Carbohydrates

Carbohydrates

1. optically active polyhydroxy aldehydes or ketones or compounds which produce such units on hydrolysis

2. Monosaccharides: cannot by hydrolysed further to give carb. 20 in nature

   • glucose, fructose, ribose
   • Aldose/ketose

3. Oligosaccharides: yield 2-10 monosac on hydrolysis

   • one molecule of sucrose gives 1 glucose and 1 fructose
   • one molecule of maltose gives 2 glucose

4. Polysaccharides: large number of monosac. Not sweet

   • Starch, cellulose, glycogen, gums

5. Glucose prep

   • sucrose + dil HCl/H2SO4 in alc soln
   • Commercially: hydrolysis of starch by boiling w dil H2SO4 at 393K,2-3 atm

6. Structure of glucose based on

   • Prolonged heating with HI forms n-hexane - straight chain
   • with hydroxylamine it forms oxime and adds HCN to give cyanohydrin. Hence carbonyl
   • Oxidised to gluconic acid on rxn w bromine water. Hence aldeyde
   • Acetylation with acetic anhydride gives glucose pentaacetate- 5 OH grps to diff C
   • On oxtn with HNO3 glucose and gluconic acid yield saccharic acid- primary alcohol grp
   • Did not explain:
     • glucose does not give Schiffs test. Does not form hydrogensulphite with NaHSO3
     • pentaacetate of glucose does not react with hydroxylamine.Absence of free CHO group
     • Two crystalline forms α and β.
       • β- crystallisation from hot saturated aq soln at 371K
       • α crystaltn from conc glucose soln at 303K

7. Fructose: ketohexose

   • ketonic func grp at C-2 in straight chain. D-(-)-fructose.
   • Has two cyclic forms by addtn of OH grp at C-5. 5 membered ring called furanose

8. Disaccharides

   • Two monosacchs formed are joined by an oxide linkage fue to loss of watercalled glycosidic linkage
   • If the reducing grps are bonded then they are non reducing sugars. eg:sucrose
   • If reducing grp is free then reducing sugar. eg: maltose, lactose
   • Sucrose: is dexto but after hydrolysis becomes laevo because laevo of fructose(beta) is more than dexto of glucose(alpha). Hence it is an invert sugar
   • Maltose: has two alpha D- glucose units
   • Lactose: β-D-galactose and β-D-glucose

9. Polysaccarides: mainly food storage or structural materials

   • Starch: main storage of plants. Polymer of α glucose
     • Amylose: water soluble component. Constitues 15-20% of starch. Long unbranches chain with 200-1000 α glucose units with C1-C4 linkage
     • Amlypectin: insoluble in water. Branched chain polymer of α glucose with C1-C4 linkage and C1-C6 branching
   • Cellulose: occurs in plants and most abundant organic subs in plants kingdom. straight chain β-D-glucose
   • Glycogen: carbs in body stored as glycogen. Similar to amylopectin but more highly branched. Present in liver, musc, brain