Michael McBride teaches this lecture from Yale's Freshman Organic Chemistry II (CHEM 125B), covering the reactions and spectroscopy of carboxylic acids and their derivatives. He works through nucleophilic substitution and decarboxylation pathways that leave behind enols, free radicals, or alkyl halides, then reviews how IR spectroscopy, including vibrational coupling, helps determine the structure of anhydrides and imides. The lecture explains how acid derivatives interconvert through tetrahedral intermediates driven by differences in acidity, examines the challenge of achieving selective reduction, and covers acidic and basic routes converting nitriles to carboxylic acids. It closes with ketene chemistry and the Baeyer-Villiger oxidation, which inserts an oxygen atom into a ketone's acyl-R bond to form an ester. Recorded in Spring 2011 as part of Open Yale Courses, the talk assumes familiarity with prior lectures in the sequence.