Title: Organic Chemistry Fifth Edition
1Chem 234 Organic Chemistry II Professor Duncan
J. Wardrop
Spring 2004
University of Illinois at Chicago
2Chapter 25Carbohydrates
3Todays 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
425.1Classification of Carbohydrates
5Carbohydrates 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
6Occurrence 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
7Industrial 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.
8Biologically Active Carbohydrates
9Monosaccharides 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.
10Ketoses Polyhydroxylated Aldehydes
aldoses aldehydic parent sugars
(polyhydroxyaldehydes HCH(OH)nC(O)H, n 3
2) and their intramolecular hemiacetals. IUPAC
Compendium of Chemical Terminology
11Ketoses 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
12Basic Carbohydrate Nomenclature
Monosaccharide Disaccharide Oligosaccharide Polysa
ccharide
13Monosaccharides
- Monoisaccarides are the simplest carbohydrates is
not cleaved to a simpler carbohydrate on
hydrolysis - glucose, for example, is a monosaccharide
14Disaccharides
Disaccharide are cleaved to two monosaccharides
on acidic hydrolysis. The constituent
monosaccharides do not necessarily have to be the
same.
15Oligosaccharides
- gives two or more monosaccharide units on
hydrolysis - is homogeneousall molecules of a particular
- oligosaccharide are the same, including
chainlength
16Polysaccharides (Glycans)
Polysaccharides contains as many as 10,000 linked
monosaccharide units. Individual chains vary in
length. Cellulose Starch Glycogen Chitin
17Monosaccharides 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
18Numbering of Aldoses
Carbons in monosaccharides are numbered such that
the CO group has lowest number
19Numbering of Ketoses
Carbons in monosaccharides are numbered such that
the CO group has lowest number
2025.2Fischer Projections and D/L Notation
21Chirality, 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
22Fisher Projections and the D/L Stereochemical
Convention
Fisher Projection
23Fisher 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.
24Glyceraldehyde 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!
25Enantiomers of Glyceraldehyde
D and L glyceraldehyde are enantiomers of one
another.
26Relationship between D/L, R/S and ()/(-)
Clear up confusion
27Representation of Monosaccharide Structures
2825.3The Aldotetroses
29The 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.
30There are Four Possible Aldotetroses
3125.4Aldopentoses and Aldohexoses
32The 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.
33Aldopentoses of the D-Series
34Aldopentoses of the L-Series
35The 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)
36Aldohexoses of the D-Series
37Aldohexoses of the L-Series
3825.6Cyclic Forms of CarbohydratesFuranose Forms
39Conformation of 6-Membered Rings
40Revision Formation of Carbonyl Hydrates
41Revision Formation of Hemiacetals
42Some Hemiacetals are More Stable than Others
43Why 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.
44Entropy 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.
45Carbohydrates Form Cyclic Hemiacetals
- equilibrium lies far to the right.
- cyclic hemiacetals that have 5-membered ringsare
called furanose forms.
46What about Ring Size?
- General slide - tetroses - five
- Pentoses - five or six
- Hexoses - five, six, seven - wait and see the
answer
47Structure 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.
48D-Erythrose
1
2
3
4
49Structure of D-Erythrose
- move oxygen atom into position by rotating about
bond between C-3 and C-4
1
4
2
3
50Structure of D-Erythrose
1
1
4
4
2
2
3
3
51Structure of D-Erythrose
- close ring by hemiacetal formation between OH at
C-4 and carbonyl group
1
4
2
3
52Structure of D-Erythrose
1
1
4
4
2
2
3
3
53Structure of D-Erythrose
- stereochemistry is variable at anomeric
carbontwo diastereomers are formed
54D-Erythrose Diastereomers (Anomers)
a- and b-D-erthyrofuranose are diastereoisomers
or anomers
55Structure 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
56Structure of D-Ribose
57D-Ribose can Exist in a Furanose Form
5825.7Cyclic Forms of CarbohydratesPyranose Forms
59Carbohydrates Form Cyclic Hemiacetals
- cyclic hemiacetals that have 6-membered ringsare
called pyranose forms
60Pyranose Forms of D-Ribose
- pyranose ring formation involves OH group at C-5
61Pyranose Forms of D-Ribose
62Anomers of D-Ribopyranose
63Anomers 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
64a (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
65a (Alpha) b (Beta) Anomers
66Structure of D-Glucose
- pyranose ring formation involves OH group at C-5
67Structure of D-Glucose
- pyranose ring formation involves OH group at C-5
68Structure of D-Glucose
- need C(4)-C(5) bond rotation to put OH in proper
orientation to close 6-membered ring
69D-Glucopyranose Adopts a Chair Conformation
- pyranose forms of carbohydrates adopt chair
conformations
70Anomers of D-Gluoopyranose
71The Big Picture
72Open-Chain Sugars are Not Favored
- Less than 1 of the open-chain form of D-ribose
is present at equilibrium in aqueous solution.
73Structure of D-Ribopyranose
- 76 of the D-ribose is a mixture of the ? and ?-
pyranose forms, with the ?-form predominating
74Structure of D-Ribofuranose
- The ? and ?-furanose forms comprise 24 of the
mixture.
?-D-Ribofuranose (18)
?-D-Ribofuranose (6)
75Todays Lecture
76Information 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.
7725.5A Mnemonic for Carbohydrate Configurations
78The Eight D-Aldohexoses
79The Eight D-Aldohexoses
- All
- Altruists
- Gladly
- Make
- Gum
- In
- Gallon
- Tanks
80The Eight D-Aldohexoses
- All Allose
- Altruists Altrose
- Gladly Glucose
- Make Mannose
- Gum Gulose
- In Idose
- Gallon Galactose
- Tanks Talose
81The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
82The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
83The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
84The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
85The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
86The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
HO
87The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
88The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
89The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
90The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
91The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
92The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
93The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
94The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
HO
95The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
96The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
97The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
98The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
99The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
100The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose
101The Eight D-Aldohexoses
- Allose
- Altrose
- Glucose
- Mannose
- Gulose
- Idose
- Galactose
- Talose