Chapter 9: Seeing Double: The Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond, and they serve as versatile intermediates and starting materials in constructing more complex molecular structures. The chapter begins by teaching students how to calculate degrees of unsaturation from both structural formulas and molecular formulas using systematic rules that account for the effects of halogens, nitrogen atoms, and oxygen atoms on saturation level. A thorough treatment of alkene nomenclature follows, covering the application of IUPAC rules for numbering carbon chains to give double bonds the lowest possible numbers, managing nomenclature when multiple double bonds are present, incorporating cyclic structures into naming conventions, and recognizing widely used common names such as ethylene and styrene. The stereochemistry section contrasts the simpler cis-trans naming system with the more powerful and unambiguous Cahn-Ingold-Prelog priority rules that enable E and Z configurational assignments. Students examine the factors that determine alkene stability, particularly how increasing substitution around the double bond and minimizing steric strain between groups affect thermodynamic favorability. The chapter then explores three major synthetic routes for forming alkenes: dehydrohalogenation, which eliminates hydrogen halides from alkyl halides to generate double bonds; dehydration, which removes water from alcohols through acid-catalyzed mechanisms; and the Wittig reaction, an exceptionally valuable transformation for constructing carbon-carbon double bonds with predictable stereochemistry. The Wittig mechanism receives detailed attention, including the formation and rearrangement of betaine intermediates and the oxaphosphetane species, illustrating how strategic understanding of mechanism leads to powerful synthetic control.