712 CARBOHYDRATES (g) Aldonic acids tend to exist as lactones. a 8-lactone has a six-membered ring H H a r OH D-Xylonic acid Eclipsed conformation of d-Lactone of D-xylonic acid (h) Ay-lactone has a five-membered ring CHO HOCH O HOCH O rotate about HO H OH C-3-C- bond OH H/C OH H H OH xylonic acid y-Lactone of D-xylonic acid () Aldaric acids have carboxylic acid groups at both ends of the chain. CHO H-OH H HO H CHOH COH D-Xylose D-Xylaric acid 2519(a) Reduction of aldoses with sodium borohydride yields polyhydroxylic alcohols called alditols. Optically inactive alditols are those that have a plane of symmetry, that is, those that are meso forms. The D-aldohexoses that yield optically inactive alditols are D-allose and D-galactose CHO CHOH CHO CHOH H HO NabH HO H H CHOH CHOH CHOH CHOH Allose (meso compound) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website(g) Aldonic acids tend to exist as lactones. A -lactone has a six-membered ring. (h) A -lactone has a five-membered ring. (i) Aldaric acids have carboxylic acid groups at both ends of the chain. 25.19 (a) Reduction of aldoses with sodium borohydride yields polyhydroxylic alcohols called alditols. Optically inactive alditols are those that have a plane of symmetry, that is, those that are meso forms. The D-aldohexoses that yield optically inactive alditols are D-allose and D-galactose. H H H OH OH CHO H OH OH CH2OH CHO H HO HO H H OH H OH CH2OH CH2OH CH2OH H HO HO H H OH H OH NaBH4 NaBH4 d-Allose Allitol (meso compound) Galactitol (meso compound) d-Galactose H H H OH OH H OH OH CH2OH CH2OH HO H H OH CHO H OH CH2OH HO H H OH H OH CO2H CO2H d-Xylose d-Xylaric acid HO H HO H H OH CH2OH C OH O  H HOCH2 OH H H OH O H d-Xylonic acid -Lactone of d-xylonic acid rotate about C-3 C-4 bond H HOCH2 O OH H H OH O C OH O -Lactone of D-xylonic acid D-Xylonic acid C HO O CH2OH HO OH OH H H H redrawn as Eclipsed conformation of D-xylonic acid H HO OH H H OH CH2O C OH O   H intramolecular ester formation H HO OH H H OH O O 712 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website