Turning Sugar Inside Out

This week's worksheet in general chemistry asks my students to analyze the chemical kinetics (the speed) of this reaction:

C₁₂H₂₂O₁₁ (sucrose or table sugar) + H₂O (water) -> C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose).

It's called the inversion of sucrose, and the resulting mix of glucose and fructose is tagged invert sugar. The name suggests that table sugar has been turned inside out or perhaps upside down, but in fact it's simply been split into two simpler sugars. The inversion is not of the sugar itself, but of the way it bends polarized light. If you pass a beam of polarized light through a solution of regular table sugar, the light will "rotate" or bend to the left. The mixture of glucose and fructose "inverts" the rotation, and bends the beam to the right.

Invert sugar is less like to crystalize than regular sugar, making it ideal for sweetening candy or jams. The reaction in the absence of a catalyst is slow at room temperature, but can be completed . The simplest way to catalyze the reaction is with an acid, often citric acid or ascorbic acid (Vitamin C), and many jam recipes call for one or the other.

So why do these compounds "bend" polarized light? Like many biological molecules, they are chiral or handed. The right handed or D form bends the polarized light toward the right, the left handed form (denoted L) left.

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Another name for the D form of glucose is dextrose, named for the direction in which it bends polarized light.