture.
 Chlorination is fast, bromination is slow while direct iodination is not possible as reaction is
reversible and highly endothermic.
F2 > Cl2 > Br2> I
Reactivity of I helogen
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2. From alkenes
Halogen acids (HCl, HBr, HI) react readily with alkenes to form haloalkanes reactivity with
halogen Acids.
HI > HBr > HCl > HF
H2C = CH2 + HI
CH3CH2 I
Ethane
Iodoethene
Br
|
CH3CH = CH2 + HBr
CH3 – CH–CH3
Propene
mark. Addn
2- Bromopropane
(unsymmetrical alkene)
(Isopropyle Bromide)
The unsymmetrical alkanes follow Markownikoff’s rules during adding and forming secondary or tertiary
alkyl halkles.
During addition of unsymmetrical reagent (HX, HoH, H2SO4) TO unsymmetrical alkenes, the negative part
of adding reagent goes to that carbon atom of the double bond which has lesser no. of hydrogen atoms.
However in presence of light, heat or peroxides (R-O-O-R) such as bezoyle peroxide (C6H5CO-O-OCOC6H5) the addition of HBr takes place against Markownikoff’s ruleand primary and normal Alkyl
bromides are formed. This is called peroxide effect or kharasch effect.
This method is well suited for industrial preparation of haloalkanes as the lower alkenes are obtained
during cracking of petroleum products.
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CH3-CH = CH2 + HBr
Propene
peroxide
CH3-CH2-CH2Br
Anti Mork-addi
I-Bromopropane
It may be noted that peroxide effect applies to the addition of HBr only.
3. From alcohols
This is the most widely used method for the preparation of haloalkanes in which the –Oh group of an
alcohol is replaced by the halogen atom.
R – OH + H-X
⟶ R-X + H20
The replacement of –OH group can be done with halogen acids phosphorus halides or thionyl chloride.
Reactivity of helogen acid sand alcohol follows them.
HI > HBr > HCl
Tertiary alcohol > Secondary alcohol > Primary alcohol.
a) Mixture of conc. HCl and Zncl (anhyd.) is known a s Lucas reagent.
This reagent is also used to distinguish 1°, 2°, 3°alcohol because their reactivity towards this reagent
lies in following seq.
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3° alcohol > 2°𝑎𝑙𝑐𝑜ℎ𝑜𝑙 > 1°alcohol
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This reaction is an example of Nucleophilic substitution. 1° alcohols.
SN2 path while 2°and 3° alcohols follows Sn path, forming stable 3°, 2°carbocation intermediates.
b) Laboratory method
C2H5OH + HBr
H2SO4 (Conc)
Ethanol
Reflux
C2H5OH + KBr + H2SO4
Heat
C2H5Br + H2O
Bromoethane
C2H5Br +kHSO4 + H2O
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 KBr AND H2SO4 is not used for 2° and 3° alcohols as their dehydration may occur.
I.
Action of phosphorus halides on alcohols
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3C2H5OH + Pcl3
3C2H5Cl + H3Po3
Ethanol
Phosphorus Acid
CH3CH2CH2OH + Pcl5
CH3 – CH2 CH2-Cl + PoCl3 + HCl
Phosphorus oxichloride.
 PBr AND PI5 are not to exist.
 The method gives good yield of primary haloalkanes but poor yields of sec. and tertiary
haloalkanes.
II.
Thionyle chloride ( Darzen’s method)
C2H5OH + SOCl2
pyridine
C2H5Cl + SO2 ↑ + HCl ↓
Thionyle chloride
4. Hunsdiecker reaction
Bromoalkanes can be easily prepared by refluxing the silver salt of a carboxylic acid with
bromine in ccl4. This reaction is called Hunsdiecker reaction.
CH3CH2CooAg + Br2
ccl4
CH3CH2Br + CO2 + AgBr
Silver propionate
Reflux
Bromoethane
Reactivity of halide is 1° > 2° > 3°
5. Halid exchange
R – Cl + NaI
Acetone R – I + NaCl
Δ
C2H5Br + NaI
Acetone C2H5I + NaBr
Δ
This reaction is called Finkelstien Reaction
CH3 – Br + AgF
Δ
(Swart Reaction)
C2H5I + NaBr
Fluoromethane
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B. General methods of preparation of haloarenes.
1. By direct halogenations of aromatic hydrocarbon
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This method is used for the preparation of chloro and bromo derivatives. Halogens react with
aromatic hydrocarbons in presence of catalysts or halogen carriers such as iron, iodine or
Aluminum halide. (lewis acid) at room temperature in absence of direct sunlight.


Ortho and Para isomers can be easily separated due to large difference in their melting point.
Iodo – derivative cannot be easily obtained, as reaction is reverible
C6H6I2 ⇌ C6H5I + HI
a. Side chain halogenations
Side chain halogenations of toluene takes place when chlorine is passed through boiling toluene
in presence of sunlight and absence of halogen carrier.
C6H5CH3 SO2Cl2 C6H5CH2Cl SO2 HCl 
2. Sandmeyer Reaction (From benzene Diazonium salt)
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Haloarenes can be obtained by decomposition of Benzene diazonium chloride in presence of copper
halide solution dissolved in halogen acid (HCl, HBr) the diazo group is replaced by a halogen atom.
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b. Gattermann reaction
In Gatterman reaction (modification of sandmeyer) the catalyst copper powder in the presence of
corresponding halogen acid (HCl or HBr) is used in place of cuprous halide (CuCl or CuBr).
3. Hundiecker Reaction
4. From phenol
5. Rasching method ( Manufacturing process)
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Benzene vapours mixed with hydrogen chloride and oxygen (air) are passed over cupric chloride at 503k.
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Physical Properties of Haloalkanes and Haloarenes
1. Physical state and smell:
i.
Haloalkanes: Haloalkanes, the lower members like CH3Cl, CH3Br, CH3 Fand ethyl chloride are
colourless gases at room temperature. The other haloalkanes upto C18 are colourless sweet
smelling liquids while higher members are colourless solids.
ii.
Haloarenes: Haloarenes are generally colourless liquids with pleasant odour or are
crystalline solids with characteristic smell.
2. Boiling Point
i.
Haloalkanes: for the same alkyl group, the boiling points of haloalkanes decreases in the
order RI > RBr > RCI. This is because with the increase in the size and mass of halogen atom,
the magnitude of van der Waal’s forces of attraction increases. Thus,
Haloalkane:
CH3l
CH3Br Ch3Cl
CH3F
Boiling point (K):
315.4 276.6 248.8 194.6
For the same halogen atom, the boiling points of haloalkanes increase with in the size of alkyl group.
Thus,
Haloalkane:
CH2Cl
C2H5Cl C3H7Cl C4H9Cl
Boiling point (K):
248.8 285.5 320.0 351.5
For isomeric haloalkanes, the boiling points decrease with branching, i.e., Primary > Secondary >
Tertiary. This is because branching of the chain makes the molecule more compact and therefore,
decreases the surface area of haloalkanes and so the magnitude of the van der Waal’s forces of
attraction decreases. Thus,
Haloalkane:
Boiling point (K):
n-Butyl
Isobutyl
tert-Butyl
bromide
bromide
bromide
374
364
346
It may be noted that haloalkanes have higher boiling point as compared to those of corresponding
alkanes. This is due to their polarity and strong dipole-dipole interactions between haloalkanes
molecules.
ii.
Haloarenes: the boiling and melting and melting points of arylhalides are nearly the same as
that of haolalkanes containing same number of carbon atoms. The boiling points of
monohalogen derivatives of benzene are in the order:
Iodo > Bromo > Chloro > Fluro
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For the same halogen atom, the boiling and melting points increases as the size of the aryl
group increase as the size of the aryl group increases. The boiling points of isomeric
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dihalobenzenes are nearly the same, but their melting points are quite different. Generally,
the melting point of the para isomer is always higher than of ortho or meta isomers. The is
due to the reason that the para isomer is more symmetrical and hence its molecules pack
closely in the crystal lattice.
para
meta
ortho
b.pt.
448 K
446 K
453 K
m.pt.
325 K
249 K
256 K
As a result, intermolecules forces of attractions are stronger and therefore, greater energy is required to
break the para isomer lattice and it melts at higher temperature.
3. Solubility
Although haloalkanes are polar in nature (dipole moment 2.05 – 2.15 D), yet they are insoluble in water.
This is because they are not able to form hydrogen bonds with water molecule. However, they are
soluble in the organic solvents like benzene, alcohol, ether, chloroform and carbon tetrachloride, etc.
Aryl halides are insoluble in water but readily miscible with organic solvents.
4. Density
Chloroalkanes are generally lighter than water whereas bromides and iodides are heavier. Their
densities are in the order R – I > R – Br > R – Cl. The density increases with increasing number and atomic
mass of the halogen. Aryl halides are heavier than water. Their densities follows the order:
Aryl iodide > Aryl bromide > Aryl chloride
5. Dipole moment
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Haloalkanes: Haolaalkanes are polar compounds and their polarity depends on the electronegativity of
halogen. Dipole moments of halomethanes are given below:
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CH3Cl > CH3F > CH3Br > CH3I
(1.86 D) (1.847 D) (1.83) (1.636 D)
Dipole moment of fluoro methane is less than that of chloro methane, it is probably due to very small
size of fluorine.
Haloarenes: The dipole moment of haloarenes lies in following sequence:
Florobenzene < Chlorobenzene
(1.60 D)
(1.69 D)
<Bromobenzene ≈ Iodobenzene
(1.70 D)
The lower dipole moment of fluorobenzene is probably due to small size of halogen.
Resultant dipole moments of isomeric dichlorobenzene are calculated by vectorial method i.e.,
The two polar bonds are 180° to each other, they
cancel the dipole moment of each other
μR = 0,
(𝜌-dichlorobenzene)
>
(𝜇= 2.54 D)
(o-dichlorobenzene)
(𝜇= 1.72 D)
(m-dichlorobenzene)
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In orthodichlorobenzene, the angle is 60° while in meta dichlorobenzene the angle is 120° thus
resultant dipole moment of ortho dichlorobenzene is greater than that of meta dichlorobenzene.