HALOALKANES AND HALOARENES

(PART 1)

By: Manpreet Sharma

Guidelines for Units 10-13 of syllabus.These units include questions on: Nomenclature : 2 marks Reasoning : 6 marks Distinguishing between compounds : 2 marks Name reactions : 2 marks Reaction Mechanism : 2 marks Word problems (conversions) coveringProperties and reactions of functional groups : 5 marks

• The monohalogen derivatives of alkanes are called alkyl halides. Their names are derived by naming the attached alkyl group to halogen and adding the name of the halide. For example

IUPAC Naming :

• Here, the monohalogen derivatives of alkanes are named as haloalkanes. The position of the halogen atom and other substituents (alkyl groups) are indicated by suitable numbers i.e., 1,2,3.....etc.

Common naming : Alkyl halide

Haloarenes or aryl halides are named by adding the prefix 'halo' (fluoro, chloro, bromo, iodo) before the name of the aromatic hydrocarbon. In case of di-substituted compounds, the relative positions of the substituents 1,2; 1,3 and 1,4 are indicated by the prefixes ortho (o-), meta (m-) and para (p-) respectively

Haloarenes

Nature of C-X Bond

Most of the reactions of halides involve the cleavage of C-X bond. Larger the bond dissociation energy of the C-X bond, smaller is the reactivity. The bond dissociation energy decreases with the increase in the size of halogen atom, i.e.,

C-Cl > C-Br > C-I.

Hence, the reactivity increases from alkyl chlorides to bromides and from bromides to iodides. In the halo derivatives, the halogen atom is attached to the carbon atom. As the halogen atom is more electronegative than carbon, the bond between carbon and halogen is polar in character .

Methods of preparation of haloalkanesFrom hydrocarbons: 

Haloalkanes can be prepared from alkanes and alkenes when the halogen atom replaces the hydrogen atom.

When pure components are needed, they can be obtained by fractional distillation.

From alkenes

Markownikoff's rule

'the negative part of the attacking reagent attaches itself to the carbon atom carrying lesser number of hydrogen atoms while the positive part goes to the carbon atom with more number of hydrogen atoms'

Anti Markownikoff's rule

It has been observed that when HBr is added to an unsymmetrical double bond in the presence of organic peroxides such as benzoyl peroxide (C6H5CO-O-O-COC6H5), the reaction takes place against Markownikoff's rule.

Anti Markownikoff's rule or peroxide effect applies to the addition of HBr only and not to the addition of HI or HCl.

From alcohols1.BY THE ACTION OF HALOGEN ACIDS

GROOVE'S PROCESS

What is function of catalyst in above reaction?

TERTIARY ALCOHOLS, ARE VERY REACTIVE AND THEREFORE, THEY REACT READILY WITH CONCENTRATED HCL EVEN IN THE ABSENCE OF THE CHLORIDE.

2.By the action of phosphorus halides

Bromoalkanes and iodoalkanes are prepared by the action of PBr3 and PI3respectively on alcohols. As PBr3 and PI3, are not very stable compounds, they are prepared in situ by the action of red phosphorus on Br2, or I2. 

3.with thionyl chloride:

This method is preferred than other methods because both the products of the reaction (SO2 and HCl) are gases and can easily escape leaving behind pure alkyl halide.

By halide exchangeFinkelstein reaction.

SWARTS REACTION

From silver salts of acidsBorodine Hundsdiecker reaction

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

Preparative methods of haloarenes

1)Direct halogenation of aromatic ring

Halogenation of arenes is an electrophilic substitution reaction. The function of the halogen carrier is to generate 'electrophile' which attacks the benzene ring to form the product

Reversible process

*Bromo compounds can be prepared in a similar way by reacting with Br2 in the presence of FeBr3.

*The reaction with fluorine is violent and cannot be controlled. therefore, fluoroarene cannot be prepared by direct fluorination of aromatic hydrocarbon.

2.SANDMEYER'S REACTION

'GATTERMANN REACTION'

4.Balz-Schiemann reaction

fluoroboric acid

Physical properties of haloalkanes & haloarenes

SolubilityHalo alkanes are not able to form hydrogen bonds with water and, even though they are polar in nature, they are practically insoluble in water. However, they are soluble in organic solvents like alcohol, ether, benzene, etc.

BOILING POINTS

The boiling points of alkyl chlorides, bromides and iodides follow the order RI > RBr > RCl where R is an alkyl group. With the increase in the size of halogen, the magnitude of Van der Waals forces increases and,, the boiling points increase. Also, for the same halogen atom, the boiling points of haloalkanes increase with increase in the size of alkyl groups.

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