Painstaking studies of his "hexaphenylethane" and its reactivity convinced Gomberg that he had prepared the first trivalent carbon compound, triphenylmethyl radical, the discovery of which marked the emergence of fundamental organic chemistry in America. Isotopic labeling could decide whether protonated cyclopropane plays a role in Friedel-Crafts alkylation. C-13 NMR spectra of aldehydes and ketones show how characteristic chemical shifts are established empirically. The carbonyl group is thermodynamically stable but kinetically reactive. Its acid- and base-catalyzed reactions often involve loss of α-proton to form an enol or enolate intermediate. Carboxylic acids display four fundamentally different reaction patterns. Acid-catalyzed hydrolysis of acetals illustrates a multistep reaction mechanism involving the carbonyl group.