Organic Compounds Containing Halogens Class 11 Notes Chemistry

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 S_N2 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    (S_N2)

• 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 →RCH₂CH_2Br

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)

S_N1 and S_N2 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
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)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:

CH₃X

Methyl

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

Bimolecular reactions only

• R₃CX - S_N1/E1 or E₂

Gives SN₂ reactions

• RCH₂X - Gives mainly S_N2 except with a hindered strong base [e.g., (CH₃)3CO^–] and then gives mainly E2.
• R₂CHX - Gives mainly S_N2 with weak bases (e.g., I^–, CN^–, RCO₂^–) and mainly E2 with strong bases (e.g., RO^–)
• R₃CX - No S_N2 reaction. In solvolysis gives S_N1/E1, and at lower temperature S­_N1 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:
  
• 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