Halogen Derivatives of Organic 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.
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:
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:
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 X2and 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.
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 +3H
5. Hydroboration-Oxidation:
6. Halogen Addition in Non-polar Solvent:
7. Halogen Addition in Aqueous Medium:
8. Syn – Hydroxylation: Formation of di-oles.
9. Ozonolysis of Alkenes:
Alkyne
• Saturated open chain hydrocarbon with general formula (CnH2n-2).
• At least one -c≡c- (triple bond) group i.e. sp hybridisation, is present throughout the chain.
• Physical properties of alkynes are similar to those of the corresponding alkenes
Preparation
1. Dehydrohalogenation of vic-Dihalides or gem-Dihalides
2. Dehalogenation of vic-Tetrahalogen Compounds
3. Alkyl Substitution in Acetylene; Acidity of º C-H
4. From Calcium Carbide:
CaC2 +2H2O → Ca(OH)2+ C2H2
5. Kolbe’s Electrolysis:
Chemical Properties
1. Hydrogenation: RC ≡ CCH2CH3 + 2H2 → CH3CH2CH2CH2CH3
2. Hydro-halogenation:
Markovnikov addition: RC≡CH +HBr → RCBr=CH2 +HBr→ RCBr2-CH3
Anti-markovnikov addition: RC≡CH +HBr +peroxide → RCH=CHBr
Aromatic Hydrocarbons:
For being aromatic a hydrocarbon should
• be a cyclic compounds.
• have planarity in geometry.
• have complete delocalization of electrons over ring.
• follow Huckel Rule i.e. number of ?? electrons in ring = (4n+2).
Benzene (C6H6)
1. Structure:
2. Chemical Reactions of Benzene:
Anti-aromatic Hydrocarbons:
Highly unstable compounds.
Number of π electrons in ring = 4n.
Example:
