8Chapter 18

8Chapter 18 - Chapter 18 Chapter Carboxylic Acids 18.1 18.1 Carboxylic Acid Nomenclature Table 18.1 Table 18.1 systematic IUPAC names

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Unformatted text preview: Chapter 18 Chapter Carboxylic Acids 18.1 18.1 Carboxylic Acid Nomenclature Table 18.1 Table 18.1 systematic IUPAC names replace "-e" systematic ending of alkane with "oic acid" ending O HCOH Systematic Name Systematic methanoic acid O CH3COH O CH3(CH2)16COH ethanoic acid octadecanoic acid Table 18.1 Table 18.1 Table common names are based on natural origin common rather than structure rather O HCOH Systematic Name Common Name methanoic acid formic acid ethanoic acid acetic acid octadecanoic acid stearic acid O CH3COH O CH3(CH2)16COH Table 18.1 Table 18.1 Table Systematic Name Common Name O CH3CHCOH OH 2-hydroxypropanoic acid O CH3(CH2)7 (CH2)7COH C H lactic acid C H (Z)-9-octadecenoic acid oleic acid 18.2 18.2 Structure and Bonding Formic acid is planar Formic acid is planar Formic Formic acid is planar Formic acid is planar Formic O H C 120 pm H O 134 pm Electron Delocalization Electron Delocalization Electron R •• C O• •O• • •H • R + C •• – O• •• • •O• • •H Electron Delocalization Electron Delocalization Electron R •• C O• R • •O• • •H stabilizes carbonyl stabilizes group group + C •• – O• •• • •O• • •H R •• – O• •• • C +O• •H 18.3 18.3 Physical Properties Boiling Points Boiling Points Boiling O OH OH O OH OH bp 31°C 80°C 99°C 141°C Intermolecular forces, especially hydrogen Intermolecular bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight molecular Hydrogen-bonded Dimers Hydrogen-bonded Dimers Hydrogen-bonded O H O CCH3 H3CC O H O Acetic acid exists as a hydrogen-bonded Acetic dimer in the gas phase. The hydroxyl group of each molecule is hydrogen-bonded to the carbonyl oxygen of the other. carbonyl Hydrogen-bonded Dimers Hydrogen-bonded Dimers Hydrogen-bonded Acetic acid exists as a hydrogen-bonded Acetic dimer in the gas phase. The hydroxyl group of each molecule is hydrogen-bonded to the carbonyl oxygen of the other. carbonyl Solubility in Water Solubility in Water carboxylic acids are similar to alcohols in respect to their solubility in water to form hydrogen bonds to water H O H O H3CC H O H O H 18.4 18.4 Acidity of Carboxylic Acids Most carboxylic acids have a pKa close to 5. Carboxylic acids are weak acids Carboxylic acids are weak acids but carboxylic acids are far more acidic than alcohols but O CH3COH CH3CH2OH Ka = 1.8 x 10-5 pKa = 4.7 Ka = 10-16 pKa = 16 Free Energies of Ionization Free Energies of Ionization CH3CH2O– + H+ ∆ G°= 64 kJ/mol ∆ G°= 91 kJ/mol O CH3CO– + H+ ∆ G°= 27 kJ/mol O CH3CH2OH CH3COH Greater acidity of carboxylic acids is attributed Greater acidity of carboxylic acids is attributed Greater stabilization of carboxylate ion by stabilization of carboxylate ion by inductive effect of carbonyl group O – O RC δ+ resonance stabilization of carboxylate ion •• O• RC • •• – O• •• • •• – • O• •• RC O• •• •• • Figure 19.4: Electrostatic potential maps of Figure 19.4: Electrostatic potential maps of Figure acetic acid and acetate ion acetic acid and acetate ion Acetic acid Acetate ion 18.5 Substituents and Acid Strength Substituent Effects on Acidity Substituent Effects on Acidity standard of comparison is acetic acid ( X = H) standard O X CH2COH Ka = 1.8 x 10-5 pKa = 4.7 Substituent Effects on Acidity Substituent Effects on Acidity O X CH2COH X Ka pKa H 1.8 x 10-5 4.7 CH3 1.3 x 10-5 4.9 CH3(CH2)5 1.3 x 10-5 4.9 alkyl substituents have negligible effect Substituent Effects on Acidity Substituent Effects on Acidity O X CH2COH X Ka pKa H 1.8 x 10-5 4.7 F 2.5 x 10-3 2.6 Cl 1.4 x 10-3 2.9 electronegative substituents increase acidity Substituent Effects on Acidity Substituent Effects on Acidity O X CH2COH electronegative substituents withdraw electronegative electrons from carboxyl group; increase K for loss of H+ loss Substituent Effects on Acidity Substituent Effects on Acidity O X CH2COH X Ka pKa H 1.8 x 10-5 4.7 Cl 1.4 x 10-3 2.9 ClCH2 1.0 x 10-4 4.0 ClCH2CH2 3.0 x 10-5 4.5 effect of substituent decreases as number of bonds effect between X and carboxyl group increases 18.6 Ionization of Substituted Benzoic Acids Hybridization Effect Hybridization Effect Ka pKa COH COH O 6.3 x 10-5 4.2 CH COH O 5.5 x 10-5 4.3 C COH 1.4 x 10-2 1.8 O H2C HC sp2-hybridized carbon is more electronwithdrawing than sp3, and sp is more electronwithdrawing sp and sp withdrawing than sp2 withdrawing sp Ionization of Substituted Benzoic Acids Ionization of Substituted Benzoic Acids X O COH COH Substituent H CH3 F Cl CH3O NO2 ortho 4.2 3.9 3.3 2.9 4.1 2.2 effect is small unless X is effect electronegative; effect is largest for ortho substituent largest pKa meta 4.2 4.3 3.9 3.8 4.1 3.5 para 4.2 4.4 4.1 4.0 4.5 3.4 18.7 Salts of Carboxylic Acids Carboxylic acids are neutralized by strong bases Carboxylic acids are neutralized by strong bases Carboxylic O RCOH + stronger acid O HO– RCO– + H2O weaker acid equilibrium lies far to the right; K is ~ 1011 equilibrium as long as the molecular weight of the acid is as not too high, sodium and potassium carboxylate salts are soluble in water carboxylate Micelles Micelles unbranched carboxylic acids with 12-18 carbons unbranched give carboxylate salts that form micelles in give micelles water O ONa sodium stearate (sodium octadecanoate) O – CH3(CH2)16CO Na+ Micelles Micelles Micelles O ONa nonpolar polar sodium stearate has a polar end (the sodium carboxylate end) and a nonpolar "tail" carboxylate the polar end is "water-loving" or hydrophilic the nonpolar tail is "water-hating" or hydrophobic iin water, many stearate ions cluster together to n form spherical aggregates; carboxylate ions on the outside and nonpolar tails on the inside the Micelles Micelles O ONa nonpolar polar Figure 19.5 A micelle Figure 19.5 A micelle Figure Micelles Micelles Micelles The interior of the micelle is nonpolar and The has the capacity to dissolve nonpolar substances. substances. Soaps clean because they form micelles, Soaps which are dispersed in water. which Grease (not ordinarily soluble in water) Grease dissolves in the interior of the micelle and is washed away with the dispersed micelle. washed 18.8 Dicarboxylic Acids Dicarboxylic Acids Dicarboxylic Acids O HOC O O COH Oxalic acid 1.2 Malonic acid 2.8 Heptanedioic acid 4.3 O HOCCH2COH O pKa O HOC(CH2)5COH one carboxyl group acts as an electronwithdrawing group toward the other; effect withdrawing decreases with increasing separation decreases 18.9 Carbonic Acid Carbonic Acid Carbonic Acid O CO2 + H2O 99.7% HOCOH 0.3% Carbonic Acid Carbonic Acid O CO2 + H2O HOCOH O H+ + HOCO– Carbonic Acid Carbonic Acid O O CO2 + H2O HOCOH H+ + HOCO– overall K for these two steps = 4.3 x 10-7 overall CO2 is major species present in a solution of "carbonic acid" in acidic media "carbonic Carbonic Acid Carbonic Acid O HOCO– Second ionization constant: Ka = 5.6 x 10-11 O H+ + OCO– – 18.10 Sources of Carboxylic Acids Synthesis of Carboxylic Acids: Synthesis of Carboxylic Acids: Review Review side-chain oxidation of alkylbenzenes (Chapter 11) oxidation of primary alcohols (Chapter 15) oxidation oxidation of aldehydes (Chapter 17) 18.11 Synthesis of Carboxylic Acids by the Carboxylation of Grignard Reagents Carboxylation of Carboxylation of Grignard Reagents Grignard Reagents O RX Mg diethyl ether RMgX converts an alkyl (or converts aryl) halide to a carboxylic acid having one more carbon atom than the starting halide than CO2 RCOMgX H3O+ O RCOH Carboxylation of Carboxylation of Grignard Reagents Grignard Reagents •• •• O• δ– R C • MgX O • •• •• • diethyl ether O• R • C • O • + MgX •• – •• •• H3O+ R O• C • • OH •• • Example: Alkyl Halide Example: Alkyl Halide CH3CHCH2CH3 Cl 1. Mg, diethyl ether 2. CO2 2. 3. H3O+ CH3CHCH2CH3 CO2H (76-86%) Example: Aryl Halide Example: Aryl Halide 1. Mg, diethyl ether CH3 CH Br 2. CO2 2. 3. H3O+ CH3 CH CO2H (82%) 18.12 Synthesis of Carboxylic Acids by the Preparation and Hydrolysis of Nitriles Preparation and Hydrolysis Preparation and Hydrolysis off Nitriles o Nitriles – •C RX • SN2 N• • RC N• • H3O+ heat O RCOH + NH4+ converts an alkyl halide to a carboxylic acid having converts one more carbon atom than the starting halide one llimitation is that the halide must be reactive toward imitation substitution by SN2 mechanism, i.e. best with primary, substitution mechanism, then secondary…… tertiary gives elimination then Example Example NaCN CH2Cl CH2CN CH DMSO (92%) O CH2COH CH (77%) H2O H2SO4 heat Example: Dicarboxylic Acid Example: Dicarboxylic Acid BrCH2CH2CH2Br NaCN H2O NCCH2CH2CH2CN (77-86%) H2O, HCl heat O O HOCCH2CH2CH2COH (83-85%) via Cyanohydrin via Cyanohydrin O 1. NaCN CH3CCH2CH2CH3 2. H+ OH CH3CCH2CH2CH3 CN H2O OH HCl, heat CH3CCH2CH2CH3 CO2H (60% from 2-pentanone) 18.13 Reactions of Carboxylic Acids: A Review and a Preview Reactions of Carboxylic Acids Reactions of Carboxylic Acids Reactions Reactions already discussed Acidity (Chapter 18) Reduction with LiAlH4 (Chapter 15) (Chapter Esterification (Chapter 15) Reaction with Thionyl Chloride (Chapter 12) Reactions of Carboxylic Acids Reactions of Carboxylic Acids Reactions New reaction in this chapter Decarboxylation But first we revisit acid-catalyzed esterification to examine its mechanism. 18.14 Mechanism of Acid-Catalyzed Esterification Acid-catalyzed Esterification Acid-catalyzed Esterification (also called Fischer esterification) O H+ COH + CH3OH COH O COCH3 + H2O Important fact: the oxygen of the alcohol is Important oxygen incorporated into the ester as shown. Mechanism of Fischer Mechanism of Fischer Esterification Esterification The mechanism involves two stages: 1) formation of tetrahedral intermediate 1) (3 steps) 2) dissociation of tetrahedral intermediate dissociation (3 steps) (3 Mechanism of Fischer Mechanism of Fischer Esterification Esterification The mechanism involves two stages: 1) formation of tetrahedral intermediate 1) (3 steps) 2) dissociation of tetrahedral intermediate dissociation (3 steps) (3 OH C OCH3 OH tetrahedral intermediate in esterification tetrahedral of benzoic acid with methanol of First stage: formation of First tetrahedral intermediate O COH + CH3OH COH H+ OH C OH OCH3 methanol adds to the methanol carbonyl group of the carboxylic acid carboxylic the tetrahedral the intermediate is analogous to a hemiacetal hemiacetal Second stage: conversion of Second tetrahedral intermediate to ester O COCH3 + H2O H+ this stage corresponds this to an acid-catalyzed dehydration dehydration OH C OH OCH3 Mechanism of formation Mechanism of formation of of tetrahedral intermediate tetrahedral intermediate Step 1 Step CH3 •• O• C •O •• • H • O• + •H H Step 1 Step CH3 •• O• H • C •O •• • •• +O •H H CH3 H C •O •• • O• + H •O • • •H Step 1 Step •• •O • H C +O H •• •• +O H C •O •• • H carbonyl oxygen is carbonyl protonated because cation produced is stabilized by electron delocalization (resonance) (resonance) Step 2 Step •• +O H C •O •• • H CH3 •O • • •H Step 2 Step •• • OH • + C CH3 O• • OH •• • •• +O •H H C •O •• • H CH3 •O • • •H Step 3 Step •• • OH • + C • OH •• • CH3 O• •H CH3 •O • • •H Step 3 Step •• • OH • + C CH3 •H • OH •• •• • •• • OH • C • OH •• • CH3 O• •O • • •H CH3 O• •• • + H O• CH3 •H Tetrahedral intermediate Tetrahedral intermediate to to ester stage ester stage Step 4 Step •• • OH • •• C H O• •• • OCH3 •• Step 4 Step •• • OH • •• C H O• • • OCH3 CH3 •• H O• + •H Step 4 Step •• • OH • •• H CH3 C OCH3 •• + O •• H •• •O• • •H •• • OH • •• C H O• •• • OCH3 CH3 •• H O• + •H Step 5 Step •• • OH • •• H C OCH3 •• + O •• H Step 5 Step •• • OH • •• H C OCH3 •• + O •• H •• • OH • + C + •• OCH3 •• •• H O •• H Step 5 Step •• • OH • C + •• OCH3 •• •• + OH C •• OCH3 •• Step 6 Step H O+ •• O• C •• CH3 H H • •• O •• •• OCH3 •• •• +O C •• H OCH3 •• CH3 Key Features of Mechanism Key Features of Mechanism Activation of carbonyl group by protonation of carbonyl oxygen carbonyl Nucleophilic addition of alcohol to carbonyl group forms tetrahedral intermediate Elimination of water from tetrahedral intermediate Elimination restores carbonyl group restores 18.15 Intramolecular Ester Formation: Lactones Lactones Lactones Lactones are cyclic esters Lactones Formed by intramolecular esterification in a compound that contains a hydroxyl group and a carboxylic acid function Examples Examples O HOCH2CH2CH2COH 4-hydroxybutanoic acid O + O 4-butanolide IUPAC nomenclature: replace the -oic acid IUPAC ending of the carboxylic acid by -olide -olide identify the oxygenated carbon by number H2O Examples Examples O HOCH2CH2CH2COH 4-hydroxybutanoic acid O + H2O O 4-butanolide O HOCH2CH2CH2CH2COH 5-hydroxypentanoic acid O + H2O O 5-pentanolide Common names Common names β α O β γ O γ -butyrolactone α γ O δ O δ-valerolactone Ring size is designated by Greek letter Ring corresponding to oxygenated carbon corresponding A γ lactone has a five-membered ring A δ lactone has a six-membered ring Lactones Lactones Reactions designed to give hydroxy acids often yield the corresponding lactone, especially if the yield resulting ring is 5- or 6-membered. Example Example O O CH3CCH2CH2CH2COH 1. NaBH4 2. H2O, H+ O O H3C 5-hexanolide (78%) Example Example O O CH3CCH2CH2CH2COH 1. NaBH4 2. H2O, H+ via: OH O CH3CHCH2CH2CH2COH O O H3C 5-hexanolide (78%) 18.16 Decarboxylation of Malonic Acid and Related Compounds Decarboxylation of Decarboxylation of Carboxylic Acids Carboxylic Acids Simple carboxylic acids do not decarboxylate readily. O RCOH RH + CO2 Decarboxylation of Decarboxylation of Carboxylic Acids Carboxylic Acids Simple carboxylic acids do not decarboxylate readily. O RH + CO2 RCOH But malonic acid does. O O HOCCH2COH 150°C O CH3COH + CO2 Mechanism of Decarboxylation Mechanism of Decarboxylation One carboxyl group assists the loss of the other. O O HO OH OH H H O HO O H H Mechanism of Decarboxylation Mechanism of Decarboxylation One carboxyl group assists the loss of the other. O O HO OH OH H O HO H O H H O OH This compound is This the enol form of HO acetic acid. acetic H H + C O Mechanism of Decarboxylation Mechanism of Decarboxylation One carboxyl group assists the loss of the other. O O HO OH OH H HO H O H O HOCCH3 O H O OH H HO H + C O Mechanism of Decarboxylation Mechanism of Decarboxylation One carboxyl group assists the loss of the other. O O HO OH OH H O HO H O H H These hydrogens play no role. O HOCCH3 O OH H HO H + C O Mechanism of Decarboxylation Mechanism of Decarboxylation One carboxyl group assists the loss of the other. O O HO OH OH R O HO R' O R R' Groups other than H may be present. O HOCCHR' R O OH R' HO R + C O Decarboxylation is a general Decarboxylation is a general rreaction for 1,3-dicarboxylic acids eaction for 1,3-dicarboxylic acids CO2H CO 185°C 185°C CO2H CO2H CO H (74%) CH(CO2H)2 CH(CO 160°C CH2CO2H CH (96-99%) Mechanism of Decarboxylation Mechanism of Decarboxylation One carboxyl group assists the loss of the other. O O HO OH OH R O HO R' O R R' This OH group plays no role. O HOCCHR' R O OH R' HO R + C O Mechanism of Decarboxylation Mechanism of Decarboxylation One carboxyl group assists the loss of the other. O O R" OH OH R O R" R" R' O R R' Groups other than OH may be present. O R"CCHR' R O OH R'' R R" R + C O Mechanism of Decarboxylation Mechanism of Decarboxylation O O R" β α OH R R' O R"CCHR' R This kind of compound iis called a β-keto acid. s Decarboxylation of a Decarboxylation β-keto acid gives a -keto ketone. ketone. Decarboxylation of a β --Keto Acid Decarboxylation of a β Keto Acid O CH3C CH3 C CH3 O CO2H 25°C CH3C CH3 C H CH3 + CO2 Section 18.17 Section Spectroscopic Analysis of Carboxylic Acids Infrared Spectroscopy Infrared Spectroscopy A carboxylic acid is characterized by peaks due to carboxylic OH and C=O groups in its infrared spectrum. C=O stretching gives an intense absorption near 1700 cm-1. OH peak is broad and overlaps with C—H OH absorptions. absorptions. Figure 19.8 Infrared Spectrum of 4-Phenylbutanoic acid Figure 19.8 Infrared Spectrum of 4-Phenylbutanoic acid C6H5CH2CH2CH2CO2H O—H and C—H stretch monosubstituted benzene C=O 3500 3000 2500 2000 2000 1500 Wave number, cm-1 1000 500 H NMR H NMR 11 proton of OH group of a carboxylic acid is normally proton the least shielded of all of the protons in a 1H the NMR spectrum: (δ 10-12 ppm; broad). O Figure 19.9 12.0 11.0 10.0 9.0 CH2CH2CH2COH 8.0 7.0 6.0 5.0 Chemical shift (δ, ppm) Chemical 4.0 3.0 2.0 1.0 0 C NMR C NMR 13 13 13 Carbonyl carbon is at low field (δ 160-185 ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone (δ 190-215 ppm). an UV-VIS UV-VIS UV-VIS Carboxylic acids absorb near 210 nm, but UV-VIS spectroscopy has not proven to UV-VIS be very useful for structure determination of carboxylic acids. carboxylic Mass Spectrometry Mass Spectrometry Aliphatic carboxylic acids undergo a variety Aliphatic of fragmentations. Aromatic carboxylic acids first form acylium ions, which then lose CO. •• O• • ArCOH •+ O• • ArCOH ArC + O• • + Ar End of Chapter 18 ...
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