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Free radical substitution

ORGANIC REACTION MECHANISMS. AS. A2. Addition – Elimination. Bond fission. Electrophilic substitution. Free radical substitution. Nitration. Eletrophilic addition. Bromination. Bromination of alkene. Friedel-Crafts. Hydration of alkene. Acylation. Addition polymerisation.

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Free radical substitution

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  1. ORGANIC REACTION MECHANISMS AS A2 Addition – Elimination Bond fission Electrophilic substitution Free radical substitution Nitration Eletrophilic addition Bromination Bromination of alkene Friedel-Crafts Hydration of alkene Acylation Addition polymerisation Nucleophilic addition Elimination Formation of polypeptides Nucleophilicsubstitution Formation of polyamides Dehydration Formation of polyesters Esterification Alkaline hydrolysis

  2. BOND FISSION Homolyticfission Br Br Br + - Br Br Br Br Br2 2 Br. Heterolytic fission Br2 Br +. + Br - Curly arrow (Breaking of the bond) HOMOLYTIC FISSION One electron moving A pair of electrons moving 2 x Free radicals (= an unpaired electron) Free radical substitution Reaction of ALKANES = HETEROLYTIC FISSION + Electrophile Nucleophile (= electron pair acceptor) (= electron pair donor) Electrophilic addition Reaction of ALKENES =

  3. FREE RADICAL SUBSTITUTION MECHANISM Br Br Br Br Br Br Br2 2 Br 2 Br 2 Br 2 Br H H H H CH4 + Br CH3+ HBr H H H C C H H H H C Br Br H H H C C C H H H Br Br Br C H H C H H H H H H H H H H H H H H H CH3+ Br2 CH3Br + Br H H Br Br Br Br Br Br H H Br Br C C H Br C H H H H C C C C H C H H H H H H H H H 2 Br Br2 H Br Br Br Br Br Br H3C + Br  CH3Br 2 CH3 H3C-CH3 Initiation Initiation Propagation Propagation + + + + Major organic product + Termination Termination Least favourable + Possible + Most favourable

  4. BROMINATION OF ALKENE Br Br H3C C H3C C H3C H Br C + Br H H3C C H3C Br C H H C H3C H H - + Nucleophile :Br - 2-methylpropene Carbocation (Electrophile) 1,2-dibromo-2-methylpropane

  5. H3C H3C C H3C H3C H C H C H C H O H H H3C H3C H3C H3C H H C O C C + C H H H H O+ H H H H HYDRATION OF ALKENE MECHANISM 2-methylpropene 2-methylpropan-1-ol H+ H+

  6. Cl H Cl H Cl H Cl H C = C C C C C C C = = = H H H H H H H H Cl H Cl H Cl H Cl H C C C C C C C C or H H H H H H H H n ADDITION POLYMERISATION MECHANISM Monomer + Chloroethene + Trigonal planar 120o Polychloroethene Repeat unit Repeat unit = Any 2 consecutive C along the C chain Dimer Trimer

  7. R - X + :Nu- R - Nu + :X- H H δ+ δ- C C Cl HO H H H H NUCLEOPHILICSUBSTITUTION MECHANISM δ+δ- + + Cl - OH - ELECTRON CLOUD from the nucleophile SHIFTS toward +C atom, and a DATIVE COVALENT BOND starts to form. As this happens, the C – X bond is WEAKENS and eventually BREAKS HETEROLITICALLY.

  8. ELIMINATION MECHANISM OH - Cl H Cl H H - C C C C H H H H H H H H Cl - C C H H H2O + +

  9. H H O H + H O H C C C C H H H H H H H H H O H H H C C H H H+ DEHYDRATION H+ + +

  10. HO HO H + + C C O H O R R HO H R O + C R O O H O + H C O H R R R H H O C O + HO H R + C O O R R R Nucleophilic attack Protonation + ESTERIFICATION MECHANISM Proton transfer + Water elimination Proton elimination + H2O

  11. H H δ+ δ- C Cl C Cl- O O- HO OH H δ+ δ- C O OH- Cl ADDITION-ELIMINATION MECHANISM (NUCLEOPHILICSUBSTITUTION) δ+δ- δ+ δ- Nucleophilic addition δ+ δ- + Nucleophilic substitution Elimination +

  12. R R C OH- C O HO O- Na+ R O R O R C O R C O R OH R O- Na+ H O Na+ O- ALKALINE HYDROLYSISMECHANISM Nucleophilic attack Break down of the tetrahedral intermediate Proton transfer

  13. HNO3 + H2SO4 H2NO3+ + HSO4- H NO2 H2NO3+ H2 O + NO2+ HNO3 + H2SO4 H2O + NO2+ + HSO4- NO2 H + NO2 H+ + HSO4-H2SO4 + H+ + NITRATION Formation of the electrophile: NO2+ nitronium ion Electrophilic substitution: HSO4- Regeneration of the catalyst: H2SO4 cat. C6H6 + HNO3 C6H5NO2 + H2O Overall equation: 50oC

  14. Br2 + FeBr3 Br+ + FeBr4- H Br Br H + Br + H+ + H+ + FeBr4- FeBr3 +HBr BROMINATION Formation of the electrophile: Br+ Electrophilic substitution: FeBr4- Regeneration of the catalyst: FeCl3 cat. C6H6 + Br2 C6H5Br + HBr Overall equation:

  15. CH3Cl + FeCl3 CH3+ + FeCl4- H CH3 CH3 H + CH3 + H+ + H+ + FeCl4- FeCl3 + HCl FRIEDEL-CRAFT MECHANISM Formation of the electrophile: CH3+ Electrophilic substitution: FeCl4- Regeneration of the catalyst: FeCl3 cat. C6H6 + CH3Cl C6H5CH3 + HCl Overall equation:

  16. CH3COCl + FeCl3 CH3CO+ + FeCl4- H COCH3 COCH3 H + COCH3 + + H+ H+ + FeCl4- FeCl3 + HCl ACYLATION MECHANISM Formation of the electrophile: CH3CO+ Electrophilic substitution: FeCl4- Regeneration of the catalyst: Overall equation: FeCl3 cat. C6H6 + CH3COCl C6H5COCH3 + HCl

  17. H CΞN H C O H H O - C NΞC H - CΞN H NΞC C O H H NUCLEOPHILIC ADDITION MECHANISM  bond weakens and breaks heterolytically  bond weakens and breaks heterolytically δ+δ- + NΞC- δ+ δ- Dative covalent bond formation Dative covalent bond formation oxoanion + 2-hydroxynitrile This reaction is useful because the chain is extended by 1 carbon.

  18. H H O H H O H H O N C C N C C N C C H R OH H R OH` H R OH H H O H O H O N C C N C C N C C H R H R H R OH H O N C C H R n FORMATION OF POLYPEPTIDES + + Monomer 2-amino acid Peptide link (amide) Peptide link (amide) + 2 H2O Trimer Repeat unit (aminoacid residue)

  19. H H H H O O O O (CH2)n N (CH2)n N N N C C C (CH2)n C (CH2)n H H OH OH H H OH OH H O O O O (CH2)n N (CH2)n N N N C C C (CH2)n C (CH2)n H H H H OH Peptide link (amide) Peptide link (amide) O C Repeat unit O O OH (CH2)n N N C C (CH2)n H H DIBASIC ACID and DIAMINE to form POLYAMIDES + + + Repeat unit Repeat unit +3 H2O Peptide link (amide) 2 monomers

  20. O O O O O O HO C C OH HO C C OH H H O O (CH2)n (CH2)n O O (CH2)n H H (CH2)n HO C C (CH2)n O O O (CH2)n (CH2)n (CH2)n O O O H H H O O O O C C C C (CH2)n (CH2)n Repeat unit (2 monomers) Repeat unit (2 monomers) REACTION OF DIBASIC ACID and DIOL to form POLYESTERS + + + + 3 H2O Ester link Ester link Ester link

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