Basic Organic Chemistry
g
y
2302202
Dr. Rick Attrill
Office MHMK 1405/5
Carbohydrates I
 Classification of Carbohydrates
 Configurations
C fi
i
off Monosaccharides:
M
h id
Fi h P
Fischer
Projections
j i
 D, L Sugars
 Configurations of the Aldoses
 Cyclic Structures of the Monosaccharides: Hemiacetal
Formation
2
I
Importance
t
off Carbohydrates
C b h d t
 Distributed widely in nature
 Key intermediates of metabolism (sugars)
 Structural components of plants (cellulose)
 Central
C t l tto materials
t i l off industrial
i d t i l products:
d t paper,
lumber, fibers
 Key component of food sources: sugars, flour,
vegetable fiber
 Contain OH groups on most carbons in linear chains
or in rings
3
Chemical Formula and Name
 Carbohydrates have roughly as many O’s
O s as C’s
Cs
(highly oxidized)
 Since H’s are about connected to each H and O the
empirical formulas are roughly (C(H2O))n

Appears to be a “carbon hydrate” from this formula,
which gave them the name “carbohydrates”
 The term carbohydrate is now used to refer to a
broad
b
d classs
l
off polyhydroxylated
l h d
l t d aldehydes
ld h d and
d
ketones commonly called sugars
4
Sources
 Glucose is produced in plants through photosynthesis
from CO2 and H2O
 Glucose is converted in plants to other small sugars
and polymers (cellulose, starch)
 Dietary carbohydrates provide the major source of
energy required
i db
by organisms
i
5
Classification of Carbohydrates
 Simple sugars (monosaccharides) can't be converted
into smaller sugars by hydrolysis.
 Carbohydrates are made of two or more simple
sugars connected as acetals (aldehyde and alcohol),
oligosaccharides and polysaccharides
 Sucrose
S
(t
(table
bl sugar):
) di
disaccharide
h id ffrom ttwo
monosaccharides (glucose linked to fructose),
 Cellulose
C ll l
iis a polysaccharide
l
h id off severall th
thousand
d
glucose units connected by acetal linkages (aldehyde
and alcohol)
A disaccharide
derived from
cellulose
6
Aldoses and Ketoses
 aldo- and keto- prefixes identify the nature of the
carbonyl
b
l group
 -ose suffix designates a carbohydrate
 Number off C’s
C in the monosaccharide indicated by
root (-tri-, tetr-, pent-, hex-)
7
Depicting Carbohydrate Stereochemistry:
Fi h P
Fischer
Projections
j ti
 Carbohydrates have multiple chiral centers
centers.
 Fischer projections are used as a quick method for
g carbohydrate
y
stereochemistry.
y
representing
 In a Fischer projection we represent a tetrahedral
carbon atom by two crossed lines.
 The
Th horizontal
h i
t l lines
li
representt bonds
b d coming
i outt off the
th
page.
 The vertical lines represent bonds going into the page
page.
8
Depicting Carbohydrate Stereochemistry:
Fi h P
Fischer
Projections
j ti
 In Fischer projections of carbohydrates the oxidized
end of the molecule is always higher on the page.
 Remember:
Horizontal lines = out of the page.
Vertical lines = into the page.
p g
9
Depicting Carbohydrate Stereochemistry:
Fi h P
Fischer
Projections
j ti
 Carbohydrates with more than one chirality centre
are shown in Fischer projections by stacking centres
p of one another.
on top
 View each carbon chiral centre individually.
e.g. D-aldopentose
10
Depicting Carbohydrate Stereochemistry:
Fi h P
Fischer
Projections
j ti
 Stacking centres on top of one another in Fischer
projections does not give an accurate picture of the
molecule,, which actuallyy is curled round on itself like
a bracelet.
11
Working With Fischer Projections
 The entire structure may only be rotated by 180 (not
by 90 or 270)
 While R, S designations can be deduced from
Fischer projections (with practice), it is best to make
molecular
l
l models
d l ffrom th
the projected
j t d structure
t t
and
d
work with the model
12
Working With Fischer Projections
 The representation can also be changed by keeping
one group steady and then rotating the other three
groups in
i either
ith a clockwise
l k i or anti-clockwise
ti l k i
direction.
 The
Th effect
ff t is
i tto rotate
t t around
d a single
i l bond,
b d which
hi h off
course does not change the stereochemistry.
13
St
Stereochemical
h i lR
Reference
f
 The reference compounds are the two enantiomers
of the simplest aldose, glyceraldehyde (C3H6O3), first
identified by their opposite rotation of plane polarized
light
 Naturally occurring glyceraldehyde rotates plane
planepolarized light in a clockwise direction, denoted (+)
g
“(+)-glyceraldehyde”
( ) gy
y
((also refered
and is designated
to as D-glyceraldehyde (D for dextrorotatory)
 The enantiomer g
gives the opposite
pp
rotation and has a
(-) or “L” (levorotatory) prefix
 The direction of rotation of light does not correlate to
any structural feature
14
D L Sugars
D,
S
 A compound is “D” if the hydroxyl group at the
chirality center farthest from the oxidized end of the
sugar is
i on th
the right
i ht or “L” if it is
i on th
the lleft.
ft
 The rest of the structure is designated in the name of
the compound
 D-glyceraldehyde is (R)-2,3-dihydroxypropanal
 L-glyceraldehyde
glyceraldehyde is (S)-2
(S) 2,3
3-dihydroxypropanal
dihydroxypropanal
15
Th D-Sugar
The
DS
Family
F il
 Correlation is always with
D-(+)-glyceraldehyde
( ) gy
y
 All sugars that have the
hydroxyl group at the
bottom pointing to the right
are called D-sugars
 (R)
( ) in C-I-P
C
sense
 Nearly all naturally
occurring
i monosaccharides
h id
are D-sugars
16
L-Sugars
g
 L-sugars have an S configuration at the lowest chirality
centre, with the –OH
OH group pointing to the left in Fischer
projections.
 An L-sugar
g is the mirror image
g ((enantiomer)) of the
corresponding D sugar and has the opposite
configuration from the D sugar at all chirality centres
17
C fi
Configurations
ti
off the
th Aldoses
Ald
 Stereoisomeric aldoses are distinguished by trivial
y systematic
y
designations
g
names, rather than by
 Enantiomers have the same names but different D,L
prefixes
 R,S designations are difficult to work with when there
are multiple similar chirality centers
 Number of stereoisomers = 2n, where n = number of
chiral centres
18
F
Four
Carbon
C b Ald
Aldoses
 Aldotetroses have two chirality centers
 There are 4 stereoisomeric aldotetroses, two pairs
p
of enantiomers: erythrose and threose
 D-erythrose is a a diastereomer of D-threose and Lthreose
Enantiomers
Enantiomers
19
Mi i l Fi
Minimal
Fischer
h P
Projections
j ti
 In order to work with structures of aldoses more
y onlyy essential elements are shown
easily,
 OH at a chirality center is “” and the carbonyl is an
arrow 
 The terminal OH in the CH2OH group is not shown
20
Ald
Aldopentoses
t
 Three chirality centers and 23 = 8 stereoisomers, four
p
pairs
of enantiomers: ribose, arabinose, xylose,
y
and
lyxose
 Only D enantiomers are shown below
21
C fi
Configurations
ti
off the
th Aldohexoses
Ald h
 Four chirality centers and 24 = 16 stereoisomers,
g p
pairs of enantiomers: allose, altrose, g
glucose,
eight
mannose, gulose, idose, galactose, talose
 Only D enantiomers are shown below
22
Cyclic S
Cy
Structures of Monosaccharides:
Hemiacetal Formation
 Alcohols add reversibly to aldehydes and ketones,
forming hemiacetals
23
Internal Hemiacetals of Sugars
 Intramolecular
I t
l
l nucleophilic
l
hili addition
dditi creates
t cyclic
li





hemiacetals in sugars
Fi
Fiveand
d six-membered
i
b d cyclic
li h
hemiacetals
i
t l are
particularly stable
Five membered rings are furanoses
Five-membered
furanoses. Six
Six-membered
membered are
pyanoses
Formation of the the cyclic hemiacetal creates an
additional chirality center giving two diasteromeric
forms, desigmated  and 
These diastereomers are called anomers
The designation  indicates that the OH at the
anomeric center is on the same side of the Fischer
projection
p
j
structure as the hydroxyl
y
y that designates
g
whether the structure is D or L
24
Fischer Projection
j
S
Structures of Anomers:
Allopyranose from Allose
25
Converting to Proper Structures
 The
Th Fischer
Fi h projection
j ti structures
t t
mustt be
b redrawn
d
tto
consider real bond lengths
 Note that all bonds on the same side of the Fischer
projection will be cis in the actual ring structure
 Groups
p on the right
g hand side of the Fischer p
projection
j
are below the plane of the ring in the Haworth projection
26
C f
Conformations
ti
off Pyranoses
P
 Pyranose rings have a chair-like geometry with axial
and equatorial substituents
 Rings are usually drawn placing the hemiacetal
oxygen atom at the right rear
27
Monosaccharide Anomers: Mutarotation
 The two anomers of D-glucopyranose can be
y
and p
purified
crystallized
 -D-glucopyranose melts at 146° and its specific
rotation, []D = 112.2°;
 -D-glucopyranose melts at 148–155°C a specific
rotation of []D = 18.7°
 Rotation of solutions of either pure anomer slowly
change due to slow conversion of the pure anomers
i t a 37:63
into
37 63 equilibrium
ilib i
mixture
i t
ott :
 called
ll d
mutarotation
28
M h i
Mechanism
off M
Mutarotation
t t ti Glucose
Gl
 Occurs by reversible ring-opening of each anomer to
the open-chain
open chain aldehyde, followed by reclosure
 Catalyzed by both acid and base
29