CARBOHYDRATES 713 (b) All the aldonic acids and their lactones obtained on oxidation of the aldohexoses are optically active. The presence of a carboxyl group at one end of the carbon chain and a Ch,OH at the other precludes the existence of meso forms. (c) Nitric acid oxidation of aldoses converts them to aldaric acids. The same D-aldoses found to yield optically inactive alditols in part(a) yield optically inactive aldaric acids CO,H CHO COH H-OH HHHH HNO HHHH HO-H H HO-H H D-Allose Alaric acid D-Galactose (d) Aldoses that differ in configuration only at C-2 enolize to the same enediol CHO CHOH CHO H CHOH CHOH D-Allose Enediol The stereogenic center at C-2 in the D-aldose becomes sp--hybridized in the enediol The other pairs of D-aldohexoses that form the same enediols D-Glucose and D-mannose D-Gulose and D-idose 25.20(a) To unravel a pyranose form, locate the anomeric carbon and mentally convert the hemiacetal and a hydroxyl function. H OH HOCH HOCH OH OH H Convert the open-chain form to a Fischer projection CHO O HoH H CHO HOCH H/.OH HoH equivale H OH HO- CH,OH Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website(b) All the aldonic acids and their lactones obtained on oxidation of the aldohexoses are optically active. The presence of a carboxyl group at one end of the carbon chain and a CH2OH at the other precludes the existence of meso forms. (c) Nitric acid oxidation of aldoses converts them to aldaric acids. The same D-aldoses found to yield optically inactive alditols in part (a) yield optically inactive aldaric acids. (d) Aldoses that differ in configuration only at C-2 enolize to the same enediol. The stereogenic center at C-2 in the D-aldose becomes sp2 -hybridized in the enediol. The other pairs of D-aldohexoses that form the same enediols are D-Glucose and D-mannose D-Gulose and D-idose D-Galactose and D-talose 25.20 (a) To unravel a pyranose form, locate the anomeric carbon and mentally convert the hemiacetal linkage to a carbonyl compound and a hydroxyl function. Convert the open-chain form to a Fischer projection. H OH CHO H OH H HO OH HOCH2 CH2OH OH CH H OH H OH O H OH equivalent to HO H HO H CHO H OH HO H CH2OH rotate about C-4 C-5 bond H H HOCH2 OH OH OH OH O HOCH2 OH CH H OH H H OH O H OH H OH H OH CHO CH2OH H OH H OH d-Allose H OH H OH CHOH C CH2OH OH H OH Enediol H OH H OH CHO CH2OH HO H H OH d-Altrose H H H OH OH CHO H OH OH CH2OH CHO H HO HO H H OH H OH CH2OH CO2H CO2H H HO HO H H OH H OH HNO3 HNO3 d-Allose Allaric acid (meso compound) Galactaric acid (meso compound) d-Galactose H H H OH OH H OH OH CO2H CO2H CARBOHYDRATES 713 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website