Alkyl and Aryl halides

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Alkyl and Aryl halides1
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Session objectives

1. Classification
2. Nature of C X bond
3. Preparation of alkyl halides
4. Physical and chemical properties of alkyl halides
5. Substitution reaction SN1 and SN2 reactions

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Elimination reaction

• E1
• E2
• E1 c B
• Ei

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E1
Mechanism
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Elimination reaction
3 gt 2 gt 1 gt CH3
Rate kRX1
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E2
Rate kRX Base kCH3CH2Cl
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E1 cB
carbanion stabilization by the participation of
R (conjugation), I effect etc.
Mechanism
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Ei
Observed for pyrolytic elimination for xanthates
(Tschugaevs reaction), esters, oxides of amines
(Copes reaction) which produces alkene.
It occurs intramolecularly through the formation
of a transition state, when they are heated.
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Comparison between E1 and E2
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Comparison between E1 and E2
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Polyhalogen compounds
Dichloromethane (CH2Cl2)
Colourless sweet smelling volatile liquid. It has
low boiling point (313 k) and inflammable used
as a solvent in extraction in food and
pharmaceutical industries.
Vicinal dihalide
When two halogen atoms are attached to adjacent
carbon atoms, the compound is called vicinal
dihalide.
e.g.,
1, 2-dichloroethane
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Polyhalogen compounds
Geminal dihalide
When two halogen atoms are attached to the same
carbon atom, the compound is called geminal
dihalide
e.g. H3C CHCl2 1,1-dichloroethane
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Polyhalogen compounds
Chloroform (CHCl3)
Methods of preparation
(1) By chlorination of methane
(2) By reduction of carbon tetra chloride
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Polyhalogen compounds
(3) From ethyl alcohol
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Polyhalogen compounds
(4) From acetone
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Properties
(1)Sweet smelling liquid, boiling point
61 C. (2) Oxidation

• Stored in dark coloured bottled because on
  oxidation chloroform gives phosgene gas which is
  poisonous

• 1 C2H5OH is added while storing chloroform

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Properties
(3) Reduction
(4) Chlorination
(5) Hydrolysis
(6) Nitration
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Properties
(7) Reaction with acetone
(8) Dehalogenation
(9) Carbylamine reaction
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Iodoform (CHI3)
Preparation
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properties

1. Yellow solid melting point 119 C.
2. Chemical properties of iodoform are same as that
   of chloroform.
3. Chlorine and bromine can replace iodine in
   iodoform reaction to give CHCl3 and CHBr3
   respectively. The reaction is
   called haloform reaction.

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Carbon tetrachloride
Preparation
Properties
(1) Colourless liquid, boiling point 350 K.
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Uses
(1) Used as fire extinguisher. Trade name is
pyrene. At 773 K it is hydrolysed by water
vapours into phosgene gas. Therefore, the
room should be well ventilated after using CCl4
as fire extinguisher. (2) Industrial solvent for
oils, fats, resins etc.
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