CHEMISTRY

CHEMISTRY HALOALKANES AND HALOARENES

HALOALKANES AND HALOARENES

Polyhalogen Compounds

Polyhalogen compounds: Carbon compounds containing more than one halogen

atom permolecule.

Polyhalogen compounds are useful in various industries and in agriculture.Some important

polyhalogen compounds:

Dichloromethane
(Methylene chloride)

p,p'-Dichlorodiphenyl- Trichloromethane
trichloroethane (DDT) (Chloroform)

Polyhalogen
Compounds

Triiodomethane
Freons
(Iodoform)

Tetrachloromethane
(Carbon tetrachloride)
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Dichloromethane (Methylene chloride)

Uses:
Dichloromethane (methylene chloride) is used as a:

1. Solvent for paint removers

2. Propellant in aerosols
3. Process solvent in the manufacture of drugs
4. Metal cleaning and finishing solvent

Harmful effects:

1. It endangers the human central nervous system.

2. Exposure to lower levels of methylene chloride in air can lead to slightly impaired
hearing andvision.
3. High levels of methylene chloride in air cause dizziness, nausea, tingling and
numbness in thefingers and toes.
4. In humans, direct skin contact with methylene chloride causes intense burning and
mild rednessof the skin.
5. Direct contact with the eyes can burn the cornea.

Trichloromethane (Chloroform)
CHEMISTRY HALOALKANES AND HALOARENES

Uses:

1. Chemically, chloroform is used as a solvent for fats, alkaloids, iodine and other
substances.
2. The major use of chloroform today is in the production of the freon refrigerant R-22.
3. It was once used as a general anaesthetic in surgery but has been replaced by less
toxic, saferanaesthetics such as ether.

Harmful effects:

1. As might be expected from its use as an anaesthetic, inhaling chloroform vapour

depresses the central nervous system.
2. Breathing about 900 parts of chloroform per million parts of air (900 ppm) for a short
time can cause dizziness, fatigue and headache.
3. Chronic chloroform exposure may cause damage to the liver (where chloroform is
metabolised to phosgene) and to the kidneys. Some people develop sores when the
skin is immersed inchloroform.
4. Chloroform is slowly oxidised by air (oxygen) in the presence of light to an extremely
poisonous gas, carbonyl chloride, also known as phosgene.

It is therefore stored in closed dark-coloured bottles which are completely filled so

that air is keptout.

Triiodomethane (Iodoform)

Uses:
CHEMISTRY HALOALKANES AND HALOARENES

 It was used earlier as an antiseptic, but the antiseptic properties are due to the
liberation of freeiodine and not due to iodoform itself.

Drawback:

 Because of its objectionable smell, it has been replaced by other formulations

containing iodine.

Tetrachloromethane (Carbon tetrachloride)

Uses:

1. It is produced in large quantities for use in the manufacture of refrigerants and

propellants foraerosol cans.
2. It is also used as feedstock in the synthesis of chlorofluorocarbons and other
chemicals, inpharmaceutical manufacturing and general solvent use.
3. Until the mid-1960s, it was also widely used as a cleaning fluid, both in industry, as a
degreasingagent, and in the home, as a spot remover and fire extinguisher.

Harmful effects:

1. There is evidence that exposure to carbon tetrachloride causes liver cancer in

humans.
2. The most common effects are dizziness, light headedness, nausea and vomiting,
which cancause permanent damage to nerve cells.
3. In severe cases, these effects can lead rapidly to stupor, coma, unconsciousness or
death. Exposure to CCl4 can make the heart beat irregularly or stop.
4. The chemical may irritate the eyes on contact. When carbon tetrachloride is released
into the air,it rises to the atmosphere and depletes the ozone layer.
5. Depletion of the ozone layer is believed to increase human exposure to ultraviolet
CHEMISTRY HALOALKANES AND HALOARENES
rays, leading to increased skin cancer, eye diseases and disorders, and possible
disruption of the immunesystem.
Freons

 The chlorofluorocarbon compounds of methane and ethane are collectively known as

freons.
 They are extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable
gases.
 They are manufactured from tetrachloromethane by Swarts reaction.
 By 1974, the total freon production in the world was about 2 billion pounds annually.

Uses:

1. These are usually produced for aerosol propellants, refrigeration and air conditioning
purposes.
2. Freon 12 (CCl2F2) is one of the most common freons in industrial use.
3. Most freons, even those used in refrigeration, eventually make their way into the
atmospherewhere it diffuses unchanged into the stratosphere.

Harmful Effect:

 In stratosphere, freons can initiate radical chain reactions which can upset the
natural ozonebalance.

p,p′-Dichlorodiphenyltrichloroethane (DDT)
CHEMISTRY HALOALKANES AND HALOARENES

DDT, the first chlorinated organic insecticide, was originally prepared in 1873.
However, it was not until 1939 that Paul Muller of Geigy Pharmaceuticals in Switzerland
discovered theeffectiveness of DDT as an insecticide.
Paul Muller was awarded the Nobel Prize in Medicine and Physiology in 1948 for this
discovery.

Paul Muller

Uses:

 The use of DDT increased enormously worldwide after World War II, primarily
because of itseffectiveness against the mosquito which spreads malaria and lice which
carry typhus.

Harmful Effects:

Problems related to extensive use of DDT began to appear in the late 1940s.
1. Many species of insects developed resistance to DDT.
2. It has a high toxicity towards fish.
CHEMISTRY HALOALKANES AND HALOARENES
3. The chemical stability of DDT and its fat solubility compounded the problem. DDT is
not metabolised very rapidly by animals. Instead, it is deposited and stored in the
fatty tissues. If ingestion continues at a steady rate, DDT builds up within the animal
over time.

The use of DDT was banned in the United States in 1973, although it is still in use in some
other parts ofthe world.

Preparation

Methods of Preparation of haloalkanes

By halogenations of alkanes in presence of light, catalyst or heat:

By halogenations of alkenes with HX :

Hunsdiecker reactions:
The Hunsdiecker reaction is an instance of a halogenation reaction which involves an organic
between silver salts of carboxylic acids and halogens resulting in the formation of organic
halides.

Allylic halogenations:
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Preparation of alkyl halides from alcohols:

Treatment of alcohol with HCl in the presence of anhydrous ZnCl2 phosphorous

pentachloride, PX3(P4 + X2)or SOCl2 leads to the preparation of alkyl halide.

By the action of halogen acids:

1 and 2 degree alcohols require anhydrous ZnCl2 while alcohols do not require ZnCl2. Mixture
of conc. HCl and anhydrous ZnCl2 is called Lucas reagent.
Primary and secondary alkyl chlorides are prepared from their respective alcohols by using
HCl gas and anhydrous ZnCl2.
By the action of phosphorous halides:
3ROH + PX3 à 3RX + H3PO3.
Alkyl chlorides can be synthesized by the action of PCl3 or PCl5.
Methods of Preparation
There are primarily 4 different types of preparation techniques of Haloalkanes and
Haloarenes. They include Preparation of Haloalkanes and Haloarenes from:
 Alcohols
 Hydrocarbons
 Alkenes by addition of hydrogen halides and halogens
 Halogen exchange reaction.
Preparation from Alcohols (Haloalkanes):
The most convenient method of preparation of haloalkane is from alcohols. R-OH when
CHEMISTRY HALOALKANES AND HALOARENES
reacts with suitable reagents, the reaction results in the formation of R − X. The suitable
reagents that help in the reaction are
Concentrated halogen acids (HX)
Phosphorus halides (PX5 or PX3)
Thionyl chloride (SOCl2)
i) The Reaction of Alcohols with Halogen Acid
An organic compound derivative of alcohol reacts with halogen acid (H-X) to form
haloalkanes as the major product.

Example-Preparation of Chloroalkanes
Preparation of chloroalkane is an example of the reaction of an alcohol with halogen acid to
form haloalkane. In this case, primary alcohol and secondary alcohol react with HCl acid gas
to form haloalkane in the presence of anhydrous ZnCl2, which act as a catalyst in this
reaction.

Preparation of Bromoalkanes
Hydrogen bromide (HBr) reacts with alcohols to form bromoalkanes. Hydrogen bromide
synthesis of the reaction takes place by the reaction of sodium bromide or potassium
bromide and H2SO4 (sulphuric acid). In the reaction below, NaBr and H 2SO4 react to form HBr
which further reacts with alcohol.
CHEMISTRY HALOALKANES AND HALOARENES
Example of Preparation of Bromoethane from Ethanol

The tertiary carbonation which occurs due to tertiary alcohols is more stable than secondary
and primary alcohols. Therefore tertiary carbocations offer more stability in comparison to
the primary and secondary form of the compound. Hence, tertiary is more reactive than
primary and secondary. The order of reactivity is 30 > 20 > 10.
Additionally, the reactivity of haloacids follows the order HI > HBr > HCl > HF. HI is easily and
highly reacting haloacid among all of them because the HI bond is weaker than the other
three. Therefore, it can be broken easily in comparison to the other halogen bonds to form
H+ and I− ions.
However, this preparation method will not be able to synthesize aryl halides/haloarenes. C-
OH bond in the phenol structure contains partial double bond character because of the
delocalization of lone pair of electrons present on the oxygen atom of the benzene ring.
Therefore, the bond formed in the structure cannot be easily broken down by any sort of
reaction with haloacids.
Ar – OH + HX → No reaction
ii) The Reaction of Alcohols with Phosphorus halides (PX5 or PX3)
This reaction helps in the formation of Chloroalkanes, bromoalkanes, and iodoalkanes. In this
reaction phosphorus halides interchange the functional group of alcohols (–OH) with the
corresponding halides. The reaction is as follows:
ROH + PCl5 → RCl + POCL3 + HCl
The above reaction is for the formation of alkyl chloride. Similarly, alkyl bromide or alkyl
iodide formation is possible by the reaction of an alcohol with phosphorus tribromide and
triiodide. To achieve the reaction, red phosphorus reacts with bromine or iodine by in-situ
preparation (during the reaction) of phosphorus tribromide and triiodide.

Note: Fresh preparation of the phosphorus tribromide and phosphorus triiodide is made with
red phosphorus and bromine or iodine due to the instability of the compounds. Thus, alcohol
reacts with phosphorus trihalides (PX3) to obtain three molecules of alkyl halide. The general
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overall reaction is.

Example: Reaction of ethanol with PCl3 for the formation of chloroethane

iii) The Reaction of Alcohols with Thionyl chloride as Suitable Reagent

This reagent is the most preferred and suitable in between the three reactions of alcohols.
Alcohol reacts with Thionyl chloride (SOCl2) to form alkyl chlorides. However, the by-
products formed in this reaction are gaseous in nature. Therefore, the by-products can easily
escape into the atmosphere, leaving the pure alkyl halide. This method helps in the
generation of pure alkyl halide.

2) Preparation of Haloalkanes & Haloarenes from Hydrocarbons

Preparation of Haloalkanes and haloarenes from hydrocarbons is possible by 3 different
methods. They are
 Free radical halogenation of haloalkanes
 Electrophilic Substitution Reactions
 Sandmeyer reaction
i) Free Radical Halogenation
Alkyl bromides and alkyl chloride formation are possible by the free radical halogenation
reaction. However, radicals are very non-selective in nature. Moreover, radicals are non-
specific and highly reactive intermediates that result in the formation of the mixture of
products.
For instance bromination or chlorination of free radical results in the formation of a number
of haloalkanes. This causes difficulty in the isolation of a single product. Therefore it is not
the preferred method for the preparation of haloalkanes. Example- When butane reacts with
chlorine in the presence of light as energy, a mixture of product formation takes place.
CHEMISTRY HALOALKANES AND HALOARENES

ii) Electrophilic Substitution Reaction

This method helps in the preparation of haloarenes such as aryl bromides and aryl chlorides.
Electrophilic substitution forms the aryl bromides and aryl chlorides by using halogens such
as chlorine and bromine in the presence of Lewis acid. However, the reaction requires the
following of certain specific condition for the generation of proper electrophile.
For example, the reaction should be conducted in presence of Lewis acid. Additionally, the
reaction must be carried out in the dark. The reactions to obtain the electrophiles are

The electrophiles in the above reactions are Cl+ and Br+ and HCl and HBr are the by-products
of the reaction. Therefore, the electrophilic substitution reaction for the preparation of aryl
bromide and aryl chloride is

Mechanism of Electrophilic Substitution Reaction

In the above reaction, two different isomers of the aryl chlorides are formed. They are Ortho
and Para isomer. The π-electron in the benzene ring attacks the Cl+ electrophile to produce
an intermediate complex. However, the H+ bond from the intermediate complex moves in
order to compensate for the positive charge of the carbon atom.
Thus the reaction forms two different isomers of the product-ortho and para. The melting
points of both the isomer differ significantly. And para-isomer has the higher boiling point
than ortho-isomer. Therefore, they can be easily separated from each other.
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Preparation of aryl chloride and bromide is possible from this reaction. Aryl fluoride
formation is not possible due to the high reactivity of the halogen fluorine. Additionally,
iodine reaction is also not possible as iodine is reversible in nature. Thus, it requires a strong
oxidizing agent such as Conc. HNO3 or HIO4 for the oxidation of HI and converting it to I2.
Therefore, driving the reaction in forwarding direction is difficult and requires a strong
oxidizing agent
iii) Sandmeyer’s Reaction
Sandmeyer’s Reaction is a two-step method which includes:
Diazonium salt formation
Diazonium salt reaction with a cuprous halide (Cu2X2)
Primary aromatic amine reacts with sodium nitrite in the presence of cold mineral acid to
form the diazonium salt. In this case, HNO 2 is prepared within the reaction by reacting
sodium nitrite and HX in the temperature of 273-278K.

Mechanism of the Sandmeyer’s Reaction

In the first step
NaNO2 + HCl → HNO2 + NaCl
The HNO2 formed in the presence of H+ undergo protonation to form NO+ as the
electrophile. The lone pair of the atom from the primary amine will react with the
electrophile.to form an intermediate compound which further gives diazonium salt after
CHEMISTRY HALOALKANES AND HALOARENES
elimination of H2O. In the second step, the diazonium salt reacts with cuprous halide to form
the respective aryl halide

3) Haloalkanes & Haloarenes from Alkenes

Haloalkanes and haloarenes preparation is possible by the addition of halogens (X2) across
the double bond of the alkene. It is also possible by the addition of hydrogen halides (HX). In
this halogen can be chlorine, bromine or even iodine.
i) Addition of HX
Alkene can be converted to haloalkane by an electrophilic addition reaction. Alkene reacts
with HX to form R-X. The order of reactivity of halides with respect to alkenes follows the
order HI > HBr > HCl > HF. The general reaction will be

The reaction, in this case, is an example of a regioselective reaction. In this type of reaction,
we get products in major and minor quantity. Additionally, the reaction follows
Markovnikov′s rule of addition for the determination of the major product by the addition of
across the double bond of the alkene.
According to Markovnikov’s rule, in an addition reaction of unsymmetrical alkenes, the
negative part of the reagent or halogen will attach itself to the carbon that contains less
CHEMISTRY HALOALKANES AND HALOARENES
number of hydrogen atoms. For example, prop-1-ene reacts with hydrogen bromide to form
2-bromopropane as a major product.
Peroxide effect (Kharash effect)
There is another possibility where the reaction contradicts Markovnikov’s rule. This effect is
known as Peroxide effect/ Kharash effect/ anti-markovnikov’s rule. In this reaction, alkene
reacts with HBr in the presence of peroxide. The Br- or the negative part of the reagent will
attach itself to the carbon having more number of hydrogen atoms. For example, Prop-1-ene
reacts with hydrogen bromide to form 1-bromopropane as a major product in the presence
of peroxide.

ii) Addition of Halogens

Similarly, alkenes can also react with halogens (X2). For example, Bromine reacts with an
alkene in the presence of carbon tetrachloride (CCl4) to form vic-dibromide. It is a common
test to determine a double bond or an alkene compound. The reaction will release reddish
brown colour during the reaction.

4) Haloalkanes and Haloarenes from Halogen exchange reaction

i) Finkelstein Reaction
The last method of preparation of haloalkane and haloarene is halogen exchange reaction. In
this reaction, an alkyl chloride or alkyl bromide reacts with sodium iodide in acetone to form
alkyl iodides.
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The reaction is an equilibrium reaction so there is a possibility of forming other products. The
solubility difference of alkyl halides in acetone is used for driving the reaction in the forward
direction. We know that sodium iodide is soluble in acetone but NaCl or NaBr are insoluble.
Therefore, they precipitate out in the reaction which is easy to remove from the reaction
mixture.
ii) Swartz Reaction
In this reaction, alkyl fluorides formation is possible by heating of Alkyl fluorides RBr/RCl. The
reaction is carried out in the presence of metallic fluoride such as SbF 3, Hg2F2, AgF, CoF2.

Darzen method: Darzens halogenation is a chemical processs involving the preparation of

alkyl halides from alcohols by treating with reflux of thionyl chloride or bromide (SOX2) in the
presence of small quantity of a nitrogen base like tertiary amine or pyridine, or the
equivalent hydrochloride.
Alkyl bromides and iodides cannot be prepared by this method. The reason behind this is
that thionyl bromide is unstable and thionyl iodide does not exist. This method is preferred
for preparing alkyl chlorides because here by-products are gaseous SO2 and HCl which escape
easily. But this does not happen in the method involving phosphorous chloride; hence it is
not used for preparing alkyl chlorides.

Methods of Preparation of aryl halides

Nuclear halogenations:

This method can be used to prepare aryl chlorides and bromides. This is done by treatment
of arene with chlorine or bromine in the absence of sunlight and in the presence of halogen
carrier like AlCl3, FeCl3 etc. at low temperatures. It is an electrophilic substitution.
CHEMISTRY HALOALKANES AND HALOARENES

From phenol:

However the yield of Chlorobenzene is very poor as the main product is Triphenyl phosphate.
By Hunsdiecker reaction:

Raschig process:

Sandmeyer reaction:
Diazonium salts are highly reactive compounds used to prepare arene derivatives. Treating
diazonium salt with copper (I) chloride (Cu2Cl2) or copper (I) bromide (Cu2Br2) leads to the
formation of corresponding haloarene. This reaction is known as Sandmeyer reaction.
CHEMISTRY HALOALKANES AND HALOARENES

Gattermann reaction:
Haloarenes can also be prepared by reaction of benzene diazonium chloride with copper
powder in the presence of corresponding halogen acid. This reaction is termed as
Gattermann reaction.

Physical Properties of haloalkanes

o Alkyl halides are colorless in pure state. Bromides and iodides cultivate colour on exposure to
light
o Volatile halogen compounds possess a sweet smell.
o Intermolecular forces of attraction of halogen derivatives are stronger due to greater polarity and
molecular mass as well compared to the parent hydrocarbon thereby resulting in higher boiling
points of chlorides, bromides and iodides compared to hydrocarbons of equivalent molecular
mass.
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o The boiling points of alkyl halides for same alkyl group follow the order: RI> RBr> RCl> RF due to
the increase in size and mass of halogen atom thereby increasing the extent of van der Waal
forces.

In isomeric haloalkanes the boiling points decreases with increase in branching.

CHEMISTRY HALOALKANES AND HALOARENES

o Para-isomers possess high melting point compared to ortho and meta-isomers due to the
symmetry of para-isomers fitting in crystal lattice better than ortho- and meta-isomers.
o Density increases with increase in number of carbon atoms, halogen atoms and atomic mass of
the halogen atoms. Consequently the density of Bromo, iodo and polychloro derivatives of
hydrocarbons is greater than water.
o The haloalkanes are only very slightly soluble in water due to release of less energy during the
setup of new attractions between the haloalkane and the water molecules which in turn is
insufficient to overcome the attractions between the haloalkane molecules and break the
hydrogen bonds between water molecules.
o Haloalkanes dissolves easily in organic solvents due to the new intermolecular forces of
attractions between haloalkanes and solvent molecules having equivalent strength as the one
that is broken in separate haloalkane and solvent molecules.

Problem: Arrange each set of compounds in order of increasing boiling points.

1. Bromomethane, Bromoform, Chloromethane, Dibromomethane.

2. 1-Chloropropane, Isopropyl chloride, 1-Chlorobutane.

Solution:
CHEMISTRY HALOALKANES AND HALOARENES

Haloalkanes are hydrocarbons in which hydrogen in a normal alkane is replaced by a halogen (group
17 elements). In this article, we will discuss the physical properties of haloalkanes. The physical
properties of haloalkanes are mostly like a normal covalent compound. Halogens not being much
reactive functional group as a carboxyl group or aldehyde doesn’t affect the overall physical
properties by much. Still, few differences can be seen as we move down in the homologous series of
haloalkanes group due to the difference in atomic masses of the compound.

Chemical Properties of Haloalkanes

Haloalkanes are quite reactive compounds. They undergo substitution, elimination and
reduction reactions. They also react with some metals to form organometallic compounds.
The reactivity of haloalkanes is mainly due to the polar nature of the carbon halogen bond.
The stability of alkyl halides generally decreases as the strength of the C-X bond decreases.
Iodides generally liberate iodine due to their low stability and acquire brown or violet colour.
2R-I → R-R + I2
Iodoform shows the antiseptic properties due to the liberation of iodine.
CHEMISTRY HALOALKANES AND HALOARENES
CHEMISTRY HALOALKANES AND HALOARENES

Important Questions
Multiple Choice questions-
1. SN1 reaction of alkyl halides lead to
(a) Retention of configuration
(b) Racemisation
(c) Inversion of configuration
(d) None of these

2. p-djchlorobenzene has higher melting point than its o- and m- isomers because
(a) p-dichlorobenzene is more polar than o- and m- isomer.
(b) p-isomer has a symmetrical crystalline structure.
(c) boiling point of p-isomer is more than o- and m-isomer.
(d) All of these are correct reasons.

3. Chloropicrin is formed by the reaction of

(a) steam on carbon tetrachloride.
(b) nitric acid on chlorobenzene.
(c) chlorine on picric acid.
(d) nitric acid on chloroform.

4. Fitting reaction can be used to prepare

(a) Toluene
(b) Acetophenon
(c) Diphenyl
(d) Chlorobenzene

5. Identify the end product (C) in the following sequence:

6.

In the above reaction, the product D is

(a) Propane
(b) 2, 3-Dimethylbutane
(c) Hexane
(d) Allyl bromide
CHEMISTRY HALOALKANES AND HALOARENES
7. Identify X and Y in the following sequence

(a) X = KCN, Y = LiAlH4

(b) X = KCN, Y = H3O+
(c) X = CH3Cl, Y = AlCl3 HCl
(d) X = CH3NH2, Y = HNO2

8. In the following sequence of reactions:

(a) n-propylamine
(b) isopropylamine
(c) ethylamine
(d) ethylmethylamine

9.

10.
Identifay Z in the series

(a) C2H5I
(b) C2H5OH
(c) CHI3
(d) CH3CHO

Very Short Questions-

1. Give IUPAC names of following compounds
(i).
CHEMISTRY HALOALKANES AND HALOARENES
(ii).

(iii).

(iv).

(v).

(vi).

(vii).

(viii).

(ix).
CHEMISTRY HALOALKANES AND HALOARENES
(x).

Short Questions-
1. Thionyl chloride is preferred for converting alcohol to haloalkane.
2. Phenol cannot be converted to chlorobenzene by reacting with HCl.
3. is added during iodination of benzene.
4. p- dichlorobenzene has higher melting point than meta – dichlorobenzene.
5. The boiling points of isomeric haloalkenes decrease with increase in branching.
6. Hydrolysis of optically active 2- bromobutane forms optically inactive
butan – 2 – ol.
7. Chlorobenzene is less reactive towards nucleophilic substitution reaction.
8. Chloroform is stored in dark coloured bottles.
9. The order of boiling points is RCl < RBr < RI.
10. Vinyl chloride is less reactive than allyl chloride.
Long Questions-
1. Write structures of the following compounds:
(i) 2-Chloro-3-methylpentane
(ii) 1-Chloro-4-ethylcyclohexane
(iii) 4-tert. Butyl-3-iodoheptane
(iv) 1,4-Dibromobut-2-ene
(v) 1-Bromo-4-sec. butyl-2-methylbenzene

2. Write structures of different dihalogen derivatives of propane.

3. Among the isomeric alkanes of molecular formula , identify the one that on photochemical
chlorination yields
(i) A single monochloride.
(ii) Three isomeric monochlorides.
(iii) Four isomeric monochlorides.

4. Draw the structures of major monohalo products in each of the following reactions:

(i)
CHEMISTRY HALOALKANES AND HALOARENES

(ii)

(iii)

(iv)

(v)

(vi)

5. Arrange each set of compounds in order of increasing boiling points.

(i) Bromomethane, Bromoform, Chloromethane, Dibromomethane.
(ii) 1-Chloropropane, Isopropyl chloride, 1-Chlorobutane.

6. Which alkyl halide from the following pairs would you expect to react more rapidly by
an mechanism? Explain your answer.

(i)

(ii)
CHEMISTRY HALOALKANES AND HALOARENES
(iii)

7. In the following pairs of halogen compounds, which compound undergoes faster

reaction?
(i)

(ii)

8. Identify A, B, C, D, E, R and in the following:

Assertion and Reason Questions-

1. In these questions, a statement of assertion followed by a statement of reason is given.
Choose the correct answer out of the following choices.

a) Assertion and reason both are correct statements and reason is correct explanation for
assertion.
b) Assertion and reason both are correct statements but reason is not correct explanation
for assertion.
c) Assertion is correct statement but reason is wrong statement.
d) Assertion is wrong statement but reason is correct statement.

Assertion: Isopropyl chloride is less reactive than CH3 Br in S N2 reactions.

Reason: SN2 reactions are always accompanied by inversion of configuration.

2. In these questions, a statement of assertion followed by a statement of reason is given.

Choose the correct answer out of the following choices.

a) Assertion and reason both are correct statements and reason is correct explanation for
assertion.
CHEMISTRY HALOALKANES AND HALOARENES
b) Assertion and reason both are correct statements but reason is not correct explanation
for assertion.
c) Assertion is correct statement but reason is wrong statement.
d) Assertion is wrong statement but reason is correct statement.

Assertion: Lower members of alkyl halides are colourless gases.

Reason: Alkyl iodides in general turn black on exposure to air and light.

Case Study Questions-

1. Read the passage given below and answer the following questions:

Haloarenes are less reactive than haloalkanes. The low reactivity of haloarenes can be
attributed to:

 Resonance effect.
 sp2 hybridisation of C - X bond.
 Polarity of C - X bond
 Instability of phenyl cation (formed by self-ionisation of haloarene).
 Repulsion between the electron rich attacking nucleophiles and electron rich arenes.

Reactivity of haloarenes can be increased or decreased by the presence of certain groups at

certain positions for example, nitro (-NO2) group at o/ p positions increases the reactivity of
haloarenes towards nucleophilc substitution reactions.

The following questions are multiple choice questions Choose the most appropriate answer:

(i) Aryl halides are less reactive towards nucleophilic substitution reaction as compared to
alkyl halides due to

a) The formation of less stable carbonium ion.

b) Resonance stabilisation.
c) Larger carbon-halogen bond.
d) Inductive effect.

(ii) Which of the following aryl halides is the most reactive towards nucleophilic
substitution?
CHEMISTRY HALOALKANES AND HALOARENES

(iii) Which one of the following will react fastest with aqueous NaOH?

(iv) Which chloro derivative of benzene among the followings would undergo
hydrolysis most readily with aqueous sodium hydroxide to furnish the corresponding
hydroxy derivative?

d. C6H5Cl

(v) The reactivity of the compounds (i) MeBr, (ii) PhCH 2Br, (iii) MeCI, (iv) p-MeOC6H4Br
decreases as:

a) (i) > (ii) > (iii) > (iv)

b) (iv) > (ii) > (i) > (iii)
c) (iv) > (iii) > (i) > (ii)
d) (ii) > (i) > (iii) > (iv)

2. Read the passage given below and answer the following questions:

A chlorocompound (A) on reduction with Zn-Cu and ethanol gives the hydrocarbon (B)
with five carbon atoms. When (A) is dissolved in dry ether and treated with sodium metal
it gave 2, 2, 5, 5 - tetramethylhexane. The treatment of (A) with alcoholic KCN gives
compound ( C).
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The following questions are multiple choice questions. Choose the most appropriate
answer:

(i) The compound (A) is:

a) 1-chloro-2, 2-dimethylpropane.
b) 1-chloro-2, 2-dimethyl butane.
c) 1-chloro-2-methyl butane.
d) 2-chloro-2-methyl butane.

(ii) The reaction of (C) with Na, C2H5OH gives:

a) (CH3)3C CH2CONH2
b) (CH3)3C NH2
c) (CH3)3C CH2CH2NH2
d) (CH3)2CHCH2NH2

(iii) The reaction of (C) with Na, C2H5OH is called:

a) Gilman reaction.
b) Mendius reaction.
c) Grooves process.
d) Swart's reaction.

(iv) The reaction of (A) with aq. KOH will preferably favour:

a) SN1 mechanism.
b) SN2 mechanism.
c) E1 mechanism.
d) E2 mechanism.

(v) Compound (B) is:

a) N-pentane.
b) 2, 2-dimethylpropane.
c) 2-methylbutane.
d) None of these.

MCQ Answers-
1. Answer: b

2. Answer: b

3. Answer: d
CHEMISTRY HALOALKANES AND HALOARENES
4. Answer: c

5. Answer: c

6. Answer: b

7. Answer: a

8. Answer: d

9. Answer: a

10. Answer: c

Very Short Answers-

(i) 1, 3- Dibromobutane
(ii). 1- Cholopropan-2-ol
(iii). 2, 3 – Dibromo-1-chloro-3-methylpentane
(iv). 2-Choloro-3-ethyl-2-methyl pentane
(v). 1-Chloro-2-phenylethane
(vi). 1-Chloro-1-phenyl ethane
(vii). 1, 2, 3, 4, 5, 6- hexachlorocyclohexane
(viii). 2, 2- Dihexyl 1, 1, 1-Trichloro ethane
(ix). 4, 4-dibromobiphenyl
(x). 1, 3-Dibromo -3- methyl butane

Short Answers-
Ans 1. Thionyl chloride is preferred for converting alcohol to haloalkane because the bi-
products formed are all gases which escape into the atmosphere.

Ans 2. In phenol, due to resonance, the carbon –oxygen bond has a partial double bond
character and is difficult to break being stronger than a single bond. Therefore, it can-not be
converted to chlorobenzene by reacting with HCl.

Ans 3. When benzene is reacted with iodine, the reaction is reversible in nature. It leads to
the formation of reactants back. Therefore, and oxidizing agent like oxidizes the HI
formed in the reaction and keeps the reaction in forward direction.
CHEMISTRY HALOALKANES AND HALOARENES
Ans 4.

p- dichlorobenzene is having symmetrical structure therefore it can fit better into the crystal
lattice which increases its melting point.

Ans 5. The boiling points of isomeric haloalkanes decreases with branching due to decrease
in surface areas with branching. As branching increasing the structure becomes more
spherical and the surface area decreases. e.g. the boiling points of isomers of Br follows
the order.

Ans 6.

The compound undergoes hydrolysis by S N1 mechanism via the formation of carbocation

which is planar.

The attack of nucleophile can result in product which is a mixture of compounds both with
same configuration and inverted configuration.
CHEMISTRY HALOALKANES AND HALOARENES

Therefore it results in the formation of racemic mixture which is optically inactive.

Ans 7. Chlorobenzene is less reactive towards nucleophilic substitution due to –

i. resonance, C- Cl bond acquires a double bond character and becomes stronger than a
single bond.

ii. hybridisation in C of C-X bond, the carbon becomes more electronegative and holds
the electron pair of C-X bond more tightly decreasing the bond length.

iii. Instability of phenyl cation.

iv. Repulsion for incoming nucleophile from electron rich ring.

Ans 8. Chloroform gets oxidsed slowly by air in the presence of light to an extremely
poisonous gas phosgene. Therefore, to avoid any exposure to air and sunlight, it is kept in
dark coloured bottles.

Ans 9. The boiling points of alkyl halides depends on dipole and van-der-waal’s interaction.
These attractions get stronger as the molecules get bigger in size and have more electrons.
As the size of halogens increases in the order –

The boiling points also follow the order

RCl < RBr <RI
CHEMISTRY HALOALKANES AND HALOARENES
Ans 10. Due to resonance C- Cl bond gets double bond character and becomes stronger than
a single bond, making vinyl chloride less reactive than allyl chloride.

Long Answers-
Ans 1. (i) 2-Chloro-3-methyl pentane

(ii) 1-Chloro-4-ethylcyclohexane

(iii) 4- tert-Butyl-3-iodoheptane

(iv) 1, 4-Dibromobut-2-ene

(v) 1-Bromo-4-sec-butyl-2-methylbenzene

Ans 2. There are four different dihalogen derivatives of propane. The structures of these
derivatives are shown below.

(i) 1, 1-Dibromopropane
CHEMISTRY HALOALKANES AND HALOARENES

(ii) 2, 2-Dibromopropane

(iii) 1, 2-Dibromopropane

(iv) 1, 3-Dibromopropane

Ans 3. (i) To have a single monochloride, there should be only one type of H-atom in the
isomer of the alkane of the molecular formula . This is because, replacement of any H-
atom leads to the formation of the same product. The isomer is neopentane.

Neopentane

(ii) To have three isomeric monochlorides, the isomer of the alkane of the molecular
formula should contain three different types of H-atoms.
Therefore, the isomer is n-pentane. It can be observed that there are three types of H atoms
labelled as a, b and c in n-pentane.
CHEMISTRY HALOALKANES AND HALOARENES

(iii) To have four isomeric monochlorides, the isomer of the alkane of the molecular formula
C5H12should contain four different types of H-atoms. Therefore, the isomer is 2-
methylbutane. It can be observed that there are four types of H-atoms labelled
as a, b, c, and d in 2-methylbutane.

Ans 4. (i)

(ii)

(iii)

(iv)

(v)

(vi)
CHEMISTRY HALOALKANES AND HALOARENES

Ans 5. (i)

For alkyl halides containing the same alkyl group, the boiling point increases with an increase
in the atomic mass of the halogen atom.
Since the atomic mass of Br is greater than that of Cl, the boiling point of bromomethane is
higher than that of chloromethane.
Further, for alkyl halides containing the same alkyl group, the boiling point increases with an
increase in the number of halides. Therefore, the boiling point of Dibromomethane is higher
than that of chloromethane and bromomethane, but lower than that of bromoform.
Hence, the given set of compounds can be arranged in the order of their increasing boiling
points as:
Chloromethane < Bromomethane < Dibromomethane < Bromoform.

(ii)

For alkyl halides containing the same halide, the boiling point increases with an increase in
the size of the alkyl group. Thus, the boiling point of 1-chlorobutane is higher than that of
isopropyl chloride and 1-chloropropane.
Further, the boiling point decreases with an increase in branching in the chain. Thus, the
boiling point of isopropyl alcohol is lower than that of 1-chloropropane.
Hence, the given set of compounds can be arranged in the increasing order of their boiling
points as:
Isopropyl chloride < 1-Chloropropane < 1-Chlorobutane

Ans 6. (i)

2-bromobutane is a alkylhalide whereas 1-bromobutane is a alkyl halide. The

approaching of nucleophile is more hindered in 2-bromobutane than in 1-bromobutane.
CHEMISTRY HALOALKANES AND HALOARENES

Therefore, 1-bromobutane reacts more rapidly than 2-bromobutane by an mechanism.

(ii)

2-Bromobutane is alkylhalide whereas 2-bromo-2-methylpropane is alkyl halide.

Therefore, greater numbers of substituents are present in alkyl halide than in alkyl
halide to hinder the approaching nucleophile. Hence, 2-bromobutane reacts more rapidly
than 2-bromo-2-methylpropane by an mechanism.
(iii)

Both the alkyl halides are primary. However, the substituent is at a greater distance to
the carbon atom linked to Br in 1-bromo-3-methylbutane than in 1-bromo-2-methylbutane.
Therefore, the approaching nucleophile is less hindered in case of the former than in case of
the latter. Hence, the former reacts faster than the latter by mechanism.

Ans 7. (i)

SN1 reaction proceeds via the formation of carbocation. The alkyl halide (I) is while (II) is .
Therefore, (I) forms carbocation while (II) forms carbocation. Greater the stability of the
carbocation, faster is the rate of SN1 reaction. Since carbocation is more stable than
carbocation. (I), i.e. 2-chloro-2-methylpropane, undergoes faster SN1 reaction than (II) i.e., 3-
chloropentane.
(ii)

The alkyl halide (I) is while (II) is . carbocation is more stable than carbocation. Therefore,
(I), 2-chloroheptane, undergoes faster reaction than (II), 1-chlorohexane.
CHEMISTRY HALOALKANES AND HALOARENES
Ans 8.

Since D of gets attached to the carbon atom to which MgBr is attached, C is

Therefore, the compound R – Br is

When an alkyl halide is treated with Na in the presence of ether, a hydrocarbon containing
double the number of carbon atoms as present in the original halide is obtained as product.
This is known as Wurtz reaction. Therefore, the halide, , is

Therefore, compound D is

And, compound E is
CHEMISTRY HALOALKANES AND HALOARENES

Assertion and Reason Answers-

1. (b) Assertion and reason both are correct statements but reason is not correct explanation
for assertion.

Explanation:

As the size of the alkyl groups increases, the S N2 reactivity decreases, further C - Cl bond is
stronger and more difficult to cleave than C - Br bond. So CH3Br is more reactive than
(CH3)2CHCl.

2. (c) Assertion is correct statement but reason is wrong statement.

Explanation:

Alkyl iodides in general turn brown due to liberation of I2 on decomposition by the action of air
and light.

Case Study Answers-

1. Answer :

(i) (b) Resonance stabilisation.

(ii) (d)
CHEMISTRY HALOALKANES AND HALOARENES

Explanation:

When in aryl halides the electron withdrawing groups are attached at ortho and para
positions to the chlorine atom then the removal of chlorine atom as Cll- ion becomes easy,
therefore, 2,4,6-trinitro chlorobenzene is the most reactive among given aryl halides.

(iii) (d)

Explanation:

(iv) (a)
CHEMISTRY HALOALKANES AND HALOARENES

Explanation:

Cl in 2,4,6-trinitrochlorobenzene is activated by three NO2 groups at o, and p-positions

and hence undergoes hydrolysis most readily.

(v) (d) (ii) > (i) > (iii) > (iv)

Explanation:

The order of reactivity follows the sequence: benzyl halides > alkyl halides> aryl halides.
Out of chlorides and bromides, bromides are more reactive. Therefore, the correct order
of reactivity is PhCH2Br (ii) > MeBr (i) > MeCl (iii) > p - MeOC6H4Br (iv).

2. Answer :
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