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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