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Haloalkanes and Haloarenes

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Halogen Derivatives

Hydrocarbons are the parent of all organic compounds. When one or more hydrogen atoms of a hydrocarbon are replaced by halogen atoms, the compound becomes a halogen derivative.

Halogen : Bromine, chlorine, fluorine and iodine are the members of halogen family. Halogens are usually represented by the letter X.

Hydrocarbon and Haloalkane

Ethane : Hydrocarbon
Chloroethane : haloalkane

In halogen derivative compounds, the halogen atom acts as a functional group. Halogen derivatives are classified as bromo, chloro, fluoro or iodo according to the type of halogen present.

Haloalkanes

The halogen derivatives of alkanes are called haloalkanes.

Haloalkanes

Chloroethane

Chloroethane

2-Iodopropane

2-Iodopropane

1-Bromo-1-iodoethane

1-Bromo-1-iodoethane

Haloarenes

In haloarenes, the halogen atom is directly attached to an aromatic ring.

Haloarenes

Chlorobenzene

Chlorobenzene

1,4-Dibromobenzene

1,4-Dibromobenzene

2-Chlorotoluene

2-Chlorotoluene

Halogen containing compounds are very useful in medical cares. For example, chloromycetin (a chlorine containing antibiotic) is used for the treatment of typhoid fever. Halothane (an anaesthetic agent) is used during surgery.

For more theory, please refer to Wikipedia.

Nature of the C-X Bond

The C-X bond (carbon-halogen bond) is polar because halogens are more electronegative than carbon atoms.

The strength of carbon-halogen bond decreases with the size of halogen atom due to increase in bond length and decrease in bond dissociation energy.

Strength of C−X bond → C−F > C−Cl > C−Br > C−I.

Nomenclature of Haloalkanes and Haloarenes.

Refer to nomenclature of haloalkanes and haloarenes. If you want to learn nomenclature from the very basics, refer to nomenclature introduction.

Questions

Question : Which of the following order regarding bond enthalpy ε(C−X) in an alkyl halide (RX) is correct?

  1. ε(C−Cl) > ε(C−Br) > ε(C−I)
  2. ε(C−Cl) < ε(C−Br) > ε(C−I)

Answer : Bond enthalpy C−X (where X is Cl, Br, I) decreases with increase in atomic number of X. Hence, the correct order is : ε(C−Cl) > ε(C−Br) > ε(C−I).

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