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JEE Main Mock Test Series 2020 & Previous Year Papers

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Organic Compounds Containing Halogens can be divided into two groups:

  • Alkyl Halides: Aliphatic carbon chain with halogen atom(s) as substitution.. Example: Chlorobutane. 
  • Aryl Halides: Aromatic carbon ring with halogen atom(s) as substitution on ring. Example: Chlorobenzene.

Methods of Preparation of Alkyl Halides:
(a) Grove’s process: Replacement of “OH” group in primary and secondary alcohols with an “X’ atom in presence of Zinc chloride.
 Notes | EduRev
The reaction follows SN2 mechanism when the concentration of zinc chloride is low.
(b) Darzen Process: Reaction of thionyl chloride with straight-chain primary alcohols without  presence or absence of pyridine.
In presence of pyridine:
ROH + SOCl→ HCl + ROSOCl
HCl+C5H5N →C5H5NH++Cl-
ROSOCl + Cl →RCl + SO   (SN2)

  • Action of a phosphorus halide on the alcohol: ROH + PCl5 →  RCl + HCl + POCl3.
  • By addition of Halogen to an alefins: R-CH=CH2 +Br2+CCl4  →R-CH(Br)CH2Br 
  • Photohalogenation: CH4 + Cl+hv → CH3Cl + HCl
  • Displacement of one halogen atom by another:RCl + NaI →RI + NaCl
  • Bonodine – Hünsdiecker Reaction: RCO2Ag + Br2→RBr + CO2 + AgBr
  • Hydrohalogenation of unsaturated hydrocarbons:
    • In absence of peroxide: RCH=CH2 +HBr→RCH(Br)CH3
    • In presence of peroxide: RCH=CH2 +HBr + Peroxide →RCH2CH2Br

Methods of Preparation of aryl halides

  • Halogenation:  Ar-H + X2 +Lewis Base → Ar-x + HX
  • From diazonium salts:
    • C6H5N2Cl + HBF4 →C6H5F               (Schiemann Reaction)
    • C6H5N2Cl + CuCl →C6H5Cl              (Sandmeyer Reaction)
    • C6H5N2Cl + Cu powder →C6H5Cl      (Gatterman Reaction)

SN1 and SN2 mechanism:

 
SN1
SN2
 
Steps
Two :  
(1) R:Xl → R+ + X-
 (2) R+ + Nu- l →RNu
One :
R:X + Nu- l → RNu + X-
 
TS of slow step
 Notes | EduRev
 Notes | EduRev
Rate
            =K [RX] (1st order)
=K[RX] [:Nu-] (2nd order)
Stereochemistry
Inversion and racemization
Inversion (backside attack)
Molecularity
Unimolecular
Bimolecular
Reactivity
structure of R
Determining Factor
Nature of X
Solvent effect on rate
3o> 2o> 1o> CH3
Stability of R+
RI> RBr> RCl> RF
Rate increases in polar solvent
CH3> 1o> 2o> 3o
Steric hindrance in R group
RI> RBr> RCl> RF
with Nu- there is a large rate increase in polar aprotic solvents.
Effect of nucleophile
No effect as it does not appear in the rate expression.
Rate depends on nucleophilicity
I-  > Br-  > Cl- ; RS-  > RO-
Catalysis
Lewis acid, eg. Ag+, AlCl3, ZnCl2
None
Competitive reaction
Elimination, rearrangement
Elimination


Reactions of Alkyl Halides:

  • Hydrolysis: : RX  + OH → ROH  +  X
  • Williamson Synthasis: R-ONa +R'X → R-R' + NaX
  • Reaction with dry silver oxide: 2R-X + Ag2O → R-O-R
  • Reaction with sodio-Alkynides: R-C≡C-Na +X-R→ R-C=C-R +NaX
  • Reaction with potassium-cyanide: KCN+X-R→ RCN +KX
  • Reaction with silver-cyanide: AgCN+X-R→ RNC +AgX
  • Reaction with silver-nitrite: AgNO2+X-R→ RNO2 +AgX
  • Reaction with potassium-nitrite: KNO2+X-R→ R-O-N=O +KX
  • Fridal Craft Reaction: R-X + C6H6 + AlCl3→C6H5-R
  • Malonic Ester Synthasis: R-X + -CH(CO2C2H5)→R-CH(CO2C2H5)2  +HX
  • Acetoacetic Ester Synthasis: R-X + -CH(CO2CH3)2 →R-CH(CO2CH3)2 +HX
  • Reaction with Ammonia: R-X +NH3→ R-NH2 +HX
  • Wurtz Reaction: 2R-I+ 2Na →R—R + 2NaI
  • Dehydrohalogenation: CH3.CH2.CH2Br + alco.KOH → CH3–CH = CH+ KBr + H2O 
  • Reaction with alcoholic AgNO3:  R-X +AgNO3 → R+ + AgX↓+HNO

Substitution Versus Elimination:

CH3X

Methyl

  • RCH2X - 1o
  • R2CHX - 2o
  • R3CX - 3o

Bimolecular reactions only

  • R3CX - SN1/E1 or E2

Gives SN2 reactions

  • RCH2X - Gives mainly SN2 except with a hindered strong base [e.g., (CH3)3CO] and then gives mainly E2.
  • R2CHX - Gives mainly SN2 with weak bases (e.g., I, CN, RCO2) and mainly E2 with strong bases (e.g., RO)
  • R3CX - No SN2 reaction. In solvolysis gives SN1/E1, and at lower temperature S­N1 is favoured. When a strong base (e.g., RO) is used. E2 predominates.

Haloform(Tri halide):

  • Preparation: It can be prepared from any alcohol having –CH(OH)CH3 group or from the aldehydes and ketones formed from above type of alcohols i.e, from a carbonyl compound having three a - hydrogen atoms by the action of X2 and an alkali or Na2CO3.
  • Laboratory Preparation of CHCl3:
     Notes | EduRev
  • Physical properties of CHCl3: colour less liquid with sweet smell and test. It is heavier  than water and insoluble in it but soluble in alcohol and ether.

Chemical Reactions of CHCl3:

  • Oxidation: CHCl+ 1/2 O2 → HCl + COCl2 (phosgene)
  • Hydrolysis: CHCl3 + 4NaOH → HCOONa + 3NaCl + 2H2O
  • Carbyl amine reactions: CHCl3 + CH3NH+ 3NaOH →CH3N≡C +3NaCl +3H2O
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