Exp 4

• Carbohydrates


  - once considered hydrates of carbon but the structures of this macromolecule proves that this view is inaccurate
• Cn(H2O)n


  what is the general formula of carbohydrates?
• Monosaccharides


  - are the simplest carbohydrates and usually consists of 3-6 carbons
• - polyhydroxy aldehydes (aldoses)

  - polyhydroxy ketones (ketoses)

  what are the types of monosaccharides
• polyhydroxy aldehydes (aldoses)

  - this type of monosaccharide has the general structure A
• polyhydroxy ketones (ketoses)


  - this type of monosaccharide has the general structure B
• True


  T or F:

  
  

  the number of carbons in a monosaccharide can be used as a way to classify them
• Trioses


  what monosaccharide has 3 carbons?
• tetroses


  what monosaccharide has 4 carbons?
• pentoses


  what monosaccharide has 5 carbons?
• hexoses


  what monosaccharide has 6 carbons?
• - ribose

  - glucose

  - fructose


  What are the three examples of monosaccharides?
• False.

  
  

  Most monosaccharides exist in a cyclic form

  T or F:

  
  

  in solution, most monosaccharides exist in a straight chain
• The aldehyde/ketone group reacts with one of the -OH groups on the other end of the same molecule to form a cyclic hemiacetal


  What causes monosaccharides to have a cyclic form?
• - a-D-glucose

  - B-D-glucose


  What are the anomers of glucose?
• True


  T or F:

  
  

  in solution, there is equilibrium between the cyclic form and the open chain or free aldehyde form. The rings are constantly opening and closing again, which is why the a and b forms can bet inter-converted.
• Disaccharide


  - 2 monosaccharides combined with the loss of 1 water molecule
• Glycosidic bond


  - the bond that connects 2 monosaccharide units in a disaccharide
• - Sucrose (table sugar)

  - Lactose (milk sugar)

  What are examples of disaccharides?
• Polysaccharide


  - consists of many monosaccharides linked together
• - starch

  - pectin

  - glycogen

  - cellulose


  What are examples of polysaccharides?
• Molisch's Test


  What is the general qualitative test for carbohydrates?
• Molisch's reagent + a few drops of conc. sulfuric acid (H2SO4)


  What are the reagents used for the molisch's test?
• Purple/Purplish-red ring at the point of contact between H2SO4 and the analyte + molisch's reagent

  What is the positive result for molisch's test?
• False.

  
  

  Almost all carbohydrates give a positive reaction to molisch's test. Tetroses and Trioses are exceptions to this test.


  T or F:

  
  

  All carbohydrates give a positive reaction to molisch's test.
• Reducing Sugar


  - a carbohydrate that is oxidized by a weak oxidizing agent in basic aqueous solutions

  - has free carbonyl groups in their structure
• Non-reducing Sugar


  - a carbohydrate that is not oxidized by a weak oxidizing agent in basic aqueous solutions
• Aldoses


  - are the true reducing sugars

  - contains the aldehyde functional group
• Ketoses


  - these do not contain aldehyde but shows reducing properties
• Aldehyde


  What functional group is present in aldoses and classifies it as a reducing sugar?
• keto-enol tautomerization


  What process do ketoses undergo to be converted into aldoses and thus develops a reducing properties?
• False.

  
  

  All monosaccharides are reducing sugars

  T or F:

  
  

  Not all monosaccharides are reducing sugars.
• True.


  T or F:

  
  

  Disaccharides may be reducing sugars or not, depending on the presence of a free anomeric carbon in the molecule.
• Disaccharides that are reducing contains a free anomeric carbon, which has the capacity to open and expose the aldehyde group


  How can you classify whether or not a disaccharide is a reducing sugar or not?
• True


  T or F:

  
  

  All polysaccharides are non-reducing.
• - Tollen's Reagent

  - Benedict's Reagent

  - Picric acid with sodium carbonate solution

  - Nylander's Reagent

  - Fehling's Reagent

  
  

  Quick Mnemonic: To Be Pretty N Friendly


  What are the weak oxidizing agents that can be used to determine a reducing sugar?
• All reagents are in basic pH conditions (>7 pH)


  What is common among all the weak oxidizing agents used to determine a reducing sugar?
• Cupric ion (Cu2+) will oxidize aliphatic aldehydes, including a-hydroxyaldehydes (aldoses)


  What fact is the Benedict's Test based on?
• formation of brick-red, orange, or green precipitate (depending on sugar amount) called cuprous oxide (Cu2O)

  
  

  brick-red color = large amount of reducing sugar

  orange = moderate amount of reducing sugar

  green = traces of reducing sugar

  What is a positive result for Benedict's Test?
• Benedict's Test


  - used to test for reducing sugars

  - used to quantitatively estimate the amount of sugar in a solution

  - also used to detect glucose in the urine