A guide for A levels by Mark McGill Organic Mechanisms A guide for A levels by Mark McGill

Mechanism Guide E.g. E.g. H Br H C C H :OH- H H Mechanisms written in bold, with thicker lines and arrows are required at AS/A2 level Mechanisms not written in bold, with thinner lines and arrows are not required at AS/A2 level E.g. E.g. H Br - O H3C + H H C C H C :OH- O :N H H H H3C C H O 2

Arrow Guide Reagent(s) Type of mechanism Type of mechanism Reagent(s) Condition(s) Condition(s) 3

Quaternary Ammonium Salts Tertiary Amines Diols Alkyl Ammonium Salts Alkanes Dihaloalkanes Secondary Amines Alkenes Primary Amines Haloalkanes Alcohols Nitriles Carbonyls Carboxylate Salts Carboxylic Acids Hydroxynitriles Acyl Chlorides Acid Anhydrides Amides

Back to Mechanisms Flow Chart Diols Dihaloalkanes KOH(aq) Nucleophilic Substitution Warm

Back to Mechanisms Flow Chart Nucleophilic Substitution Example: Dibromo-1,2,-ethane + Potassium hydroxide -> Ethane-1,2-diol + Potassium bromide C2H4Br2 + 2KOH -> C2H4(OH)2 + 2KBr H - Br H OH H C + C H :OH- H C + C H :OH- Br H H Br - The hydroxide ion attacks the slightly positive carbon atom, breaking the carbon-halogen bond Again, the hydroxide ion attacks the slightly positive carbon atom, breaking the carbon-halogen bond H OH H C C H + 2 :Br- H OH Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Diols Dihaloalkanes Alkenes KOH(aq) Cl2, Br2, etc Nucleophilic Substitution Electrophilic Addition Warm

Electrophilic Addition Example: Ethene + Bromine -> Dibromo-1,2-ethane C2H4 + Br2 -> C2H4Br2 H H H + + - C C Br Br H C C H :Br- H H H Br The remaining halogen ion bonds with the carbocation A pair of electrons from the C=C bond, bond with the slightly positive halogen atom, breaking the double bond and forming a carbocation H Br H C C H Br H Back to Mechanisms Flow Chart

Alkanes Dihaloalkanes Alkenes Haloalkanes Alcohols Elimination Back to Mechanisms Flow Chart Alkanes Hydrogenation Ni catalyst 150ºC 2 atm H2 Dihaloalkanes Alkenes Electrophilic Addition HCl, HBr, etc Haloalkanes Electrophilic Addition Cl2, Br2, etc KOH(ethanol) Elimination Electro- philic Addition Elimin- ation H2SO4 180ºC H2SO4 H2 Warm Alcohols

Electrophilic Addition Example: Ethene + Hydrogen bromide-> Bromoethane C2H4 + HBr -> C2H5Br H H H + + - C C H Br H C C H :Br- H H H H The remaining halogen ion bonds with the carbocation A pair of electrons from the C=C bond, bond with the slightly positive hydrogen atom, breaking the double bond and forming a carbocation H Br H C C H H H Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Hydrogenation Example: Ethene + Hydrogen -> Ethane C2H4 + H2 -> C2H6 H H H C C H H H H H H H C C H H Ni Ni The ethene and hydrogen molecules are adsorbed to the catalyst, breaking the C=C and H-H bonds When hydrogen atom diffuses near to the adsorbed ethene molecule, it bonds with the carbon atom. This occurs twice, forming two more C-H bonds. Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Elimination Example: Bromoethane + Sodium hydroxide -> Ethene + Sodium bromide + Water C2H5Br + NaOH -> C2H4 + NaBr + H2O H Br H H H C C H :OH- C C + :Br- + H2O H H H H The hydroxide ion attacks a hydrogen atom bonded to a carbon atom, bonded to the carbon atom bonded to the halogen. This causes the C-H bond to collapse onto the C-C bond, forming a double bond. This causes the carbon-halogen bond to collapse Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Electrophilic Addition Example: Ethene + Water -> Ethanol C2H4 + H2O -> C2H5OH H H H + - + :OH2 C C H OSO3H H C C H H H H H A water molecule bonds with the carbocation, forming another intermediate A pair of electrons from the C=C bond, bond with the slightly positive hydrogen atom, breaking the double bond and forming a carbocation H H - H O H OH H C C H H2SO4 + H C C H H H H H An O-H bond collapses, forming an alcohol and reforming the sulfuric acid Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Elimination Example: Ethanol -> Ethene + Water C2H5OH -> C2H4 + H2O H H H - H O H O: + - H OSO3H H C C H The C-O bond collapses, releasing a water molecule, and leaving a carbocation H C C H H H H A lone pair on the oxygen atom steals a hydrogen atom from a sulfuric acid molecule, forming an intermediate H H H H + H H2SO4 + C C C H2O C H + H H H H :OSO3H- Back to Mechanisms Flow Chart The hydrogensulfate ion takes a hydrogen atom from a carbon bonded to the carbocation, causing the C-H bond to collapse onto the C-C bond, forming a double bond whilst reforming the sulfuric acid

Back to Mechanisms Flow Chart Alkenes Alkanes Haloalkanes H2 Free Radical Substitution Cl2 Hydrogenation Ni catalyst 150ºC 2 atm UV light

Back to Mechanisms Flow Chart Free Radical Substitution Example: Ethane + Chlorine -> Chloroethane + Hydrogen chloride UV light C2H6 + Cl2 -> C2H5Cl + HCl Initiation Step: Cl2 -> 2Cl• Propagation Step 1: C2H6 + Cl• -> C2H5• + HCl Propagation Step 2: C2H5• + Cl2 -> C2H5Cl + Cl• Termination Step 1: C2H5• + Cl • -> C2H5Cl Possible termination Step 2: 2C2H5• -> C4H10 Back to Mechanisms Flow Chart

Alkanes Nitriles Haloalkanes Alkenes Alcohols Amines Elimination Back to Mechanisms Flow Chart Alkanes Free Radical Substitution Cl2 UV light KOH(ethanol) Nitriles Elimination Haloalkanes Alkenes Nucleophilic Substitution KCN(ethanol) HCl, HBr, etc Electrophilic Addition Heat Nucleophilic Substitution Nucleophilic Substitution KOH(aq) Warm NH3 Alcohols Amines

Back to Mechanisms Flow Chart Nucleophilic Substitution Example: Bromoethane + Potassium hydroxide -> Ethanol + Potassium bromide C2H5Br + KOH -> C2H5OH + KBr H - Br H OH H C + C H :OH- H C C H + :Br- H H H H The hydroxide ion attacks the slightly positive carbon atom, breaking the carbon-halogen bond Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Nucleophilic Substitution Example: Bromoethane + Potassium cyanide -> Propanenitrile + Potassium bromide C2H5Br + KCN -> C2H5CN + KBr H - Br H CN H C + C H :CN- H C C H + :Br- H H H H The cyanide ion attacks the slightly positive carbon atom, breaking the carbon-halogen bond Back to Mechanisms Flow Chart

Nucleophilic Substitution Example: Chloroethane + Ammonia -> Aminoethane + Ammonium chloride C2H5Cl + 2NH3 -> CH3CH2NH2 + NH4Cl H H H N + H - Cl H + H C C H :NH3 H C C H :NH3 H H H H The ammonia attacks the slightly positive carbon atom, breaking the carbon-halogen bond Another ammonia molecule attacks the intermediate, producing the amine H H H H C C N + NH4Cl H Back to Mechanisms Flow Chart H H

Haloalkanes Alkenes Alcohols Carbonyls Elimination Oxidation Reduction Back to Mechanisms Flow Chart Haloalkanes Nucleophilic Substitution KOH(aq) Warm H2SO4 Elimination K2Cr2O7 Oxidation Alkenes Alcohols Carbonyls 180ºC Acidic Conditions H2SO4 + H2 LiAlH4 / NaBH4 Electrophilic Addition Reduction Warm

Oxidation Primary alcohols: Secondary alcohols: Tertiary alcohols: Example: Methanol + [O] -> Methanal + Water [O] = K2Cr2O7 CH3OH + [O] -> CH2O + H2O Secondary alcohols: Example: Propan-2-ol + [O] -> Propanone + Water C3H7OH + [O] -> C3H6O + H2O Tertiary alcohols: Example: Methylpropan-2-ol + [O] -> No reaction Aldehydes will quickly be further oxidised to carboxylic acids Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Reduction Aldehydes: Example: Methanal + 2[H] -> Methanol [H] = NaBH4 or LiAlH4 CH2O + 2[H] -> CH3OH Ketones: Example: Propanone + 2[H] -> Propan-2-ol C3H6O + 2[H] -> C3H7OH Nucleophilic Addition Mechanism: H H H O:- OH + C :H- C C O - H+ H H H H H NaBH4 contains the BH4- ion, which acts as a source of H- ions. These attack the carbon atom, breaking the double bond, forming an intermediate The intermediate takes a H+ ion from the acidic conditions to form an alcohol Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Hydroxynitriles Nucleophilic Addition KCN Acidic Conditions K2Cr2O7 K2Cr2O7 Oxidation Oxidation Alcohols Carbonyls Carboxylic Acids Acidic Conditions Acidic Conditions LiAlH4 / NaBH4 LiAlH4 Reduction Reduction

Back to Mechanisms Flow Chart Nucleophilic Addition Example: Ethanal + Hydrogen Cyanide -> 2-hydroxypropanenitrile C2H4O + HCN -> CH3CH(OH)CN H3C H3C H3C O:- OH + C C C - :CN- H+ O H CN H CN H The intermediate takes a H+ ion from the acidic conditions to form an a hydroxyl group The cyanide ions attack the carbon atom, breaking the double bond, forming an intermediate Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Oxidation Aldehydes: Example: Methanal + [O] -> Methanoic Acid [O] = K2Cr2O7 CH2O + [O] -> CHOOH + H2O Ketones: Example: Propanone + [O] -> No reaction Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Reduction Example: Methanoic Acid + 2[H] -> Methanal [H] = LiAlH4 CHOOH + 2[H] -> CH3O Nucleophilic Addition-Elimination Mechanism: - - O O O OH C + O H :H- C C H H H H H LiAlH4 contains the AlH4- ion, which acts as a source of H- ions. These attack the carbon atom, breaking the C=O bond, forming an intermediate The intermediate reforms the C=O bond, expelling the hydroxyl group in the process Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Hydroxynitriles Carbonyls Nucleophilic Addition KCN Acidic Conditions

Nitriles Carbonyls Carboxylic Acids Carboxylate Salts Acid Anhydrides Back to Mechanisms Flow Chart Nitriles Hydrolysis Acidic Conditions H2O K2Cr2O7 A base e.g. NaOH Carbonyls Oxidation Carboxylic Acids Carboxylate Salts Acid-Base Acidic Conditions An acid e.g. HCl LiAlH4 Reduction Acid-Base Nucleo- philic Addition- Elimination Nucleo- philic Addition- Elimination H2O H2O Acid Anhydrides Acyl Chlorides

Back to Mechanisms Flow Chart Hydrolysis Example: Ethanenitrile + Water -> Ethanoic Acid + Ammonium ion CH3CN + 2H2O + H+ -> CH3COOH + NH4+ O H H H H + H O H+ H O + + H3C C N H3C C N H H3C C N H O H H H The C=N bond takes a hydrogen from the oxygen intermediate The acidic conditions allow a water molecule to take a H+ ion to form a H3O+ ion The highly negative triple-bond steals a hydrogen from the ion, reforming water The water attacks the carbocation H H H H O H + H H O O: O + H3C C N H + H3C C N: H3C C N H H3C C :N H H H + O O H H O O H H H H H H An oxygen’s lone pair collapse to form a C=O bond, expelling the ammonia molecule The lone pair on the nitrogen takes a hydrogen from the oxygen intermediate The ammonia takes the hydrogen from the oxygen intermediate, leaving a carboxylic acid and an ammonium ion. Another water molecule attacks the newly-formed carbocation H O + H3C C + H N H Back to Mechanisms Flow Chart HO H

Back to Mechanisms Flow Chart Nucleophilic Addition-Elimination Example: Ethanoyl chloride + Water -> Ethanoic acid + Hydrogen chloride CH3COCl + H2O -> CH3COOH + HCl Nucleophilic Addition-Elimination Mechanism: - - O O O Cl H C + Cl :O C C OH + H O H + H3C H3C H H HCl The double bond on the water molecule attacks the carbon atom, causing the C=O bond to break. Both intermediates collapse in; the negative dispelling a chlorine ion in remaking the double bond, and the positive dispels a hydrogen ion Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Nucleophilic Addition-Elimination Example: Ethanoic Anhydride + Water -> Ethanoic acid CH3COOCOCH3 + H2O -> 2CH3COOH Nucleophilic Addition-Elimination Mechanism: - O - O H H3C + C O H H3C C O + O H C OH + CH3COOH :O O H H3C C H O H3C C O Both intermediates collapse in; the negative causing the -OCOCH3 to be dispelled, and the positive dispels a hydrogen ion The double bond on the water molecule attacks a carbon atom bonded to two oxygens, causing the C=O bond to break. Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Acid-Base reaction Example: Methanoic acid + Sodium hydroxide -> Sodium methanoate + Water CHOOH + NaOH -> CHOO-Na+ + H2O There isn’t a mechanism to this reaction as it is ionic, so no electrons are being shared, and hence, there are no curly arrows to draw This reaction also occurs with sodium carbonate Example: Methanoic acid + Sodium carbonate -> Sodium methanoate + Water + Carbon dioxide 2CHOOH + Na2CO3 -> 2CHOO-Na+ + H2O + CO2 Reverse reaction Example: Sodium methanoate + Hydrochloric acid -> Methanoic acid + Sodium chloride CHOO-Na+ HCl -> CHOOH + NaCl Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart A base e.g. NaOH Acid-Base Carboxylic Acids Carboxylate Salts Nitriles H2O A base e.g. NaOH Hydrolysis An acid e.g. HCl Acid-Base

Back to Mechanisms Flow Chart Hydrolysis Example: Ethanenitrile + Sodium hydroxide + Water -> Sodium ethanoate + Ammonia CH3CN + NaOH + H2O -> CH3COO-Na+ + NH3 H H O H O H O - - :OH H3C C N H3C C N H3C C N H The negative nitrogen intermediate steals a hydrogen from a water molecule. This stage forms a hydroxide ion The hydroxide ion attacks the carbon bonded to the nitrogen, breaking the triple bond The C=N bond takes a hydrogen from the hydroxyl group, creating a double bond O H - H O O O H H3C C N H :N - H3C C N H3C C H3C C + N H H H H O O - H O :OH H H The result is the carboxylate ion, which will ionically bond with a sodium ion to complete the salt The nitrogen configuration takes a hydrogen from the oxygen forming ammonia The hydroxide ion from stage 2 attacks the carbon atom, breaking the C=O bond The oxygen intermediate reforms the double bond, expelling the nitrogen configuration H O H3C C + N H Back to Mechanisms Flow Chart O-Na+ H

Back to Mechanisms Flow Chart Haloalkanes Nucleophilic Substitution KCN(ethanol) Heat Carboxylate Salts H2O A base e.g. NaOH Nitriles Carboxylic Acids Hydrolysis Hydrolysis H2O Acidic Conditions Reduction In solution with ethoxyethane Acidic Conditions LiAlH4 Primary Amines

Back to Mechanisms Flow Chart Nucleophilic Addition Example: Ethanenitrile + 4[H] -> Aminoethane [H] = LiAlH4 CH3CN + 4[H] -> CH3CH2NH2 H H+ H H :H- - 2- H3C C N :H- H3C C N H3C C N H3C C N H H H H H+ The hydride ions attack the carbon atom, breaking the triple bond, forming an intermediate Another hydride ion attacks the carbon atom, creating a dianion The dianion is protonated by an acid to form the amine Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Secondary Amines Nucleophilic Substitution A haloalkane e.g. CH3Cl An acid e.g. HCl Nitriles Primary Amines Acid-Base Alkyl Ammonium Salts Nucleophilic Addition LiAlH4 A base e.g. NaOH Acid-Base In solution with ethoxyethane Acidic Conditions Nucleophilic Substitution NH3 Haloalkanes

Back to Mechanisms Flow Chart Nucleophilic Substitution Example: Aminoethane + Chloromethane -> N-methyl-1-aminoethane + Hydrogen chloride CH3CH2NH2 + CH3Cl -> CH3NHCH2CH3 + HCl :Cl- Cl - H H H H H H H C + H H H C C N: H C C N + C H H H H H H H H H The lone pair on the nitrogen attacks the carbon, breaking the carbon-halogen bond The halogen ion steals a hyrogen from the nitrogen, forming the secondary amine H H CH3 H C C N + HCl H H H Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Acid-Base reaction Primary amine Example: Aminomethane + Hydrochloric acid -> Methylammonium chloride CH3NH2 + HCl -> [CH3NH3]+Cl- There isn’t a mechanism to this reaction as it is ionic, so no electrons are being shared, and hence, there are no curly arrows to draw Secondary amine Example: N-methyl-1-aminomethane + Hydrochloric acid -> Dimethylammonium chloride (CH3)2NH + HCl -> [(CH3)2NH2]+Cl- Tertiary amine Example: N,N-dimethyl-1-aminomethane + Hydrochloric acid -> Trimethylammonium chloride (CH3)3NH + HCl -> [(CH3)3NH2]+Cl- Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Acid-Base reaction Alkyl ammonium salt Example: Methylammonium chloride + Sodium hydroxide -> Aminomethane + Sodium chloride + Water [CH3NH3]+Cl- + NaOH -> CH3NH2 + NaCl + H2O There isn’t a mechanism to this reaction as it is ionic, so no electrons are being shared, and hence, there are no curly arrows to draw Dialkyl ammonium salt Example: Dimethylammonium chloride + Sodium hydroxide -> N-methyl-1-aminomethane + Sodium chloride + Water [(CH3)2NH3] +Cl- + NaOH -> (CH3)2NH2 + NaCl + H2O Trialkyl ammonium salt Example: Trimethylammonium chloride + Sodium hydroxide -> N,N-dimethyl-1-aminomethane + Sodium chloride + Water [(CH3)3NH3] +Cl- + NaOH -> (CH3)3NH2 + NaCl + H2O Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Primary Amines Acid- Base An acid e.g. HCl Alkyl Ammonium Salts Secondary Amines Acid- Base A base e.g. NaOH Tertiary Amines

Back to Mechanisms Flow Chart Alkyl Ammonium Salts Acid- Base Acid- Base A base e.g. NaOH An acid e.g. HCl Primary Amines A haloalkane e.g. CH3Cl A haloalkane e.g. CH3Cl Secondary Amines Tertiary Amines Nucleophilic Substitution Nucleophilic Substitution

Back to Mechanisms Flow Chart Nucleophilic Substitution Example: N-methyl-1-aminoethane + Chloromethane -> N,N-dimethyl-1-aminoethane + Hydrogen chloride C2H5NHCH3 + CH3Cl -> C2H5NH(CH3)2 + HCl :Cl- Cl - H H H H H H H C + H H H C C N: H C C N + C H H CH3 H H H H H CH3 The lone pair on the nitrogen attacks the carbon, breaking the carbon-halogen bond The halogen ion steals a hyrogen from the nitrogen, forming the tertiary amine H H CH3 H C C N + HCl H H CH3 Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Alkyl Ammonium Salts Acid- Base Acid- Base A base e.g. NaOH An acid e.g. HCl Secondary Amines A haloalkane e.g. CH3Cl Tertiary Amines A haloalkane e.g. CH3Cl Quaternary Ammonium Salts Nucleophilic Substitution Nucleophilic Substitution

Back to Mechanisms Flow Chart Nucleophilic Substitution Example: N,N-dimethyl-1-aminoethane + Chloromethane -> Trimethylethylammonium chloride C2H5NHCH3 + CH3Cl -> C2H5NH(CH3)2 + HCl + Cl - H H CH3 H H CH3 + H C H C N + H C CH3 Cl- H C C N: H H H CH3 CH3 H H The lone pair on the nitrogen attacks the carbon, breaking the carbon-halogen bond, and forming the salt Back to Mechanisms Flow Chart

Back to Mechanisms Flow Chart Quaternary Ammonium Salts A haloalkane e.g. CH3Cl Tertiary Amines Nucleophilic Substitution

Carboxylic Acids Acid Anhydrides Amides Nucleophilic Back to Mechanisms Flow Chart Carboxylic Acids Acid Anhydrides Amides Nucleophilic Addition-Elimination H2O NH3 Nucleophilic Addition-Elimination

Back to Mechanisms Flow Chart Nucleophilic Addition-Elimination Example: Ethanoic Anhydride + Ammonia -> Ethanamide Ethanoic acid CH3COOCOCH3 + NH3 -> CHONH2 + CH3COOH Nucleophilic Addition-Elimination Mechanism: - O - O H H3C + H C O H3C C N H + O C NH2 + CH3COOH :N H H O H H3C C H O H3C C O Both intermediates collapse in; the negative causing the -OCOCH3 to be dispelled, and the positive dispels a hydrogen ion The double bond on the water molecule attacks a carbon atom bonded to two oxygens, causing the C=O bond to break. Back to Mechanisms Flow Chart

Carboxylic Acids Acyl Chlorides Amides Nucleophilic Back to Mechanisms Flow Chart Carboxylic Acids Acyl Chlorides Amides Nucleophilic Addition-Elimination Nucleophilic Addition-Elimination H2O NH3

Back to Mechanisms Flow Chart Nucleophilic Addition-Elimination Example: Ethanoyl chloride + Ammonia -> Ethanamide + Hydrogen chloride CH3COCl + NH3 -> CHONH2 + HCl Nucleophilic Addition-Elimination Mechanism: H - - O O O Cl H C + Cl :N C C NH2 H + N H + H3C H H3C H H HCl The double bond on the ammonia molecule attacks the carbon atom, causing the C=O bond to break. Both intermediates collapse in; the negative dispelling a chlorine ion in remaking the double bond, and the positive dispels a hydrogen ion Back to Mechanisms Flow Chart

Acyl Chlorides Amides Acid Anhydrides Nucleophilic Back to Mechanisms Flow Chart Acyl Chlorides Amides Acid Anhydrides Nucleophilic Addition-Elimination NH3 Nucleophilic Addition-Elimination NH3