Carboxylic Acids: Properties, Nomenclature, and Preparation - Prof. Jonathan L. Sessler, Study notes of Organic Chemistry

Download Carboxylic Acids: Properties, Nomenclature, and Preparation - Prof. Jonathan L. Sessler and more Study notes Organic Chemistry in PDF only on Docsity! Carboxylic Acids R C O OH C O O- R + H+ R C O- O Chapter 17 No Office Hours Today (Sorry!) Recommended Problems (old): EVERYTHING IN CH 16! But if pressed for time, start with: 16.19-16.22, 16.24-16.26 16.30-16.34, 16.37, 16.38 16.40-16.42, 16.43-16.44 16.50, 16.54-16.62, 16.65 Recommended Problems (new): EVERYTHING IN CH 17! But if pressed for time, start with: 17.7-17.8, 17.12-17.13 17.17, 17.18-17.23 17.26-17.27 17.29, 17.32-17.40 Exam II will cover chapters 15.1 & 15.2-18, inclusive. Details soon. Benzoic acid Sodium benzoate Butyric acid Ammonium butyrate Naming the Salts • To name the salt of the carboxylic acid, name the cation followed by the name of the anion (two words). • The anion is named by removing -oic acid and adding ate Boiling Points • Intermolecular forces, especially hydrogen bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight bp (1 atm) 31°C 80°C 99°C OH 141°C OH OO Physical Properties • In the liquid and solid states, carboxylic acids are associated by hydrogen bonding into dimeric structures δ- δ+ δ-δ+ C O H O C OHO CH3H3C • A carboxylic acid is characterized by peaks due to 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 absorptions. Infrared Spectroscopy - Review 20003500 3000 2500 10001500 500 Wave number, cm-1 Infrared Spectrum of 4-Phenylbutanoic acid C=O O—H and C—H stretch monosubstituted benzene C6H5CH2CH2CH2CO2H 1H NMR of Carboxylic acids - Review The acidic proton in the HO- group of a carboxylic acid is normally the least shielded of all protons in a 1H-NMR spectrum: (δ 10-13 ppm; broad)…it moves and it is subject to exchange Mass Spectrometry - Review – The McLafferty rearrangement gives a characteristic peak at m/z = 60, at least for a,a- unsubstituted alkanoic acid systems. •+ •+ + McLafferty rearrangement m/z 60 H H2 C H2 C CH 2 C O C OH O CH 2 HOH H2 C H2 C ic aci = J = = ce f= Mass spectrum of butano aouepuNqy eaAnepPYy 100 60 70 50 20 10 mlz Acidity • Carboxylic acids are weak acids – The pKa of typical aliphatic and aromatic carboxylic acids falls within the range 4 to 5 R C O OH C O O- R + H+ R C O- O •The greater acidity of carboxylic acids relative to alcohols, both of which have oxyanions conjugate bases is because: • The carboxylate anion is stabilized by resonance pKa: 4.52 3.98 2.83 • The inductive effect of an electron- withdrawing substituent falls off rapidly with its distance from the carboxyl group 4-Chlorobutanoic 3-Chlorobutanoic 2-Chlorobutanoic Cl Cl Cl CH2 CH2 CH2 CO2 H CH3 CH2 CHCO2 HCH3 CHCH 2 CO2 H Acid Strength Acidity -- Review Preparation of Carboxylic Acids Recall: Carboxylic acids can be prepared from the oxidation of alcohols and aldehydes (9.8 and 16.13A). OH CrO3 H2SO4, H2O OH O butanol butyric acid H O H2CrO4 OH O H O Ag2O+ OH O 1. EtOH, NaOH 2. HCl, H2O Ag(I) is also used as the oxidant in the Tollen’s reagent which was used to generate a “silver mirror” earlier this year. Review: Grignard reagents add to CO2 to give acids. This is done in 210C to generate benzoic acid. Review: Benzoic acids may be made by oxidation of aryl alkanes. Review: Methyl ketones may be converted to carboxylic acids with loss of a carbon atom via haloform reaction. CH2 CH2 CH2CH3 CO2H H2CrO4, KMnO4, etc. + oxidized products from alkyl chain Br Mg0, I2 (cat) MgBr O C O O O [MgBr] HCl, H2O OH O + Mg2+ R C O CH3 3I2 NaOH R C O C I I I HO- R C O O- + HCI3 Iodoform Example: Reactions of Acids • Reduction • Decarboxylation • Esterification • Formation of Acid Halides Reduction • The carboxyl groups is one of the organic functional groups most resistant to reduction – it is not affected by catalytic hydrogenation under conditions that easily reduce aldehydes and ketones to alcohols, and reduce alkenes and alkynes to alkanes – it is not reduced by NaBH4 Reduction by LiAlH4 • Lithium aluminum hydride reduces a carboxyl group to a 1° alcohol – reduction is carried out in diethyl ether, THF, or other nonreactive, aprotic solvent 1 . LiAlH4 , ether 2 . H 2 O COH O + +LiOH Al(OH) 3CH2 OH Mechanism of Fischer esterification o © to form an oxoniumion, ~ o d CHa R— “H toe “OH HH then CH - 2 ie HH” \\e 7 CH3 RCE db: rs cl 20: •Activation of carbonyl group by protonation of carbonyl oxygen •Nucleophilic addition of alcohol to carbonyl group forms tetrahedral intermediate •Elimination of water from tetrahedral intermediate restores carbonyl group Key Features of Mechanism Diazomethane R O OH CH2N2 R O OCH3 N2+ ether diazomethanecarboxylic acid + nitrogen gasmethyl ester H C H N N H C H N N - -++ An easy, but dangerous, way to make methyl esters Diazomethane Fischer De-esterification • Esters can be hydrolyzed by treatment with aqueous acid. This is just the reverse of the esterification reaction. • Be sure you know mechanism! ethanol (ethyl alcohol) ethanoic acid (acetic acid) + O CH3 COH CH3 CH2 OH H2 SO4 Ethyl ethanoate (ethyl acetate) + O CH3 COCH2 CH3 H2 O Base Hydrolysis of Esters • Hydrolysis of an esters is aqueous base is called saponification • Each mole of ester hydrolyzed requires 1 mole of base; for this reason, ester hydrolysis in aqueous base is said to be base-promoted (not catalyzed) • Hydrolysis of an ester in aqueous base involves formation of a tetrahedral carbonyl addition intermediate followed by its collapse and proton transfers ++ O O RCOCH 3 NaOH RCO - Na + CH3 OH H2 O Saponification • Base-promoted ester hydrolysis R O OH R O OCH3 OH OCH3 R O OCH3 OH R O O CH3OH methyl ester (example for study of mech) + - - + - + - carboxylate anion methanol in this exampleNote: Consumption of base makes reaction essentially irreversable Soap micelle Nat Polar “head” Nonpolar “tail” ( . ¢ > A soap Cross section of a soap micelle in water Soap micelle with a dissolved grease droplet Soap micelle with “dissolved” grease Grease Soap “the solution to pollution is dilution” Decarboxylation • Decarboxylation: loss of CO2 from a carboxyl group • Most carboxylic acids, if heated to a very high temperature (fried), undergo thermal decarboxylation • Most carboxylic acids, however, are quite resistant to reasonable heat and melt or even boil without decarboxylation Decarboxylation • Thermal decarboxylation of a β-ketoacid involves rearrangement of six electrons in a cyclic six-membered transition state (A cyclic six-membered transition state) C H O C C O H H3 C O H enol of a ketone H O C C C O H3 C O H H + O CH3 -C-CH 3 CO2 This reaction will be really important when we get to chapter 19. Decarboxylation • Thermal decarboxylation of malonic acids also involves rearrangement of six electrons in a cyclic six-membered transition state enol of a carboxylic acid C HH C H C O C HO H OO H C O C HO OO H + O CH 3 -C-OH CO2 This reaction will be really important when we get to chapter 19.