optical activity

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optical activity The ability of some compounds to rotate the plane of polarized light because of the asymmetry of the molecule. If the plane of light is rotated to the right, the substance is dextrorotatory and is designated by the prefix (+); if laevorotatory (rotated to the left), the prefix is (−). A mixture of the two forms is optically inactive and is termed racemic.

Sucrose is dextrorotatory but is hydrolysed to glucose (dextrorotatory) and fructose, which is more strongly laevorotatory so hydrolysis changes optical activity from (+) to (−); hence, the mixture of glucose and fructose is termed invert sugar.

(The obsolete notation for (+) was d‐ and for (−) was l‐; this is distinct from D‐ and L‐, which are used to designate stereo‐isomerism; see D‐.)

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