Haloalkanes and Haloarenes

Classification and Nomenclature

Classification

• Based on number of halogen atoms

• Monohalogen

• Dihalogen

• Polyhalogen (tri-, tetra-, penta- etc.)

• Examples of haloalkanes:

• Examples of haloarenes:

• Based on the hybridisation of C−atom of C−X bond of monohalocompounds

• Compounds containing sp3 C−X bond (X = F, Cl, Br, I)

(i) Alkyl halides or haloalkanes (R−X) → They form homologous series of general formula CnH2n+1X.
They are further classified into primary, secondary, and tertiary.

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 (ii) Allylic halides → Compounds containing halogen atom bonded to an allylic carbon

(iii) Benzylic halides → Compounds containing halogen atom bonded to an sp3 hybridised carbon
atom next to an aromatic ring

To test your knowledge of this concept, solve the following puzzle.

• Compounds containing sp2 C−X bond (X = F, Cl, Br, I)

(i) Vinylic halides → Compounds containing halogen atom bonded to a vinylic carbon

(ii) Aryl halides → Compounds containing halogen atom bonded to an sp2 hybridised carbon atom
of an aromatic ring

Nomenclature

• For haloalkanes

• Common name → Name of alkyl group followed by name of the halide

• IUPAC name → Named as halo-substituted hydrocarbon in IUPAC

• Examples:

Structure Common Name IUPAC Name

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 CH3CH2CH2CH2Br n−Butyl bromide 1-Bromobutane

Isobutyl chloride 1-Chloro-2-methyl-propane

sec-butyl bromide 2-Bromobutane

CH3CH2CH(Cl)CH3 sec-Butyl chloride 2-Chlorobutane

(CH3)3CCH2Br neo-Pentyl bromide 1-Bromo-2,2-

dimethylpropane

(CH3)3CBr tert-Butyl bromide 2-Bromo-2-methylpropane

CH2Cl2 Methylene chloride Dichloromethane

• For haloarenes

• Named as halo-substituted hydrocarbon (for both common and IUPAC name)

• For dihalogen derivatives:

In common names → prefixes o-, m-, p- are used

In IUPAC names → numerals 1, 2; 1, 3; 1, 4 are used

• Examples:

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 Structure Common name IUPAC name

Chlorobenzene
Chlorobenzene

1,2-Dibromobenzene
o-Dibromobenzene

1,3,5-Trichlorobenzene
sym-Trichlorobenzene

1-Chloro-2-
methylbenzene
o-Chlorotoluene
or

2-Chlorotoluene

Chlorophenylmethane
Benzyl chloride

Nature of C−X bond

• C-atom bears partial positive charge and X-atom bears partial negative charge.

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 • C−X bond length increases down the group.

• Reason − Size of halogen atom increases down the group.

Methods of Preparation

From Alcohols

• 3R − OH + PX3 3R − X + H3PO3 (X = Cl, Br)

• R − OH + PCl5 R − Cl + POCl3 + HCl

• R − OH + SOCl2 R − Cl + SO2 + HCl

• R − OH + NaCl + H2SO4 R − Cl + NaHSO4 + H2O

• For this reaction, the increasing order of reactivity of alcohols is

1° < 2° < 3°

• This reaction cannot be applied to produce aryl halides.

Reason − It is difficult to break carbon-oxygen bond in phenols as it possesses a partial double

bond character.

From Hydrocarbons

• Free radical halogenation

• Gives a complex mixture of isomeric mono- and poly- haloalkanes This method is not of much
practical use as it is difficult to separate the complex mixture.

• When only mono-substitution is carried out

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 • Electrophilic Substitution

• However, iodination is a reversible reaction. Oxidising agent such as HNO3 or HIO4 is required to
oxidise HI formed during the reaction.

• Sandmeyer’s Reaction: The Cl , Br and CN nucleophiles can easily be introduced in the benzene
ring of benzene diazonium salt in the presence of Cu(I) ion. This reaction is called Sandmeyer
reaction. It is applicable to primary aromatic amine.

Step I: Preparation of diazonium salt

Step II: Formation of haloarene with release of nitrogen gas

For Cl and Br:

For I:

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 • From Alkenes

• By adding hydrogen halides

• By adding halogens

This method is used to detect double bond in a molecule as reddish brown colour of bromine is
discharged during the reaction.

Halogen Exchange

• Finkelstein Reaction

• This reaction in forward direction can be favoured by precipitating NaX formed in dry acetone
(according to Le Chatelier’s principle).

• Swarts Reaction

• Preparation of alkyl fluoride

• Requires metallic fluoride such as AgF, Hg2 F2, CoF2, or SbF3

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 Physical Properties

• Pure alkylhalides are colourless. However, in the presence of light, bromides and iodides become
coloured.

• Some halides are sweet in smell.

Melting and Boiling Points

• Chlorides, bromides, and iodides have higher boiling points than hydrocarbons of comparable
molecular mass.

• Reason − Chlorides, bromides, and iodides are polar in nature whereas hydrocarbons are non-
polar. Therefore, these halides have greater intermolecular forces of attraction (dipole − dipole)
than their parent hydrocarbons and hence, have higher boiling points.

• Reason − Vander Waals forces increase with increase in size and mass of halogen atoms and hence,
boiling point also increases.

• Boiling points of isomeric haloalkanes decrease with increase in branching.

• For example,

• The boiling points of isomeric dihalobenzenes are nearly the same.

• However, the melting point of para-isomer is higher than those of ortho- and meta- isomers.

Reason − Better fit of para-isomer in crystal lattice due to its symmetry

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 • Lower members (such as CH3Cl, CH3Br, C2H5Cl) are gases whereas higher members are liquids or
solids at room temperature.

Density

• Increases with the increase in

• number of carbon atoms

• number of halogen atoms

• atomic mass of the halogen atoms

• Very slightly soluble in water, but soluble in organic solvents

• Reason − The energy required to overcome the intermolecular attraction between the haloalkane
molecules is greater than the energy released during dissolution in water.

Reactions of Haloalkanes

Nucleophilic Substitution Reactions

Mechanism

• Substitution nucleophilic bimolecular (SN2)

• Inversion of configuration takes place.

• The increasing order of reactivity is

3° halide < 2° halide < 1° halide

Reason − Due to the presence of bulky substituents in 3° and 2° halides, nucleophile cannot
approach.

• Substitution nucleophilic unimolecular (SN1)

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 • Carried out in polar protic solvents such as water, alcohol, acetic acid, etc.

• The increasing order of reactivity is

1° halide < 2° halide < 3° halide

Reason − Greater the stability of carbocation, more easily the alkyl halide is formed and hence,
faster is the reaction rate. The increasing order of stability of carbocation is 1° < 2° < 3°. Since 1°
halide forms 1° carbocation, 2° halide forms 2° carbocation, and 3° halide forms 3° carbocation.
Therefore, the increasing order of reactivity is 1° halide < 2° halide < 3° halide.

• Allylic and benzylic halides are very reactive towards SN1 reaction because of stabilisation of their
carbocations through resonance.

• For both SN1 and SN2 reaction, the order of reactivity of halides is

R−F << R−Cl < R−Br < R−I

Stereochemical Aspects of Nucleophilic Substitution

• In SN2 reaction, complete stereochemical inversion takes place.

• In SN1 reaction, racemisation takes place.

Some Stereochemical Terms:

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 • Compounds that rotate the plane polarised light are called optically active compounds.

• Angle of rotation of plane polarised light is measured by an instrument called polarimeter.

• Dextrorotatory or d-form − Compounds that rotate plane polarised light to right

• Laevorotatory or l-form − Compounds that rotate plane polarised light to left

• d- and l- forms of a compound are called optical isomers and the phenomenon is called
optical isomerism.

• Asymmetric carbon or stereocentre − Carbon atom with all the four substituents attached to
it are different.

• The objects which are non-superimposable on their mirror images are known as chiral and
the property is known as chirality.

• The objects which are superimposable on their mirror images are known as achiral.

• Enantiomers are stereoisomers which are non-superimposable mirror images.

• There are three outcomes for a reaction at an asymmetric carbon atom.

• Inversion

• Retention

• Racemisation

Elimination Reactions

• On heating a haloalkane containing β-hydrogen atom with alcoholic KOH solution, elimination of H
from β carbon and a halogen from α takes place.

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 • Also called β-elimination as β-hydrogen is eliminated

• In case of more than one product:

• Saytzeff’s rule − In dehydrohalogenation reactions, the alkene with the greater number of alkyl
groups attached to the doubly bonded carbon atoms is preferably formed.

Reaction with Metals

• Chlorides, bromides, and iodides react with certain metals to form organo-metallic compounds
such as Grignard reagents.

CH3CH2Br + Mg CH3CH2MgBr

Grignard reagent

• Wurtz Reaction

2RX + 2Na R−R + 2NaX

• Hydrocarbon containing double the number of carbon atoms present in the halide is formed.

Reactions of Haloarenes

The aryl and vinyl halides are much less reactive than the alkyl halides. Let us undersatnd this by
means of the following video.

Nucleophilic Substitution Reactions

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 • Haloarenes are much less reactive towards nucleophilic substitution reactions due to the following
reasons:

• In haloarenes, the benzene ring undergoes resonance and as a result, the C−X bond acquires a
partial double bond character. Therefore, it becomes difficult to break the C−X bond.

• In haloalkanes, the halogen atom is attached to an sp3 hybridised carbon atom while in haloarenes,
it is attached to an sp2 hybridised carbon atom. Since an sp2 hybridised carbon has more s-
character than sp3 hybridised carbon, the former is more electronegative than the latter. As a
result, the electron pair of C−X bond is held by carbon atom more tightly in haloarenes than
haloalkanes. Therefore, the C−X bond becomes shorter in haloarenes and hence, becomes stronger.

• The phenyl cation formed by the self ionisation is unstable and hence, SN1 mechanism is avoided.

• Electron-rich nucleophile cannot approach electron-rich arenes due to repulsion.

• Replacement by hydroxyl group

• Reactivity increases if an electron withdrawing group (−NO2) is present at ortho- and para-
positions. This can be observed as the temperature required to carry out the reaction decreases in
the presence of −NO2 group at o-and p-positions.

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 Electrophilic Substitution Reactions

• Substitution occurs at o- and p-direction due to availability of electrons at these positions because
of resonance.

• Halogenation

• Nitration

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 • Sulphonation

• Friedel−Crafts Alkylation

• Friedel-Crafts Acylation

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 Reaction with Metals

• Wurtz−Fittig Reaction

• Fittig reaction

Polyhalogen Compounds

• Polyhalogen compounds are carbon compounds containing more than one halogen atom.

Dichloromethane or Methylene Chloride (CH2Cl2)

• Uses

• As propellant in aerosols

• As metal finishing and cleaning solvent

• As a process solvent in the manufacture of drugs

• Toxicity

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 • Harms the human central nervous system

• Lower levels of CH2Cl2 in air can lead to slightly impaired hearing and vision.

• Higher levels of CH2Cl2 in air can cause dizziness, nausea, tingling and numbness in the fingers and
toes.

• Direct skin contact causes intense burning and mild redness of the skin.

Trichloromethane or Chloroform (CHCl3)

• Uses

• As a solvent for fats, alkaloids, iodine, and other substances

• In the production of the freon refrigerant R-22

• Earlier used as anaesthetic, but has been replaced due its toxicity

• Toxicity

• Inhaling of CHCl3 vapours can cause depression of the central nervous system, dizziness, fatigue,
and headache.

• Chronic chloroform exposure may lead to damage of the liver and kidneys.

• Immersion of skin in CHCl3 leads to development of sores.

• CHCl3 is oxidised to an extremely poisonous gas phosgene (COCl2) in the presence of light.

That is why chloroform is stored in closed dark-coloured bottles completely filled so that no air is
left inside the bottles.

Triiodomethane or Iodoform (CHI3)

Uses

• Earlier used as an antiseptic, but has been replaced due to its objectionable smell

• Its antiseptic properties are not due to iodoform itself, but due to the liberation of free iodine.

Tetrachloromethane or Carbon Tetrachloride (CCl4)

• Uses

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 • In the manufacture of refrigerants and aerosol propellants

• As feedstock in the synthesis of chlorofluorocarbons

• In pharmaceutical manufacturing

• As industrial solvent

• As cleaning fluid

• As fire extinguisher

• Toxicity

• Causes lives cancer, dizziness, light headedness, nausea, vomiting These effects may lead to stupor,
coma, unconsciousness, or death.

• Leads to irregularity in heart beat or even stop

• Causes irritation of eyes on contact

• Causes depletion of ozone layer, leading to increase in skin cancer, eye diseases

Freons

• Chlorofluorocarbon compounds of methane and ethane are collectively called freons.

• Physical properties:

• Stable and unreactive

• Non-toxic and non-corrosive

• Easily liquefiable gas

• Uses

• In aerosol propellants, refrigeration, and air conditioning purposes

• Toxicity

• Upsets the natural ozone balance

• Freon 12 (CCl2F2) is one of the most common freons that have industrial use.

-Dichlorodiphenyltrichloroethane (DDT)

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 • First Chlorinated Organic Insecticide

• Effective against mosquitoes that spread malaria and lice that carry typhus

• Toxicity

• DDT is highly stable and is not metabolised very rapidly by animals. Rather it is deposited and
stored in the fatty tissues. It is proved to be toxic to living beings. It is banned in many countries
due to its toxicity.

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