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Organic Chemistry II Carboxylic Acids and Nitriles

Organic Chemistry II Carboxylic Acids and Nitriles. Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University. Chapter Objectives. Nomenclature Properties Preparation Reactions Spectroscopy. The Importance of Carboxylic Acids (RCO 2 H).

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Organic Chemistry II Carboxylic Acids and Nitriles

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  1. Organic Chemistry IICarboxylic Acids and Nitriles Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University

  2. Chapter Objectives • Nomenclature • Properties • Preparation • Reactions • Spectroscopy

  3. The Importance of Carboxylic Acids (RCO2H) • Abundant in nature from oxidation of aldehydes and alcohols in metabolism • Acetic acid (CH3CO2H) - vinegar • Butanoic acid (CH3CH2CH2CO2H) – smell of rancid butter • Caproic acid (CH3CH2CH2CH2CH2CO2H) - smell of sweaty gym socks • Long-chain aliphatic acids from the breakdown of fats • Starting materials for acyl derivatives (esters, amides, and acid chlorides)

  4. Nomenclature • 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 C1

  5. Nomenclature • Compounds with CO2H bonded to a ring are named using the suffix -carboxylic acid • The CO2H carbon is not itself numbered in this system

  6. Common Names • Many common names in use

  7. Common Names

  8. Nitriles • Closely related to carboxylic acids • Named by adding -nitrile as a suffix to the alkane name, • Nitrile carbon is numbered 1 • Complex nitriles are named as derivatives of carboxylic acids. • Replace -ic acid or -oic acid ending with -onitrile

  9. Common Names

  10. Structural and Physical Properties • Carboxyl carbon sp2 hybridized • Carboxylic acid groups are planar with C–C=O and O=C–O bond angles of approximately 120° • Carboxylic acids form hydrogen bonds, existing as cyclic dimers held together by two hydrogen bonds • Strong hydrogen bonding causes much higher boiling points than the corresponding alcohols • Acetic acid (bp = 117.9oC) • Ethanol (bp = 78.3oC)

  11. Hydrogen Bonding

  12. 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

  13. Acidity Constant and pKa • Carboxylic acids transfer a proton to water to give H3O+ and the carboxylate anion, RCO2 • H3O+ is a much stronger acid • The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid • pKa ~ 5

  14. Acidity of Carboxylic Acids • Carboxylic acids – stronger acids than ROH • The pKa of ethanol is ~16, compared to ~5 for acetic acid • In an alkoxide ion, the negative charge is localized on oxygen while in a carboxylate ion the negative charge is delocalized over two equivalent oxygen atoms, giving resonance stabilization

  15. Resonance

  16. Henderson-Hasselbalch Equation • Covered in General Chemistry • Be certain you know Section 15.3 • You must know how to use the equation

  17. Substituent Effects on Acidity • Electron withdrawing groups increase the acidity • Promote formation of the carboxylate ion • Electron donating groups decrease the acidity

  18. Preparation of Carboxylic Acids • Oxidation of a substituted alkylbenzene with KMnO4 or Na2Cr2O7 gives a substituted benzoic acid • 1° and 2° alkyl groups can be oxidized, but tertiary groups do not react

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

  20. From Alcohols • Oxidation of a primary alcohol or an aldehyde with CrO3 in aqueous acid

  21. Hydrolysis of Nitriles • Hot acid or base yields carboxylic acids • Conversion of an alkyl halide to a nitrile (with cyanide ion) followed by hydrolysis produces a carboxylic acid with one more carbon • RBr  RCN  RCO2H • Best with primary halides because elimination reactions occur with secondary or tertiary alkyl halides

  22. Carboxylation of Grignard Reagents • Grignard reagents react with dry CO2 to yield a carboxylic acid after acid hydrolysis • Limited to alkyl halides that can form Grignard reagents

  23. Reactions of Carboxylic Acids

  24. Reactions of Carboxylic Acids • Carboxylic acids transfer a proton to a base to give carboxylic anions, which are good nucleophiles in SN2 reactions • Like ketones, carboxylic acids undergo addition of nucleophiles to the carbonyl group • In addition, carboxylic acids undergo other reactions characteristic of neither alcohols nor ketones

  25. Reduction of Carboxylic Acids • Reduced by LiAlH4 to yield primary alcohols • The reaction is difficult • Generally requires heating in THF

  26. Reduction with Borane • Borane in tetrahydrofuran (BH3/THF) converts carboxylic acids to primary alcohols selectively • Preferable to LiAlH4 because of its relative ease, safety, and specificity

  27. Chemistry of Nitriles • Nitriles and carboxylic acids both have a carbon atom with three bonds to an electronegative atom, and both contain a  bond • Both are electrophilic centers

  28. Preparation of Nitriles • Primary alkyl halides + cyanide • SN2 reaction – same constraints

  29. Preparation of Nitriles • Primary amides RCONH2 react 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)

  30. Aromatic Nitriles • Conversion of anilines into diazonium salt • Followed by CuCN/KCN

  31. Reactions of Nitriles • RCºN is strongly polarized • Electrophilic carbon atom • Attacked by nucleophiles to yield sp2-hybridized imine anions

  32. Hydrolysis of Nitriles • Hydrolyzed by acid or base to the carboxylic acid • Ammonia or an amine is other product

  33. Reduction of Nitriles • Reduction of a nitrile with LiAlH4 gives a primary amine

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

  35. Infrared Spectroscopy • Carboxylic Acids • OH and C=O absorptions • OH is very broad over 2500 – 3300 cm-1 • C=O absorbs over 1710 – 1760 cm-1 • See next spectrum

  36. Infrared Spectroscopy • Nitriles intense C-N triple bond absorption • 2250cm-1 for saturated compounds • 2230cm-1 for aromatic and conjugated compounds • Highly diagnostic for nitriles • See next spectrum

  37. NMR Spectroscopy • Carboxyl carbons resonate between 165 – 185d • Nitrile carbons resonate between 115 – 130d • CO2H resonates near 12d • Proton is exchangeable with Deuterium

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