NIOS Lesson 25 - COMPOUNDS OF CARBON CONTAINING HALOGENS (HALOALKANES AND HALOARENES)

 COMPOUNDS OF CARBON CONTAINING HALOGENS

(HALOALKANES AND HALOARENES)

You have studied about the hydrocarbons in the previous lesson. When a hydrogen attached to a carbon atom in the hydrocarbons is replaced by a halogen atom (i.e. F,C1,Br or I), the compounds formed are called haloalkanes or haloarenes. The halogen derivatives do not occur in nature and they are synthesized in the laboratory. These compounds have wide applications in industry and domestic uses. They are used as industrial solvents, in the manufacture of pharmaceuticals, as dry cleaning agents, as pesticides, as anesthetics in medicine, as refrigerants, as fire extinguishers and as antiseptics. In this lesson, you will study the nomenclature, methods of preparation and properties of this important class of carbon compounds.

OBJECTIVES

􀁺 define haloalkanes and haloarenes;

􀁺 name haloalkanes and haloarenes according to IUPAC rules;

􀁺 explain methods of preparation, physical properties, chemical properties and uses of haloalkanes and haloarenes;

􀁺 distinguish between haloalkanes and haloarenes, and

􀁺 explain the preparation, properties and uses of some important polyhalogen compounds.

25.1 NOMENCLATURE OF HALOALKANES AND HALOARENES

You have learnt the nomenclature of hydrocarbons in lesson 25. In this section, you will learn the nomenclature of halogen derivatives of both aliphatic and aromatic hydrocarbons i.e. haloalkanes and haloarenes.

Nomenclature of Haloalkanes

The following rules are used for naming haloalkanes according to the IUPAC system.

1. The longest chain of the carbon atoms bearing the halogen atom is selected.

The longest chain of carbon atoms in the above example is shown in the

box.

2. Numbering of the carbon atoms in the chain is done in such a way that the

carbon atom bearing the halogen atom gets the lowest number.

For example.

In the above example, numbering shown in (I) is correct while in (II), it is incorrect since the carbon atom bearing halogen atom gets lower number in I than in II.

3. The word chloro, is prefixed to the parent hydrocarbon name.

So, the correct name for the above halocompound is

4. In case of alkyl substituted haloalkanes, the longest chain containing halogen

atom is selected for numbering.

For example :

In structure I, the selection of chain is shown by two different ways. Both the ways of selection are correct since they include chloro group. In structure II numbering shown is not correct since it does not include chloro group.

5. When two or more halogen atoms are present in a compound, the longest chain selected must contain the maximum number of halogen atoms. The multiplicative prefixes (di, tri, tetra, etc.) are added before the name of halogen atom to indicate the number of halogen atoms. The following examples illustrate this rule.

Nomenclature of Haloarenes

Haloarenes are those aromatic halogen compounds in which the halogen atom is directly linked to an aromatic ring. Their general formula is Ar-X where Arrepresents an aromatic ring and X denotes the halogen. In naming a haloarene, the prefix chloro, bromo-or iodo- etc. is are added to name of arene according to halogen(s) present. The relative positions of halogen atoms are indicated by appropriate numbers. The prefixes ortho (o-), meta(m-) and para (p-) are also commonly used respectively to indicate the relative positions i.e. 1,2- ;1,3 − and

1,4- of substituents in a benzene ring. Following examples illustrate the nomenclature of some haloarenes.

INTEXT QUESTIONS 25.1

1. Write the IUPAC names of each of the following compounds :

(i) 2-Chloropentane

(ii) 3-Iodo-2-methylpentane

(iii) l-Chloro-2,3-dimethylpentane

(iv) 1,3,4-Trichloro-6-methylbenzene or 2,4,5 - Trichlorotoluene

(v) 1,3-Dibromo-5-ethylbenzene

(vi) 2,4-Dibromohexane

2. Draw structural formulae of the following compounds:

(i) 2-Bromo-3-methylbutane

(ii) 3-Chloro-4-methylhexane

(iii) 3-Bromochlorobenzene

(iv) 2,4-Dibromotoluene

25.2 PREPARATION OF HALOALKANES AND

HALOARENES

25.2.1 Preparation of Haloalkanes

(i) From Hydrocarbons : Direct halogenation of hydrocarbons takes place in the presence of sunlight or at high temperature in dark. For example, chloroethane is prepared by monochlorination of ethane.

This reaction follows a free radical mechanism. You have already learnt the

mechanism of chlorination of methane in lesson 26.

Bromo derivatives of alkanes are also prepared by direct bromination

Direct iodination is not possible with iodine as the reaction is reversible.

Direct fluorination is also not possible because due to the high reactivity of

the fluorine, the reaction cannot be controlled.

(ii) From Alcohols : Alcohols are converted into haloalkanes by treating with

(a) hydrogen halides (b) phosphorus halides or (c) thionyl chloride.

(a) Reaction with Hydrogen Halides : Hydrogen halides react with an alcohol

in presence of a dehydrating agent such as anhydrous zinc chloride to produce

a haloalkane.

Chloroethane is conveniently prepared by the reaction of ethanol with

concentrated hydrochloric acid in presence of anhydrous zinc chloride.

Zinc chloride absorbs water from the reaction mixture and thus prevents the reverse reaction.

Bromoethane is obtained by refluxing ethanol with HBr in presence of little concentrated H2SO4 as the catalyst.

(b) Reaction with Phosphorus Halides : Haloalkanes are conveniently prepared Compounds

by the reaction of an alcohol with a phosphorus halide ( ) PCl3, PCl5 or PBr3

according to the following equations.

(c) Reaction with Thionyl Cholride : Thionyl chloride (SOCl2) is another

reagent which reacts with an alcohol to yield a chloroalkane.

As both the byproducts, SO2 and HC1 are gases, the purification of final

product is not required.

25.2.2 Preparation of Haloarenes

(i) From Aromatic Hydrocarbons : Haloarenes are obtained by direct halogenation of aromatic hydrocarbons in the presence of a catalyst. Usually iron filings or iron (III) halide is used as the catalyst.

The direct iodination of aromatic hydrocarbons is not a useful reaction since

the HI produced reduces the aryl iodide back to the aromatic hydrocarbon.

However, in the presence of an oxidizing agent such as nitric acid, iodic acid

(HIO3), mercury oxide, the HI produced is either oxidized to iodine or is

eliminated as mercuric iodide and, thus, iodobenzene is obtained.

Fluorobenzene cannot be prepared by direct fluorination of aromatic

hydrocarbons since the reaction is very violent and cannot be controlled.

(ii) From Diazonium Salts : Benzene diazonium salt is formed by treating an

aromatic primary amine with NaNO2 and dil. HCl at low temperature. The

process is known as diazotisation.

Diazonium salts are highly reactive compounds. They are used in the preparation of a large number of arene derivatives. When a diazonium salt is treated with copper (I) chloride (Cu2Cl2 ) or copper (I) bromide (Cu2Br2), the corresponding haloarene is formed.

This reaction is known as Sandmeyer reaction. It is used for introducing a chloro or bromo group in the benzene ring.

Haloarenes can also be prepared by reacting benzene diazonium chloride with copper powder in presence of corresponding halogen acid. This reaction is called

Gattermann reaction and is shown below :

INTEXT QUESTIONS 25.2

1. Write the structure of the main product obtained by treating 1-propanol with excess of hydrogen chloride in the presence of anhydrous zinc chloride.

2. What will be the product obtained on treatment of 1-propanol with thionyl chloride?

3. Give one example of preparation of chlorbenzene using Sandmeyer reaction?

4. Complete the following reaction :

25.3 PROPERTIES OF HALOALKANES AND HALOARENES

Before we discuss the important physical and chemical properties of alkylhalides

and aryl halides, let us consider the nature of C—X bond.

25.3.1 The Nature of C–X Bond

In alkyl halides, the carbon – halogen bond is formed by the overlap of the sp3

hybrid orbital of carbon atom with the p-orbital of the halogen atom.

As one moves from fluorine to iodine, the size of the halogen atom increases and hence the overlap decreases. Hence, the C–X bond becomes longer and weaker on going from alkyl fluorides to alkyl iodides.

Also, the halogens are more electronegative than carbon. Thus, the electron density along the C–X bond is displaced in the direction of the halogen. Thus, the C–X bond in polar in nature. The carbon atom bears a partial positive charge (δ+) and the halogen atoms bears a partial negative charge (δ–).

You will now study that this bond polarity has important impact on the physical and chemical properties of alkyl halides.

The partially positively charged carbon in haloalkanes can be easily attached by anions and electron rich species which are called nucleophiles. On the other hand, the partially negatively charged halogen atom can be attacked by the cations and electron deficient species.

25.3.2 Physical Properties

3. The melting and boiling points of haloalkanes and haloarenes are higher

than those of their parent hydrocarbons (Table 25.2) This is due to (i) the

greater molecular mass and hence greater magnitude of van der Waals forces

of attraction in halocompounds than in the parent hydrocarbons and (ii) the

existence of intermolecular dipole-dipole interaction, as shown below:

For a given alkyl or aryl group, the boiling points increase from fluoro to

iodo compounds as the size of halogen atom increases. The boiling points of

halomethanes, haloethanes and halobenzene given in the Table 25.2 to show

this variation.

4. All monohalobenzenes are liquids at room temperature. Among

dihalobenzenes, the para isomers have the highest melting points. It is due

to the greater symmetry that causes a better packing of molecule in the para

isomer.

1. Substitution : Substitution reactions are those in which an atom or a group of

atoms from the reactant molecule is displaced by another atom or a group of

atoms. For example, on treating chloroethane with sodium hydroxide, the chlorine

atom of chloroethane is substituted by the hydroxyl group and ethanol is formed

as the reaction product.

In this reaction, it is to be noted that the hydroxide ion (nucleophile) displaces the

chlorine atom from C2H5C1 as chloride ion (another nucleophile). Such reactions

which are initiated by the attack of a nucleophile are known as nucleophilic

reactions. In haloalkanes, the carbon atom carrying the halogen atom is electron

deficient due to – I effect of halogen atom. This electron deficient carbon atom

is susceptible to attack by a nucleophile. Thus, you may conclude that haloalkanes undergo nucleophillic substitution reactions. Following are a few examples of

nucleophilic substitution reactions of haloalkanes.

Also, note that it is a one step process and the transition state involves two

species. The formation of this transition state is the rate determining step in this

mechanism.

Here, the bond making and the bond breaking takes place simultaneously. The

nucleophilie (–OH) attacks from one side of the carbon atom whereas the leaving

group (Cl–) leaves from the opposite direction. Hence, there is an inversion of

configuration at the carbon atom.

Primary alkyl halides undergo substitution by SN2 mechanism. Compounds

However, in case of tertiary alkyl halides, the substitution takes place by an

alternative mechanism, i.e. substitution nucleophilic, unimolecular or SN1

mechanism. For example, in the hydrolysis of 2-bromo-2-methylpropane, any

one molecule participates in the rate determining step which is the dissociation

of the alkyl halide to alkyl cation and bromide ion.

This alkyl cation is a carbocation and its formation is a slow and rate determining step.

After this, as soon as this carbocation is formed, the nucleophile, which is water (solvent) molecule, attacks on it which is a fast step.

A carbocation is classified as primary, secondary or tertiary depending upon whether the positively charged carbon atom is linked to one, two or three carbon atoms, respectively.

Here, as the number of alkyl groups attached to the positively charged carbon atoms increases, the stability of the carbocation also increases. This is because alkyl groups are electron releasing in nature and help in the stabilization of the positive charge on the carbon atom of the carbonation.

Thus, a tertiary carbocation is more stable than a secondary carbocation which, in turn, is more stable than a primary carbonation.

The above order of stability of carbocations is also explained on the basis of hyperconjugation. Hyperconjugation results from the overlap of a p orbital with a neighbouring bonding molecular orbital. In a carbocation, the p orbital on the carbon carrying positive charge is vacant.

This vacant p orbital can overlap with the neighboring orbital of C–H bond and

stabilize the charge. The more the number of such neighboring orbitals, the move

will be the stabilization.

If we see the extent of hyper conjugation possible in primary, secondary and

tertiory carbocation we can observe that in a primary carbocation cation 3 C–

H bonds are available for hyperconjugation and in secondary carbocation 6 C–

H bonds are available for hyperconjugation. Similarly, in a tertiary carbocation,

9 C–H bonds are available for hyperconjugation.

Hence, a tertiary carbocation is more stable than a secondary carbocation which

is, in turn, more stable than a primary carbocation.

Thus, this also explains why tertiary halides undergo nucleophilic substitution

reactions by SN1 mechanism.

Haloalkanes are highly reactive compounds due to the presence of a polar carbonhalogen

bond in their molecules. The bond energy values of C−X bonds in

haloalkanes and haloarenes are given in Table 25.3.

These bond energy values show that C-I bond is the weakest bond and C-F bond

is the strongest bond. Therefore, the order of reactivity of haloalkanes is iodoalkane

>bromoalkane > chloroalkane> fluoroalkane.

Comparing the haloalkanes and haloarenes, haloalkanes are found to more reactive

than haloarenes in reactions involving the breaking of C−X bond (X = F,C1,Br,

or I). It is due to the existernce of the phenomenon of resonance which cause

carbon–halogen bond to acquire double bond character in haloarenes. The

resonating structures of chlorobenzene are shown below :

2. Elimination reactions : When haloalkanes are heated with aqueous solution

of potassium or sodium hydroxide, the major product formed is the alcohol,

produced by nucleophilic displacement of the halogen atom by HO–.

In this reaction, the OH− ion acts and a base removes a proton from the molecule.

If the structure of alkyl halide is such that it can undergo elimination in two

different ways, then the more highly substituted alkene (i.e. having lesser number

of hydrogen atoms on the doubly bonded carbon atoms) is the major product of

elimination. This is known as Saytzeff's rule. For example, elimination reaction

When haloarenes are treated with sodium, diaryls are produced. This reaction is

called Fittig reaction.

INTEXT QUESTIONS 25.3

1. Although haloalkanes are polar in nature, they are immiscible in water. Explain.

2. Which one of the following isomers has the higher boiling point and why?

(i) o-dichlorobenzene (ii) p-dichlorobenzene

3. What will be the products of nitration of chlorobenzene?

4. What products will obtained when ethylbromide reacts with

(i) aq. KOH and (ii) alc. KOH

5. What is the major product of elimination reactions of 2- bromobutane?

25.4 SOME USEFUL POLY HALOGEN COMPOUNDS Compounds

A large number of polyhalogen aliphatic and aromatic halogen compounds are

known. These are extensively used as solvents, pesticides, anaesthetics etc. Some

of the important compounds are chloroform (CHCl3), iodoform (CHI3), carbon

tetrachloride (CCl4), benzene hexachloride (BHC), DDT, etc.

Let us now study some of these compounds.

25.4.1 Chloroform

Chlorofom is a derivative of the simplest hydrocarbon, methane. Its IUPAC name

is trichloromethane.

1. From Ethanol

Chlorofom is prepared in the laboratory by treating ethanol or propanone with

chlorine gas in the presence of an alkali :

Chlorofom is a colourless sweet smelling liquid (b.p. 334K). It is slowly oxidized by air in the presence of light to a poisonous gas, phosgene. Chemically phosgene is carbonyl chloride, (COCl2 ). Therefore, chloroform is stored in dark coloured bottles to protect it from light. The bottle are completely filled so that the air is kept out. A small amount of ethanol is added to chloroform to convert toxic phosgene, if formed, into a nontoxic compound, ethyl carbonate.

Chloroform is used in isocyanide test for the detection of primary amines. In this

test, a mixture of amine and chloroform is heated with alcoholic NaOH. A foul

smelling isocyanide is obtained. This test is also known as carbylamine test. It

can be used to test aliphatic and aromatic primary amines.

25.4.2 Iodoform

Iodoform is a pale-yellow solid with a distinct smell. Its IUPAC name is triiodomethane.

Preparation

Iodoform is prepared by heating ethanol or acetone with iodine in the presence of alkali.

25.4.3 Dichlorodiphenyltrichloroethane (DDT)

It is available in several different forms : powder, aerosols, granules, etc.

Uses : It is used mainly to control mosquito-borne malaria. It is also used as an

argicultural insecticide. The use of DDT has been banned in many countries because

being non-biodegradable, it accumulates in environment. It is toxic to other living

organisms such as: mammals, birds, fishes, etc.

INTEXT QUESTIONS 25.4

1. Write IUPAC names of chloroform and iodoform.

2. Why is chloroform stored in dark coloured bottles?

3. What type of of compounds will give a positive iodoform test?

4. Name two commonly used polyhalogen compounds.

WHAT YOU HAVE LEARNT

􀁺 Haloalkanes and haloarenes are important organic compounds having wide

industrial and household applications.

􀁺 Various rules for IUPAC naming of haloalkanes and haloarenes.

􀁺 Methods of preparation and chemical properties of haloalkanes and

haloarenes which are summarized below :

􀁺 Due to the polar nature, halo compounds have higher melting and boiling

points than the corresponding hydrocarbons.

􀁺 Chemically, fluoro compounds are comparatively least reactive and iodo

compounds are the most reactive. Also, haloalkanes are more reactive than

haloarenes in reactions involving cleavage of C–X bond.

􀁺 Haloalkanes undergo nucleophilic substitution reactions. But in haloarenes,

the substitution in the benzene ring is an electrophilic substitution reaction.

􀁺 Grignard reagents are produced by the reaction of a haloarene or haloalkane

with magnesium metal.

􀁺 Chloroform and idoform are useful trihalo derivatives of methane.

Chloroform is prepared in the laboratory from ethanol or propanone by

reacting with chlorine in presence of alkali.

􀁺 Iodoform test is given by compounds containing either

|

3 CH –CO - Oor

|

3 CH -CH-OH group.

TERMINAL EXERCISE

1. Give IUPAC names of the following compounds:

(i) sec-butyl chloride

(ii) iso- propyl bromide

2. Name the product obtained by treating 2-propanol with hydrogen chloride in presence of anhydrous zinc chloride. Also write reaction involved.

3. Alkyl halides are more reactive towards nucleophilic reagents than aryl halides. Discuss briefly.

4. Write chemical equations for the reactions of :

(i) n-propanol with PCl5 .

(ii) chlorine gas with benzene in presence of FeCl3 as catalyst.

(iii) bromoethane with aqueous KOH solution. Compounds

(iv) nitrous acid with aniline at 278 K.

(v) chlorobenzene with magnesium.

(vi) chlorobenzene with a mixture of conc. HNO3 and H2SO4 .

5. Give reason for the following :

(i) Haloalkanes undergo nucleophilic substitution reactions.

(ii) Haloarenes undergo electrophilic substitution reactions.

6. What is a Grignard reagent ? How is it prepared?

7. Disucss briefly the following :

(i) Iodoform test

(ii) Carbylamine test

(iii) Diazotization

(iv) Relative reactivities of chloroethane and bromoethane

8. How is chloroform prepared in the laboratory? 

Write the reaction for its preparation from ethanal.