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Reduction with sodium borohydride (NaBHâ‚„) is a fundamental concept in carbohydrate chemistry. NaBHâ‚„ acts as a reducing agent, converting carbonyl groups in ketones and aldehydes into alcohol groups. This transformation is particularly important when dealing with sugars. The reducing process involves the addition of hydrogen to the carbonyl carbon, turning it into a hydroxyl group.

In the context of sugars, NaBHâ‚„ reduces the ketone group present in ketohexoses like fructose, transforming them into alcohols. Unlike aldohexoses, such as glucose or mannose, which primarily possess aldehyde groups, ketohexoses have ketone functional groups. As a result, when fructose is reduced, the ketone group on the sugar is transformed, hence creating new chiral centers and resulting in different alcohol products.

Chiral centers play a critical role in determining the properties and behavior of molecules in chemistry, especially in sugars. A chiral center is a carbon atom bonded to four distinct groups, leading to molecules that are non-superimposable mirror images, known as enantiomers.

When reducing fructose with NaBHâ‚„, a chiral center is introduced at the site of the former ketone group. This happens because the reduction creates two possible alcohol products depending on the direction from which the hydride ion (source of hydrogen from NaBHâ‚„) attacks the carbonyl group. This leads to the formation of two different epimers, sorbitol and mannitol.

• Epimers are a type of stereoisomer differing only at one chiral center.
• In this scenario, the creation of these two epimers is why we see an equimolar mixture of sorbitol and mannitol when fructose is reduced.

Understanding the difference between ketohexoses and aldohexoses is integral to grasping their behavior under reduction. Ketohexoses, such as fructose, contain a ketone group in their structure, while aldohexoses, like glucose and mannose, contain an aldehyde group.

When a ketohexose like fructose undergoes reduction with NaBH₄, the ketone at the second carbon (C2) is targeted. The reduction at this site creates two stereoisomers—D-mannitol and D-sorbitol—due to the creation of a new chiral center. This process results in an equimolar mixture of both stereoisomers, distinguishing it from aldohexoses and aldopentoses like glucose, mannose, or xylose, which do not form such equimolar mixtures under the same conditions. Thus, the reduction of aldoses typically does not yield multiple stereoisomers in equimolar ratios as seen with the ketohexose fructose.