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Organic Chemistry

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William H. Brown & Christopher S. Foote Carbohydrates Chapter 24 Carbohydrates Carbohydrate: a polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives ... – PowerPoint PPT presentation

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Title: 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
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