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Summary of Organic Chemistry Lecture 2-10

Summary of Organic Chemistry Lecture 2-10 . Isomers with several chiral centers. 2,3,4-Trihydroxybutanal two stereocenters ; 2 2 = 4 stereoisomers are possible. Stereo chemistry. Meso compound: an achiral compound possessing two or more stereocenters tartaric acid

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Summary of Organic Chemistry Lecture 2-10

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  1. Summary of OrganicChemistryLecture 2-10

  2. Isomers with several chiral centers • 2,3,4-Trihydroxybutanal two stereocenters; 22 = 4 stereoisomers are possible

  3. Stereo chemistry • Meso compound: an achiral compound possessing two or more stereocenters • tartaric acid • two stereocenters; 2n = 4, but only three stereoisomers exist

  4. Enantiomers & Diastereomers

  5. SN2 Inversion of configuration!

  6. C H C H 6 5 6 5 + CH O C C OCH 3 3 H H Cl Cl (S)-Enantiomer (R)-Enantiomer A racemic mixture SN1

  7. Elimination • What favors Elimination reactions: • attacking nucleophil is a strong and large base • steric crowding in the substrate • High temperatures and low polarity of solvent E1 Mechanism Nucleophile -> acting as a strong base

  8. Nucleophilic reactions of carbonyl groups

  9. Oxygen as a nucleophile Addition of 1 mol of Alcohol to Aldehyde -> Hemiacetal+ 1 mol Alcohol -> Acetal Addition of 1 mol of Alcohol to Keton -> Hemiketal+ 1 mol Alcohol -> Ketal

  10. Hydride as a nucleophileReduction of aldehydes and ketons Complex metal hydrides (LiAlH4 or NaBH4) can deliver hydride -> they act like nucleophiles

  11. Carbon as a nucleophile Cyanide -> Cyanohydrin Organometallics: Grignard reagents

  12. Nitrogen as a nucleophile Addition of primary amine to carbonyl -> Imine (Schiff base)

  13. Nucleophilic substitution on carbonyl groups – Carboxylic acid derivates Carboxylic acids react with base (NaOH, KOH) -> water-soluble salts

  14. Oxygen as nucleophile Synthesis of anhydrides Acid + Alcohol -> Ester base-catalysedesterfication -> does not work with acid + alcohol

  15. Oxygen as nucleophile Hydrolysis of esters + amids: acid + base catalysed

  16. Sulfur as nucleophile • Thiols + Carboxylic acid derivates -> Thioacids and Thioesters • Reactivity increased (compared to formation of ester): • Sulfur better nucleotide than oxygen • RS- better leaving group than RO-

  17. Nitrogen as nucleophile Amines (1º, 2º, 3º) + Carboxylic acid (derivates) -> Amides -> with esters N (aminogroup) betternucleophilthan O (hydroxyl/ester) ! Ester betterleavinggroup !!!

  18. Hydride as nucleophile Reduction of carboxylic acid derivates Reduction of carboxylic acid derivates -> Alkohols Lithium aluminium hydride (LAH) -> strong hydrid donor Sodium borohydride (NaBH4) -> weaker hydriddonor Lithium aluminium hydride (LAH) -> reduces almost all carboxylic acid derivates -> primary alcohol Sodium borohydride (NaBH4) -> reduces acyl halides, ketons, aldehydes -> primary alcohol

  19. Hydride as nucleophile Reduction of carboxylic acid derivates Reduction of Amides -> Amines ->different mechanism!!!

  20. Carbon as nucleophile Carboxylic acid derivates + Grignard -> Alcohols

  21. Electrophilic Reactions Alkenes –Addition of hydrogen halides Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens

  22. ElectrophilicReactions Alkynes –Addition of halides Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens

  23. ElectrophilicReactions Alkenes –Addition of water Alkynes – Hydration gives a keton

  24. ElectrophilicReactions Alkenes –Addition of halogens Alkenes –Addition of halogens in water -> Halo alcohol (halohydrin) Halo alcohols can be used to make -> epoxides

  25. Electrophilic ReactionsAddition to conjugated systems Alkenes – addition of halides Conjugate addition Alkenes – addition of halogens

  26. Electrophilic ReactionsCarbocations as electrophiles Cationic polymerization Polystyrene

  27. Catalytic hydration -> NOT a radical reaction (Chapter 9: Radical reactions) Involvement of atomic hydrogen -> mechanism has more in common with radical reaction than with ionic reaction Catalytic HydrationAddition of hydrogen to alkenes and alkynes

  28. Homolysis of peroxides Radical reactionsForming of a radical Homolytic cleavage by Photolysis Reactions of molecules with other radicals

  29. Radical substitution reactionsHalogenation

  30. Chlorine rather unselective !!! (number of hydrogens versus radical position) Radical substitution reactionsHalogenation Bromine less reactive - More selective !!!

  31. Radical addition reactionsAddition of HBr to alkenes Anti- Markovnikov’srule: in additions of HX, H adds to the carbon with the lower number of hydrogens Addition to conjugated dienes

  32. Radical addition reactionsAddition of HBr to alkenes

  33. Radical polymerization reactionspolymerization of alkenes

  34. Electrophilic ReactionsAromatic substitution Electrophilic reactions with aromatic substrates -> substitution

  35. Electrophilic ReactionsAromatic substitution Halogenation

  36. Electrophilic ReactionsAromatic substitution Nitration HNO3 + H+ (strongacid) -> NO2+ Sulfonation

  37. Electrophilic ReactionsAromatic substitution Electrophilicalkylations -> Friedel-Craft alkylation

  38. Electrophilic ReactionsAromatic substitution Electrophilic alkylations -> Friedel-Craft alkylation -> alkylation of aromatic ring can be achieved by any system that generates carbocation

  39. Electrophilic ReactionsAromatic substitution Electrophilic acylation -> Friedel-Craft Acylation

  40. Electrophilic ReactionsAromatic substitution Effect of substituents -> influence rate and position of substitution Electron withdrawing substituents -> deactivate and hinder further substitution (-> destabilize ortho and para -> substitution preferable in meta position) Electron donating substituents -> activate for further substitution (-> preferable in ortho and para) Substituent has maximum effect on reaction if it is in ortho or para position -> electron releasing substituents (ortho and para) -> electron withdrawing substituents (destabilize arenium cation ->meta)

  41. Electrophilic ReactionsAromatic substitution Effect of substituents -> influence rate and position of substitution

  42. Electrophilic ReactionsAromatic substitution Effect of substituents

  43. Electrophilic ReactionsAromatic substitution Effect of substituents -> reaction is regioselective

  44. Nucleophilicreactionsinvolvingenolate anions Enolate anion Conjugated base

  45. Halogenation Nucleophilic reactions involving enolate anions Alkylation of enolate anion

  46. Addition-dehydration: Aldol reaction Carbonyl compounds reacting as electrophiles -> aldol reaction Nucleophilic reactions involving enolate anions Aldehydes are better electrophiles than ketons -> aldol reaction more favourable with aldehydes

  47. Addition-dehydration: Aldol reaction mixed aldol reaction Nucleophilic reactions involving enolate anions Base oracidcatalysed!

  48. Other stabilized anions as nucleophiles: nitriles and nitromethane Nucleophilic reactions involving enolate anions

  49. Enamines as nucleophiles Nucleophilic reactions involving enolate anions

  50. Enolate anions from carboxylic acid derivates Nucleophilic reactions involving enolate anions

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