1 Chapter 20: Carboxylic Acids I. Introduction: Carboxylic acid structure: Classification of carboxylic acids: A carboxylic acid donates protons by the heterocyclic cleavage of the O-H bond, generating a carboxylate ion. II. Nomenclature A. IUPAC Nomenclature of Carboxylic Acids 1) Find the longest carbon chain which contains the carboxylic acid (COOH) carbon. a) this is your parent b) number nearest the -COOH 2) Add the suffix -anoic acid. 3) Name substituents and assign stereochemistry where necessary.
2 Example(s) B. Common Names of Carboxylic Acids (MEMORIZE) Methanoic Ethanoic Acid acid Formic Acid Acetic acid Propanoic acid Propionic acid Butanoic acid Butyric acid Hexanoic acid Caprioc acid
3 C. IUPAC Names of Dicarboxylic Acids 1) Find the longest carbon chain which contains both carboxylic acid (COOH) carbons. (number nearest the substituents Ha!) 2) Count the number of carbon in this chain and name it as the corresponding alkane. 3) Add the suffix -dioic acid. D. Common Names of Dicarboxylic Acids (MEMORIZE) E. Nomenclature of Carboxylate Salts 1) Name the related carboxylic acid. 2) Replace the -ic acid suffix with -ate. Add the name of the positive counter ion in front to complete the name of the carboxylate salt:
4 III. Structure of the Carboxyl Group A. Lewis Structure B. Physical Properties C. Solubility
5 D. Acidity Substituents that decrease the electron density at the neighboring carbon will increase the acidity of the compound. What about benzoic acid derivatives? Same idea! E. Acidity of Dicarboxylic Acids
6 F. Physical Properties of Salts IV. Syntheses of Carboxylic Acids A. Oxidation of 1 o Alcohols and Aldehydes Observed Reactions B. Oxidative Cleavage of Alkenes Alkenes react with concentrated, warm, KMnO 4 to produce intermediate glycols which react further to produce either carboxylic acids or ketones (depending on the original alkene substituents).
7 Alkynes also react with conc, warm,.kmno 4 to give carboxylic acids, and the same transformation can be achieved by the use of ozonolysis. C. Oxidation of Alkylbenzenes Benzoic acid derivatives can be made by the oxidation of alkylbenzenes with either hot KMnO 4 or hot chromic acid. The vigorous conditions means this can only be used when there are no oxidizable groups present in the molecule. D. Reduction of CO 2 with Grignard Reagents Grignard reagents can react as nucleophiles and attack carbon dioxide. This results in magnesium salts of carboxylic acids, and treatment with dilute acid produces a carboxylic acid.
8 E. Hydrolysis of Nitriles (Heteroatom Exchange) Nitriles are easily made by using the cyanide ion as a nucleophile on alkyl halides (or tosylates). Mechanism:
9 V. Reactions of Carboxylic Acids (and Derivatives) Ketones and aldehydes have a carbonyl group and undergo nucleophilic addition. Carboxylic acids (and their derivatives) react via nucleophilic acyl substitution - S N Ac. A. S N Ac Nucleophilic acyl substitution can interconvert all of these different acid derivatives. Two Conditions: Case #1 Basic Conditions
10 Case #2: Acidic Conditions B. The Fischer Esterification Observed Reaction : Mechanism:
11 Esterification reactions such as this one are equilibrium reactions, and the equilibrium normally will lie to the. How can we obtain more product? C. Esterification using Diazomethane Diazomethane is a toxic, explosive yellow that dissolves in ether, and requires special glassware. Observed Reaction Mechanism
12 D. Condensations of Acids with Amines ( Amides) Although the acid chloride/amine reaction generates amides, it is also possible to synthesize amides directly from carboxylic acids. The direct reaction of an amine and a carboxylic acid initially forms a carboxylate anion and an ammonium cation. Normally the reaction would stop as this point since the carboxylate anion is a poor electrophile, but by heating the reaction to over 100 C, the water can be driven off as steam, and amide products are formed. This is an important commercial process. E. Reduction of Carboxylic Acids 1. Use of Lithium aluminum hydride and followed by H 2 O.
13 2. Use of NaBH 4 (only works for aldehydes and ketones) 3. Use of LiAl(tBuO) 3 H 4. Borane as BH 3 (reacts with COOH fastest)
14 F. Alkylation of Carboxylic Acids to form Ketones A general method to form ketones is the reaction of a carboxylic acid with two equivalent of organolithium reagent. CASE #1 CASE #2
15 G. Synthesis of Acid Chlorides The carbonyl oxygen and the chlorine are electron withdrawing, and thus make the carbonyl carbon very electrophilic. The mechanism of formation of acid chloride is similar to the reaction of alcohols with thionyl chloride. Note that this mechanism is slightly different from the text.
16 H. Reaction with Alcohols Acid chlorides react with alcohols to give esters through a nucleophilic acyl substitution by the addition elimination mechanism. The overall transformation of this two step scheme is a carboxylic acid is converted into an acid chloride, then into an ester. I. Reaction with Ammonia Ammonia and amines react with acid chlorides to give amides, also through this addition-elimination version of nucleophilic acyl substitution.