Edexcel 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 Nucleophilic Substitution SN2 SN1 Electrophilic Substitution Nitration Br2 Alkylation Acylation Nucleophilic Addition

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

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

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

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

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

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

Electrophilic addition mechanism bromine with propene reaction equation CH3 H C CH3 H C Br + carbocation + - Br Br - CH3 H C Br Br 1,2-dibromopropane

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

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

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

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

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

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

CN- ion with 2-bromo,2-methylpropane (SN1) Nucleophilic substitution mechanism CN- ion with 2-bromo,2-methylpropane (SN1) Br - Br - CH3 Br C CH3 C + CH3 CN C + - SN1 CN - 2,2-dimethyl propanenitrile S (substitution) 1(species reacting in the slowest step) N(nucleophilic) reaction equation

Electrophilic Substitution 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

electrophilic substitution mechanism (nitration) 1. Formation of NO2 + the nitronium ion HNO3 + 2H2SO4 NO2 + + 2HSO4- + H3O+ 2. Electrophilic attack on benzene NO2 + + NO2 H O SO3H- NO2 3. Forming the product and re-forming the catalyst H O SO3H reaction equation

Bromination of benzene Where an H atom attached to an aromatic ring is replaced by a Br atom electrophilic substitution C6H6 + Br2 C6H5Br + HBr R = alkyl group Conditions / Reagents Br2 and anhydrous AlBr3 25oC

Electrophilic substitution mechanism 1. Formation of the electrophile Br AlBr3 - Br + AlBr3 Br Br 2. Electrophilic attack on benzene Br + Br AlBr3 - + H Br Br 3. Forming the products H Br and re-forming the catalyst AlBr3 bromobenzene

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

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

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

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

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

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

Nucleophilic Addition addition of hydrogen cyanide to carbonyls to form hydroxynitriles RCOR + HCN RC(OH)(CN)R RCHO + HCN RCH(OH)CN Conditions / Reagents NaCN (aq) and H2SO4(aq) supplies H+ supplies the CN- nucleophile Room temperature and pressure

Nucleophilic Addition Mechanism hydrogen cyanide with propanone CH3COCH3 + HCN CH3C(OH)(CN)CH3 NaCN (aq) is a source of cyanide ions C N + - H+ from H2SO4 (aq) CH3 C O H+ CH3 C O CN CH3 C O CN H CN 2-hydroxy-2-methylpropanenitrile

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References Steve Lewis for the Royal Society of Chemistry