AQA organic reaction mechanisms Click a box below to go to the mechanism Click here for advice AS Free Radical Substitution Electrophilic Addition Nucleophilic Substitution Elimination of HX from haloalkanes A2 Electrophilic Substitution Friedel-Crafts Alkylation Acylation Nucleophilic Addition Nucleophilic Addition Elimination

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 Cl2 Cl + Cl initiation step two propagation steps CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl termination step CH3 + Cl CH3Cl CH3 + CH3 CH3CH3 minor termination step

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

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 concentrated sulphuric acid with but-2-ene mech CH3CH=CHCH3 + HOSO3H CH3CH2CH(OSO3H)CH3 2-butylhydrogensulphate

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

concentrated H2SO4 with cis but-2-ene Electrophilic addition mechanism concentrated H2SO4 with cis but-2-ene reaction equation CH3 H C CH3 H C + carbocation OSO3H - OSO3H H + - CH3 H C OSO3H 2-butylhydrogensulphate

Nucleophilic substitution hydroxide ion with bromoethane mechanism CH3CH2Br + OH- (aqueous) CH3CH2OH + Br- ethanol cyanide ion with iodoethane mechanism CH3CH2I (ethanol) + CN-(aq) CH3CH2CN + I- propanenitrile ammonia with bromoethane mechanism CH3CH2Br + NH3 2 CH3CH2NH2 + NH4+Br- aminoethane

hydroxide ion with bromoethane Nucleophilic substitution mechanism hydroxide ion with bromoethane CH3 H Br C CH3 H OH C + - Br - - OH ethanol reaction equation

cyanide ion with iodoethane Nucleophilic substitution mechanism cyanide ion with iodoethane CH3 H Br C CH3 H CN C + - Br - CN - propanenitrile reaction equation

ammonia with bromoethane Nucleophilic substitution mechanism ammonia with bromoethane CH3 H Br C H CH3 NH2 C + Br - + - NH3 NH3 CH3 H NH2 C H NH3+Br - aminoethane reaction equation

Elimination of HX from haloalkanes Elimination of HBr from 2-bromopropane CH3CHBrCH3 + OH- CH3CH=CH2 + H2O + Br- (in ethanol) CH3 H C Br CH3 H C propene Br - OH - H OH acting as a base

Haloalkanes and hydroxide ions alcohol nucleophilic substitution RCH3CH2OH + Br- + OH- (aqueous) hydroxide acts as a nucleophile RCH2CH2X + OH- (ethanol) hydroxide acts as a base elimination RCH=CH2 + H2O + X- alkene

Electrophilic Substitution Nitration of benzene 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

Friedel-Crafts alkylation Where an H atom attached to an aromatic ring is replaced by a C atom Alkylation of benzene 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

Friedel-Crafts acylation An H atom attached to an aromatic ring is replaced by a C atom where C is part of C=O Acylation of benzene 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 product CH3C O H Cl and re-forming the catalyst AlCl3 phenylethanone

Nucleophilic Addition Reduction of carbonyls primary alcohol RCHO + 2[H] RCH2OH secondary alcohol RCOR + 2[H] RCH(OH)R Conditions / Reagents NaBH4 and H2SO4(aq) Room temperature and pressure

Nucleophilic Addition Mechanism RCOR + 2[H] RCH(OH)R alcohol reduction of propanone NaBH4 is a source of hydride ions H H+ from H2SO4 (aq) + - CH3 C O H+ CH3 C O H CH3 C O H H propan-2-ol

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

Nucleophilic Addition Elimination Acylation of water to give carboxylic acids RCOCl + H2O RCOOH + HCl carboxylic acid R C O OH Conditions room temperature and pressure

Formation of ethanoic acid ethanoyl chloride CH3COCl + H2O CH3COOH + HCl ethanoic acid CH3 C O OH mechanism

Nucleophilic Addition Elimination Mechanism + - CH3 Cl C O OH H + nucleophilic addition CH3 Cl C O OH H elimination CH3 C O OH H + CH3 C O OH Cl Cl H reaction equation

Acylation of primary amines to N-alkyl amides RCOCl + 2 R’NH2 RCONHR’ + R’NH3+Cl- N-alkylamide R C O NHR’ Conditions room temperature and pressure CH3 C O N-propylethanamide NHCH2CH2CH3

Formation of N-propyl ethanamide from ethanoyl chloride 1-aminopropane CH3COCl + CH3CH2CH2NH2 2 CH3CONHCH2CH2CH3 + CH3CH2CH2NH3+Cl- N-propylethanamide mechanism

Nucleophilic Addition Elimination Mechanism + - nucleophilic addition CH3 Cl C O NHCH2CH2CH3 H + CH3 Cl C O NHCH2CH2CH3 H elimination CH3 C O NHCH2CH2CH3 H + CH3 C O NHCH2CH2CH3 Cl NH2CH2CH2CH3 H + NH2CH2CH2CH3 Cl reaction equation

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References Steve Lewis for the Royal Society of Chemistry