Exam 4

D-Fructose

6 Carbons C-3 has hydroxyl on the left ketone

D-Glucose

6 carbons 3rd Hydroxyl is on the left

D-galactose

6 carbons 3rd and 4th hydroxyls are on left

D-Mannose

6 carbons C-2 and C-3 have their hydroxyls on the left

How are maltose and sucrose similar and different with regards to their structural features?

Both are disaccharides But: Maltose has two (alpha1->4)- linked D-glucose units; sucrose has (alpha1<->2beta)- linked D-glucose and D-fructose

How are D-glucose and D-fructose similar and different with regards to their structural features?

Both are hexoses But: D-glucose is an aldohexose D-furanose is a ketohexose

How are cellulose and glycogen similar and different with regards to their structural features?

Both are polymers of D-glucose Have different glycosidic linkages: Cellulose has beta 1->4 linkage Glycogen has alpha 1->4 linkage

What is the critical distinction between configuration and conformation of sugars?

Conversion between chair conformations does not require bond breakage

In sugar alcohols, the carbonyl oxygen is reduced to a hydroxyl group. However, this sugar alcohol is no longer designated D or L. Why? Give an example.

D-glyceraldehyde can be reduced to glycerol. However, when it's reduced, the stereochemistry at the C-1 and C-3 carbons are the same; therefore glycerol is not chiral and D and L assignments no longer apply.

What are epimers?

Each of two isomers with different configurations of atoms around one of several asymmetric carbon atoms present. A pair of stereoisomers

What classifies a sugar as a reducing sugar?

Only if it has an open-chain form with an aldehyde group or a free hemiacetal group. Monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses.

Why does the specific rotation of a freshly prepared solution of the alpha form gradually decrease with time? Why do solutions of the alpha or beta forms reach the same specific rotation at equilibrium?

Solutions of fresh alpha or beta glucose undergo mutatrotation to reach an equilibrium mix in which both the alpha and beta forms are present.

A dissacharide is treated with Fehling's solution and a red color is formed. Is it sucrose or maltose and how do you know?

The red color is indicative that a reducing sugar is present. In this case our only choice is maltose. It can't be sucrose because both of its anomeric carbons are involved in the glycosidic bonds.

What is required if you're going to couple two reactions?

There must be a common intermediate

Which bonds in alpha-D-glucose must be broken to change its configuration to beta-D-glucose? Which bonds to convert one chair form of D-glucose to the other?

To go to beta-D-glucose, the C-1 and C-5 carbon bonds must be broken and reformed in the opposite configuration. To convert it to D-mannose, the H or the -OH on C-2 must be broken and reformed in the opposite configuration.

aldol

a beta-hydroxy ketone or aldehyde

aldol addition

an abbreviation of aldehyde and alcohol. When the enolate of an aldehyde or a ketone reacts at the α-carbon with the carbonyl of another molecule under basic or acidic conditions to obtain β-hydroxy aldehyde or ketone

enol

an alkene (olefin) with a hydroxyl group attached to one end of the alkene double bond

Aldose condensation

an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to give a conjugated enone.

enolate

the conjugate bases or anions of enols