In the last five chapters, we have seen a variety of reaction mechanisms - nucleophilic substitutions, phosphoryl transfers, carbonyl additions, and acyl substitutions - in which bonds were broken and new bonds were formed. In each case, these events involved bonds between carbon (or phosphorus) and a heteroatom - usually oxygen or nitrogen, but occasionally sulfur or a halide. We have yet to see a reaction where a carbon-carbon bond is formed or broken. In this chapter, the focus will be on two very important carbon-carbon bond-forming / bond breaking events known as 'aldol' and 'Claisen' reactions, in which stabilized carbanions act as nucleophiles or leaving groups. First, however, we will be introduced to the concept of tautomerization, and then see some examples of biochemical isomerization reactions in which two constitutional isomers or stereoisomers are interconverted via stabilized carbanion intermediates.