IIT JEE Chemistry - Study Guide - 17. Organic Compounds with Functional Groups Containing Halogens

Sections in the Chapter of Jauhar

17.1 Classification of halogen Derivatives of Hydrocarbons
17.2 Nomenclature of Haloalkanes
17.3 Nomenclature of Aryl Halides
17.4 isomerism in Haloalkanes
17.5 Methods of Preparation of Haloalkanes
17.6 General Methods of Preparation of Haloarenes
17.7 Physical Properties of haloalkanes
17.8 Physical Properties of haloarenes
17.9 Nature of C-X Bond
17.10 Chemical Properties of Haloalkanes
17.11 Chemical Properties of Haloarenes
17.12 Some Commercially Important Compounds
17.13 Analysis and Difference Between Haloalkanes and Haloarenes



Conceptual Questions with Answers: 21
Additional Numerical Problems for Practice:
Revision Exercises
Very Short Answer questions 31
Short Answer Questions: 38
Long Answer Questions: 11

Competition File
Numerical Problems
Objective Questions: 53
Fill in the blanks: 10
True or False: 10

Study Plan

Day 1

17.1 Classification of halogen Derivatives of Hydrocarbons
17.2 Nomenclature of Haloalkanes
17.3 Nomenclature of Aryl Halides

Day 2

17.4 isomerism in Haloalkanes
Ex. 17.1 to 17.4
PP. 17.1 to 17.6
Day 3

Revision Exercises; Very Very Short Answer questions 1,2, 7,9

17.5 Methods of Preparation of Haloalkanes
Ex. 17.5 to 17.8

Day 6
PP. 17.7 to 17.8
17.6 General Methods of Preparation of Haloarenes
Ex. 17.9

Day 7

17.7 Physical Properties of haloalkanes
17.8 Physical Properties of haloarenes
17.9 Nature of C-X Bond

Day 8

17.10 Chemical Properties of Haloalkanes

Day 9
Revision of th material
Ex. 17.10 to 17.15
PP. 17.9 to 17.18

Day 10

17.11 Chemical Properties of Haloarenes

Day 11
PP. 17.19 to 17.24

17.12 Some Commercially Important Compounds

Day 12
17.12 contd.

Day 13
17.13 Analysis and Difference Between Haloalkanes and Haloarenes
PP. 17.25 to 17.29

Day 14
Miscellaneous Problems 17.17 to 17.21
Conceptual questions 1 to 21

Day 15
Revision Exercises: Very Short Answer questions 31
Revision Exercises: Short Answer Questions: 38

Revision period

Day 16


Competition File: Objective Questions: 1 to 25

Day 17
Competition File: Objective Questions: 26 to 53

Day 18
Competition File: Fill in the blanks: 10
Competition File: True or False: 10

Day 19

Revision of 171. to 17.4

Day 20
Revision of 17.5 and 17.6

Day 21
Revision of 17.7 to 17.9

Day 22
Revision of 17.10,17.11

Day 23
Revision of 17.12, 17.13

Alkyl halides- Study Guide - IIT JEE

Preparation, properties and reactions

Characteristic reactions of

rearrangement reactions of alkyl carbocation, Grignard reactions, nucleophilic substitution reactions;

IIt JEE Revision Ch.26 Alkyl and Aryl Halides - Core Points

syllabus


METHODS OF PREPARATION
PHYSICAL PROPERTIES
CHEMICAL REACTIONS
charateristic reactions

Specially highlighed topics

Rearrangement reactions of alkyl carbocation,
Grignard reactions,
Nucleophilic substitution reactions;
-------------

1. When hydrogen atom or atoms of alkanes are replaced by the corresponding number of halogen atoms, the compounds are called halogen derivatives of alkanes.



2. Methods of preparation

1. From hydrocarbons
a) from alkanes: halogens react with alkanes in the presence of uv light to form haloalkanes.
b) from alkenes: by the electrophylic addition of halogen acids (HBr, HCl, or HI)

3. The only methyl halide which is a liquid is iodomethane.chloroethane is a gas.

4. Nucleophilic substitution in primary halogenoalkanes
The nucleophilic substitution reaction - an SN2 reaction - S stands for substitution, N for nucleophilic, and the 2 is order of reaction. It is because the initial stage of the reaction involves two species - the bromoethane and the Nucleophilic (Nu-) ion.

5. Nucleophilic substitution in tertiary halogenoalkanes - The nucleophilic substitution reaction - an SN1 reaction (1 denotes 1st order)

6.

Alkyl and Aryl Halides - Introduction, Nomenclature

When hydrogen atom or atoms of alkanes are replaced by the corresponding number of halogen atoms, the compounds are called halogen derivatives of alkanes.

They are classified according to the number of halogen atoms that replace hydrogen atoms in the alkane.

Monohalogen derivatives: They contain only one halogen atom.

e.g. CH-3Cl Methyl chloride
CH-3-CH-CH-3 2-Bromopropane
!
Br
Monohalogen derivatives of alkane are called alkyl halides

Dihalogen alkanes contain two halogen atoms.
Trihalog alkanes contain three halogen atoms.

Monohalo alkanes

The general formula is RX where is a alkyl group and X is a halogen.

Classification of Haloalkanes

A. Type of halogen atoms; Fluorides, chlorides, bromides, iodides
B. Number of halogena atoms, monohalo, dihalo, trihalo, tetra halo.

Classification of Haloalkanes
1. Compounds containing sp3 hybridization
2. Compounds containing sp2 hybridization

Compounds containing sp3 hybridization
a) halo alkances or alkyl halides
(i)Primary halogenoalkanes

In a primary (1°) halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group.

Secondary halogenoalkanes

In a secondary (2°) halogenoalkane, the carbon with the halogen attached is joined directly to two other alkyl groups, which may be the same or different.

Tertiary halogenoalkanes

In a tertiary (3°) halogenoalkane, the carbon atom holding the halogen is attached directly to three alkyl groups, which may be any combination of same or different.

b) Allylic halides: Halogens attached to alkenes to a carbon atom next to carbon-carbon double bond.

c) Benzylic halide (aralalkyl halide): Halogens attached to alkenes to a carbon atom next to to an aromatic ring (not to a carbon atom in the aromatic ring). It is attached to a carbon atom which is inturn attached to a carbon atom in the aromatic ring.

2. Compounds containing sp2 hybridization

a) Vinylic halides: Halogens attached to alkenes to a carbon atom of one of the carbon atoms of a double bond.

B) Aryl halides: Halogens attached to alkenes to a carbon atom of an aromatic ring


Nomenclature – Alkyl and Aryl halides

Nomenclature of alkyl halides

Common system

The monohalogen derivatives of alkanes are called alkyl halides. These are named by naming the alkyl group attached to halogen and adding the name of the halide.
Ex: Methyl halide, Isobutyl halide
The name of the alkyl group and halide are written as two separate words. The prefixes used to distinguish alkanes like n-, iso-, sec-, tert, etc. are also written.

IUPAC System

The monohalogen derivatives of alkanes are called haloalkanes.
The names of haloalkanes are written by prefixing the word ‘halo’ (bromo or chloro or iodo or fluoro) to the name of the alkane corresponding to the longest continuous carbon chain holding the halogen atom.
Ex: Bromoethane

Rules for naming haloalkanes having branches in carbon chains

1. The longest continuous chain containing the carbon attached to halogen group is selected as the parent alkane (principal chain or parent chain). In naming alkanes all rules that apply to alkane names are to be followed.
2. The carbon atoms are numbered in such a way that the carbon atom carrying the halogen atom gets the lowest number.
3. The position of the halogen atom and other substituents are indicated by numbers 1,2,3… etc.
Ex: 1-Iodo-2-methylpropane

Dihalo derivatives

1. When both the halogen atoms are attached to the same C-atom, these are called geminal dihalides. Alkylidene dihalides or alkylidene halides are also names used for such compounds.
Ex: ethlydine dichloride
2. When the two halogen atoms are in adjacent C-atoms, they are called vicinal dihalides. They are prepared from alkenes and hence they are named as the dihalide of the alkene from which they are prepared.
Ex: ethylene dichloride

Polyhalo derivatives

Ex: Trichloromethane
Fully halogenated hydrocarbons are also called perhalohydrocarbons under common system.

Nomenclature of aryl halides

Aryl halides are termed Haloarenes in IUPAC systems. The prefix ‘halo” ((bromo or chloro or iodo or fluoro) is placed before the name of the aromatic hydrocarbon. In case of disubstituted compounds, the relative positions are indicated by (1,2), (1,3) or (1,4). Ortho, meta and para are also used to indicate the positions.

Ex: Chlorobenzene, Bromobenzene

IIT JEE Revision - Alkyl and Aryl Halides - Methods of preparation

Methods of preparation

1. From hydrocarbons
a) from alkanes: halogens react with alkanes in the presence of uv light to form haloalkanes.
b) from alkenes: by the electrophylic addition of halogen acids (HBr, HCl, or HI)

Markownikov rule and anti Markownikov rule are applicable in the reaction between alkene and halogen acids.

2. From alcohols:

This is the most widely used method for the preparation of haloalkanes in the laboratory.

Methods preparing alkyl halides from alcohols

(i) By the action of halogen acids on alcohols

(a) Chloro alkanes: Primary and secondary alcohols form chloroalkanes when hydrochloric acid gas is passed through alcohol in the presence of anhydrous zinc chloride (Groove's process).

ZnCl2 is a Lewis acid.

(b) Tertiary alcohols are very reactive and therefore, they react readily with conc. HCl even in the absence of zinc chloride.

(c) Bromo alkanes: Bromoalkanes are obtained by heating an alcohol with hydrobromic acid (48%) in the presence of a little conc. H2SO4 whihc acts as a catalyst.

Hydrobromic acid (HBR) can be generated in situ (during the reaction itself) by the action of conc. H2SO4 on KBr or NaBr.

(d) Iodoalkanes: are obtained by heating alcohols with constant boiling hydroiodic acid (57%).

Hydroiodic acid (HI) can be generated in situ (during the reaction itself) by the action of phosporic acid on potassium iodide.

Note: Unlike alkyl chlorides, secondary andd tertiary bromides and iodides cannot be prepared from respective alcohols because the secondary and tertiary alcohols on heating with H2SO4 undergo dehydration to form alkenes.

Hydrogen fluoride is the least reactive of the hydrogen halides and the preparation of fluroalkanes is not practical by using alcohol and hydrogen fluoride.

(ii) Reaction of alcohols with phosphorus halides

Phosphorus halides such as PCl5, PCl3, PBr3 and PI3 react with alcohols to form corresponding haloalkanes.

(a) Chloroalkanes can be prepared by the action of phosphorus pentachloride or phosphorous trichloride on alcohols.

(b)Bromoalkanes and iodoalkanes are prepared by the action of phosphorous bromide and phosphorous tri-iodide respectively on alcohols.

phosphorous bromide and phosphorous iodide are not very stable compounds. Hence there are generally prepared in situ (during the reaction) by the action of red phosphorous on

Haloalkanes : Chloroethane, Bromoethane, Iodoethane

Reaction with ZnCl2/conc.-HCl

This is a reaction or test to distinguish various categories of alcohols and is termed Lucas test.

In this test, an alcohol is treated with an equimolar mixture of concentrated hydrochloric acid and anhydrous ZnCl2 (called Lucas reagent).

Alcohols get converted into alkylhalides. As alkyl halides are insoluble in water, their presence is indicated by the appearance of turbidity in the reaction mixture. Br2 or I2.

(iii) By the action of thionyl chloride on alcohol

Chloroalkanes can be prepared from alcohols by refluxing alcohols with thionyl chloride in the presence of pyridine.


3. By Halide Exchange

This reaction is particularly useful for preparing iodoalkanes.

The iodoalkanes are obtained by heating chloro or bromo alkanes with a concentrated solution of sodium iodide in acetone.

The reaction gives the best results with primary halides. This reaction is known as Finkelstein reaction.

Fluoro alkanes are difficult to prepare by other methods. They are prepared by treating alkyl chlorides or bromides with salts such as mecurous fluoride (Hg2F2), silver fluoride (AgF), cobalt fluoride (CoF3) or antimony trifluoride (SbF3). This reaction is known as Swarts reaction.

cobalt fluoride (CoF3) or antimony trifluoride (SbF3) are used when the organic halides contain two or three halogen atoms on the same carbon atom.

4. Preparation of alkyl halides from silver salts of acids.

The silver salts of the carboxylic acids dissolved in CCl4 are decomposed by bromine to form bromoalkanes. This reaction is called Borodine Hundsdiecker reaction.

Chloroalkanes can also be obtained by this method by using Cl2 instead of Br2, but the yield of chloroalkanes is very poor.

Iodo alkanes cannot be obtained by this reaction.

Alkyl and Aryl Halides - Physical Properties

Physical properties of halogenoalkanes

Boiling Points

the only methyl halide which is a liquid is iodomethane;

chloroethane is a gas.

The examples show that the boiling points fall as the isomers go from a primary to a secondary to a tertiary halogenoalkane. This is a simple result of the fall in the effectiveness of the dispersion forces.

Solubility in water

The halogenoalkanes are at best only very slightly soluble in water.

In order for a halogenoalkane to dissolve in water you have to break attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and break the hydrogen bonds between water molecules. Both of these cost energy.

Solubility in organic solvents

Halogenoalkanes tend to dissolve in organic solvents because the new intermolecular attractions have much the same strength as the ones being broken in the separate halogenoalkane and solvent.



bond strength falls as you go from C-F to C-I, and notice how much stronger the carbon-fluorine bond is than the rest.

In order for anything to react with the halogenoalkanes, the carbon-halogen bond has got to be broken. Because that gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order.

Iodoalkanes are the most reactive and fluoroalkanes are the least. In fact, fluoroalkanes are so unreactive that we shall pretty well ignore them completely in discussion on reactions.

Alkyl and Aryl Halides - CHEMICAL REACTIONS

Haloalkanes are one of the most reactive classes of organic compounds. Many organic compounds are prepared using alkyl halides. So alkyl halides are considered as synthetic tools in the hands of an organic chemist to synthesize various compounds.

The reactions of alkyl halides can be grouped under the following heads for a study of them.

Nucleophilic substitution reactions
Elimination reactions
Reactions with metals
Reduction



Nucleophilic substitution reactions

1. Substitution by hydroxylgroup (OH) leads to the formation of alcohols
2. Substitution by alkoxy group leads to the formation of ether.
3. Substitution by cyano group leads to the formation of cyanides or nitriles.

4. Substitution by isocyanide group leads to the formation of isocyanides.
When haloalkane is treated with alcoholic silver cyanide (AgCN), isocynaides (R-N≡C) are obtained. They are also called carbyl amines.

RX + AgCN ---> RNC + AGX

C2H5Br + AgCN ---> C2H5NC + Ag Br

5. Substitution by amino group leads to the formation of amines.
6. Substitution by nitrite group leads to the formation of nitrite.
7. Substitution by nitro group leads to the formation of nitro alkanes.
8. Substitution by carboxyl group leads to the formation of esters.
9. substitution by hydrosulphide group leads to the formation of thioalcohols.
10. Substitution by mercaptide group leads to the formation of thioethers.
11. Substitution by alkyl group leads to the formation of alkynes.

Elimination reactions (Dehydrohalogenation)

In this reaction alkenes are formed.
This reaction occus when alkyl halides are boiled iwth alcoholic solution of potassium hydroxide. One hydrogen atom and one halogen atom are removed and a double bond forms.

Reaction with active metals
Active metals like sodium, magnesium, cadmium, lithium combine with alkyl halides to give compounds containing carbon metal bonds (called as organometallic compounds).

i) With magnesium, alkyl magnesium halide is formed. This is called as Grignard reagent.

ii)Two molecules of alkyl halides react with sodium in the presence of ether and form alkanes. The reaction is termed Wurtz reaction and is used to prepare symmetrical alkanes

Reduction

Haloalkanes can be reduced to alkanes by some reagents. The reagents include
i) Hydrogen in the presence of a metal catalyst such as nickel, palladium or platinum.
ii)zinc copper couple and ethyl alcohol
iii) Hydroiodic acid in the presence of red phosphorus

Rearrangement
When a haloalkance is heated at 573 K or at a slightly lower temperature in the presence
of anhydrous aluminium chloride as catalyst, the halide undergoes rearrangement to form isomeric haloalkane. So this reaction is an isomerism reaction.

Alkyl and Aryl Halides - Rearrangement reactions of alkyl carbocation

When a haloalkance is heated at 573 K or at a slightly lower temperature in the presence of anhydrous aluminium chloride as catalyst, the halide undergoes rearrangement to form isomeric haloalkane. So this reaction is an isomerism reaction.

1-Chloropropane CH3CH2CH2Cl rearranges to 2-Chloropropane. Chlorine is attached to the 2nd carbon (or middle carbon) instead of the 1st carbon or terminal carbon

If there is no hydrogen atom on the carbon adjacent to the C-X group, rearrangment occurs and methyl group migrates providing the opportunity for the halogen atom to occupy the position.

1-Chloro-2,2 dimethyl propane rearranges to 2-Chloro-2-methylbutane

Alkyl and Aryl Halides Grignard reactions

A solution of an alkyl halide in dry ether reacts with magnesium to form alkyl magnesium halide.

RX + Mg in dry ether --> RMGX

RMgX compounds are known as Grignard reagents. They were discovered by Victor Grignard, a French National. He got the Nobel prize in Chemistry in 1912.

These compounds have many synthetic applications and are used in the preparation of many organic compounds.

Vinyl and aryl halides also form magnesium halides.

Alkyl and Aryl Halides - Nucleophilic substitution reactions

Nucleophilic substitution in primary halogenoalkanes
The nucleophilic substitution reaction - an SN2 reaction - S stands for substitution, N for nucleophilic, and the 2 is order of reaction. It is because the initial stage of the reaction involves two species - the bromoethane and the Nucleophilic (Nu-) ion.



Nucleophilic substitution in tertiary halogenoalkanes - The nucleophilic substitution reaction - an SN1 reaction (1 denotes 1st order)



Many organic compounds are prepared using alkyl halides

Nucleophilic substitution reactions

1. Substitution by hydroxylgroup (OH) leads to the formation of alcohols
2. Substitution by alkoxy group leads to the formation of ether.
3. Substitution by cyano group leads to the formation of cyanides or nitriles.

4. Substitution by isocyanide group leads to the formation of isocyanides.
When haloalkane is treated with alcoholic silver cyanide (AgCN), isocynaides (R-N≡C) are obtained. They are also called carbyl amines.

RX + AgCN ---> RNC + AGX

C2H5Br + AgCN ---> C2H5NC + Ag Br

5. Substitution by amino group leads to the formation of amines.
6. Substitution by nitrite group leads to the formation of nitrite.
7. Substitution by nitro group leads to the formation of nitro alkanes.
8. Substitution by carboxyl group leads to the formation of esters.
9. substitution by hydrosulphide group leads to the formation of thioalcohols.
10. Substitution by mercaptide group leads to the formation of thioethers.
11. Substitution by alkyl group leads to the formation of alkynes.