CHEM 30 B  Dr. R. Rinehart  
Chapter 17   CARBOHYDRATES

 At last! The long-awaited transition from organic chemistry to biochemistry begins with the important class of compounds called carbohydrates - sugars, starches, and fibers. We’ll see that there is no clean-cut division between  organic chemistry and biochemistry - both will come into play.

I.  What is a carbohydrate?
            A.  Composition
                         1. Treat with heat or sulfuric acid --> carbon + water
                         2. General formula [CH₂O]_n or C_n(H₂O)_m

B.  Structure and Functionality of monosaccharides
            1. Polyhydroxy aldehydes or ketones with 3 to 9 carbons
            2. Classification by:
                      a.  Functionality: Aldoses or Ketoses
                      b.  # of C: trioses, tetroses, pentoses, hexoses, etc.
                      c.       Prefixes can be combined: e.g., glucose is an aldohexose 

C. Larger molecules assembled from monosaccharides
            1. Disaccharides: 2 monosaccharides in “glycosidic”  [acetal/ketal] linkage
            2. Oligosaccharides: 3-10 monosaccharide units 
            3. Polysaccharides: may contain thousands of monosaccharide units 

II.  Through the looking glass: STEREOCHEMISTRY
            A.  Nonidentical [nonsuperimposible] mirror-image molecules = enantiomers
            B.  Chiral centers: C with 4 different substituents [may be >1 in a molecule]
            C.  Fischer projections: a way to show stereochemistry
                        1. D- vs. L-  structures
            D.  for a structure with n chiral centers, there are 2ⁿ possible stereoisomers
            E.  Detection by rotation of plane-polarized light
                        1. Clockwise = dextrorotatory;  angle is +
                        2. Counterclockwise = levorotatory;  angle is - 

III.  Monosaccharides or “simple sugars”
            A. Physical properties
                        1. Highly soluble in water
                        2. Most taste sweet  

            B. Chemical properties
                        1. Formation of cyclic hemiacetals or hemiketals
                        2. New chiral center => 2 “anomers”  designated  a and b

                        3. Oxidation: Benedict’s test; “reducing sugars”
                        4. Formation of phosphate esters
                        5. Acetal/ketal formation with alcohols yields  nonreducing “glycosides” 

            C. Important monosaccharides [you should learn these structures]
                        1. D-glyceraldehyde [aldotriose] and dihydroxyacetone [ketotriose]
                        2. Aldopentoses: D-ribose and D-2-deoxyribose
                        3. Aldohexoses
                                    1. D-glucose [glc]
                                    2. D-galactose [gal]
                                    3. D-mannose [man]
                        3. Ketohexose: D-fructose [fru] 

IV.   Disaccharides:  glycosides [acetals or ketals] where the alcohol comes from a second monosaccharide
            A. Many possible combinations; we only care about 3
            B. Maltose : D-glc a(1à4) D-glc; formed by breakdown of starch
            C. Lactose: D-gal b(1à4) D-glc: found only in milk
            D. Sucrose: D-glc(a1↔b2)D-fru: cane or beet sugar; nonreducing

V.   Polysaccharides:  many possible; we’ll mainly look at 3
            A. Starch: 2 forms
                        1. Amylose: All D-glc with a(1à4) linkages
                        2. Amylopectin: amylose chains with a(1à6) branches every ~20 units
            B. Glycogen: “animal starch” like amylopectin, but branches more frequent
            C. Cellulose: All D-glc with b(1à4) linkages; the most abundant organic substance

VI.  Carbohydrate derivatives [probably not in your text, but you will be hearing about them at some point]
                        A. Sugar alcohols:
                                    1.  sorbitol
                                    2.  mannitol
                                    3.   inositol
                        B.  Sugar acids:
                                    1.   -onic
                                     2.  -uronic
                                    3.  –aric
                        C.  Amino sugars
                                    1. D-glucosamine
                        D. Heteropolysaccharides

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© Ronald W. Rinehart, 2002-2007