ACS Organic Chemistry Exam Study Questions and Answers 2024 Graded A

What is the general formula for a carbohydrate? - Cx(H2O)y How do you determine the number of stereoisomers for a given carbohydrate? - 2^n where n= number of stereocenters Distinguish between the D and L designations for carbohydrates. - In the L form the hydroxy group is on the bottom left closest to the CH2OH group in the frame of a Fischer projection In the D form the OH is on the bottom right What are anomers? - Diastereomers formed by the cyclization of carbohydrates (*note they have different configurations at the new stereocenter-this is what makes them diastereomers) Distinguish between alpha and beta anomers. - beta anomers have the OH belonging to the hemiacetal carbon equatorial alpha anomers have the OH belonging to the hemiacetal carbon axial What is the definition of mutarotation? - the process for which alpha and beta anomers come to equilibrium (aqueous) in a specific ratio. It is the same ratio is observed when you start with the alpha anomer in water versus the beta anomer in water What agent is most often used for complete oxidation of a carbohydrate? - HNO3 What kind of agents are used for selective or partial oxidation of carbohydrates? - most commonly Br or some other MILD oxidative agent What are reducing sugars? - Carbohydrates that yield positive results of the oxidative tests, because in being oxidized they reduce the metal ion (or other reducing agent) Carbohydrates undergo reactions similar to which functional group? - OH (hemiacetals and acetals) How does the Tollen's test work? - Carbohydrate is oxidized by Ag+ and positive result shows a silvery mirror as Ag+ is reduced to metallic (elemental) silver How do the Benedict's and Fehling's tests work? - Both employ a solution of aqueous Cu2+ in base. When reduced it forms Cu2O which is a red ppt. What is a glycoside? - a sugar bonded to another oxygen containing functional group to make the carbohydrate a full acetal rather than a hemiacetal What does a glycosidic bond consist of? - A glycosidic bond is between C4 (the acetal carbon) and another oxygen atom What is the difference between an alpha and a beta glycosidic bond? - the alpha bond is axial the equatorial bond is equatorial What is a nucleophile? - An electron rich species than seeks an electron poor site What is an electrophile? - An electron poor species which seeks an electron rich species