HALOALKANES

UNIT – 8

Haloalkanes

Introduction: 

Haloalkanes are the halogen derivatives of alkanes, which are formed by replacing one or more hydrogen atoms number of halogen atoms. 

Classification of Haloalkanes: 

Depending upon the number of halogen atoms on present in haloalkane, they are classified into, 1. Monohaloalkanes: Haloalkanes containing only one halogen atom. 

Monohaloalkanes are further classified into primary haloalkanes (1ohaloalkanes), secondary haloalanes (2ohaloalkanes) and tertiary haloalkanes (3ohaloalkanes). 

a)Primary haloalkanes (1o haloalkanes): Halogen atom attached with primary carbon atom.

b) Secondary haloalkanes (2o haloalkanes): Halogen atom attached with secondary carbon atom.

c) Tertiary haloalkanes (3o haloalkanes): Halogen atom attached with the tertiary carbon atom. 

2. Dihaloalkanes: Haloalkane containing two halogen atoms. 

Dihaloalkanes are further classified into vicinal dihalides, geminal dihalides and polymethylene dihalides. 

a) Vicinal dihalides: Two halogen atoms are attached on the adjacent carbon atoms.

b) Geminal dihalides: Two halogen atoms are attached on the same carbon atom.

c) Polymethylene dihalides: Two halogen atoms are attached on terminal carbon atoms of the chain. 

2. Polyhaloalkanes: Haloalkanes containing more than two halogen atoms. 

Nomenclature: 

 IUPAC name : Halo + alkane = Haloalkane 

 Common name: Alkyl + halide = Alkyl halide

Isomerism: 

Haloalkanes exhibit two types of structural isomerism. 

1. Chain isomerism: 

Two or more organic compounds having same molecular formula but differ in length of carbon chain are called chain isomers and the phenomenon is called chain isomerism. 

Haloalkanes containing 4 or more carbon atoms shows chain isomerism. 

Chain isomers of C4H9Cl are, 

 1-chlorobutane 

2. Position isomerism: 

Two or more organic compounds with the same molecular formula but differ in the position  of functional group in parent carbon chain are called position isomers. The phenomenon  is called position isomerism. 

Haloalkanes containing 3 or more carbon atom shows position isomerism. 

Position isomers of C3H7Br are, 

 1-bromopropane 

Monohaloalkanes: 

General Methods of Preparation of Monohaloalkanes: 

1. From alkanes: 

Alkanes reacts with limited amount of halogens in presence of light, heat or catalyst to give haloalkanes. 

I.e.

NOTE: HI is strong reducing agent so to remove it from reaction and to increase the yield of  iodoalkanes strong oxidizing agent like HNO3 or HIO3 are used. 

➢ Generally, this is not a good method for the preparation of haloalkanes due to following reasons: ➢ Excess halogen, X2 (Cl2 or Br2, etc.) leads to the formation of a mixture of haloalkanes. ➢ Higher alkanes give a mixture of all possible isomeric alkyl halide which are difficult to separate. 

Reactivity order of hydrogen atom towards halogenation is 3o > 2o > 1o > CH4 Reactivity order of halogen towards halogenation is F2 > Cl2 > Br2 > I2 

• Fluorine is most reactive. It reacts with alkanes explosively under most conditions. • Chlorine reacts with alkanes rapidly in presence of light or heat. Therefore, chlorination is done indiffused sunlight. 

• Bromine reacts with alkanes slowly; heat is required to increase the rate of reaction. • Iodine reacts with alkanes reversibly, to make irreversible, iodination is done in presence of oxidizingagents like, conc. HNO3 or HIO3 or HgO. 

2. From alkenes: 

Alkenes reacts with hydrogen halide (HX = HCl, HBr, HI) to give respective alkyl halide. i.e.

Addition of hydrogen halide into unsymmetrical alkene: Markovnikov’s addition

According to Markovnikov’s rule, “When an unsymmetrical reagent is added to  unsymmetrical alkenes then the positive part of the reagent goes to that double bonded  carbon having greater number of hydrogen atoms.” 

i.e.

Addition of hydrogen bromide (HBr) into unsymmetrical alkene: Anti-Markovnikov’s  addition / Peroxide effect / Kharasch effect

According to Anti-Markovnikov’s rule, “When HBr is added to unsymmetrical alkene in  presence of organic peroxide then the positive part of the reagent goes to that double bonded  carbon having less number of hydrogen atoms.” 

i.e.

3. From alcohols

Haloalkanes are prepared from alcohols by following ways; 

a) Reaction with halogen acids: 

Reactivity order of halogen acids is HI > HBr > HCl 

Reactivity order of different alcohols is 3o > 2o > 1o 

i) Reaction with HI: 

HI reacts with 1o, 2oand 3oalcohols without any catalyst.

ii) Reaction with HBr: 

HBr reacts with 1oalcohols in presence of H2SO4 as catalyst but it reacts with 2oand 3oalcohols  without any catalyst. 

iii) Reaction with HCl: 

HCl reacts with 1oand 2oalcoholsin presence of H2SO4 or anhydrous ZnCl2 (Lucas reagent) as catalyst but it reacts with 3oalcohols without any catalyst.

b) Reaction with phosphorous halides: 

PBr3 and PI3 are unstable so they are prepared in-situ by reacting Red P with Br2 and I2 respectively. 

Examples: 

Note: Reaction with PBr3 and PI3 is similar to PCl3

c) Reaction with Thionyl chloride (SOCl2): 

When alcohol is heated with thionyl chloride in presence of pyridine then chloroalkane is formed.  

Note: This is the best way to prepare chloroalkanes because by-products are easily removed and pure  chloroalkanes can be obtained. 

Physical Properties of Monohaloalkanes: 

1. Physical Character: 

State: Lower members like CH3Cl, CH3Br, CH3CH2Cl, etc. are colorless gases, other alkyl halide up to C18 are liquid andbeyond C18 are solid. 

2. Solubility: 

Haloalkanes are insoluble in water because they cannot form intermolecular hydrogen bond with  water molecules. 

But, haloalkanes are soluble in organic solvents. 

3. Boiling point: 

Haloalkanes have higher boiling point than corresponding alkanes because haloalkanes are polar  compounds. So, they are held together by dipole-dipole interaction but alkane is non polar and  lacks such interaction.

I) For same alkyl group but different halogen atom: 

Increasing order of boiling point is R ̶Cl < R ̶Br < R ̶I 

This is because with increase in the size and molecular mass of halogen atom, magnitude of Van der Waal’s force of attraction increases and boiling point also increases. 

Example: CH3CH2 ̶ Cl < CH3CH2 ̶Br < CH3CH2 ̶I 

II)For different alkyl group but same halogen atom: 

Boiling point of haloalkanes increases with increase in number of carbon atoms due to increase in  molecular mass. 

 i.e. CH3 ̶ Cl < CH3CH2 ̶Cl < CH3CH2CH2 ̶Cl 

III)Comparison between isomeric haloalkane

Boiling point decrease with the increase in the branching in alkyl group because with the increase in branching,the haloalkane molecule attain spherical shape with less surface area. Thus, strength of Van der Waal’s force of attraction also decreases and boiling point also decreases. Example: 

Chemical Properties of Haloalkanes

Haloalkanes are more reactive class of organic compounds but are less reactive than alcohol, carboxylic acids, etc. 

The reactivity is due to polar C ̶ X bond. This polarity arises due to the electronegativity difference between the carbon and halogen atoms. 

Reactivity also depends on the bond dissociation energy of C ̶X bond. 

 C ̶ Cl > C ̶ Br > C ̶ I 

(326.5 KJ) (284.5 KJ) (213.5 KJ) 

Hence reactivity is in the order of C ̶Cl < C ̶Br < C ̶I 

i.e. C ̶ I bond is easier to break and C ̶ Cl bond is difficult to break. 

Nature of alkyl group also determines the reactivity: 

Methyl halide < Primary haloalkane < Secondary haloalkane < Tertiary haloalkane 

Haloalkanes shows following types of reactions: 

A. Nucleophilic substitution reactions 

B. Elimination Reactions 

C. Reduction reaction 

D. Reactions with metals

[A]Nucleophilic substitution Reactions: 

The reaction in which nucleophile is substituted by another nucleophile is called nucleophilic  substitution reaction. 

General representation: 

Where, Nu ̶= ̶OH, NH3, H2O, etc. 

X̶ = Cl ̶, Br ̶, I ̶, etc. 

Here, incoming nucleophile attacks the electron deficient center having halogen atom while the  leaving group leaves with bonding pair of electrons. 

There are 2 types of Nucleophilic substitution reaction. They are: 

1. SN1 reaction and 

2. SN2 reaction 

1) SN1 Reaction: It stands for substitution nucleophilic unimolecular or (Unimolecular nucleophilic  substitution). 

It is a two-step process. First step is the slowest step and thus determines the rate of reaction. Mechanism: 

Step 1: Formation of carbocation

Step 2: Nucleophilic attack on carbocation 

Kinetics: 

Since the rate determining step involves the participation of haloalkane only. That’s why rate of  reaction depends only on concentration of haloalkane. 

i.e. Rate ∝ [R−X] 

Rate = K [R−X] 

Hence, SN1 reaction is of first order. 

Reactivity order of haloalkanes towards SN1 reaction: 

Tertiary (30) haloalkane > Secondary (20) haloalkane > Primary (10) haloalkane > Methyl halide 

1) SN2 Reaction: It stands for substitution nucleophilic bimolecular or (Bimolecular nucleophilic  substitution). 

It is a single step process in which both haloalkane as well as nucleophile are present in the slowest 

Kinetics: 

Since the rate determining step involves the participation of both haloalkane as well as nucleophile. That’s  why rate of reaction depends on concentration of both haloalkane and nucleophile. 

i.e. Rate ∝ [R−X] [Nu

Rate = K [R−X] [Nu

Hence, SN2 reaction is of second order. 

Reactivity order of haloalkanes towards SN2 reaction: 

Methyl halide > Primary (10) haloalkane > Secondary (20) haloalkane > Tertiary (30) haloalkane  

Some examples of nucleophilic substitution reactions: 

1) Reaction with aq. NaOH or aq. KOH or with moist Ag2

Nucleophile is hydroxide ion,OH 

– Oxygen carrying lone pair electrons & negative charge serves as nucleophile which attacks theelectrophilic carbon atom of haloalkane. 

– Old C-X bond is broken 

– New C-O bond formed. In this reaction, Alcohol is formed. 

Example: 

Note: Moist Ag2O +H2O 2AgOH, it can produce OH nucleophile. 

Although water contains OH ion which is not powerful enough to reacts with haloalkane. Hence, aqueous solution o f strong bases are used for the reactions. 

2) Reaction with alcoholic solution of NH3 (Hofmann’s Ammonolysis reaction) – Nucleophile is nitrogen atom with a lone pair electron which attacks the electrophilic carbon atom ofhaloalkane. 

– Old C─X bond is broken 

– New C─N bond is formed. In this reaction, primary amine is formed.

Example: 

Note: If haloalkane is used in excess then mixtures of primary, secondary, tertiary amines and quaternary salts are formed. 

3) Reactions with sodium or potassium alkoxide (Williamson’s Ether synthesis) Nucleophile is alkoxide ion (OR) 

– Oxygen atom with lone pair electrons and negative charge acts as a nucleophile which attacks the  e lectron deficient carbon atom of haloalkane. 

– Old C─X bond is broken 

New C─O bond is formed. In this reaction, ether is formed. 

(Dialkyl ether) 

Example:

4) Reactions with sodium or potassium thiolate: 

– Nucleophile is thiolate ion (RS

– Negative charge containing S-atom acts as nucleophile. 

– Old C─X bond is broken 

New C─S bond is formed. In this reaction, thioether is formed. 

i.e. Example,

5) Reactions with alcoholic sodium or potassium cyanide (NaCN and KCN): – NaCN or KCN is ionic compound. 

– Nucleophile is CN ion 

– Negative charge containing c-atom acts as nucleophile. 

– Old C─X bond is broken 

New C─C bond is formed. In this reaction, alkyl cyanide or alkane nitrile is formed. 

Application of alkyl cyanide in organic synthesis: 

6) Reactions with alcoholic silver cyanide (AgCN): 

– AgCN is covalent compound. 

– Lone pair containing N-atom acts as nucleophile. 

– Old C─X bond is broken 

New C─N bond is formed. In this reaction, alkyl isocyanide or alkyl carbylamine or alkane isonitrile is formed. 

i.e. 

Application of alkyl isocyanide:

6. Reactions with alcoholic potassium or sodium nitrite (KNO2 or NaNO2): 

– Alkyl nitrite is formed. Nucleophile is NO2– 

– negatively charged oxygen atom with lone pair electrons acts as nucleophile. 

 7. Reactions with alcoholic silver nitrite (AgNO2): 

– Nitroalkane is formed. Nucleophile is NO2– 

– lone pair containing N-atom acts as nucleophile. 

i.e.

NOTE: CN and NO2are ambident nucleophiles (nucleophiles in which more than one atom can acts as nucleophilic site are called ambident nucleophiles.) 

[B] Elimination reaction (Dehydrohalogenation or β-elimination or 1,2 –elimination) – Removal of HX (HCl, HBr, HI) from adjacent carbon atoms of haloalkane in presence of alcoholic solution of strong bases like KOH or NaOH is called dehydrohalogenation.  

– This is also called β-elimination because hydrogen is removed from β-carbon or also called 1,2- elmination because H and X are removed from adjacent c-atoms. 

– Alkene is formed as major product. 

i.e.

Example:  

Saytzeff’s Rule or Zaitsev’s Rule 

This rule was given by Russian chemist Alexander Zaitsev 

“In dehydrohalogenation reaction, highly substituted alkene is major product”. 

[C] Reaction with metals: 

a) Reactions with Na metal (Wurtz reaction)  

When haloalkane is heated with sodium metal in presence of dry ether then it gives alkane (having  double number of carbon atoms than the haloalkane). 

Example: 

b) Reaction with Mg metal (Grignard reaction): 

Question: What are Grignard reagents? How are they prepared? What are precautions for the preparation of Grignard reagents? 

Ans: Alkyl magnesium halide (RMgX) or aryl magnesium halide (ArMgX) are commonly known as Grignard reagents.  

They are prepared by reacting alkyl halide or aryl halide with magnesium metal in presence dry ether. This reagent is discovered by Victor Grignard in 1900.

Precautions for the preparation of Grignard reagent 

Apparatus and reagents must be dried and reaction must be carried out in dry and inert condition because if carbon dioxide and moisture are present then Grignard reagent will be decomposed  into different products. 

Example: 

Applications of Grignard Reagent in organic synthesis: 

[D] Reduction reactions: 

When haloalkane is treated with reducing agents like LiAlH4 or H2/Ni or H2/Pt or Na/C2H5OH, etc.  then alkane is produced.

Polyhaloalkane 

Those haloalkanes which contains more than two halogen atoms in a molecule are called  polyhaloalkanes. 

Example:  CHCl3 CHI3 CCl4 

Chloroform Iodoform Carbon tetrachloride 

Chloroform 

It is also called trichloromethane. 

Laboratory preparation of Chloroform (Trichloromethane) 

Principle: 

In laboratory, chloroform is prepared by heating bleaching powder paste with ethanol or  propanone. Here, bleaching powder acts as oxidizing, chlorinating and hydrolyzing agents. 

From Ethanol

From Propanone 

Chemical Properties of Chloroform 

1) Oxidation: (Action with air) 

Chloroform reacts with oxygen in presence of sunlight to oxidize into poisonous phosgene 

gas. i.e. 

Question: 

Question: 

Why 1% ethyl alcohol is added to chloroform during storage? 

Ans: If any phosgene gas has formed during storage then ethyl alcohol reacts with poisonous  phosgene gas and converts it into non-poisonous, non-volatile solid substance diethyl carbonate.

Question: 

Why pure chloroform does not give white precipitate with aq. AgNO3 solution? Ans: In chloroform C─Cl bond is covalent in nature, which does not ionize easily to give free Cl ions. Therefore, in absence of free Clions pure chloroform does not give white precipitate with  aq. AgNO3 solution. 

But, impure chloroform contains HCl due to following reaction 

Therefore, HCl ionizes to give free Clions. These chloride ions reacts with aq. AgNO3 solution to gives white precipitate of AgCl. 

2) Reduction

3) Reaction with Ag Powder

When chloroform is heated with silver powder then ethyne (acetylene) is produced. i.e. 

Ethyne Note: Iodoform also gives similar reaction to produce ethyne.

4) Reaction with Conc. HNO3 : 

When chloroform reacts with concentrated nitric acid then chloropicrin (nitrochloroform) is  formed. 

– Chloropicrin is used as a tear gas. 

5) Reaction with Propanone (Acetone): 

When chloroform reacts with acetone in presence of alkali then chloretone is formed. – Chloretone is used in medicine as hypnotic drug (sleep inducing drug) 

6) Reaction with aqueous alkali

When chloroform is heated with aq. NaOH or KOH solution then sodium or potassium formate  is formed. 

i.e. 

Potassium formate 

7) Carbylamine reaction

When chloroform is heated with primary aliphatic or aromatic amine in presence of alcoholic KOH or NaOH then very offensive smelling alkyl isocyanide (alkyl  carbylamines) is formed. 

– This reaction is used for the distinction of primary amine from other amines.

Example: 

8) Reimer-Tiemann reaction

When chloroform is heated with phenol in presence of aqueous NaOH or KOH solution  followed by acidification then salicylaldehyde (o-hydroxybenzaldehyde) is formed. i.e.  

Uses of Chloroform 

– Used as solvent, anesthetic agent, used for the preparation of chloretone, chloropicrin, formic acid,Salicyaldehyde etc.

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