Haloalkanes and Haloarenes Notes and Definition

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Haloalkanes and Haloarenes Notes and Definition
Haloalkanes and Haloarenes Notes and DefinitionHaloalkanes and Haloarenes Notes and Definition

Haloalkanes and Haloarenes Notes

Halogens, being among the most electronegative elements, have grate tendency to form various derivatives of organic compounds. These derivatives are of grate importance in terms of day to day applications, synthesis of other useful compounds, chemical research, etc. They can be synthesised in laboratories and also found in nature.

Halogen Derivatives

On substituting one or more than one hydrogen atoms of hydrocarbon by halogen ( X = Xl, Br, I ) the compound is known as halogen derivatives.

Classification– On the basis of nature if hydrocarbon they are classified into haloalkane (alkyl halide like CH3X, C2H5X) and haloarene (like C6H5X). Alkyl halides show chain, position and optical isomerism.

Alkyl halide can be mono, di or tri etc. Haloalkanes depends on number of halide groups, present in the molecule. Monohalo compounds are further classified on the basis of hybridisation of carbon atom, to which halogen atoms is attached. It can be sp3, sp2 or sp and are known as primarym, secondary and tertiary respectively.

Method of Preparation of Haloalkanes

  • From Alcohols

    Haloalkanes and Haloarenes Notes and Definition
    Method of Preparation of Haloalkanes

In the preparation of alkyl halides from alcohols, the best method is the reaction of alcohols with SOCl2 as it yields gaseous products leaving behind pure haloalkanes.

  • From Alkenes 

(i) From Halogen acids

{RCH} = {CH}_{2} + {HX} \longrightarrow \underset {Markownikoff's  Rule}{{R}—{CHX}— {CH}_{3} }

{RCH}={CH}_{2} + {HBr} \overset {Peroxide}{\longrightarrow} \underset { Anti-Markownikoff's  Rule }{ { R }—{ CH }_{ 2 }—{ CH }_{ 2 }—Br }

(ii) Allylic Halogenation

{CH}_{3}—{CH}={CH}_{2} + {X}_{2} \overset {773K}{\longrightarrow}\underset { \begin{matrix} | \\ X \end{matrix} }{ { CH }_{ 2 } } —{CH} = {CH}_{2} + {HX}

  • Fluoroalkanes are prepared by  treating alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg2F2, CoF2. This reaction is known as Swarts reaction.

\underset { Methyl   bromide }{ { CH }_{ 3 }Br } +\underset { Silver   fluoride }{ AgF } \longrightarrow \underset { Fluoromethane }{ { CH }_{ 3 }F } +\underset { Silver   bromide }{ AgBr }

Method of Preparation of Haloarenes

Haloalkanes and Haloarenes Notes and Definition
Method of Preparation of Haloarenes

Physical Properties 

  • For a given halogen atom, the boiling points of halides increases with increases in the size of alkyl group.
  • Alkyl halides are slightly soluble in water because they do not form H-bond with water.
  • The stability of haloalkanes decreases as the strength of C—X bond decreases, (RF > RBr > RCl > RI) due to increase in size of halogen atom down the group. Alternatively, bond dissociation enthalpy for C—X bond decreases down the group.
  • C—X bond in aryl halides is shorter (due to partial double bond character acquired by    C—X bond due to resonance), stronger and less polar than in alkyl halides.
  • Amongst isomeric dichlorobenzenes, p-isomer has highest melting point due to symmetry. For isomeric alkyl halides, boiling points decrease with branching.

Chemical Properties of Haloarenes

As compared to haloalkanes, haloarenes are less reactive towards nucleophilic substitution reactions, due to resonance. C—X single bond attains partial double bond characteristic in haloarenes, X bonded to sp2-hybridised carbon. which is more electonegative. Thus, it is difficult to break C—X bond in haloarenes. Haloarenes generally give electophilic reaction and halogen atoms are ortho and para-directing.

Polyhalogen Compounds

Those carbon compounds in the molecules of which more than one halogen atoms are present are known as polyhalogen compounds.

Dichloromethane

This is a good solvent and used for removing paints. It is used as rocket propellent in aerosols. It is used as solvent in food and medicine industries.

Trichloromethane

This is a medicinal solvent which is used for dissolving fat, iodine, etc. To prevent its oxidation it is strored in dark in coloured bottles, otherwise in presences of sunlight it gets oxidised to produce toxic phosgene (COCl2).

Iodoform

It is used as antiseptic, this is due to free I2 present in it. It is yellow coloured solid compound.

Iodoform reaction

Reaction in which ethanol or propanone is made to react with base to obtain yellow coloured crystals of Iodoform is known as Iodoform reaction.

Freon

Chlorofluoro  derivative of methane and ethane are known as freon. Freon is a neutral, unreactive, easily liquefiable gas. Freon-12 is used in refrigerator air conditioners as refrigerant form and in aerosol.

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