A reducing sugar will undergo the Fehling reaction, which requires a (potential) free aldehyde group. Which of the disaccharides shown in Figure 9.14 are reducing and which are nonreducing?  

Figure 9.14

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Disaccharides with a-connections Maltose: a-D-glucopyranosyl (1-4)a-D-glucopyranose a,a-Trehalose: a-D-glucopyranosyl (1-1) -D-glucopyranose Sucrose: a-D-glucopyranosyl (1-2)B-D-fructofuranoslde Н HO H HO Н HO CH₂OH H OH H Н OH a-D-Glc CH₂OH Н OH H Н a-D-Glc OH CH₂OH H OH H Н OH a-D-Glc H Н H Н Н CH₂OH Н OH H Н «-D-GIC Н U OH OH H OH CH₂OH HO Н a-D-Glc H HO OH H B-D-Fru Н OH H т. OH Н CH₂OH Disaccharides with ß-connections Celloblose: B-D-glucopyranosyl (1-4)B-D-glucopyranose Lactose: B-D-galactopyranosyl (1-4)B-D-glucopyranose Gentloblose: B-D-glucopyranosyl (1-6)B-D-glucopyranose Н HO HO H H HO CH₂OH H OH H Н B-D-GIC OH CH₂OH H OH H Н OH B-D-Gal CH₂OH H OH H Н OH B-D-GIC H Н Н Н Н Н HO CH₂OH H OH Н Н OH B-D-GIC CH₂OH H OH H Н OH B-D-GIC O H OH H Н OH B-D-GIC OH Н OH Н OH I