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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Halogen Derivatives of Alkanes Page 2

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SN2 Reaction


Bimolecular nucleophilic substitution reaction.
The order of reactivity of alkyl halide towards SN1 reaction is tertiary > secondary > primary.
Example:
CH3
- Br + CH3 - OH + KBr

  1. Kinetic Expression:
    Rate = k[CH3Br] [OH
    -]
  2. Mechanism:
    (i) Approach of the nucleophile from the opposite side of the C
    -Br bond.
    (ii) Transition state (activated complex)

 

 

 

SN1 Reaction


Unimolecular nucleophilic substitution reaction.
The order of reactivity of alkyl halide towards SN2 reaction is primary > secondary > tertiary.
Example:
(CH3)3C
- Br + (CH3)3 C - OH + KBr

  1. Kinetic Expression:
    Rate = k [(CH3)3 C
    - Br]
  2. Mechanism:
    (i) Formation of Carbonium Ion:

(ii)Formation of the Product:


In SN1 reaction, racemization takes place.
Plane-Polarised Light
A ray of monochromatic light which contains all the vibrations restricted to a single plane is called plane-polarised light.

Optical Activity
The property of a substance to rotate the plane of plane polarised light to the left or right is called as optical activity.

Optical Isomers or Enantiomers or Optical Antipodes
Two compounds that have the same structural formula but each one has a structure which is a non
-superimposable mirror image of the other are called optical isomers. One isomer rotates the plane of plane-polarised light to the left (l-isomer), while the other to the right (d-isomer).

The number of optical isomers can be calculated by the formula:

a =

2n

where, a =

optical isomers

n =

no. of asymmetric carbon atoms.


d-Isomer or Dextrorotatory Compound
The optical isomer which rotates the plane of plane
-polarised light to the right hand side (clockwise rotation or dextrorotation) is called a d-isomer.

l-Isomer or Laevorotatory Compound:
The optical isomer which rotates the plane of plane
-polarised light to the left hand side (anticlockwise rotation or laevorotation) is called a l-isomer.

Racemic Mixture (dl)
The equimolar mixture of the dextro isomer and laevo isomer of a compound, which is optically inactive due to external compensation, is called a racemic mixture.
Separation of enantiomers is called resolution.

Asymmetric or Chiral Carbon Atom
The carbon atom which is attached to four different substituents is called an asymmetric carbon atom.

 

 

 

 

Dihalogen Derivatives


Dichloroethane, C2H4Cl2:

  1. Preparation of Ethylene Dichloride (1, 2 - dichloroethane):
    1. By Addition of Chlorine to Ethane:
      H2C = CH2 + Cl2 Cl
      - CH2 - CH2 - Cl
      It is an electrophilic addition reaction.
    2. From Ethylene Glycol and PCl5:
      HO
      - CH2 - CH2 - OH + 2PCl5 Cl - CH2 - CH2 - Cl + 2POCl3 + 2HCl
      It is a nucleophilic substitution reaction.

 

  1. Preparation of Ethylidene Dichloride (1, 1 - dichloroethane):
    1. From acetaldehyde:
      CH3
      - CHO + PCl5 CH3 - CHCl2 + POCl3
      It is a displacement reaction.
    2. From acetylene:
      HC = CH + HCl CH2 = CHCl
      CH2 = CH
      - Cl + HCl CH3 - CHCl2
      HC = CH + 2HCl CH3
      - CHCl2
      It is an electrophilic addition reaction.

 

 

Trihalogen Derivatives

  1. Chlorofrom (Trichloromethane):
    1. Preparation from Ethanol:

CaOCl2 + H2O Ca(OH)2 + Cl2

 

bleaching
powder

 

    1.  
      1. Oxidation:
        CH3
        - CH2OH + Cl2 CH3-CHO + 2HCl
      2. Chlorination:

CH3 - CHO + 3Cl2

 Cl3C- CHO

+ 3HCl

 

chloral

 

      1. Hydrolysis:
        2Cl3C
        - CHO + Ca(OH)2 2CHCl3 + (HCOO)2 Ca

 

    1. Physical Properties:
      1. Chloroform is a volatile liquid with a b.p. 334 K.
      2. It is colourless and has a sweet odour.
      3. It is immiscible with water and is heavier than water (hence, sinks in water).
      4. On inhalation, it can produce unconsciousness.
      5. It is inflammable.

 

    1. Chemical Properties:

1.      Reduction
(i) Partial reduction:

CHCl3 + 2[H] 

 CH2Cl2

+ HCl

 

methylene
dichloride

 

2.     
(ii) Complete Reduction:
CHCl3 + 6[H] CH4 + 3HCl

3.      Oxidation

2CHCl3 + O2

 2COCl2

+ 2HCl

 

phosgene
(carbonyl
chloride)

 

4.     
Phosgene is a poisonous gas. To prevent the formation of phosgene, a small amount of ethanol (about 1%) is added to chloroform. This stabilises chloroform from undergoing oxidation.

5.      Hydrolysis



(HCOOH + KOH HCOOK + H2O)

CHCl3 + 4KOH HCOOK + 3KCl + 2H2O

6.      Carbylamine Reaction

CHCl3 +

C6H5-NH2+ 3KOH

C6H5NC + 3KCl + 3H2O

 

aniline

 

phenyl
isocyanide

7.     
This reaction is used to detect primary amine and haloform. Carbylamine has an offensive odour.

8.      Nitration (Action of Concentrated HNO3):

 

    1. Uses of Chloroform:

                                                        .            As a solvent for oils, fats, rubber, etc.

                                                        i.            As a general anaesthetic in surgery.

                                                       ii.            For preparation of chloropicrin - an insecticide and also used in teargas.

                                                     iii.            For preparation of chloretone - a sedative drug.

 

  1. Iodoform
    1. Preparation from Ethanol:
      I2 + 2NaOH NaI +NaOI + H2O
      or
      Sodium hypoiodite
      I2 + Na2CO3 NaOI + NaI + CO2

                                                        .            Oxidation:
CH3
- CH2 - OH + NaOI CH3 - CHO + NaI + H2O

                                                        i.            Iodination:

CH3 - CHO + 3NaOI I3C-CHO + 3NaOH

 

triiodoacetaldehyde

                                                       ii.             

                                                     iii.            Hydrolysis:
I3C
- CHO + NaOH CHI3 + HCOONa

 

    1. Physical Properties:

                                                        .            It is a pale yellow crystalline solid.

                                                        i.            It has a melting point, 392K.

                                                       ii.            It is insoluble in water but soluble in chloroform and acetone.

                                                     iii.            It sublimes at ordinary temperature.

                                                    iv.            It decomposes when exposed to air in the presence of light.

 

    1. Uses:

                                                        .            As an antiseptic for dressing of wounds.

                                                        i.            For washing surgical instruments.

 

 

 

 

 

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