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Chapter 20: Carboxylic Acids and Nitriles

Chapter 20: Carboxylic Acids and Nitriles. The Importance of Carboxylic Acids (RCO 2 H). Abundant in nature from oxidation of aldehydes and alcohols in metabolism Acetic acid, CH 3 CO 2 H, - vinegar Butanoic acid, CH 3 CH 2 CH 2 CO 2 H (rancid butter)

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Chapter 20: Carboxylic Acids and Nitriles

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  1. Chapter 20: Carboxylic Acids and Nitriles

  2. The Importance of Carboxylic Acids (RCO2H) • Abundant in nature from oxidation of aldehydes and alcohols in metabolism • Acetic acid, CH3CO2H, - vinegar • Butanoic acid, CH3CH2CH2CO2H (rancid butter) • Long-chain aliphatic acids from the breakdown of fats • Carboxylic acids present in many industrial processes and most biological processes • An understanding of their properties and reactions is fundamental to understanding organic chemistry

  3. The Importance of Carboxylic Acids (RCO2H) • They are the starting materials from which other acyl derivatives are made

  4. 20.1 Naming Carboxylic Acids & Nitriles • Carboxylic Acids, RCO2H • If derived from open-chain alkanes, replace the terminal -e of the alkane name with -oic acid • The carboxyl carbon atom is C #1 Propanoic acid 4-Methylpentanoic acid 3-Ethyl-6-methyloctanedioic acid

  5. Alternative Names • Compounds with CO2H bonded to a ring are named using the suffix -carboxylic acid • The CO2H carbon is not itself numbered in this system trans-4-Hydroxycyclohexanecarboxylic acid 1-cyclopentenecarboxylic acid

  6. Common Names: • Carboxylic acids some of the 1st compound studied so some of their common names are still used. Especially for those with biological interest. Names accepted by IUPAC

  7. Common Names:

  8. Nitriles, RCN • Closely related to carboxylic acids named by adding -nitrileas a suffix to the alkane name, with the • nitrile carbon numbered C #1 4-Methylpentanenitrile

  9. Nitriles, RCN • Complex nitriles are named as derivatives of carboxylic acids. • Replace -ic acidor -oic acidending with -onitrile Acetonitrile (From acetic acid) Benzonitrile (From benzoic acid) 2,2-Dimethylcyclohexanecarbonitrile (From 2,2-dimethylcyclohexanecarboxylic acid)

  10. Learning Check: • Give IUPAC names for the following:

  11. Solution: • Give IUPAC names for the following: 2-ethylpentanoic acid 3-Methylbutanoic acid 4-Bromopentanoic acid 2,4-Dimethylpentanenitrile cis-1,3-cyclopentanedicarboxylic acid) cis-4-hexenoic acid

  12. 20.2 Structure & Properties Carboxyl carbon sp2 hybridized: planar with bond angles of approximately 120°

  13. 20.2 Structure & Properties: • Carboxylic acids form hydrogen bonds, • exist as cyclic dimers held together by 2 hydrogen bonds • Strong hydrogen bonding causes much higher boiling points than the corresponding alcohols

  14. Dissociation of Carboxylic Acids • Carboxylic acids are proton donors toward weak and strong bases, producing metal carboxylate salts, RCO2+M • Carboxylic acids with more than six carbons are only slightly soluble in water, but their conjugate base salts are water-soluble

  15. Acidity Constant and pKa • Carboxylic acids transfer a proton to water to give H3O+ and carboxylate anions, RCO2, but H3O+ is a much stronger acid pKa = -1.7 Lower pKa means stronger acid so equilibrium favors starting materials pKa = 15.7 (acts as a base here) pKa = ~5 • The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid (pKa ~ 5)

  16. Substituent Effects on Acidity Electron withdrawing (electronegative) substituents (like F, Cl, Br, I) promote formation of the carboxylate ion so raise the Ka (lower the pKa) The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid (pKa ~ 5) Ka,, ~10-15 to -16 for alcohols and water (pKa ~ 15 to 16) CH3OH 15.5 H2O 15.7 CH3CH2OH 15.9

  17. Resonance Effects on Acidity Phenols Carboxylic acid Alcohols Inorganic acids

  18. Resonance Effects on Acidity Alcohols weaker acids since O- not stabilized by resonance C=O Bond length (120 pm) Negative charge spread over O-C-O C-O Bond length (134 pm) C-O Bond lengths same (127 pm) Carboxylic acids more acidic since O- stabilized by resonance

  19. 20.3 Biological Acids and the Henderson-Hasselbalch Equation • If pKa of given acid and the pH of the medium are known, % of dissociated and undissociated forms can be calculated using the Henderson-Hasselbalch eqn

  20. Learning Check: • Calculate the % of acid form (undissociated = HA) and carboxylate ion form (dissociated = A-) present in a 0.0020 M propanoic acid solution at pH = 5.30. (pKa = 4.87)

  21. Solution: • Calculate the % of acid form (undissociated = HA) and carboxylate ion form (dissociated = A-) present in a 0.0020 M propanoic acid solution at pH = 5.30. (pKa = 4.87) HA A- and

  22. 20.4 Substituent Effects on Acidity Electron withdrawing (electronegative) substituents (like F, Cl, Br, I) promote formation of the carboxylate ion so raise the Ka (lower the pKa) They stabilize the carboxylate anion by induction

  23. Aromatic Substituent Effects electron-donating (activating) group (like OCH3) decreases acidity by destabilizing the carboxylate anion electron-withdrawing ( activating) groups (like -NO2) increase acidity by stabilizing the carboxylate anion

  24. Aromatic Substituent Effects

  25. Learning Check: Rank the following in order of increasing acidity: (#1 is least acidic) (Don’t look at a table of pKa data to help you.) A. B.

  26. Solution: Rank the following in order of increasing acidity: (#1 is least acidic) (Don’t look at a table of pKa data to help you.) A. 3 2 4 1 B. 2 3 1

  27. 20.5 Preparation of Carboxylic Acids:From Oxidation of Benzylic Carbons • Oxidation of a substituted alkylbenzene with KMnO4 or Na2Cr2O7 gives a substituted benzoic acid • 1° and 2° alkyl groups can be oxidized, but 3° are not 1o 2o 3o 3o

  28. From Oxidative Cleavage of Alkenes • Oxidative cleavage of an alkene with KMnO4 gives a carboxylic acid if the alkene has at least one vinylic hydrogen

  29. From Oxidation of 1o Alcohols & Aldehydes • Oxidation of a 1o alcohols or aldehydes with CrO3 in aqueous acid

  30. Hydrolysis of Nitriles • Hot acid or base yields carboxylic acids

  31. Halides Nitriles Carboxylic Acids • Conversion of an alkyl halide to a nitrile (by an SN2 with cyanide ion) followed by hydrolysis produces a carboxylic acid with one more carbon (RBr  RCN  RCO2H) • Best with 1o halides because competing elimination reactions occur with 2o or 3o alkyl halides

  32. Carboxylation of Grignard Reagents • Grignard reagents react w/ dry CO2 to yield a metal carboxylate • Limited to alkyl halides that can form Grignard reagents The organomagnesium halide adds to C=O of carbon dioxide • Protonation by addition of aqueous HCl in a separate step gives the free carboxylic acid

  33. Halides Nitriles Carboxylic Acids Halides  Grignard  Carboxylic Acids

  34. 20.6 Reactions of Carboxylic Acids: An Overview Like ketones, carboxylic acids undergo addition of nucleophiles to the carbonyl group Carboxylic acids transfer a proton to a base to give anions, which are good nucleophiles in SN2 reactions NaOH • LiAlH4 • H3O+ Carboxylic acids undergo substitutions reactions characteristic of neither alcohols nor ketones

  35. Learning Check: Prepare 2-phenylethanol from benzyl bromide.

  36. Solution: Prepare 2-phenylethanol from benzyl bromide.

  37. 20.7 Chemistry of Nitriles • Nitriles and carboxylic acids both have a carbon atom with three bonds to an electronegative atom, and contain a  bond • C’s of nitriles and carboxylic acids are electrophilic d- d- d+ d+ d+ d-

  38. Preparation of Nitriles:Dehydration of Amides • Reaction of primary amides RCONH2 with SOCl2 or POCl3 (or other dehydrating agents) • Not limited by steric hindrance or side reactions (as is the reaction of alkyl halides with NaCN)

  39. Mechanism: Dehydration of Amides • Nucleophilic amide oxygen atom attacks SOCl2 followed by deprotonation and elimination Nucleophilic amide oxygen atom attacks SOCl2 Deprotonation of acidic H on N Elimination of the SO2 and Cl leaving groups

  40. Reactions of Nitriles • RCºN is strongly polarized and with an electrophilic carbon • Attacked by nucleophiles to yield sp2-hybridized imine anions

  41. Hydrolysis: NitrilesCarboxylic Acids • Hydrolyzed in with acid or base catalysis to a carboxylic acid and ammonia or an amine

  42. Mechanism: Hydrolysis of Nitriles • Nucleophilic addition of hydroxide to CN bond • Protonation gives a hydroxy imine, which tautomerizes to an amide • A second hydroxide adds to the amide carbonyl group and loss of a proton gives a dianion • Expulsion of NH2gives the carboxylate

  43. Mechanism: Step 4: Hydrolysis of Amides

  44. Reduction: Nitriles 1oAmines • Reduction of a nitrile with LiAlH4 gives a primary amine Nucleophilic addition of hydride ion (H-) to the polar CN bond, yields an imine anion The C=N bond undergoes a second nucleophilic addition of hydride (H-) to give a dianion, which is protonated by water

  45. Reaction of Nitriles with Organometallic Reagents • Grignard reagents add to give an intermediate imine anion that is hydrolyzed by addition of water to yield a ketone

  46. Reactions of Nitriles: Overview

  47. Learning Check: Prepare 2-methyl-3-pentanone from a nitrile.

  48. Solution: Prepare 2-methyl-3-pentanone from a nitrile. A couple of possibilities:

  49. 20.8 Spectroscopy of Carboxylic Acids and Nitriles Infrared Spectroscopy • O–H bond of the carboxyl group gives a very broad absorption 2500 to 3300 cm1 • C=O bond absorbs sharply between 1710 and 1760 cm1 • Commonly encountered dimeric carboxyl groups absorb in a broad band centered around 1710 cm1 Free carboxyl groups absorb at 1760 cm1

  50. Infrared Spectroscopy O–H bond of the carboxyl group gives a very broad absorption 2500 to 3300 cm1 • Commonly encountered dimeric carboxyl groups absorb in a broad band centered around 1710 cm1

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