Haloalkanes and Haloarenes

                 Haloalkanes and Haloarenes

Topics: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.

(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

• 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
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:

Structure	Common name	IUPAC name
	Chlorobenzene	Chlorobenzene
	o-Dibromobenzene	1,2-Dibromobenzene
	sym-Trichlorobenzene	1,3,5-Trichlorobenzene
	o-Chlorotoluene	1-Chloro-2-methylbenzene or 2-Chlorotoluene
	Benzyl chloride	Chlorophenylmethane

Nature of C−X bond

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

• C−X bond length increases down the group.

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

Example :

Hybridizations of carbon which is bonded directly to Br in aryl halide and benzyl halide respectively are

• A )

sp, sp2

• B )

sp2, sp3

• C )

sp3, sp

• D )

sp, sp3

In aryl halide, Br is attached to a double bonded C-atom which is in sp2 hybrid state.

In benzyl halide, Br is attached to a single bonded C-atom which is in sp3 hybrid state.

Topics: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 + H2SO4R − 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

• 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:

                

• 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

Example :

Consider the reaction.

X is major product which is

• A )

• B )

• C )

• D )

In the given reactant,

Topics: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

• 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.

Example :

The correct order of increasing boiling points of the given alkyl halides is

• A )

I < II < III

• B )

II < III < I

• C )

III < I < II

• D )

I < III < II

The given alkyl halides are isomeric form of C4H9Br. The boiling point of isomeric alkyl halides decreases with increase in branching. This is because branching decreases the surface area, which further decreases the van der Waals forces of attraction. Consequently, the heat required to overcome these forces is also less. Hence, the order of boiling point for the given compounds is:

Topics: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)

• 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: 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.

• 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.

Example :

Consider the following reaction mechanism.

Which of the following statements is not correct regarding the given mechanism?

• A )

Step 1 is the slowest step.

• B )

The reaction is proceeding via SN1 mechanism.

• C )

First order kinetics is followed by the reaction.

• D )

The reaction is carried out in non-polar solvent.

The step 1 of the reaction involves the breaking of C−Br bond for which the energy is obtained through solvation of halide ion (Br−) with the protons given by polar protic solvent such as water, alcohol, acetic acid etc.

Example :

Consider the given structures.

Which of the following statements is correct regarding structures I and II?

• A )

Nucleophilic substitution reaction proceeds via SN1 mechanism for both structures I and II.

• B )

Nucleophilic substitution reaction proceeds via SN2 mechanism for both structures I and II.

• C )

Nucleophilic substitution reaction proceeds via SN2 mechanism for structure I and SN1 mechanism for structure II.

• D )

Nucleophilic substitution reaction proceeds via SN1 mechanism for structure I and SN2 mechanism for structure II.

The above figureshows nucleophilic substitution reaction via SN2 mechanism because I is attached to a primary carbon atom and does not have any bulky group attached to it. Besides this, the large size of I makes it a better leaving group.

The above figure shows nucleophilic substitution reaction via SN1 mechanism. It is an allylic halide. The carbocation formed here gets stabilized through resonance. Greater the stability of the carbocation, faster will be SN1 rate of the reaction.

Topics:Reactions of Haloarenes

Nucleophilic Substitution Reactions

• 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.

Electrophilic Substitution Reactions

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

• Halogenation

• Nitration

• Sulphonation

• Friedel−Crafts Alkylation

• Friedel-Crafts Acylation

Reaction with Metals

• Wurtz−Fittig Reaction

• Fittig reaction

Example :

Consider the following reaction mechanisms.

Which of the following statements is not correct for the given reaction mechanism?

• A )

Cl stabilizes as well as destabilizes the intermediate carbocation.

• B )

Inductive effect stabilizes while resonance effect destabilizes the intermediate carbocation.

• C )

Inductive effect controls reactivity while resonance decides orientation.

• D )

E⊕ in halogenations, nitration and sulphonation are X−,  and respectively.

Resonance stabilizes carbocation while inductive effect destabilizes it. However, due to stronger inductive effect over resonating effect, there is net electron withdrawal from the ring resulting in net ring deactivation.

Topics: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 

• 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 

• 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)

• 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.

Example :

Which of the following statements is true regarding polyhalogen compounds?

• A )

DDT is an effective antiseptic.

• B )

Chloroform with air and light gives carbonyl chloride gas.

• C )

Antiseptic property of iodoform is due to iodoform itself.

• D )

Chloroform and chlorofluoro carbon of methane are known as freons.

Chloroform with air and light slowly oxidises and produces extremely poisonous gas, carboxyl chloride, also known as phosgene.

The incorrect statements can be corrected as follows.

DDT is a pesticide.

Antiseptic property of iodoform is due to free iodine.

Chlorofluoro carbon of methane and ethane are known as freons.

Example :

Freon 12 is manufactured by

• A )

Sandmeyer reaction

• B )

Friedel Crafts reaction

• C )

Fittig reaction

• D )

Swarts reaction

Freon 12 i.e. CCl2F2 is one of the most common freons. It is manufactured from tetra chloromethane by Swarts reaction. The following chemical reaction takes place in this process.