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Carboxylic Acid Nomenclature Introduction The functional group of carboxylic acids consists of a C═O with —OH bonded to the same carbon. Carboxyl group is usually written —COOH. Aliphatic acids have an alkyl group bonded to —COOH. Aromatic acids have an aryl group. Fatty acids are long-chain aliphatic acids. © 2013 Pearson Education, Inc. Chapter 20 2 Table 18.1 Systematic IUPAC names replace "-e" ending of alkane with "oic acid" Systematic Name O HCOH methanoic acid O CH3COH ethanoic acid O CH3(CH2)16COH octadecanoic acid Table 18.1 Common names are based on natural origin rather than structure. Systematic Name Common Name O HCOH methanoic acid formic acid ethanoic acid acetic acid octadecanoic acid stearic acid O CH3COH O CH3(CH2)16COH Table 18.1 Systematic Name Common Name O CH3CHCOH 2-hydroxypropanoic acid lactic acid O OH CH3(CH2)7 (CH2)7COH C H C H (Z)-9-octadecenoic acid oleic acid or (Z)-octadec-9-enoic acid Aromatic Acids Aromatic acids are named as derivatives of benzoic acid. Ortho-, meta-, and para- prefixes are used to specify the location of a second substituent. Numbers are used to specify locations when more than two substituents are present. © 2013 Pearson Education, Inc. Chapter 20 6 Structure of the Carboxyl Group The sp2 hybrid carbonyl carbon atom is planar, with nearly trigonal bond angles. The O—H bond also lies in this plane, eclipsed with the C═O bond. The sp3 oxygen has a C—O—H angle of 106°. © 2013 Pearson Education, Inc. Chapter 20 7 Boiling Points Carboxylic acids boil at considerably higher temperatures than do alcohols, ketones, or aldehydes of similar molecular weights. The high boiling points of carboxylic acids result from formation of a stable, hydrogen-bonded dimer. © 2013 Pearson Education, Inc. Chapter 20 8 Solubility in Water Carboxylic acids are similar to alcohols in respect to their solubility in water. They form hydrogen bonds to water. H O H O H3CC H O H O H Solubility Water solubility decreases with the length of the carbon chain. Acids with more than 10 carbon atoms are nearly insoluble in water. Very soluble in alcohols. Also soluble in relatively nonpolar solvents like chloroform because the hydrogen bonds of the dimer are not disrupted by the nonpolar solvent. © 2013 Pearson Education, Inc. Chapter 20 10 Acidity of Carboxylic Acids Most carboxylic acids have a pKa close to 5. Carboxylic Acids are Weak Acids But carboxylic acids are far more acidic than alcohols. O CH3COH CH3CH2OH pKa = 4.7 pKa = 16 Energy Diagram of Carboxylic Acids and Alcohols © 2013 Pearson Education, Inc. Chapter 20 13 Acetate Ion Structure Each oxygen atom bears half of the negative charge. The delocalization of the negative charge over the two oxygens makes the acetate ion more stable than an alkoxide ion. © 2013 Pearson Education, Inc. Chapter 20 14 Substituent Effects on Acidity The magnitude of a substituent effect depends on its distance from the carboxyl group. © 2013 Pearson Education, Inc. Chapter 20 15 Aromatic Carboxylic Acids Electron-withdrawing groups enhance the acid strength, and electron-donating groups decrease the acid strength. Effects are strongest for substituents in the ortho and para positions. © 2013 Pearson Education, Inc. Chapter 20 16 Hybridization Effect O pKa COH 4.2 O H2C HC CH COH O 4.3 C COH 1.8 sp2-hybridized carbon is more electronwithdrawing than sp3, and sp is more electron-withdrawing than sp2. Salts of Carboxylic Acids Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Deprotonation of Carboxylic Acids The hydroxide ion completely deprotonates the acid to form the carboxylate salt. © 2013 Pearson Education, Inc. Chapter 20 19 Protonation of Carboxylic Acids Salts Adding a strong acid, like HCl, regenerates the carboxylic acid. © 2013 Pearson Education, Inc. Chapter 20 20 Naming Carboxylic Acid Salts First name the cation. Then name the anion by replacing the -ic acid with -ate. © 2013 Pearson Education, Inc. Chapter 20 21 Properties of Acid Salts Usually solids with no odor. Carboxylate salts of Na+, K+, Li+, and NH4+ are soluble in water. Soap is the soluble sodium salt of a long-chain fatty acid. Salts can be formed by the reaction of an acid with NaHCO3, releasing CO2. © 2013 Pearson Education, Inc. Chapter 20 22 Basic Hydrolysis of Fats and Oils • The basic hydrolysis of fat and oils produces soap (this reaction is known as saponification). © 2013 Pearson Education, Inc. Chapter 20 23 Micelles Unbranched carboxylic acids with 12-18 carbons give carboxylate salts that form micelles in water. O ONa sodium stearate (sodium octadecanoate) O – CH3(CH2)16CO Na+ Micelles O ONa nonpolar polar Micelles O ONa nonpolar polar Sodium stearate has a polar end (the carboxylate end) and a nonpolar "tail“. The polar end is hydrophilic ("water-loving”). The nonpolar tail is hydrophobic ("water-hating”). In water, many stearate ions cluster together to form spherical aggregates; carboxylate ions are on the outside and nonpolar tails on the inside. Figure 18.5: A micelle Micelles The interior of the micelle is nonpolar and has the capacity to dissolve nonpolar substances. Soaps clean because they form micelles, which are dispersed in water. Grease (not ordinarily soluble in water) dissolves in the interior of the micelle and is washed away with the dispersed micelle. Dicarboxylic Acids Dicarboxylic Acids O HOC O COH Oxalic acid 1.2 Malonic acid 2.8 Heptanedioic acid 4.3 O HOCCH2COH O pKa O O HOC(CH2)5COH One carboxyl group acts as an electronwithdrawing group toward the other; effect decreases with increasing separation. Synthesis of Carboxylic Acids: Review side-chain oxidation of alkylbenzenes (Section 11.12) oxidation of primary alcohols (Section 15.9) oxidation of aldehydes (Section 17.15) Side Chain Oxidation of Alkylbenzenes © 2013 Pearson Education, Inc. Chapter 20 32 Oxidation of Primary Alcohol to Carboxylic Acids Primary alcohols and aldehydes are commonly oxidized to acids by chromic acid (H2CrO4 formed from Na2Cr2O7 and H2SO4). Potassium permanganate is occasionally used, but the yields are often lower. © 2013 Pearson Education, Inc. Chapter 20 33 Oxidation of Aldehydes Aldehydes are easily oxidized to carboxylic acids. © 2013 Pearson Education, Inc. Chapter 18 34 Cleavage of Alkenes Using KMnO4 Warm, concentrated permanganate solutions oxidize the glycols, cleaving the central C═C bond. Depending on the substitution of the original double bond, ketones or acids may result. © 2013 Pearson Education, Inc. Chapter 20 35 Alkyne Cleavage Using Ozone or KMnO4 With alkynes, either ozonolysis or a vigorous permanganate oxidation cleaves the triple bond to give carboxylic acids. © 2013 Pearson Education, Inc. Chapter 20 36 Synthesis of Carboxylic Acids by the Carboxylation of Grignard Reagents Carboxylation of Grignard Reagents Grignard reagents react with CO2 to produce, after protonation, a carboxylic acid. This reaction is sometimes called “CO2 insertion,” and it increases the number of carbons in the molecule by one. © 2013 Pearson Education, Inc. Chapter 20 38 Example: Alkyl Halide CH3CHCH2CH3 Cl 1. Mg, diethyl ether 2. CO2 3. H3O+ CH3CHCH2CH3 CO2H (76-86%) 2-methylbutanoic acid Example: Aryl Halide 1. Mg, diethyl ether 2. CO2 CH3 3. H O+ 3 Br CH3 CO2H (82%) Synthesis of Carboxylic Acids by the Preparation and Hydrolysis of Nitriles Hydrolysis of Nitriles Basic or acidic hydrolysis of a nitrile (—CN) produces a carboxylic acid. The overall reaction, starting from the alkyl halide, adds an extra carbon to the molecule. A limitation is that the halide must be reactive toward substitution by SN2 mechanism. © 2013 Pearson Education, Inc. Chapter 20 42 Example NaCN CH2Cl CH2CN DMSO (92%) O CH2COH (77%) H2O H2SO4 heat Example: Dicarboxylic Acid BrCH2CH2CH2Br NaCN H2O NCCH2CH2CH2CN H2O, HCl O (77-86%) heat O HOCCH2CH2CH2COH (83-85%) via Cyanohydrin OH O 1. NaCN CH3CCH2CH2CH3 2. H+ CH3CCH2CH2CH3 CN H2O HCl, heat OH CH3CCH2CH2CH3 CO2H (60% from 2-pentanone) Reactions of Carboxylic Acids: A Review and a Preview Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Reactions of Carboxylic Acids Reactions already discussed Reduction with LiAlH4 (Section 15.3) Formation of acyl chlorides (Section 12.7) Esterification (Section 15.8) LiAlH4 Reduction of Carboxylic Acids LiAlH4 reduces carboxylic acids to primary alcohols. © 2013 Pearson Education, Inc. Chapter 20 48 Synthesis of Acid Chlorides The best reagents for converting carboxylic acids to acid chlorides are thionyl chloride (SOCl2) and oxalyl chloride (COCl2). They form gaseous by-products that do not contaminate the product. © 2013 Pearson Education, Inc. Chapter 20 49 Mechanism of Acid Chloride Formation Step 1 Step 2 Step 3 © 2013 Pearson Education, Inc. Chapter 20 50 Fischer Esterification Reaction of a carboxylic acid with an alcohol under acidic conditions produces an ester. Reaction is an equilibrium; the yield of ester is not high. To drive the equilibrium toward the formation of products, use a large excess of alcohol. © 2013 Pearson Education, Inc. Chapter 20 51 Mechanism of the Fischer Esterification Step 1: The carbonyl oxygen is protonated to activate the carbon toward nucleophilic attack. The alcohol attacks the carbonyl carbon. Deprotonation of the intermediate produces the ester hydrate. © 2013 Pearson Education, Inc. Chapter 20 52 Mechanism of the Fischer Esterification (Continued) Step 2: Protonation of one of the hydroxide groups creates a good leaving group. Water leaves. Deprotonation of the intermediate produces the ester. © 2013 Pearson Education, Inc. Chapter 20 53 Key Features of Mechanism Protonation of carbonyl group activates carbonyl oxygen. Nucleophilic addition of alcohol to carbonyl group forms tetrahedral intermediate. Elimination of water from tetrahedral intermediate restores carbonyl group. Intramolecular Ester Formation: Lactones Lactones Lactones are cyclic esters. Formed by intramolecular esterification in a compound that contains a hydroxyl group and a carboxylic acid function Examples O HOCH2CH2CH2COH 4-hydroxybutanoic acid O + O 4-butanolide IUPAC nomenclature: replace the -oic acid ending of the carboxylic acid by –olide. Identify the oxygenated carbon by number. H2O Examples O HOCH2CH2CH2COH 4-hydroxybutanoic acid O + O H2O 4-butanolide O HOCH2CH2CH2CH2COH 5-hydroxypentanoic acid O + H2 O O 5-pentanolide Common names O O -butyrolactone O O -valerolactone Ring size is designated by Greek letter corresponding to oxygenated carbon A lactone has a five-membered ring. A lactone has a six-membered ring. Lactones Reactions designed to give hydroxy acids often yield the corresponding lactone, especially if the resulting ring is 5- or 6-membered. Example O O CH3CCH2CH2CH2COH 1. NaBH4 2. H2O, H+ via: OH O CH3CHCH2CH2CH2COH O O H3C 5-hexanolide (78%) Decarboxylation of Malonic Acid and Related Compounds Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Decarboxylation of 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 of Malonic Acid One carboxyl group assists the loss of the other. O O O H O HO OH H HO H O H H O OH This compound is HO the enol form of acetic acid. H H + C O Mechanism of Decarboxylation of Malonic Acid One carboxyl group assists the loss of the other. O O O H O HO OH H HO H O H H These hydrogens play no role. O HOCCH3 O OH H HO H + C O Mechanism of Decarboxylation of Malonic Acid One carboxyl group assists the loss of the other. O O O H O HO OH HO R’ 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 reaction for 1,3-dicarboxylic acids CO2H 185°C CO2H CO2H H (74%) 160°C CH(CO2H)2 CH2CO2H (96-99%) Mechanism of Decarboxylation of Malonic Acid One carboxyl group assists the loss of the other. O O O H O HO OH HO R’ R O R R’ This OH group plays no role. O HOCCHR' R O OH R’ HO R + C O Mechanism of Decarboxylation of Malonic Acid One carboxyl group assists the loss of the other. O O O H O R" OH R R" R' O R R' Groups other than OH may be present. O R"CCHR' R O OH R' R" R + C O Mechanism of Decarboxylation of Malonic Acid O O R" OH R This kind of compound is called a -keto acid. R' O R"CCHR' R Decarboxylation of a -keto acid gives a ketone. Decarboxylation of a -Keto Acid O CH3 O CH3 25°C CH3C C CH3 CO2H CH3C C H CH3 + CO2 Some Important Acids Acetic acid is in vinegar and other foods, used industrially as a solvent, catalyst, and reagent for synthesis. Fatty acids from fats and oils. Benzoic acid is found in drugs and preservatives. Adipic acid is used to make nylon 66. Phthalic acid is used to make polyesters. © 2013 Pearson Education, Inc. Chapter 20 72 Spectroscopic Analysis of Carboxylic Acids IR Bands of Carboxylic Acids There will be two features in the IR spectrum of a carboxylic acid: the intense carbonyl stretching absorption (1710 cm–1) and the OH absorption (2500–3500 cm–1). Conjugation lowers the frequency of the C═O band. © 2013 Pearson Education, Inc. Chapter 20 74 IR Spectroscopy © 2013 Pearson Education, Inc. Chapter 20 75 NMR of Carboxylic Acids Carboxylic acid protons are the most deshielded protons we have encountered, absorbing between 10 and 13. The protons on the carbon atom absorb between 2.0 and 2.5. © 2013 Pearson Education, Inc. Chapter 20 76 NMR Spectroscopy © 2013 Pearson Education, Inc. Chapter 20 77 13C NMR Carbonyl carbon is at low field ( 160-185 ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone ( 190-215 ppm). Mass Spectrometry Aliphatic carboxylic acids undergo a variety of fragmentations. Aromatic carboxylic acids first form acylium ions, which then lose CO. •• O •• ArCOH •+ O •• ArCOH ArC + O •• Ar +