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UNIT 4

UNIT 4. A2 ORGANIC CHEMISTRY MECHANISMS AND REAGENTS. ALDEHYDES & KETONES. Identified by Tollens reagent Fehling’s Solution Aldehydes are oxidised to carboxylic acids. Therefore, the Tollens ’ and Fehling’s must be reduced. Tollens ’ Reagent

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UNIT 4

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  1. UNIT 4 A2 ORGANIC CHEMISTRY MECHANISMS AND REAGENTS.

  2. ALDEHYDES & KETONES • Identified by • Tollens reagent • Fehling’s Solution • Aldehydes are oxidised to carboxylic acids. Therefore, the Tollens’ and Fehling’s must be reduced. Tollens’ Reagent Sodium hydroxide, aqueous silver nitrate this produces a precipitate. Ammonia is then added to make aqueous [Ag(NH3)2]+ This is reduced to silver when an aldehyde is added. Fehling’s Solution This is a blue solution containing Cu2+ ions. When aldehydes are added and heated, these ions are reduced to cooper (I) oxide Cu2O, producing a brick-red ppt. AQA A2 text book, p64

  3. ALCOHOLS FROM ALDEHYDES & KETONES Lithium tetrahydridoaluminate LiAlH4 ALDEHYDE PRIMARY ALCOHOL [H] Sodium tetrahydridoborate NaBH4 SECONDARY ALCOHOL KETONE • Mechanism • Nucleophilic addition • The reducing agents produce the H- ion (the nucleophile). • The carbonyl bond is polar, due to oxygen being more electronegative. • First Step is the hydride ion being attracted to the ƍ+ carbon. • Second step, the product of first step gains a proton. AQA A2 text book, p66

  4. HYDROXYNITRILES FROM ALDEHYDES & KETONES ALDEHYDE HCN HYDROXYNITRILE KETONE KCN/H2SO4 • Mechanism • Similar to making alcohols! • Nucleophilic addition • The reducing agents produce the CN- ion (the nucleophile). • The carbonyl bond is polar, due to oxygen being more electronegative. • First Step is the cyanide ion being attracted to the ƍ+ carbon. • Second step, the product of first step gains a proton from the solvent or hydrogen cyanide. AQA A2 text book, p68

  5. HYDROLYSIS OF THE NITRILE GROUP The nitrile is hydrolysed in acidic conditions, producing a carboxylic group. H2O/ H+ HYDROXYNITRILE CARBOXYLIC ACID AMMONIUM AQA A2 text book, p69

  6. ESTERS FROM CARBOXYLIC ACIDS & ALCOHOLS H2SO4 or HCl cat, reflux CARBOXYLIC ACID ESTER ALCOHOLS WATER • No need for Mechanism • But you do need to be able to draw and name ester. • OH from carboxylic acid and H from OH in alcohol bond to produce water. • First part of name comes from the alcohol, second from the carboxylic acid. AQA A2 text book, p71

  7. ESTER HYDROLYSIS • Acid Catalysed Hydrolysis • Reverse of esterification. • Dilute sulfuric acid or hydrochloric acid. • Heat under reflux. ESTER CARBOXYLIC ACID ALCOHOLS WATER • Base-Catalysed Hydrolysis • Form a salt • Dilute strong base, sodium hydroxide. • Heat under reflux. • This method is quicker than acid catalysed. Heat with excess dilute acid to produce carboxylic acid ESTER SODIUM CARBOXYLATE SODIUM HYDROXIDE ALCOHOLS AQA A2 text book, p74

  8. VEGETABLE OILS & ANIMAL FATS Alkyl chain in Vegetable oils are unsaturated. Alkyl chain in Animal fats are saturated. TRIACYLGLYCEROLS or TRIGLYCERIDES (three “esters”) TRIESTERS No need for Mechanism Hydrolysed by heating with aqueous sodium hydroxide or potassium hydroxide. Produces glycerol (propane-1,2,3-triol) and sodium/potassium salt. TRIACYGLYCEROL PROPANE-1,2,3-TRIOL SODIUM HYDROXIDE SODIUM CARBOXYLATE Three moles Three moles produced AQA A2 text book, p74-75

  9. BIODIESEL • No need for Mechanism • Base catalysed Transesterification. • Heating vegetable oil with methanol and sodium hydroxide. • Produces methyl esters with long chains and propane-1,2,3-triol. PROPANE-1,2,3-TRIOL TRIACYGLYCEROL SODIUM HYDROXIDE METHYL ESTER METHANOL Three moles Three moles AQA A2 text book, p75

  10. ACYL CHLORIDES • Mechanism • Nucleophilic Substitution reactions. (two steps addition + elimination) • Greater ƍ+on carbon due to Cl and O bond. • 1st Step (addition) nucleophile attacks carbon ƍ+. Pair of electrons in C=O is transferred to oxygen. • 2nd Step (elimination) lone pair of electrons on oxygen forms C=O, chlorine is eliminated. Hydrogen attached to oxygen atom breaks. alcohol water CARBOXYLIC ACID ESTER ACYL CHLORIDE AMIDE N-SUBSTITUTED AMIDE Primary amine ammonia AQA A2 text book, p76-81 Hydrogen chloride, HCl is the secondary product.

  11. ACID ANHYDRIDE Synthesis Dehydrate a carboxylic acid (P4O10 cat) • Mechanism • Same as acyl chlorides. • Secondary product is a carboxylic acid. alcohol water CARBOXYLIC ACID ESTER ACID ANYDRIDE AMIDE N-SUBSTITUTED AMIDE Primary amine ammonia Preferred in synthesis, cheaper, less violent reactions, no fumes of HCl AQA A2 text book, p76-81

  12. NITRO COMPOUNDS FROM BENZENE • Mechanism • Generate the nitronium ion. Nitric acid and sulfuric acid. • Protonation of nitric acid, then protonated nitric acid breaks down. • Electrophilic substitution(addition then elimination). • Electrophile is attracted to the delocalized electron. • C-H bond breaks and restores the stable delocalized electron structure. UNSTABLE POSITIVELY CHARGED INTERMEDIATE BENZENE NITROBENZENE Temperature should be kept at 50oC. Further substitution occurs at 100oC (1,3-dinitrobenzene). AQA A2 text book, p90-91

  13. ICE DYES & DIAZONIUM SALTS Ice Dyes No need for mechanism but need to be able to draw compounds. Sodium chloride and water is also formed. Sodium nitrate(III), HCl Ice bath (chilled) PHENYLAMINE DIAZONIUM SALT WATER Coupling Reaction (azo dyes) No need for mechanism but you need to be able to draw compounds. Diazonium salt reacts with phenylamine or other aromatic compounds to produce azo dyes. HCl is also produced. SODIUM CHLORIDE AQA A2 text book, p92-93

  14. TRINITROTOLUENE, TNT No need for mechanism but need to be able to draw compounds. conc. HNO3 conc. H2SO4 METHYLBENZENE 2-METHYL-1,3,5-TRINITROBENZENE AQA A2 text book, p93

  15. FIEDEL-CRAFTS ACYLATION • Mechanism • Generate the electrophile. Aluminium chloride and acyl chloride. • Electrophilic substitution(addition then elimination). • Electrophile is attracted to the delocalized electron. • C-H bond breaks and restores the stable delocalized electron structure. • Aluminium chloride is regenerated whren [AlCl4]- reacts with H+. UNSTABLE POSITIVELY CHARGED INTERMEDIATE PHENYLETHANONE BENZENE HYDROGEN CHLORIDE AQA A2 text book, p94

  16. MAKING ALIPHATIC AMINES From Haloalkanes Excess ammonia reacting with primary haloalkanes. base NH3 nucleophile NH3 AMMONIA AMINE AMMONIUM SALT HALOALKANE HALIDE ION AMMONIUM HALIDE • Mechanism • If you can do this you can describe • Amine acting as a nucleophile • Amine acting as a base • How to make secondary, tertiary amines and quarternary ammonium salts From Nitrile Amine can be made by reducing nitrile LiAlH4 NITRILE 4[H] AMINE H2/Ni AQA A2 text book, p100-107

  17. MAKING AROMATIC AMINES Reduction of aromatic nitro compounds WATER AMINE NITROBENZENE 6[H] NaOH conc.HCl/Sn or conc. HCl/Fe NH3+ INTERMEDIATE AQA A2 text book, p101

  18. ZWITTERIONS Acting as a Base Accepting protons. NITROGEN IS PROTONATED AMINO ACID H+ HCl SALT With Acid Acting as an Acid Donating protons. NEGATIVE CHARGE ON OXYGEN AMINO ACID OH- NaOH SALT With Base WATER AQA A2 text book, p108-109

  19. PROTEINS FROM AMINO ACIDS Amino group reacting with carboxylic group of another amino acid. PEPTIDE LINK Each link produces one water molecule. Condensation reaction. Can form ploymers(amino acids) AMINO ACID WATER DIPEPTIDE AMINO ACID AQA A2 text book, p110

  20. AMINO ACIDS FROM PROTEINS Hydrolysis of peptide link. Heat Water 6 moldm-3 HCl for 24 hours AMINO ACID DIPEPTIDE AMINO ACID AQA A2 text book, p112

  21. ADDITION POLYMERS • Mechanism • Addition polymerisation • Involves one functional groups. • Carbon, carbon double bond. • Need to be able to identify repeating units • Taught in AS Chemistry Unit 2. POLY(ALKENE) ALKENE Monomer Polymer AQA A2 text book, p114-115 AQA AS text book, p218-219

  22. POLYESTERS • Mechanism • Condensation polymerisation • Involves two functional groups. • Polyesters are formed from a dicarboxylic acid and a diol. • Carboxyl group forms an ester link with the hydroxygroup. • Need to identify ester link. • Identify the repeating units. DICARBOXYLIC ACID POLYESTER WATER DIOL AQA A2 text book, p116-117

  23. POLYAMIDES • Mechanism • Condensation polymerisation • Involves two functional groups. • Polyamide are formed from a dicarboxylic acid and a diamines. • Hydroxyl group from dicarboxylic acid reacts with hydrogen attached to nitrogen of polyamide. • Forms an amide link. • Need to identify amide link. • Identify the repeating units. DICARBOXYLIC ACID POLYAMIDE WATER DIAMINE AQA A2 text book, p118-119 Questions on Nylon often appear on exams.

  24. BIODEGRADABLE POLYMERS Esters and amides can be hydrolysed by water, acids, alkalis and enzymes. This is the same for polyesters and polyamides. Break ester link or amide link! DICARBOXYLIC ACID DIAMINE Water SODIUM SALT OF DICARBOXYLIC ACID NaOH POLYAMIDE DIAMINE DICARBOXYLIC ACID DIAMINE ACID With polyesters the secondary product is a diol. You need to be able to identify repeating units. AQA A2 text book, p120-121

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