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Organic Chemistry Fifth Edition

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Title: Organic Chemistry Fifth Edition


1
Chem 234 Organic Chemistry II Professor Duncan
J. Wardrop
Spring 2004
University of Illinois at Chicago
2
Chapter 25Carbohydrates
3
Todays Lecture
  • Topics Covered (Structure of Carbohydrates)
  • Classification of Carbohydrates
  • Viewing Carbohydrates through Fischer Projections
  • Stereochemistry and C-4, C-5 and C-6 Aldoses
  • Anomers
  • Conformation of D-Glucose

4
25.1Classification of Carbohydrates
5
Carbohydrates Defined
carbohydrates originally, compounds such as
aldoses and ketoses, having the stoichiometric
formula Cn(H2O)n, hence hydrates of carbon. The
generic term carbohydrate includes
monosaccharides, oligosaccharides and
polysaccharides as well as substances derived
from monosaccharides through various
transformations. Cn(H2O)n .. a hydrate of
carbon? Perhaps, but more accurately described
as polyhydroxy aldehydes and ketones
6
Occurrence and Role of Carbohydrates in Nature
Carbohydrates form the most abundant group of
compounds found natural sources they are present
in animals and plants. The main source of
carbohydrates is plant, which produce these
important molecules through photosynthesis
energy (hu) CO2 H2O ? carbohydrates O2
Animals consume carbohydrates and transform them
into CO2, water and the energy required for work
carbohydrates O2 ? energy CO2 H2O
7
Industrial Uses of Carbohydrates
  • Carbohydrates have numerous industrial
    applications
  • Food Industry - sucrose (a disaccharide of
    D-glucose and D-fructose is used as a sweetening
    agent and preservative
  • Textile Industry - cotton (cellulose) is
    extensively used as a textile.
  • Paper Plastics - in addition to being used in
    the manufacture of paper, cellulose is also used
    as to make semi-synthetic polymers
  • Packaging - cellulose derived from plants is used
    to make paper
  • Pharmaceutical Industry - numerous antibiotics as
    well as vitamin C are carbohydrates. Also used as
    starting materials for the synthesis of
    non-carbohydrate pharmaceuticals.

8
Biologically Active Carbohydrates
9
Monosaccharides Defined
monosaccharides A term which includes aldoses,
ketoses and a wide variety of derivatives.
Derivation includes oxidation, deoxygenation,
introduction of other substituents, alkylation
and acylation of hydroxy groups, and chain
branching.
10
Ketoses Polyhydroxylated Aldehydes
aldoses aldehydic parent sugars
(polyhydroxyaldehydes HCH(OH)nC(O)H, n 3
2) and their intramolecular hemiacetals. IUPAC
Compendium of Chemical Terminology
11
Ketoses Polyhydroxylated Ketones
ketoses ketonic parent sugars (polyhydroxy
ketones HCHOHn C(O)CHOHmH with three or
more carbon atoms) and their intramolecular
hemiacetals. The oxo group is usually at C-2.
e.g. D-fructose
12
Basic Carbohydrate Nomenclature
Monosaccharide Disaccharide Oligosaccharide Polysa
ccharide
13
Monosaccharides
  • Monoisaccarides are the simplest carbohydrates is
    not cleaved to a simpler carbohydrate on
    hydrolysis
  • glucose, for example, is a monosaccharide

14
Disaccharides
Disaccharide are cleaved to two monosaccharides
on acidic hydrolysis. The constituent
monosaccharides do not necessarily have to be the
same.
15
Oligosaccharides
  • gives two or more monosaccharide units on
    hydrolysis
  • is homogeneousall molecules of a particular
  • oligosaccharide are the same, including
    chainlength

16
Polysaccharides (Glycans)
Polysaccharides contains as many as 10,000 linked
monosaccharide units. Individual chains vary in
length. Cellulose Starch Glycogen Chitin
17
Monosaccharides are Defined According to Chain
Length and Type of Carbonyl Group
  • No. of carbons Aldose Ketose
  • 4 Aldotetrose Ketotetrose
  • 5 Aldopentose Ketopentose
  • 6 Aldohexose Ketohexose
  • 7 Aldoheptose Ketoheptose
  • 8 Aldooctose Ketooctose

18
Numbering of Aldoses
Carbons in monosaccharides are numbered such that
the CO group has lowest number
19
Numbering of Ketoses
Carbons in monosaccharides are numbered such that
the CO group has lowest number
20
25.2Fischer Projections and D/L Notation
21
Chirality, Chiral Stereogenic Centers
Chirality the term describing an object that is
not superimposable on its mirror image
Chiral Center an atom that has four
nonequivalent atoms or groups attached to it. At
various times, chiral centers have been and are
called asymmetric centers or stereogenic centers
22
Fisher Projections and the D/L Stereochemical
Convention
Fisher Projection
23
Fisher Projections of Carbohydrates
In the Fischer convention, the carbon chain is in
vertical arrangement with the carbonyl group at
the top (for aldoses), or nearest to the top (for
ketoses). The horizontal bonds (C-H and C-O)
project towards viewer. These projections
represent stereochemistry not conformation.
24
Glyceraldehyde The Simplest
Carbohydrate
molecules with an OH group to the right are in
the D-series those with the OH group to the left
belong to the L-series. Show enantiomer!
25
Enantiomers of Glyceraldehyde
D and L glyceraldehyde are enantiomers of one
another.
26
Relationship between D/L, R/S and ()/(-)
Clear up confusion
27
Representation of Monosaccharide Structures
28
25.3The Aldotetroses
29
The Hudson-Rosanoff Convention
The letters D and L at the beginning of a
carbohydrate name indicate the absolute
configuration of the chiral atom most remote from
the carbonyl group. Molecules with an OH group to
the right are in the D-series those with the OH
group to the left belong to the L-series.
30
There are Four Possible Aldotetroses
31
25.4Aldopentoses and Aldohexoses
32
The Aldopentoses
  • There are 8 aldopentoses.
  • Four belong to the D-series four belong to the
    L-series, i.e. four pairs of enantiomers.
  • Their names are ribose, arabinose, xylose, and
    lyxose.

33
Aldopentoses of the D-Series
34
Aldopentoses of the L-Series
35
The Aldohexoses
  • There are 16 aldopentoses.
  • 8 belong to the D-series 8 belong to the
    L-series.
  • Their names and configurations are best
    remembered with the aid of the mnemonic described
    in Section 25.5 (see end of slide show)

36
Aldohexoses of the D-Series
37
Aldohexoses of the L-Series
38
25.6Cyclic Forms of CarbohydratesFuranose Forms
39
Conformation of 6-Membered Rings
40
Revision Formation of Carbonyl Hydrates
41
Revision Formation of Hemiacetals
42
Some Hemiacetals are More Stable than Others
43
Why are Cyclic Hemiacetal Stable?
  • Aldehydes and ketones that contain an OH group
    elsewhere in the molecule can undergo
    intramolecular hemiacetal formation. The
    equilibrium favors the cyclic hemiacetal if the
    ring is 5- or 6-membered.

44
Entropy is the Key!
A B ? C DS -ve larger DG ve small
A ? B DS smaller DG -ve large
DG DH - TDS
In the first reaction, two molecules become one
and this results in a larger drop in entropy than
the cyclic case. Reactions with negative DS tend
to have a positive DG and accordingly, are less
favorable.
45
Carbohydrates Form Cyclic Hemiacetals
  • equilibrium lies far to the right.
  • cyclic hemiacetals that have 5-membered ringsare
    called furanose forms.

46
What about Ring Size?
  • General slide - tetroses - five
  • Pentoses - five or six
  • Hexoses - five, six, seven - wait and see the
    answer

47
Structure of D-Erythrose
  • Stereochemistry of the C2 and C3 centers is
    maintained during cyclic hemiacetal formation.
  • Cyclization generates a new stereogenic center at
    C1, a.k.a the anomeric center.

48
D-Erythrose
1
2
3
4
49
Structure of D-Erythrose
  • move oxygen atom into position by rotating about
    bond between C-3 and C-4

1
4
2
3
50
Structure of D-Erythrose
1
1
4
4
2
2
3
3
51
Structure of D-Erythrose
  • close ring by hemiacetal formation between OH at
    C-4 and carbonyl group

1
4
2
3
52
Structure of D-Erythrose
1
1
4
4
2
2
3
3
53
Structure of D-Erythrose
  • stereochemistry is variable at anomeric
    carbontwo diastereomers are formed

54
D-Erythrose Diastereomers (Anomers)
a- and b-D-erthyrofuranose are diastereoisomers
or anomers
55
Structure of D-Ribose
  • Rotation og the C(3)-C(4) bond places the C-5
    hydroxyl group in an orientation to allow
    addition to the aldehyde group and formation of
    5-membered ring furan ring

56
Structure of D-Ribose
57
D-Ribose can Exist in a Furanose Form
58
25.7Cyclic Forms of CarbohydratesPyranose Forms
59
Carbohydrates Form Cyclic Hemiacetals
  • cyclic hemiacetals that have 6-membered ringsare
    called pyranose forms

60
Pyranose Forms of D-Ribose
  • pyranose ring formation involves OH group at C-5

61
Pyranose Forms of D-Ribose
62
Anomers of D-Ribopyranose
63
Anomers Defined
Anomers (n) diastereoisomers of glycosides,
hemiacetals or related cyclic forms of sugars, or
related molecules differing in configuration only
at C-1 of an aldose, C-2 of a 2-ketose,
etc. IUPAC Compendium of Chemical Terminology
64
a (Alpha), b (Beta) Defined
a (Alpha), b (Beta) Relative stereodescriptors
used in carbohydrate nomenclature to describe the
configuration at the anomeric carbon by relating
it to the anomeric reference atom. For simple
cases the anomeric reference atom is the same as
the configurational reference atom. Thus in a-D-
glucopyranose the reference atom is C-5 and the
OH at C-1 is on the same side as the OH at C-5 in
the Fischer projection. IUPAC Compendium of
Chemical Terminology
65
a (Alpha) b (Beta) Anomers
66
Structure of D-Glucose
  • pyranose ring formation involves OH group at C-5

67
Structure of D-Glucose
  • pyranose ring formation involves OH group at C-5

68
Structure of D-Glucose
  • need C(4)-C(5) bond rotation to put OH in proper
    orientation to close 6-membered ring

69
D-Glucopyranose Adopts a Chair Conformation
  • pyranose forms of carbohydrates adopt chair
    conformations

70
Anomers of D-Gluoopyranose
71
The Big Picture
72
Open-Chain Sugars are Not Favored
  • Less than 1 of the open-chain form of D-ribose
    is present at equilibrium in aqueous solution.

73
Structure of D-Ribopyranose
  • 76 of the D-ribose is a mixture of the ? and ?-
    pyranose forms, with the ?-form predominating

74
Structure of D-Ribofuranose
  • The ? and ?-furanose forms comprise 24 of the
    mixture.

?-D-Ribofuranose (18)
?-D-Ribofuranose (6)
75
Todays Lecture
  • Topics Covered

76
Information Suggested Problems
Suggested Problems 25.19-25.30 ------------------
----------------------------------------------- Of
fice Hour Thursday, 3.15 P.M., SES
4446 ---------------------------------------------
-------------------- Note that solutions to
on-line homework for Chapter 27 were incorrect
and have now been corrected.
77
25.5A Mnemonic for Carbohydrate Configurations
78
The Eight D-Aldohexoses
79
The Eight D-Aldohexoses
  • All
  • Altruists
  • Gladly
  • Make
  • Gum
  • In
  • Gallon
  • Tanks

80
The Eight D-Aldohexoses
  • All Allose
  • Altruists Altrose
  • Gladly Glucose
  • Make Mannose
  • Gum Gulose
  • In Idose
  • Gallon Galactose
  • Tanks Talose

81
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

82
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

83
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

84
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

85
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

86
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

HO
87
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

88
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

89
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

90
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

91
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

92
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

93
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

94
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

HO
95
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

96
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

97
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

98
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

99
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

100
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose

101
The Eight D-Aldohexoses
  • Allose
  • Altrose
  • Glucose
  • Mannose
  • Gulose
  • Idose
  • Galactose
  • Talose
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