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Chapter 20: Carboxylic Acids 259 Class I carbonyl are known as Carboxylic Acids and Carboxylic acid Derivatives. Their reactions follow the same pattern and it is easy to go from one structure to the next within the class itself by Nucleophilic Acyl Substitution. cannot be replaced by a nucleophile O O R H R R ketone aldehyde Class II compounds with groups that can be replaced by a nucleophile O O O R OH R OR carboxylic ester acid R O O O R anhydride Class I R O X R NH2 acyl halides amides 260 • Physical Properties (20(20-3) Boiling point of carbonyl usually follow this trend: Amide > acid > nitrile >> ester > acid chloride > aldehyde > ketone The high boiling points of amide, acid and nitrile is due to strong dipole interactions that exist in all three types. 261 Acidity: pKa of carboxylic acid is about 5. They can therefore be easily deprotonated forming the carboxylate anion. 262 The acidity of carboxylic acids can be influenced by the nature of the substituent bonded to the carboxyl group: 263 Synthesis of Carboxylic Acids (Review) • Oxidation Other reagents such as KMnO4, HNO3 etc… will also oxidize a 1o alcohol to the corresponding carboxylic acid. 264 • Oxidative cleavage Alkenes and Alkynes can give carboxylic acid by oxidative cleavage: either with ozone or KMnO4. 265 • Oxidation of alkyl benzene Also provides carboxylic acids as the product. Must have at least one benzylic hydrogen. 266 New Synthesis of Carboxylic Acids • Carboxylation of Grignard Reagents (20-8) Grignard reagents react with CO2 like they do with any other carbonyl compound. Once protonated, the product is a carboxylic acid. 267 • Hydrolysis of Nitriles (20-8) Nitriles can be hydrolyzed to the corresponding carboxylic acid, either under acidic or basic conditions. (mechanisms will be discussed in Chapter 21) We have seen that they can easily formed by SN2 reactions with primary halides. 268 Practice Questions Complete the following reactions. 1) KMnO4 2) H3O+ CN H3O+ heat 269 Provide a detailed stepwise mechanism for the last reaction from the previous slide. 270 Reactions of Carboxylic Acids (Review) • Alkylation of Acids to form Ketones (20-14) 271 • Reduction of Acids to Primary Alcohols (20-13) 272 Nucleophilic Acyl Substitution (20-9) Most reactions of carboxylic acids are classified as “acyl substitutions” and usually produce acid derivatives. 273 The hydrolysis of an ester under basic conditions is a good example of acyl substitution. 274 The Fisher Esterification (20-10) 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 to the formations of products use a large excess of alcohol. 275 Mechanism of Fisher Esterification 276 Esterification Using Diazomethane (20-11) One of the best way to make methyl ester is to react the corresponding carboxylic acid with Diazomethane. The reaction is quantitative and very easy…but diazomethane has a tendency to explode…. 277 Mechanism of Diazomethane Esterification 278 Condensation of acids with amines (20-12) The reaction of carboxylic acids with amines is a simple acid/base reaction forming an ammonium salt. However, under more extreme conditions, when heat is applied it is possible to eliminate water from this salt and form amide. 279 Synthesis and Use of Acid Chlorides (20-15) Acid chlorides are the most versatile acid derivatives because they can be transformed into all other acid derivatives by acyl substitution. This is due to the presence of a good leaving group, chloride ion. 280 The best reagent for converting carboxylic acids to acid chlorides are thionyl chloride (SOCl2) and oxalyl chloride (COCl2) because they form gaseous by-products that do not contaminate the product. Thionyl chloride reaction produces SO2 while the oxalyl chloride reaction produces HCl, CO, and CO2 (all gaseous). 281 Mechanism of Acid Chloride Formation 282 As mentioned before, reactions of acid chlorides lead to acid derivatives such as esters, amide and others. 283