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Halogenoalkanes are compounds in which one

or more hydrogen atoms in an alkane have

been replaced by halogen atoms (fluorine,

chlorine, bromine or iodine).

The different kinds of halogenoalkanes

Halogenoalkanes fall into different classes depending on how the halogen atom is positioned on the chain of carbon atoms. There are some chemical differences between the various types.

Primary

Secondary

Tertiary

Reactions of the halogenoalkanes

Nucleophilic substitution

Elimenation reaction

Nucleophilic Substitution In a substitution reaction, one atom or group of atoms,

takes the place of another in a molecule.

A nucleophile is a molecule or ion that has a high

electron density. It is attracted to atoms in molecules

with a lower electron density.

The general form for the reaction is

Nuc: + R-LG → R-Nuc + LG:

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1.With aqueous alkali

When an aqueous solution of NaOH or KOH added to haloalkane alcohol is produced.

propan-2-ol

2.With Cyanide ion

The halogenoalkane is boiled under reflux with potassium cyanide dissolved in ethanol.

Notice :

Reaction added an extra carbon atom to the chain.

+ CN-(aq)

propanenitrile

CH3CH2CN + Br-CH3CH2Br

3.With alcoholic ammonia (NH3 )

The nitrogen atom carries a partial negative charge and has alone

pair of electrons. This makes it nucleophile.

concentrated ammonia (in excess) in ethanol solution.heat in a sealed tube.

Reaction :CH3CH2Br + NH3 ---------> CH3CH2 NH2 + HBr

Ethylamine (amine)

The amine formed is also a nucleophile.

It will attack any unreacted bromoethane in the mixture.

CH3CH2Br + CH3CH2 NH2 (CH3CH2) 2NH + HBr

diethylamine

(a secondary amine)

The secondary amine formed can also act as a nucleophile

CH3CH2Br + (CH3CH2) 2NH (CH3CH2) 3N + HBr

triethylamine

( a tertiary amine)

If any unreacted bromoethyane remain , the tertiary amine attack it.

CH3CH2Br + (CH3CH2) 3N (CH3CH2)4N+Br -

tetraethylammonium bromide

Elimination reaction

Halogenoalkanes also undergo elimination reactions in

the presence of sodium or potassium hydroxide which

dissolved in ethanol.

The mechanism

In elimination reactions, the hydroxide ion acts as a base - removing a hydrogen as a hydrogen ion from the carbon atom next door to the one holding the bromine.

The resulting re-arrangement of the electrons expels the bromine as a bromide ion and produces propene.

The role of the hydroxide ion in a substitution reaction

In the substitution reaction between a halogenoalkane and OH- ions, the hydroxide ions are acting as nucleophiles. For example, one of the lone pairs on the oxygen can attack the slightly positive carbon. This leads on to the loss of the bromine as a bromide ion, and the -OH group becoming attached in its place.

The role of the hydroxide ion in an elimination reaction

Hydroxide ions have a very strong tendency to combine with hydrogen ions to make water - in other words, the OH- ion is a very strong base. In an elimination reaction, the hydroxide ion hits one of the hydrogen atoms in the CH3 group and pulls it off. This leads to a cascade of electron pair movements resulting in the formation of a carbon-carbon double bond, and the loss of the bromine as Br-.

Thank You For Watching

Bibliography -http://www.yteach.com/page.php/resources/view_all?id=hydrocarbon_haloalkanes_nomenclature_isomeris

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