Basic Organic
g
Chemistry
y
2302202
Dr. Rick Attrill
Office MHMK 1405/5
Carbohydrates II
 Reactions of Monosaccharides
 Disaccharides
Di
h id and
dP
Polysaccharides
l
h id
2
Reactions of Monosaccharides
 OH groups can be converted into esters and
ethers, which are often easier to work with than the
free sugars and are soluble in organic solvents.


Esterification by treating with an acid chloride or acid
anhydride
h d id iin th
the presence off a b
base
All OH groups react
3
Ethers
Eth
 Treatment with an alkyl halide in the presence of
y
base—the Williamson ether synthesis
 Use silver oxide as a catalyst with base-sensitive
compounds
4
Gl
Glycoside
id F
Formation
ti
 Treatment of a monosaccharide hemiacetal with an
y yields
y
an acetal in which
alcohol and an acid catalyst
the anomeric OH has been replaced by an OR
group

-D-glucopyranose with methanol and acid gives a
mixture of  and  methyl D-glucopyranosides
5
Gl
Glycosides
id
 Carbohydrate acetals are named by first citing the
alkyl
y g
group
p and then replacing
p
g the -ose ending
g of the
sugar with –oside
 Stable in water,, requiring
q
g acid for hydrolysis
y
y
6
S l ti F
Selective
Formation
ti off C1-Acetal
C1 A t l
 Synthesis requires distinguishing the numerous OH
groups
g
p
 Treatment of glucose pentaacetate with HBr converts
anomeric OH to Br
 Addition of alcohol (with Ag2O) gives a  glycoside
(Koenigs–Knorr reaction)
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Koenigs-Knorr
g
Reaction Mechanism
  and  anomers of tetraacetyl-D-glucopyranosyl
bromide give  -glycoside
 Suggests either bromide leaves and cation is
stabilized by neighboring acetyl nucleophile from 
side
 Incoming
I
i alcohol
l h l di
displaces
l
acetyl
t l oxygen tto give
i 
glycoside
R d ti off M
Reduction
Monosaccharides
h id
 Treatment of an aldose or ketose with NaBH4
reduces it to a polyalcohol (alditol)
 Reaction via the open-chain form in the
aldehyde/ketone hemiacetal equilibrium
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Oxidation of Monosaccharides
 Aldoses are easily oxidized to carboxylic acids by:
Tollens' reagent (Ag+,aq. NH3) (gives a Ag “metallic
mirror”), Fehling's reagent (Cu2+, sodium tartrate) (forms a
red Cu20 ppt.),
ppt ) Benedict
Benedict`ss reagent (Cu2+, sodium citrate)
(forms a red Cu20 ppt.)
 Ketoses are reducing sugars if they can isomerize to
aldoses
10
Oxidation of Monosaccharides
O
with Bromine
 Br2 in water is an effective oxidizing reagent for
converting aldoses to carboxylic acid, called aldonic
acids (the metal reagents are for analysis only)
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Formation of Dicarboxylic Acids
 Warm dilute HNO3 oxidizes aldoses to dicarboxylic
acids, called aldaric acids
 The CHO
CHO group and the terminal CH
CH2OH group
are oxidized to COOH
C
Chain
Lengthening:
g
g
The Kiliani–Fischer Synthesis
 Lengthening aldose chain by one CH(OH), an
aldopentose
ld
t
i converted
is
t d iinto
t an aldohexose
ld h
13
Kili i Fi h Synthesis
Kiliani-Fischer
S th i Method
M th d
 Aldoses form cyanohydrins with HCN
Follow by hydrolysis,
hydrolysis ester formation
formation, reduction
 Modern improvement: reduce nitrile over a palladium
catalyst yielding an imine intermediate that is
catalyst,
hydrolyzed to an aldehyde

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Stereoisomers from Kiliani-Fischer Synthesis
 Cyanohydrin is formed as a mixture of stereoisomers
at the new chirality center
center, resulting in two aldoses
15
Chain Shortening: The Wohl Degradation
 Shortens aldose chain by one CH2OH
16
Di
Disaccharides
h id
 A disaccharide combines a hydroxyl of one
monosaccharide in an acetal linkage with another
 A glycosidic bond between C1 of the first sugar ( or
) and the OH at C4 of the second sugar is
particularly common (a 1,4 link)
17
M lt
Maltose
and
d Cellobiose
C ll bi
 Maltose: two D-glucopyranose units with
a 1,4--glycoside
gy
bond ((from starch hydrolysis)
y
y )
 Cellobiose: two D-glucopyranose units with a
1,4--glycoside bond (from cellulose hydrolysis)
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H i
Hemiacetals
t l iin Di
Disaccharides
h id
 Maltose and cellobiose are both reducing sugars
 The  and  anomers equilibrate,
equilibrate causing
mutarotation
19
Y C
You
Can’t
’t E
Eatt Cellobiose
C ll bi
 The 1-4’--D-glucopyranosyl linkage in cellobiose is
not attacked by any digestive enzyme
 The
Th 1-4’-1 4’ D-glucopyrnaosyl
l
l lilinkage
k
iin maltose
lt
is
i a
substrate for digestive enzymes and cleaves to give
glucose
20
L t
Lactose
 A disaccharide
di
h id th
thatt occurs naturally
t ll iin milk
ilk
 Lactose is a reducing sugar. It exhibits mutarotation
 It is 1,4’--D-galactopyranosyl-D-glucopyranoside
 The structure is cleaved in digestion
g
to g
glucose and
galactose
21
Sucrose
 “Table
“T bl Sugar”
S
” is
i pure sucrose, a disaccharide
di
h id th
thatt
hydrolyzes to glucose and fructose
 Not a reducing sugar and does not undergo
mutarotation (not a hemiacetal)
 Connected as acetal from both anomeric carbons
(aldehyde to ketone)
22
P l
Polysaccharides
h id
 Complex carbohydrates in which very many simple
sugars are linked
 Cellulose and starch are the two most widely
occurring polysaccharides
23
C ll l
Cellulose
 Consists of thousands of D-glucopyranosyl 1,4--
glucopyranosides as in cellobiose
 Cellulose
C ll l
molecules
l
l fform a llarge aggregate
t
structures held together by hydrogen bonds
 Cellulose
C ll l
iis th
the main
i componentt off wood
d and
d plant
l t
fiber
24
Starch and Glycogen
 Starch is a 1,4
1,4--glupyranosyl-glucopyranoside
 glupyranosyl glucopyranoside
polymer
 It is digested
g
into g
glucose
 There are two components
 amylose, insoluble in water – 20% of starch
 1,4’--glycoside polymer
 amylopectin,
amylopectin soluble in water – 80% of starch
25
Amylopectin
 More
M
complex
l iin structure
t t
than
th amylose
l
 Has 1,6--glycoside branches approximately every
25 glucose units in addition to 1,4--links
26
Glycogen
Gl
 A polysaccharide that serves the same energy
g function in animals that starch serves in
storage
plants
 Highly branched and larger than amylopectin—up to
100,000 glucose units
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Oth Important
Other
I
t t Carbohydrates
C b h d t
 Deoxy sugars have an OH group is replaced by an
H.
H.

Derivatives of 2-deoxyribose are the fundamental units
of DNA (deoxyribonucleic acid)
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Amino Sugars
 OH
OH group is replaced by an NH
NH2
 Amino sugars are found in antibiotics such as
streptomycin and gentamicin
 Occur in cartilage
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