Reactions of Monosaccharides

1. Reactions of Monosaccharides

2. Reactions of Monosaccharides Oxidation of sugars • Oxidation with Cu+ The Benedict’s test • Sugars that are capable of producing a positve Benedicts test are called reducing sugars.

3. Oxidation of Aldehydes Aldehydes can be oxidized with the copper(II) ion (Cu2+) • The Cu2+ ion forms a clear blue solution • The Cu+ that is produced in the reaction forms an orange/red precipitate. • The reaction is called the Benedict’s reaction, and has been used for years in a clinical setting to test for the presence of glucose in the urine. Cu2+ Cu2+ + Cu+ Cu+

4. Reactions of Monosaccharides Oxidation of sugars • Even though ketones should not give a positive Benedict’s test, ketoses do. • This is because under the basic conditions of the test, the ketoses can isomerize to form aldoses, which react. • Benedict’s test has been replaced with a more specific, enzyme catalyzed, glucose oxidase test.

5. Reactions of Monosaccharides Reactions with alcohols to form hemiacetals and hemiketals • Since monosaccharides contain both hydroxyl groups along with either aldehyde or ketone groups, they can react to form hemiacetals and hemiketals.

6. Reactions of Alcohols with Aldehydes and Ketones The first reaction, which is similar to the reduction of aldehydes and ketones, involves adding an alcohol across the carbonyl to form a hemiacetal (from aldehydes) or a hemiketal (from ketones).

7. Reactions of Alcohols with Aldehydes and Ketones In carbohydrates, the carbonyl group and the alcohol that react, can come from the same molecule. • This will produce a ring molecule.

8. Cyclic Form of Monosaccharides Monosaccharides form rings by reacting one of the hydroxyls with the carbonyl to form a hemiacetal or hemiketal:

9. Cyclic Form of Monosaccharides Usually these are drawn using a Haworth project: • The OH’s that were on the right in the Fisher projection are placed in the down position on the Haworth projection • The OH’s that were on the left in the Fisher projection are placed in the up position on the Haworth projection • The CH2OH on the number 5 carbon points up for D sugars and down for L sugars.

10. Cyclic Form of Monosaccharides The hemiacetal or hemiketal carbon that forms in the ring is called the anomeric carbon. • The anomeric carbon is also chiral, which increases the number of chiral carbons by 1 and increases the number of stereoisomers. • The two forms of the anomeric carbon are designated as α or β. • The α-anomer has the -OH pointing down in the ring form. • The β-anomer has the -OH pointing up in the ring form.

11. Cyclic Form of Monosaccharides When naming the ring forms of monosaccharides, the endings -pyranose and -furanose to designate the six-member and five-member rings, respectively.

12. Cyclic Form of Monosaccharides In general, the -OH on the chiral carbon furthest from the the carbonyl is the one that reacts to from the pyranose or furanose ring. This means that • Aldohexoses will form pyranose rings: • Aldopentoses and ketohexoses will form furanose rings: D-glucose(aldohexose) D-ribose(aldopentose) D-fructose(ketohexose)

13. إن الله وملائكتــه يصلــون على النبي يــأيــها الذيـن آمنوا صلــوا عليه وسلمـــوا تسليمـــا