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Chapter 20: Carboxylic Acids and Nitriles

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Chapter 20: Carboxylic Acids and Nitriles Chapter 20: Carboxylic Acids and Nitriles 20.1 Naming Carboxylic Acids (please read) In general, the same for alkanes; replace the terminal -e of the alkane name with -oic acid The carboxyl carbon atom is C1 Compounds with -CO2H group on a ring are named using the suffix -carboxylic acid The -CO2H carbon is not numbered in these cases Many non-systematic names: Table 20.1 174 20.1 Naming Nitriles (please read) If derived from open-chain alkanes, replace the terminal -e of the alkane name -nitrile as a suffix The nitrile carbon numbered C1 Complex nitriles are named as derivatives of carboxylic acids. Replace -ic acid or -oic acid ending with -onitrile 175 88 20.2 Structure and properties of carboxylic acids The carboxylic acid function group contains both a hydrogen bond donor (-OH) and a hydrogen bond acceptor (C=O) Carboxylic acids exist as hydrogen bonded dimers H-bond aceptor O O H O H-bond C H H3C C C CH3 H C O donor 3 O H O acetic acid 176 20.3 Dissociation of Carboxylic acids Acidity Constant and pKa Carboxylic acids react with base to give carboxylate salts O O C H Na + NaOH C + H2O R O R O Bronsted Acidity (Chapter 2.7): + Carboxylic acids transfer a proton to water to give H3O and − carboxylate anions, RCO2 O O C + H2O C + H3O R OH R O - + [RCO2 ] [H3O ] Ka= pKa= - log Ka [RCO2H] typically ~ 10-5 typically ~ 5 for for carboxylic acid carboxylic acid 177 89 CH3CH3 CH3CH2OH PhOH CH3CO2H HCl pKa ~50-60 16 10 4.75 -7 Increasing acidity The negative charge of a carboxylate is resonance stabilized H2O H3C-H2C-OH H3C-H2C-O + H3O pKa~ 16 O H2O O O + H O C C C 3 H3C OH H3C O H C O pKa~ 4.7 3 ! 4 electron delocalized O O C over three p-prbitals H3C C O O ! C-O bond length of a carboxylates are the same 178 20.4 Substituent Effects on Acidity Substituents on the α-carbon influence the pKa of carboxylic acids largely through inductive effects. Electron- withdrawing groups increase the acidity (lower pKa) and electron-donating groups decrease the acidity (higher pKa) also see Table 20.4 Inductive effects work through σ-bonds, and the effect falls off dramatically with distance 179 90 20.5 Substituent Effects in Substituted Benzoic Acids An aromatic or substituted aromatic rings has dramatically less influence on the acidity of benzoic acids, pKa ~ 4.2 (when compared to acetic acid) than in the case of phenols OH CO2H H3CH2C-OH H3C-CO2H R R pKa ~ 16 R=H, pKa ~9.9 pKa ~ 4.75 R=H, pKa ~ 4.2 Cl 9.4 Cl 4.0 NO2 7.1 NO2 3.4 CH3 10.2 CH3 4.3 OCH3 10.2 OCH3 4.5 The charge of the carboxylate ion cannot be delocalize into the aromatic ring Electron-donating groups decrease the acidity Electron-withdrawing groups increase the acidity 180 20.6 Preparation of carboxylic acids 1. Benzoic acids are prepared from the oxidation of alkyl benzene with KMnO4 (Chapter 16.10) CH CH CH 2 3 3 CO2H KMnO4 O N 2 O2N Alkyl benzene 2. Oxidation of primary alcohols with chromic acid (CrO3/HCl) (Chapter 17.18) 3. Oxidation of aldehydes with chromic acid or Ag2O (Chapter 19.3) CrO3/HCl OH CO2H 1° alcohol CrO3/HCl CHO CO2H -or- Ag O Aldehyde 2 181 91 20.6 Preparation of carboxylic acids 4. Acid or basic hydrolysis of a nitrile (mechanism, Fig. 20.4) cyanide ion is an excellent nucleophile and will react with 1° and 2° alkyl halides and tosylates to give nitriles. This reaction add one carbon. + NaC!N H3O H CH CH CH C-Br H3CH2CH2CH2C-C!N H3CH2CH2CH2C-CO2H 3 2 2 2 DMSO -or- NaOH C4 C5 C5 + NaC!N H3O Br C!N CO2H DMSO -or- PhO NaOH PhO + O NaC!N H3O . and don’t forget HO CN HO CO2H 182 20.6 Preparation of carboxylic acids 5. Carboxylation of Grignard reagents Reaction of a Grignard reagent with CO2 gives a carboxylic acid O O Br Mg(0) H O+ MgBr CO2 C 3 C O OH MgBr O _ C O 183 92 20.7 Reactions of carboxylic acids: an overview O base Deprotonation H O (chapters 20.3 - 20.5) H H [H] Reduction H OH (chapters 17.5 & 20.8) O H OH O α-Substitution R OH (chapter 22) O Nucleophilic acyl H Y substitution (chapter 21) 184 20.8 Reductions of carboxylic acids Carboxylic acids are reduced to primary alcohols by LiAlH4 (Chapter 17.5) and borane (BH3) but not by NaBH4 a. LiAlH4, THF + b. H3O R-CO2H RCH2OH Lithium aluminium hydride will also reduce ketones, aldehydes esters, nitriles, amides, acid chlorides alkyl halides, epoxides and nitro groups Borane reacts most rapidly with carboxylic acids to give primary alcohols and it is possible to reduce carboxylic acids in the presence of some other functional groups a. BH3 , THF + OH b. H3O CO2H O2N O2N 185 93 20.9 Chemistry of Nitriles: Preparation of nitriles 1. Reaction of cyanide ion with 1° and 2° alkyl halides- this is an SN2 reaction. 2. Cyanohydrins- reaction of cyanide ion with ketones and aldehydes (Chapter 19.7) 3. Dehydration of primary amides with POCl3 or SOCl2 O SOCl2 -or- POCl C 3 Dehydration: formal loss R NH2 R C N benzene, 80°C of H2O from the substrate Primary amide nitrile Mechanism (p. 751, please read): the carbonyl oxygen of an amide is nucleophilic, and can react with electrophiles O O A very important resonance form of an C C amide, which contributes significantly R NH 2 R NH2 to their properties and reactivity 186 O O O Cl S S S O O Cl Cl O Cl O Cl C C C C H R NH2 R NH2 R NH2 R N O H Cl S O Cl - HCl C - SO2, - HCl R N R C N H Cl - Cl - R C N H 20.9 Chemistry of Nitriles: Reactions of Nitriles The C≡N bond has a large dipole moment like carbonyls, making the carbon a potential site for nucleophilic attack O O H-A OH aldehydes C C O ! - C Nu & ketones Nu ! + C µ~ 2.8 D :Nu - N N N N NH2 N ! nitriles H-A C C C C C C ! + µ ~ 3.9 D :Nu Nu Nu R Nu R Nu :Nu R R R R Nu Nu NH + O H-A H3O C -or- C 187 R Nu HO R Nu 94 1. Hydrolysis of nitriles to carboxylic acids and amides: Nitriles are hydrolyzed in either aqueous acid or aqueous base to give carboxylic acids. The corresponding primary amide is an intermediate in the reaction. Mechanism of the base-promoted reaction (Figure 20.4) 188 Please try the acid-promoted mechanism 2. Reduction of nitriles to primary amines Nitriles can be reduced by LiAlH4 or BH3 (but not NaBH4) to give primary amines H 2 N N + NH2 LiAlH4 H3O R C N C C C ether R H R H R H H H H Reduction of a nitrile with Diisobutylaluminium hydride (DIBAH) give an imine, which easily hydrolyzes to the aldehyde (not in book) Al(iBu) H 2 DIBAH + NH O N H3O R C N C toluene C C R H R H R H 189 95 3. Reaction of nitriles with Grignard reagents: general method for the preparation of ketones H3C MgBr MgBr N + NH O THF H3O R C N C C C R CH R CH3 R CH3 3 Must consider functional group compatibility; there is wide flexibility in the choice of Grignard reagents. Summary: primary amines CH2NH2 CHO aldehydes LiAlH4 DIBAH Br C!N O Na+ -CN MgBr ketones SN2 + H3O CO2H 190 carboxylic acids 20.10 Spectroscopy of Carboxylic Acids and Nitriles Infrared Spectroscopy Carboxylic acids: Very broad O-H absorption between 2500 - 3300 cm−1 Strong C=O absorption bond between 1710 - 1760 cm−1 Usually broader than the O-H absorption of an alcohol H O carboxylic acid alcohol 191 96 Nitriles show an sharp C≡N absorption near 2250 cm−1 for alkyl nitriles and 2230 cm−1 for aromatic and conjugated nitriles (highly diagnostic) C N H3CH2CH2C C N 192 1H NMR The -CO2H proton is a broad singlet near δ ~12 When D2O is added to the sample the -CO2H proton is replaced by D causing the resonance to disappear (same for alcohols) -CO2H 193 97 The nitrile function group is invisible in the proton NMR. The effect of a nitrile on the chemical shift of the protons on the α-carbon is similar to that of a ketone. H3CH2CH2C C N δ= 2.3 (2H, t) 1.7 (2H, m) 1.1 (3H, t) 194 13C NMR The chemical shift of the carbonyl carbon in the 13C spectrum is in the range of ~165-185. This range is distinct from the aldehyde and ketone range (~190 - 220) The chemical shift of the nitrile carbon in the 13C spectrum is in the range of ~115-130 (significant overlap with the aromatic region). Ph-H2C-CO2H H3CH2CH2C C N 133.2 129.3 128.5 41.1 19.3, 13.3 127.3 19.0 CDCl3 CDCl 119.8 TMS 178.2 3 195 98 1.92 C10H11N 3.72 (2H, dt, J=7.4, 7.1) (1H, t, J=7.1) 7.3 1.06 (5H, m) (3H, t, J=7.4) 129.0 128.0 127.3 29.2 38.9 11.4 CDCl3 135.8 120.7 TMS 196 99.
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