Lecture Ten : Carbohydrates Convener : Dr. Fawaz Aldabbagh First Year Organic Chemistry Biological Chemistry

Carbohydrates C x (H 2 O) y – hydrates of carbon Alternative name is saccharides Glyceraldehyde is the simplest Monosaccharide

All Sugars from Natural Sources are D-Sugars That is, the stereocenter most remote from the aldehyde group is on the right No Correlation between D- and L- and optical rotation 2 n stereoisomers Absolute configuration can only be determined by X-ray crystallography

Professor Emil Fischer Nobel Prize 1902 One of the Fathers of modern chemistry Fischer was lucky with his assignment 16 stereoisomers

Aldohexose sugar ketohexose sugar

Hemi-acetal formation creates a new stereocentre at position 1 Anomeric carbon

Professor W. N. Haworth Nobel Prize 1937 Five and Six –membered rings are easily formed and are often more stable than the open forms Five membered ring sugars are called Furanoses Six membered ring sugars are called Pyranoses

All equitorial –OH conformation is the most stable –  anomer

Like acetals, glycosides are stable in basic solutions but are hydrolysed by acids

DISACCHARIDES Maltose is a Reducing Sugar Positive Test for Fehlings, Benedicts, Tollen’s One mole Acid-Hydrolysed or by  glycosidase enzyme to 2 moles of D-glucose  -Glycosidic Linkage Sucrose is Table sugar & is a Non-Reducing Sugar Negative Test for Fehlings, Benedicts, Tollen’s One mole Acid-Hydrolysed or by sucrase enzyme to 1 mole of D-glucose and D-fructose

Lactose is present in milk Lactose is a reducing sugar  -glycosidic linkage between D-glucose and D-galactose POLYSACCHARIDES Polymers made of many sugar units held by glycosidic linkages Glucans are the most common consisting of only glucose units Glucans include starch (principle food reserve of plants), Glycogen (principle food reserve for animals), and Cellulose principle structural material of plants Hydrolysed to maltose then D-glucose