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Chapter 20

Carboxylic Acids and Nitriles - Lecture and Textbook Notes Chapter 20

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Cecilia Kutas

CHM247H1 Jasmyn Lee Chapter 20: Chemistry of Carboxylic Acids and Nitriles Carboxylic Acids in Nature Some Biologically active Nitriles  Both are aromatase inhibitors  Aromatase is the enzyme responsible for estrogen synthesis  So these are anti-tumor drugs for estrogen stimulated tumors 20.1 Naming Carboxylic Acids and Nitriles Naming Carboxylic Acids, R-COOH  Replace “e” in parent alkane name with “oic acid”  Eg/ (S)-3-ethyl-3-(p-methylphenyoxy) hexanoic acid o Note: carbon count in hexanoic includes the COOH carbon  Eg/ 1-cyclopentene carboxylic acid o Note: cyclopentene does not include the –COOH o “1-“ gives the position of alkene “Oh My Such Good Apple Pie” Oxalic, Malonic, Succinic, Glutaric, Adipic, Pim(heptanedioic) 1 CHM247H1 Jasmyn Lee Naming Nitriles, RC≡N  Complex nitriles are named as derivatives of carboxylic acids  Replace –ic acid or –oic acid or –oxylic acid ending with –onitrile 20.2 Structure and Properties of Carboxylic Acids  Carbonyl carbon is sp hybridized  carboxylic acid group is planar with C-C=O and O=C-O bond angles ~120°  Strongly associated because of H-bonding o Most carboxylic acids exist as cyclic dimers held together by two H-bonds o High boiling point (higher than corresponding alcohol)  Acidic – react with bases to give metal carboxylate salts,2RCO M  Carboxylic acids with more than 6 carbons are only slightly soluble in water – but the alkali metal salts are highly soluble in water o Possible to purify an acid by extracting its salt into aqueous base, then reacidifying and extracting the pure acid back into an organic solvent The Acidity of Carboxylic Acids  Carboxylic acids dissociate slightly in dilute aqueous solution to 3ive H O and the corresponding carboxylate anions, RCO  extent of dissociate given by acidity constant, K 2 a  Carboxylic acids are more acidic than alcohols o Alcohol dissociates to give an alkoxide ion – negative charge is localized on a single EN atom o Carboxylic acid dissociates to give a carboxylate ion – stabilized resonance hybrid of two equivalent structures 2 CHM247H1 Jasmyn Lee 20.4 Substituent Effects on Acidity  Dissociation of a carboxylic acid is an equilibrium process  Any factor that stabilizes the carboxylate anion relative to undissociated carboxylic acid will drive the equilibrium toward increased dissociation – result in increased acidity o Electron Withdrawing Effect o Inductive Effects (Eg/ halogen; closer to C=O, more stable anion, more acidic) o Eg/ CF COOH a stronger acid than CH COOH – 3 electron withdrawing F atoms delocalize negative 3 3 charge in the trifluoroacetate anion; stabilizes anion and increases acidity  Which acid is stronger? Why? O O OH OH CH3 NC Deactivating (EWG) Acidity of p-substituted benzoic acids  Electron Withdrawing Substituents o Increase the acidity of benzoic acid o Decreases SEAr reaction rates  Electron Donating Substituents o Decrease the acidity of benzoic acid o Increase E Ar reaction rates Activating (EDG) Eg/ Use resonance structures to show which acid is more acidic COOH COOH O N 2 NO 2 20.5 Preparing Carboxylic Acids Synthesis of Carboxylic Acids 1. Oxidation of 1° alcohols or aldehydes 2. Ozonolysis of alkynes 3. Hydrolysis of esters, amides, acid chlorides… 4. Grignard + CO  carboxylation – bubbling CO gas through solution of Grignard reagent 2 2
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