Ch. 19-20 Lect. 1 Carboxylic Acids and Derivatives • Naming Carboxylic Acids • Common Names used for the simplest acids • Rules for naming carboxylic acids • Assign number 1 to carboxy carbon and number longest chain including it • Replace –ane ending of an alkane with –oic acid ending (or –dioic acid if two) • Number and name any other substituents • Carboxylic acids have priority over any other functional group studied • Cyclic (cycloalkanecarboxylic acids) and aromatic (benzoic acids)
Structure and Properties of Carboxylic Acids • Planar sp2 hybridized structure • Hydrogen bonded dimers • High melting and boiling points because of hydrogen bonding (Table 19-2) • NMR of Carboxylic Acids • Proton NMR • Hydroxy proton strongly deshielded and variable 10-13 ppm • CH2 next to carbonyl is deshielded due to electronegativity: 2-3 ppm • Carbon NMR • Carbonyl carbon similar to aldehydes and ketones, but more shielded 180ppm
IR of Carboxylic Acids • Carbonyl stretch at 1700 cm-1 • O—H stretch at 2500—3300 is broad due to hydrogen bonding
Acid-Base Characteristics of Carboxylic Acids • Fairly strong acids • Electropositive carbonyl carbon pulls electrons away from O—H bond • Resulting carboxylate anion is stabilized by resonance • pKa’s usually between 4 and 5 • Electron withdrawing substituents increase the acidity: F3CCOOH pKa = 0.23 • Carboxylate anion named as an alkanoate: formate, acetate, pentanoate • Can be bases if protonated by other strong acids • Carbonyl oxygen is the most basic (first one protonated) • Resulting cation is stabilized by resonance (not so if hydroxy is protontated)
Synthesis of Carboxylic Acids • Oxidation of primary alcohols and aldehydes by Cr(VI) reagents • KMnO4 and HNO3 can also oxidize alcohols and adehydes to carboxylic acids 2 HNO3 + ClCH2CH2CHO ClCH2CH2COOH + 2 NO2 + H2O • Organometallic reagents attack carbon dioxide to give carboxylic acids CH3CH2Br + Mg CH3CH2MgBr CH3CH2MgBr + O=C=O CH3CH2COO- CH3CH2COO- + H+/H2O CH3CH2COOH • Hydrolysis of a Nitrile is preferred if other functional groups react with Grignard CH3CH2Br + -C≡N CH3CH2C≡N CH3CH2C≡N 1. OH- 2. H+/H2O CH3CH2COOH Mechanism Later!
The Addition-Elimination Mechanism • The carboxy carbon is attacked by nucleophiles (like other carbonyls) • In aldehydes and ketones, addition is followed by aqueous workup to give alcohol • In Carboxylic Acids and derivatives, there is a potential leaving group: elimination • Acid Catalyzed Addition-Elimination Mechanism • Base Catalyzed Addition-Elimination Mechanism
Notes on Addition-Elimination Reactions • Hydroxide (and other derivitives) are generally poor leaving groups • Acidic Carboxylic Acid will protonate most basic nucleophiles • Formation of Carboxylate anion with strongly basic nucleophilies is irreversible • Weak bases (alcohols, amines, other neutral nucleophiles) can proceed in substitution without deprotonating the acid • Relative Reactivities • Alkanoyl halides > Anhydrides > Esters > Acids > Amides • The potential leaving group decides the reactivity • Electronegative leaving group activates the carbonyl carbon • Resonance stabilizes the carbonyl group