10. HALOGEN DERIVATIVES - part 07 - Reaction with active metals

10. HALOGEN DERIVATIVES - part 07 - Reaction with active metals

Reaction with active metals :
  • Active metals like sodium, magnesium,cadmium readily combine with alkyl chlorides, bromides and iodides to form compounds containing carbon-metal bonds. These are known as organometallic compounds.

a. Reaction with magnesium : 
  • When alkyl halide is treated with magnesium in dry ether as solvent, it gives alkyl magnesium halide. It is known as Grignard reagent.
  • Grignard reagents are very reactive compounds. 
  • They react with water or compounds containing hydrogen attached to electronegative element.
b. Wurtz reaction : 
  • Alkyl halides react with metallic sodium in dry ether as solvent, and form higher alkanescontaining double the number of carbon atoms present in alkyl halide. This reaction is called Wurtz reaction. 
  • When a mixture of two different alkyl halides is used, all the three possible alkanes are formed.
Do you know ?

  • Carbon-magnesium bond in Grignard reagent is a polar covalant bond. 
  • The carbon pulls electrons from the electropositive magnesium. Hence carbon in Grignard reagent has negative polarity and acts as a nucleophite.
  • Victor Grignard received Nobel Prize in 1912 for synthesis and study of organomagnesium compounds. 
  • Grignard reagent is a very versatile reagentused by organic chemist. 
  • Vinyl and aryl halides also form Grignard reagent.
1 Reaction of haloarenes :
a. Reactions of haloarene with metals :
  • The reaction of aryl halide with alkyl halide and sodium metal in dry ether to give substituted aromatic compounds is known as Wurtz- Fittig reaction. 
  • This reaction is an extension of Wurtz reaction and was carried out by Fittig. This reaction allows alkylation of aryl halides.
  • In case only aryl halide takes part in the reaction, the product is biphenyl and the reaction is known as Fittig reaction.
b. Nucleophilic substitution SN of haloarenes:
  • Aryl halides show low reactivity towards nucleophilic substitution reactions.
  • The low reactivity of aryl halides is due to :L
  1. Resonance effect and
  2. sp2 hybrid state of C .
  • One of the lone pairs of electrons on halogen atom is in conjugation with π -electrons of the ring. 
  • For example the following different resonance structures can be written for chlorobenzene.
  • Resonance structures II, III and IV show double bond character to carbon-chlorine bond.
  • Thus carbon-chlorine bond in chlorobenzene is stronger and shorter than chloroalkane molecule, C-Cl bond length in chlorobenzene is 169 pm as compared to C-Cl bond length in alkyl chloride 178 pm. Hence it is difficult to break. 
  • Phenyl cation produced due to selfionization of haloarene will not be stabilised by resonance, which rules out possibility of SN1 mechanism. 
  • Back side attack of nucleophile is blocked by the aromatic ring, which rules out SN2mechanism.
  • Thus nuclophilic substitution reaction involving cleavage of C-X bond in haloareneproceeds with difficulty. 
  • However, the presence of certain groups at certain positions of the ring, markedly activate the halogen of aryl halides towards substitutuion. 
  • For example, presence of electron withdrawing group at ortho and/or para postion greatly increases the reactivity of haloarenes towards subsitution of halogen atom. 
  • Greater the number of electron withdrawing groups at o/p position, greater is the reactivity. 
  • Electron withdrawing group at meta positionhas practically no effect on reactivity.
More Information :
  • Conversion of chlorobenzene to phenol by aqueous sodium hydroxide requires high temperture of about 623K and high pressure.
Do you know ?

  • Occurrence of nucleophilic substitution in p-nitrochlorobenzene can be explained on the basis of resonance stabilization of the intermediate.
  • The resonance structure (III) shows that the electron withdrawing nitro group (-NO2) in the p-position extends the conjugation. 
  • As a result, the intermediate carbanion is better stabilized which favours nucleophilic substitution reaction.
c. Electrophilic substitution (SE) in arylhalides :
  • Aryl halides undergo electrophilic substitution reaction slowly as compared to benzene.
  • In resonance structures of chlorobenzeneelelctron density is relatively more at ortho and para position.
  • Therefore incoming electrophilic group is more likely to attack at these positions. 
  • But due to steric hinderance at ortho position, para product usually predominates. 
  • In haloarenes, halogen atom has strong electron withdrawing inductive effect (-I). This deactivates the ring and electrophilic substitution reaction occurs slowly.

Remember...
  • The -I effect of Cl is more powerful than its +R effect.Therefore Cl is o-/p- directing but ringdeactivating group.
i. Halogenation : 
  • It is carried out by reacting haloarene with halogen in presence of ferric salt as Lewis acid catalyst.
ii. Nitration :
  • It is carried out by heating haloarene with conc. HNO3 in presence of conc. H2SO4.
iii. Sulfonation : 
  • It is carried out by heating haloarene with fuming H2SO4.
iv. Friedel Craft’s reaction : 
  • It is carried out by treating haloarene with alkyl chloride or acyl chloride in presence of anhydrous AlCl3 as a catalyst.
 

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