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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.
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
C6H5N2Cl + HBF4 →C6H5F (Schiemann Reaction)
C6H5N2Cl + CuCl →C6H5Cl (Sandmeyer Reaction)
C6H5N2Cl + Cu powder →C6H5Cl (Gatterman Reaction)
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
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
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
CH3X
RCH2X
R2CHX
R3CX
Methyl
1°
2°
3°
Bimolecular reactions only
SN1/E1 or E2
Gives SN2 reactions
Gives mainly SN2 except with a hindered strong base [e.g., (CH3)3CO–] and then gives mainly E2.
Gives mainly SN2 with weak bases (e.g., I–, CN–, RCO2–) and mainly E2 with strong bases (e.g., RO–)
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.
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:
Physical properties of CHCl3: colourless liquid with sweet smell and test. It is heavier than water and insoluble in it but soluble in alcohol and ether.
Oxidation: CHCl3 + 1/2 O2 → HCl + COCl2 (phosgene)
Hydrolysis: CHCl3 + 4NaOH → HCOONa + 3NaCl + 2H2O
Carbyl amine reactions: CHCl3 + CH3NH2 + 3NaOH →CH3N≡C +3NaCl +3H2O
ROSOCl + Cl– RCl
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