HALOGEN CONTAINING COMPOUNDS
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:
Grove’s process: Replacement of “OH” group in primary and secondary alcohols with an “X’ atom in presence of Zinc chloride.
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 + SOCl2 → HCl + ROSOCl
HCl+C5H5N →C5H5NH++Cl-
ROSOCl + Cl– →RCl + SO2 (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 + Cl2 +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-
Rate
=K [RX] (1st order)
=K[RX] [:Nu-] (2nd order)
TS of slow step
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)2 →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 = CH2 + KBr + H2O
* Reaction with alcoholic AgNO3: R-X +AgNO3 → R+ + AgX↓+HNO3
