Organic Chemistry

1
Organic Chemistry

• William H. Brown Christopher S. Foote

2
Carbohydrates
Chapter 25

• Chapter 24

3
Carbohydrates

• Carbohydrate a polyhydroxyaldehyde or
  polyhydroxyketone, or a substance that gives
  these compounds on hydrolysis
• Monosaccharide a carbohydrate that cannot be
  hydrolyzed to a simpler carbohydrate
• they have the general formula CnH2nOn, where n
  varies from 3 to 8
• aldose a monosaccharide containing an aldehyde
  group
• ketose a monosaccharide containing a ketone group

4
Monosaccharides

• Monosaccharides are classified by their number of
  carbon atoms

5
Monosaccharides

• There are only two trioses
• often aldo- and keto- are omitted and these
  compounds are referred to simply as trioses
  although this designation does not tell the
  nature of the carbonyl group, it at least tells
  the number of carbons

6
Monosaccharides

• Glyceraldehyde contains a stereocenter and exists
  as a pair of enantiomers

7
Fischer Projections

• Fischer projection a two dimensional
  representation for showing the configuration of
  tetrahedral stereocenters
• horizontal lines represent bonds projecting
  forward
• vertical lines represent bonds projecting to the
  rear

8
D,L Monosaccharides

• In 1891, Emil Fischer made the arbitrary
  assignments of D- and L- to the enantiomers of
  glyceraldehyde

9
D,L Monosaccharides

• According to the conventions proposed by Fischer
• D-monosaccharide a monosaccharide that, when
  written as a Fischer projection, has the -OH on
  its penultimate carbon on the right
• L-monosaccharide a monosaccharide that, when
  written as a Fischer projection, has the -OH on
  its penultimate carbon on the left

10
D,L Monosaccharides

• Here are the two most common D-aldotetroses and
  the two most common D-aldopentoses

11
D,L Monosaccharides

• And the three common D-aldohexoses

12
Physical Properties

• Monosaccharides are colorless crystalline solids,
  very soluble in water, but only slightly soluble
  in ethanol
• sweetness relative to sucrose

13
Cyclic Structure

• Monosaccharides have hydroxyl and carbonyl groups
  in the same molecule and exist almost entirely as
  five- and six-membered cyclic hemiacetals
• anomeric carbon the new stereocenter resulting
  from cyclic hemiacetal formation
• anomers carbohydrates that differ in
  configuration at their anomeric carbons

14
Haworth Projections

• Haworth projections
• five- and six-membered hemiacetals are
  represented as planar pentagons or hexagons, as
  the case may be, viewed through the edge
• most commonly written with the anomeric carbon on
  the right and the hemiacetal oxygen to the back
  right
• the designation ?- means that -OH on the anomeric
  carbon is cis to the terminal -CH2OH ?- means
  that it is trans

15
Haworth Projections
16
Haworth Projections

• six-membered hemiacetal rings are shown by the
  infix -pyran-
• five-membered hemiacetal rings are shown by the
  infix -furan-

17
Conformational Formulas

• five-membered rings are so close to being planar
  that Haworth projections are adequate to
  represent furanoses

18
Conformational Formulas

• for pyranoses, the six-membered ring is more
  accurately represented as a strain-free chair
  conformation

19
Conformational Formulas

• if you compare the orientations of groups on
  carbons 1-5 in the Haworth and chair projections
  of ?-D-glucopyranose, you will see that in each
  case they are up-down-up-down-up respectively

20
Mutarotation

• Mutarotation the change in specific rotation
  that occurs when an ? or ? form of a carbohydrate
  is converted to an equilibrium mixture of the two

21
Mutarotation
22
Formation of Glycosides

• Glycoside a carbohydrate in which the -OH of the
  anomeric carbon is replaced by -OR
• methyl ?-D-glucopyranoside (methyl ?-D-glucoside)

23
Glycosides

• Glycosidic bond the bond from the anomeric
  carbon of the glycoside to an -OR group
• Glycosides are named by listing the name of the
  alkyl or aryl group attached to oxygen followed
  by the name of the carbohydrate with the ending
  -e replaced by -ide
• methyl ?-D-glucopyranoside
• methyl ?-D-ribofuranoside

24
N-Glycosides

• The anomeric carbon of a cyclic hemiacetal also
  undergoes reaction with the N-H group of an amine
  to form an N-glycoside
• N-glycosides of the following purine and
  pyrimidine bases are structural units of nucleic
  acids

25
N-Glycosides
26
Reduction to Alditols

• The carbonyl group of a monosaccharide can be
  reduced to an hydroxyl group by a variety of
  reducing agents, including NaBH4 and H2/M

27
Oxidation to Aldonic Acids

• The -CHO group can be oxidized to -COOH

28
Oxidation to Aldonic Acids

• among the mild oxidizing agents used for this
  purpose is Tollens solution if the test is done
  properly, silver metal precipitates as a silver
  mirror

29
Oxidation to Aldonic Acids

• 2-Ketoses are also oxidized by these reagents
• under the conditions of the oxidation, 2-ketoses
  equilibrate with isomeric aldoses

30
Oxidation to Uronic Acids

• Enzyme-catalyzed oxidation of the terminal -OH
  group gives a -COOH group

31
Oxidation by HIO4

• Periodic acid cleaves the C-C bond of a glycol

32
Oxidation by HIO4

• it also cleaves ?-hydroxyaldehydes

33
Oxidation by HIO4

• and ?-hydroxyketones

34
Oxidation by HIO4

• Oxidation of methyl ?-D-glucoside consumes 2
  moles of HIO4 and produces 1 mole of formic acid,
  which indicates 3 adjacent C-OH groups

35
Oxidation by HIO4

• this is evidence that methyl ?-D-glucoside is a
  pyranoside

36
Glucose Assay

• The analytical procedure most often performed in
  the clinical chemistry laboratory is the
  determination of glucose in blood, urine, or
  other biological fluid
• this need stems from the high incidence of
  diabetes in the population

37
Glucose Assay

• The glucose oxidase method is completely specific
  for D-glucose

38
Glucose Assay

• the enzyme glucose oxidase is specific for
  ?-D-glucose
• molecular oxygen, O2, used in this reaction is
  reduced to hydrogen peroxide H2O2
• the concentration of H2O2 is determined
  experimentally, and is proportional to the
  concentration of glucose in the sample
• in one procedure, hydrogen peroxide is used to
  oxidize o-toluidine to a colored product, whose
  concentration is determined spectrophotometrically

39
Ascorbic Acid (Vitamin C)

• L-Ascorbic acid (vitamin C) is synthesized both
  biochemically and industrially from D-glucose

40
Ascorbic Acid (Vitamin C)

• L-Ascorbic acid is very easily oxidized to
  L-dehydroascorbic acid. Both are physiologically
  active and are found in most body fluids

41
Maltose

• From malt, the juice of sprouted barley and other
  cereal grains

42
Lactose

• The principle sugar present in milk
• about 5 - 8 in human milk, 4 - 5 in cows milk

43
Sucrose

• Table sugar, obtained from the juice of sugar
  cane and sugar beet

44
Starch

• Starch is used for energy storage in plants
• it can be separated into two fractions amylose
  and amylopectin each on complete hydrolysis
  gives only D-glucose
• amylose is composed of continuous, unbranched
  chains of up to 4000 D-glucose units joined by
  ?-1,4-glycoside bonds
• amylopectin is a highly branched polymer of
  D-glucose. Chains consist of 24-30 units of
  D-glucose joined by ?-1,4-glycoside bonds and
  branches created by ?-1,6-glycoside bonds

45
Glycogen

• Glycogen is the reserve carbohydrate for animals
• like amylopectin, glycogen is a nonlinear polymer
  of D-glucose units joined by ?-1,4- and
  ?-1,6-glycoside bonds bonds
• the total amount of glycogen in the body of a
  well-nourished adult is about 350 g (about 3/4 of
  a pound) divided almost equally between liver and
  muscle

46
Cellulose

• Cellulose is a linear polymer of D-glucose units
  joined by ?-1,4-glycoside bonds
• it has an average molecular weight of 400,000
  g/mol, corresponding to approximately 2800
  D-glucose units per molecule
• both rayon and acetate rayon are made from
  chemically modified cellulose

47
Acidic Polysaccharides

• Hyaluronic acid an acidic polysaccharide present
  in connective tissue, such as synovial fluid and
  vitreous humor

48
Acidic Polysaccharides

• Heparin
• its best understood function is as an
  anticoagulant

49
Prob 25.11

• Which are D-monosaccharides? Which are
  L-monosaccharides?

50
Prob 25.20

• Convert each Haworth projection to an open-chain
  form and then to a Fischer projection. Name each
  monosaccharide.

51
Prob 25.21

• Convert each chair conformation to an open-chain
  form and then to a Fischer projection. Name each
  monosaccharide.

52
Prob 25.24

• Draw the cyclohexane ring in topiramate in its
  more stable chair conformation. Which groups are
  axial and which are equatorial? Draw the cyclic
  structure after hydrolysis of all amide, ester,
  and acetal bonds. Name the monosaccharide unit in
  topiramate.

53
Prob 25.30

• One pathway for the metabolism of glucose is
  this enzyme-catalyzed reaction. Show that is can
  be regarded as two enzyme-catalyzed keto-enol
  tautomerizations.

54
Prob 25.31

• Describe the type of reaction involved in each
  step in the biochemical synthesis of L-fucose.

55
Prob 25.31 (contd)

• Steps 4-5 are repeated here

56
Prob 25.38

• Name the two monosaccharide units in trehalose.
  Is trehalose a reducing sugar? Does it undergo
  mutarotation?

57
Prob 25.39

• Name the three monosaccharide units in raffinose.

58
Prob 25.40

• From what monosaccharide is laetrile derived?
  Assign an R or S configuration to the
  stereocenter bearing the -CN group. Draw
  structural formulas for the products of
  hydrolysis of laetrile in aqueous acid.

59
Prob 25.43

• Draw a chair conformation for the a- and
  b-pyranose forms of this monosaccharide and for
  the chair conformation of the disaccharide formed
  of joining two units of the pyranose form by a
  b-1,4-glycosidic bond.

60
Prob 25.44

• (a) Alginic acid is a polymer of D-mannuronic
  acid in the pyranose form joined by
  b-1,4-glycosidic bonds. Draw the repeating unit.
• (b) Pectic acid is a polymer of D-galacturonic
  acid in the pyranose form joined by
  a-1,4-glycosidic bonds. Draw the repeating unit.

61
Prob 25.45
62
Prob 25.46

• Name each monosaccharide unit and describe each
  glycosidic bond.

63
Prob 25.49

• Keratin sulfate is an important component of the
  cornea of the eye. From what monosaccharides is
  keratin sulfate derived? Describe the glycosidic
  bond.

64
Carbohydrates
End Chapter 25