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Organic reaction mechanisms

Organic reaction mechanisms. Click a box below to go to the mechanism. Click here for advice. Homolytic. Free Radical Substitution. Free Radical Addition. Heterolytic. Electrophilic Addition. S N 2. S N 1. Nucleophilic Substitution. Nitration. Br 2. Electrophilic Substitution.

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Organic reaction mechanisms

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  1. Organic reaction mechanisms Click a box below to go to the mechanism Click here for advice Homolytic Free Radical Substitution Free Radical Addition Heterolytic Electrophilic Addition SN2 SN1 Nucleophilic Substitution Nitration Br2 Electrophilic Substitution Alkylation Acylation Nucleophilic Addition

  2. Free radical substitution chlorination of methane i.e. homolytic breaking of covalent bonds Overall reaction equation CH4 + Cl2 CH3Cl + HCl Conditions ultra violet light excess methane to reduce further substitution

  3. Free radical substitution mechanism Cl Cl Cl Cl H Cl H3C H3C H3C H3C Cl Cl Cl H3C H Cl Cl H3C Cl H3CCl H3CCH3 CH3 ultra-violet initiation step two propagation steps termination step minor termination step

  4. Further free radical substitutions Overall reaction equations CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl Conditions ultra-violet light excess chlorine

  5. Free radical addition [ CH2CH2 ]n addition polymerisation of ethene i.e. homolytic breaking of covalent bonds Overall reaction equation n H2C=CH2 ethene polyethene Conditions free radical source (a species that generates free radicals that allow the polymerisation of ethene molecules)

  6. Free radical addition mechanism R R R R H2C CH2 H2C CH2 R H2C CH2 R H2C CH2 CH2CH2R H2C CH2R H2C(CH2)mR R(CH2)nCH2 R(CH2)nCH2 CH2(CH2)mR initiation step chain propagation steps Addition of H2C=CH2 repeats the same way until: termination step polyethene

  7. Electrophilic addition bromine with propene mechanism CH3CH=CH2 + Br2 CH3CHBrCH2Br 1,2-dibromopropane hydrogen bromide with but-2-ene mechanism CH3CH=CHCH3 + HBr CH3CH2CHBrCH3 2-bromobutane

  8. bromine with propene H H C C H H CH3 H C C H CH3 + Br - Br Br Br + Br Br H H - C C H CH3 Br Br Electrophilic addition mechanism reaction equation Cyclic intermediate 1,2-dibromopropane

  9. hydrogen bromide with trans but-2-ene CH3 H C C H H CH3 H CH3 C C CH3 + + H H - Br Br H H - CH3 C C CH3 Br H Electrophilic addition mechanism reaction equation Carbonium ion 2-bromobutane

  10. Nucleophilic substitution hydroxide ion with bromoethane mechanism CH3CH2Br + OH- (aqueous) CH3CH2OH + Br- ethanol hydroxide ion with 2-bromo,2-methylpropane mechanism (CH3)3CBr + OH- (aqueous) (CH3)3COH + Br- 2-methylpropan-2-ol

  11. hydroxide ion with bromoethane (SN2) H - + OH C CH3 H H - Br Br C CH3 - H OH Nucleophilic substitution mechanism ethanol SN2 reaction equation S(substitution) 2(species reacting in the slowest step) N(nucleophilic)

  12. OH- ion with 2-bromo,2-methylpropane (SN1) CH3 CH3 CH3 - + OH + C C CH3 Br C CH3 CH3 CH3 CH3 CH3 - - Br Br - OH Nucleophilic substitution mechanism 2-methylpropan-2-ol SN1 reaction equation S(substitution) 1(species reactingin the slowest step) N(nucleophilic)

  13. Nucleophilic substitution cyanide ion with iodoethane mechanism CH3CH2I (ethanol) + CN-(aq) CH3CH2CN + I- propanenitrile cyanide ion with 2-bromo,2-methylpropane mechanism (CH3)3CBr (ethanol) + CN- (aqueous) (CH3)3CCN + Br- 2,2-dimethylpropanenitrile

  14. cyanide ion with iodoethane (SN2) H - + CN C CH3 H H - I I C CH3 H - CN Nucleophilic substitution mechanism propanenitrile SN2 S(substitution) 2(species reacting in the slowest step) N(nucleophilic) reaction equation

  15. CN- ion with 2-bromo,2-methylpropane (SN1) CH3 CH3 CH3 - + CN + C C CH3 Br C CH3 CH3 CH3 CH3 CH3 - - Br Br - CN Nucleophilic substitution mechanism SN1 2,2-dimethylpropanenitrile S(substitution) 1(species reactingin the slowest step) N(nucleophilic) reaction equation

  16. Nitration of benzene Where an H atom attached to an aromatic ring is replaced by an NO2 group of atoms C6H6 + HNO3 C6H5NO2 + H2O Conditions / Reagents concentrated HNO3 and concentrated H2SO4 50oC mechanism

  17. electrophilic substitution mechanism (nitration) NO2 + 1. Formation of NO2 H + + NO2 + NO2 NO2 the nitronium ion (nitryl cation) HNO3 + H2SO4 + HSO4- + H2O 2. nitronium ion attack on benzene 3. Forming the product + H+ H+ reacts to reform catalyst reaction equation

  18. Bromination of benzene C6H6 + Br2 C6H5Br + HBr Conditions / Reagents Br2 and CATALYST: Fe, FeBr3 or anhydrous AlBr3 25oC

  19. Electrophilic substitution mechanism - - Br FeBr3 Br FeBr3 + + Br Br Br H Br H + BrBr Br 1. Formation of the bromine cation FeBr3 2. Attack on benzene 3. Forming the organic product Forming the minor product and re-forming the catalyst FeBr3 bromobenzene

  20. Alkylation of benzene Where an H atom attached to an aromatic ring is replaced by a C atom electrophilic substitution C6H6 + RCl C6H5R + HCl R = alkyl group Conditions / Reagents RCl (haloakane) and anhydrous AlCl3 0 - 25oC to prevent further substitution

  21. Alkylation example - Cl AlCl3 + CH3CH2 Cl CH3CH2 With chloroethane overall reaction equation C6H6 + CH3CH2Cl C6H5CH2CH3 + HCl Three steps in electrophilic substitution mechanism 1. Formation of the electrophile (a carbocation) AlCl3

  22. Alkylation electrophilic substitution mechanism 2 - Cl AlCl3 + CH3CH2 H Cl H + CH3CH2 CH3CH2 2. Electrophilic attack on benzene 3. Forming the product and re-forming the catalyst AlCl3 ethylbenzene

  23. Acylation of benzene An H atom attached to an aromatic ring is replaced by a C atom where C is part of C=O electrophilic substitution C6H6 + RCOCl C6H5COR + HCl Conditions / Reagents RCOCl (acyl chloride) and anhydrous AlCl3 50 oC

  24. Acylation example - Cl AlCl3 O + CH3C O CH3C Cl With ethanoyl chloride overall reaction equation C6H6 + CH3COCl C6H5COCH3 + HCl Three steps in electrophilic substitution mechanism 1. Formation of the electrophile (an acylium ion) AlCl3

  25. Acylation electrophilic substitution mechanism 2 - Cl AlCl3 O CH3C H Cl O + CH3C O H CH3C + 2. Electrophilic attack on benzene 3. Forming the products and re-forming the catalyst AlCl3 phenylethanone

  26. Nucleophilic Addition addition of hydrogen cyanide to carbonyls to form hydroxynitriles + HCN RCOR RC(OH)(CN)R RCH(OH)CN + HCN RCHO Conditions / Reagents NaCN (aq) andH2SO4(aq) supplies H+ supplies the CN- nucleophile Room temperature and pressure

  27. Nucleophilic Addition Mechanism C N - O + O H O C CH3 C CN CH3 CH3 C CN CH3 CH3 CH3 CN hydrogen cyanide with propanone + HCN CH3COCH3 CH3C(OH)(CN)CH3 NaCN (aq) is a source of cyanide ions fromH2SO4 (aq) H+ H+ 2-hydroxy-2-methylpropanenitrile

  28. Advice To get back to the mechanism links page from anywhere in the presentation, click the button at the top right corner of the screen. This version provides the organic mechanisms specified (2002/3) by the Edexcel exam board. Each stage of a reaction equation, its conditions and mechanism are revealed in turn on a mouse click or keyboard stroke. Note that there is another version available where each reaction and mechanism play automatically after an initiating click or key stroke. The number of ways of navigating through this presentation may depend on the version of PowerPoint being used and how it is configured. Some possible ways of advancing: left mouse click or return key or right arrow key or up arrow key. Some possible ways of reversing: backspace key or left arrow key or down arrow key.

  29. References Steve Lewis for the Royal Society of Chemistry

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