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AQA organic reaction mechanisms

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Presentation on theme: "AQA organic reaction mechanisms"— Presentation transcript:

1 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 (nitrile hydrolysis reaction) Elimination of HX from haloalkanes Elimination of water from alcohols A2 Electrophilic Substitution Friedel-Crafts Alkylation Acylation Nucleophilic Addition Nucleophilic Addition Elimination

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
ultra-violet Cl2 Cl + Cl initiation step two propagation steps CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl termination step CH3 + Cl CH3Cl CH CH3 CH3CH3 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 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

6 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

7 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

8 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

9 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

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

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

12 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

13 Nitrile hydrolysis Acid hydrolysis of nitriles: CH3CH2CN + H2O + H+ 2
CH3CH2COOH + NH4+ propanoic acid Reflux with strong acid eg HCl (aq)

14 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

15 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

16 Elimination of water from alcohols
Elimination of H2O from propan-1-ol CH3CH2CH2OH CH3CH=CH2 + H2O Heat with concentrated H2SO4

17 Elimination mechanism – water from propan-1-ol
+ CH3 H OH C CH3 H OH C + protonated alcohol H OH CH3 H C + H OH CH3 H C propene carbocation H + reaction equation

18 Electrophilic Substitution
Nitration of benzene C6H6 + HNO3 C6H5NO2 + H2O Conditions / Reagents concentrated HNO3 and concentrated H2SO4 50oC mechanism

19 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

20 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

21 - 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

22 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

23 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

24 - 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

25 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

26 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

27 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

28 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

29 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

30 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

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

32 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

33 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

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

35 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

36 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 AQA 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.

37 References Steve Lewis for the Royal Society of Chemistry

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