Revision Notes on Organic Compounds Containing Halogens

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 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-
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 

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 X₂ and an alkali or Na₂CO₃.

• Laboratory Preparation of CHCl₃:
  
  

• Physical properties of CHCl_3: 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 +3H₂O