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20.11 Decarboxylation of Carboxylic Acids

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20.11 Decarboxylation of Carboxylic Acids 20_BRCLoudon_pgs4-2.qxd 11/26/08 9:12 AM Page 976 976 CHAPTER 20 • THE CHEMISTRY OF CARBOXYLIC ACIDS PROBLEMS 20.20 Give the structure of a compound with the indicated formula that would give the following diol in a LiAlH4 reduction followed by protonolysis. HOCH2 CH2OH LcL (a) C8H6O3 (b) C8H6O4 20.21 Propose reaction sequences for each of the following conversions: (a) benzoic acid into phenylacetic acid, PhCH2CO2H (b) benzoic acid into 3-phenylpropanoic acid 20.11 DECARBOXYLATION OF CARBOXYLIC ACIDS The loss of carbon dioxide from a carboxylic acid is called decarboxylation. O S R C O H R H OCOAA (20.37) L L L L + Although decarboxylation is not an important reaction for most ordinary carboxylic acids, cer- tain types of carboxylic acid are readily decarboxylated. Among these are 1. b-keto acids 2. malonic acid derivatives 3. carbonic acid derivatives b-Keto acids—carboxylic acids with a keto group in the b-position—readily decarboxylate at room temperature in acidic solution. O O O S S S H O b 3 | C a C C CO2 (20.38) H C CH OH 25 °C H C CH 3 % % 2 % % 3 % % 3 + acetoacetic acid acetone (a b-ketoacid) Decarboxylation of a b-keto acid involves an enol intermediate that is formed by an inter- nal proton transfer from the carboxylic acid group to the carbonyl oxygen atom of the ketone. The enol is transformed spontaneously into the corresponding ketone (Sec. 14.5A). H O O OH O ( S % ( S CO2 (Sec. 14.5A) C "C - "C C (20.39) H C C O H C CH H C CH 3 % 3 % 2 3 % % 3 %H2 % acetone enol acetone The acid form of the b-keto acid decarboxylates more readily than the conjugate-base car- boxylate form because the latter has no acidic proton that can be donated to the b-carbonyl oxygen. In effect, the carboxy group promotes its own removal! Malonic acid and its derivatives readily decarboxylate upon heating in acidic solution. 20_BRCLoudon_pgs4-2.qxd 11/26/08 9:12 AM Page 977 20.11 DECARBOXYLATION OF CARBOXYLIC ACIDS 977 H O HO C CH CO H 3 | HO C CH CO (20.40) 2 2 135 °C 2 2 2 L L L + "CH3 "CH3 methylmalonic acid propionic acid This reaction, which also does not occur in base, bears a close resemblance to the decarboxy- lation of b-keto acids because both malonic acids and b-keto acids have a carbonyl group b to the carboxy group. O O S S HO C CH2 CO2H R C CH2 CO2H LLba L LLba L malonic acid a b-keto acid Because decarboxylation of malonic acid and its derivatives requires heating, the acids themselves can be isolated at room temperature. Carbonic acid is unstable and decarboxylates spontaneously in acidic solution to carbon dioxide and water. (Carbonic acid is formed reversibly when CO2 is bubbled into water; car- bonic acid gives carbonated beverages their acidity, and CO2 gives them their “fizz.”) O S C CO2 H2O (20.41) HO OH % % + carbonic acid Similarly, any carbonic acid derivative with a free carboxylic acid group will also decarboxy- late under acidic conditions. O S H3O| C CH3OH CO2 (20.42) CH O OH 3 % % + methyl carbonate O S H3O| H3O| C CO2 NH3 NH| 4 (20.43) H N OH 2 % % + carbamic acid Under basic conditions, carbonic acid and its derivatives exist as carboxylate salts and do not decarboxylate. For example, the sodium salts of carbonic acid, such as sodium bicarbon- ate (NaHCO3) and sodium carbonate (Na2CO3), are familiar stable compounds. Carbonic acid diesters and diamides are stable. Dimethyl carbonate (a diester of carbonic acid) and urea (the diamide of carbonic acid) are examples of such stable compounds. Like- wise, the acid chloride phosgene is also stable. O O O S S S C C C CH O OCH 3 % % 3 H2N % %NH2 Cl % %Cl dimethyl carbonate urea phosgene 20_BRCLoudon_pgs4-2.qxd 11/26/08 9:12 AM Page 978 978 CHAPTER 20 • THE CHEMISTRY OF CARBOXYLIC ACIDS PROBLEMS 20.22 Give the product expected when each of the following compounds is treated with acid. (a) O CH3 (b) LCO2H (c) CH3CH2NHCO2 Na S _ | L Ph C "C CO2H CO2H L LL "CH3 ( heat) + 20.23 Give the structures of all of the b-keto acids that will decarboxylate to yield 2-methyl- cyclohexanone. 20.24 One piece of evidence supporting the enol mechanism in Eq. 20.39 is that b-keto acids that cannot form enols are stable to decarboxylation. For example, the following b-keto acid can be distilled at 310 C without decomposition. Attempt to construct a model of the enol that would be formed when° this compound decarboxylates. Use your model to explain why this b-keto acid resists decarboxylation. (Hint: See Sec. 7.6C.) CO H )2 @O KEY IDEAS IN CHAPTER 20 I The carboxy group is the characteristic functional I Because of their acidities,carboxylic acids dissolve not group of carboxylic acids. only in aqueous NaOH but also in aqueous solutions of weaker bases such as sodium bicarbonate. I Carboxylic acids are solids or high-boiling liquids, and carboxylic acids of relatively low molecular mass are I The reaction of Grignard reagents with CO2 serves as very soluble in water. both a synthesis of carboxylic acids and a method of carbon–carbon bond formation. I The carbonyl and OH absorptions are the most impor- tant infrared absorptions of carboxylic acids.In proton I The reactivity of the carbonyl carbon toward nucle- NMR spectra the a-hydrogens of carboxylic acids ab- ophiles plays an important role in many reactions of sorb in the d 2.0–2.5 region, and the OH protons, carboxylic acids and their derivatives. In these reac- tions, a nucleophile approaches the carbonyl carbon which can be exchanged with D2O, absorb in the d 9–13 region. The carbonyl carbon resonances in the from above or below the plane of the carbonyl group 13C NMR spectra of carboxylic acids occur at d and reacts to form a tetrahedral addition intermedi- 170–180,about 20 ppm higher field than the carbonyl ate, which then breaks down by loss of a leaving carbon resonances of aldehydes and ketones. group. The result is a net substitution reaction at the carbonyl carbon. Acid-catalyzed esterification and I Typical carboxylic acids have pKa values between 4 lithium aluminum hydride reduction are two exam- and 5,although pKa values are influenced significantly ples of nucleophilic carbonyl substitution. In lithium by polar effects. Sulfonic acids are even more acidic. aluminum hydride reduction, the substitution prod- The acidity of carboxylic acids is due to a combination uct,an aldehyde,reacts further in an addition reaction of polar and resonance effects.The conjugate base of to give, after protonolysis, an alcohol. a carboxylic acid is a carboxylate ion. I The nucleophilic reactivity of the carboxylate oxygen is I Carboxylic acids with long unbranched carbon chains important in some reactions of carboxylic acids, such as are called fatty acids. The alkali-metal salts of fatty ester formation by the alkylation of carboxylic acids with acids are soaps. Soaps and other detergents are sur- diazomethane or the alkylation of carboxylate salts with factants; they form micelles in aqueous solution. alkyl halides..
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