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.
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
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:
(1) R:Xl → R+ + X-
(2) R+ + Nu- l →RNu
R:X + Nu- l → RNu + X-
TS of slow step
=K [RX] (1st order)
=K[RX] [:Nu-] (2nd order)
Inversion and racemization
Inversion (backside attack)
structure of R
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-
Lewis acid, eg. Ag+, AlCl3, ZnCl2
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
Substitution Versus Elimination:
- RCH2X - 1o
- R2CHX - 2o
- R3CX - 3o
Bimolecular reactions only
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.
- 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: 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: CHCl3 + 1/2 O2 → HCl + COCl2 (phosgene)
- Hydrolysis: CHCl3 + 4NaOH → HCOONa + 3NaCl + 2H2O
- Carbyl amine reactions: CHCl3 + CH3NH2 + 3NaOH →CH3N≡C +3NaCl +3H2O