Carbohydrates

Question 1

Describe the 2 smallest and simplest monosaccharides

Answer 1

The smallest and simplest monosaccharides are trioses (3 carbon sugars/C3)

• 2 trioses:

Question 2

How is the [α] of light rotated by a molecule determined?

What is expected for enantiomers, diastereomers, meso compounds?

Answer 2

1. Enantiomers rotate plane polarised light to the same magnitude but in opposite directions
2. Diastereomers are unpredictable
3. Meso cannot rotate light

Question 3

How do I determine whether a sugar is D or L?

What is this difference at one stereocenter called?

Answer 3

• Penultimate carbon
  
  • The furthest stereocenter (from C₁) is usually the penultimate one
  • If the OH is to the right, it is D; if left, it is L
• Epimer: a difference at just one stereocenter

Question 4

What does the following nomenclature indiciate?

• # Deoxy - no OH at specified location
• # Amino - an NH2 replaces the OH at specified location
• # N-acetylamino - an NHCOCH3 replaces the OH at specified location
• Glucose –> glucuronic acid - CH2OH is replaced by COOH
• Furanose
• Pyranose

Answer 4

• # Deoxy - no OH at specified location
• # Amino - an NH2 replaces the OH at specified location
• # N-acetylamino - an NHCOCH3 replaces the OH at specified location
• Glucose –> glucuronic acid - CH2OH is replaced by COOH
• 5 membered sugar: furanose
• 6 membered sugar: pyranose

Question 5

What happens when monosaccharide hydroxyl and carbonyl groups react?

Answer 5

• They form a hemiacetal ring (intramolecular)

Question 6

Mechanism for acid-catalysed hemiacetal formation?

Answer 6

Question 7

Mechanism for base-catalysed hemiacetal formation?

Answer 7

Question 8

How can this sugar be drawn as a Haworth projection?

• How do I determine D and L in this form?

Answer 8

C6 (CH2OH group) is up for D, down for L

Question 9

What is the anomeric carbon?

Answer 9

• Anomeric carbon: C₁, the most oxidised carbon
  
  • Exception: D-fructose, where C₁ is swapped with C₂

Question 10

What are anomers?

Answer 10

• Created during the formation of the Haworth projection; the carbonyl group is sp² and can freely rotate, so when the penultimate OH is attacked it creates 2 stereoisomers
  
  • Note: if an OH group other than the penultimate carbon is involved, assignment is not the same