Chemistry 2223B Lecture Notes - Lecture 5: Anomer, Glucoside, Mutarotation
10 Feb 2016
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Carbohydrates (cid:121) 20 (cid:120) because hemiacetals readily revert back to their carbonyl and alcohol components, anomers interconvert in aqueous solutions. Even at neutral ph, there is sufficient acid present to catalyze the reaction. (cid:120) thus, when one anomer is dissolved, an equilibrium mixture of both will form over time. This process can be monitored by optical rotation and is called mutarotation. Note that the rotations cannot be predicted and must be experimentally measured. Monosaccharide (cid:68)-d-glucose (cid:69)-d-glucose (cid:68)-d-galactose (cid:69)-d-galactose pure anomer [(cid:68)] 72: formation of glycosides (acetals of sugars) Carbohydrates (cid:121) 21 (cid:120) hemiacetals can react with another alcohol to form glycosides, which are acetals of sugars. The alcohol is an oxygen nucleophile, so these are also called o-glycosides. Carbohydrates (cid:121) 22 (cid:120) the mechanism of hemiacetal to glycoside conversion is an acid-catalyzed sn1, which involves a carbocation intermediate. A mixture of (cid:68) and (cid:69) glycosides is formed regardless of which anomer of the hemiacetal we start with.
