Carbohydrates

Question 1

D-fructose

Answer 1

• Ketohexose.

Question 2

D-glucose

Answer 2

• Aldohexose.

Question 3

D-galactose

Answer 3

• Aldohexose.

Question 4

D-mannose

Answer 4

• Aldohexose.

Question 5

Stereoisomer Equation

Answer 5

• 2^n, where n = number of chiral C’s in molecule.

Question 6

Aldoses form cyclic ___, and ketoses form cyclic ___. Only five-membered ___ and six-membered ___ are stable in solution.

Answer 6

• hemiacetals, hemiketals.

* furanose, pyranose.

Question 7

What is the anomeric carbon? Define alpha and beta anomers.

Answer 7

• Carbonyl carbon in Fischer Projection that becomes chiral in Haworth Projection.
• Alpha-anomer: OH group of C-1 is trans to CH2OH substituent and is in axial position.
• Beta-anomer: OH group of C-1 is cis to CH2OH and is in equatorial position.

Question 8

Mutarotation

Answer 8

• Spontaneous change from alpha-anomer to beta-anomer and vice versa.
• Occurs when hemiacetal rings are exposed to water, causing them to spontaneously cycle between the open and closed form. OH group can then freely rotate about single bond between C-1 and C-2.

Question 9

Oxidation and Reduction of Monosaccharides

Answer 9

• Oxidation of monosaccharides occurs via Tollen’s Reagent (Ag) or Benedict’s Reagent (Cu).
• Oxidation of linear aldose yields aldonic acid (aldehyde to carboxylic acid). Oxidation of hemiacetal ring yields lactone (cyclic ester with carbonyl group on anomeric carbon).
• Oxidation of ketose doesn’t occur directly; ketose tautomerizes to aldose under basic conditions via keto-enol shifts, which can then undergo oxidation.
• Aldoses and ketoses are both reducing sugars.
• Reduction of aldose yields alditol (aldehyde to alcohol).

Question 10

Esterification of Monosaccharides

Answer 10

• Hydroxyl groups can participate in reactions with carboxylic acids and carboxylic acid derivatives to form esters.
• Phosphorylation of glucose yields a phosphate ester and is similar to esterification.

Question 11

Nucleophilic Attack of Monosaccharides

Answer 11

• Hemiacetals or hemiketals react with alcohol to form acetals or ketals.
• Resulting C-O bonds are called glycosidic bonds; the acetals formed are glycosides.
• Monosaccharides joined via glycosidic linkages form disaccharides and polysaccharides.
• Glycosides derived from furanose rings are referred to as
furanosides and those derived from pyranose rings are called pyranosides.
• Glycoside formation is a dehydration reaction; breaking a glycosidic bond requires hydrolysis.

Question 12

___ + ___ = Sucrose
___ + ___ = Lactose
___ + ___ = Maltose

Answer 12

• Glucose + Fructose.
• Galactose + Glucose.
• Glucose + Glucose.