1
Chapter 16
Carbohydrates
- most abundant class of compounds
found in nature
- contain the elements C, H and O
- term “CARBOHYDRATES” came from the
fact that these compounds could be written
as “hydrates of carbon”
e.g.
C6H12O6
C12H22O11
IUPAC name of C6H12O6?
2
- They are polyhydroxyaldehydes,
polyhydroxyketones or compounds that give
these on hydrolysis
- “chemistry” is a combination of what we
saw for hydroxyl and carbonyl groups
Ways of classifying carbohydrates?
Carbohydrates are classified according to:
-
location of C=O
length of the carbon chain
stereochemistry
size of ring formed
number of sugar units
3
1. Based on location of carbonyl functional
group (C=O), they could be aldehydes or
ketones
H
H
H C OH
C O
C O
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
Name these?
4
2. Classified According to Chain Length
General:
H
H
H
H
C O
C O
C O
C O
CHOH
CHOH
CHOH
CHOH
CH2OH
CHOH
CHOH
CHOH
CH2OH
CHOH
CHOH
CH2OH
CHOH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
C O
C O
C O
C O
CH2OH
CHOH
CHOH
CHOH
CH2OH
CHOH
CHOH
CH2OH
CHOH
CH2OH
5
3.
Stereochemistry of Carbohydrates
H
C O
CHOH
CH2OH
The D- and L- system of Stereochemistry
- if the first stereogenic centre from the
bottom (when carbonyl is towards the top) is:
to the RIGHT, then Dto the LEFT, then L-
6
e.g.
H
H
C O
C O
H C OH
HO C
H
H C OH
HO C
H
H C OH
HO C
H
H C OH
HO C
H
CH2OH
CH2OH
D-(+)-allose
L-(-)-allose
Specific Naming:
H
H
C O
C O
H C OH
CH2OH
HO C H
CH2OH
CH2OH
C O
CH2OH
7
Aldotetroses
H
H
C O
C O
H C OH
HO C H
H C OH
H C OH
CH2OH
CH2OH
D-erythrose
D-threose
Aldopentoses
H
H
H
H
C O
C O
C O
C O
H C OH
HO C H
H C OH
HO C H
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
CH2OH
CH2OH
D-ribose
D-arabinose
HO C H
D-xylose
HO C H
D-lyxose
8
Aldohexoses
H
H
H
H
C O
C O
C O
C O
H C OH
HO C H
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
CH2OH
CH2OH
D-(+)-allose
D-(+)-altrose
HO C H
HO C H
D-(+)-glucose
HO C H
D-(+)-mannose
H
H
H
H
C O
C O
C O
C O
H C OH
H C OH
HO C H
HO C H
H C OH
HO C H
H C OH
HO C H
HO C H
HO C H
HO C H
HO C H
H C OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
CH2OH
CH2OH
D-(-)-gulose
D-(-)-idose
D-(+)-galactose
D-(-)-talose
9
Cyclization of D-Glucose
CH2OH
..
OH
H
H
..
H
O
OH H
OH
H
OH
CH2OH
O H
H
anomeric
H
carbon
OH H
OH
OH
H
OH
α -D-glucopyranose
(mp = 146 oC)
[α] = +112 o
anomeric
carbon
CH2OH
O OH
H
H
OH H
OH
H
H
OH
β -D-glucopyranose
(mp = 150 oC)
[α] = +19 o
10
Ketohexose
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
D-(-)-fructose
Cyclization of D-Fructose
HOH2C
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
D-(-)-fructose
HOH2C
H
H
OH
..
CH2OH
OH
..
HO
O
H
O CH2OH
H
HO
H
OH
OH
H
α -D-fructofuranose
HOH2C
O OH
H
HO
H
CH2OH
OH
H
β -D-fructofuranose
11
4. According to number of sugar units in a
chain:
Monosaccharides
Dissaccharide
Oligosaccharides
Polysaccharides
- the term saccharide from the Latin
(saccharum) refers to the sweetness of
some simple sugars
- examples of monosaccharides:
glucose
galactose
fructose
12
- examples of disaccharides:
maltose
CH2OH
CH2OH
O H
H
O OH
H
H
H
OH H
OH H
HO
H
O
H
OH
H
OH
β
α-glycosidic bond
cellobiose
CH2OH
O
H
H
OH H
H
HO
H
OH
CH2OH
O OH
H
H
OH H
O
H
H
OH
β
β
lactose
CH2OH
O
HO
H
OH H
H
H
H
OH
CH2OH
O H
H
H
OH H
O
OH
H
OH
β
α
13
sucrose
CH2OH
O H
H
H
OH H
HO
H
OH
O
HOH2C
O
H
HO
H
OH
H
CH2OH
α
β
14
- examples of polysaccharides:
starch
- the energy-storage carbohydrate found in plants
- polymer composed solely of glucose units linked
through 1,4- and 1,6-linkages
- partial hydrolysis of starch gives maltose and
complete hydrolysis gives glucose
2 Components:
amylose & amylopectin
Amylose
- composes about 20% of starch
- 50 - 300 glucose units in a continuous chain
- 1,4-linkages
O
CH2OH
O H
H
H
OH H
H
OH
O
H
OH
O
α-glycosidic bond
H
OH
CH2OH
O H
H
H
OH H
CH2OH
O H
H
H
OH H
CH2OH
O H
H
H
OH H
CH2OH
O H
H
H
OH H
O
H
OH
O
H
OH
O
15
Amylopectin
- 300 - 5000 glucose units per molecule
- 1,4-linked portions 25 - 30 units in length
- chains connected by 1,6-linked branching
O
CH2OH
O H
H
H
OH H
H
O
OH
CH2OH
O H
H
H
OH H
H
OH
O
H
OH
H
OH
O
H
O
H
OH
O
OH
CH2
CH2OH
O H
H
H
OH H
CH2OH
O H
H
H
OH H
O
CH2OH
O H
H
H
OH H
CH2OH
O H
H
H
OH H
H
O
H
OH
H
CH2OH
O H
H
H
OH H
O H
H
OH
O
H
OH
O
glycogen
- the energy-storage carbohydrate found in animals
- similar structure to amylopectin but more highly
branched (ie. glucose units linked through 1,4- and
1,6-linkages)
- branch points about every 8 - 12 glucose units
- produced and stored in the liver from blood glucose
provided by digestion
- helps maintain “glucose balance” within the body
16
cellulose
O
CH2OH
O
H
H
OH H
H
O
H
OH
CH2OH
O
H
H
OH H
H
O
H
OH
CH2OH
O
H
H
OH H
H
O
CH2OH
O
H
H
OH H
H
H
OH
O
H
OH
17
Other modified sugars
HO
CH2OH
O
H
OH
OH
OH
α-D-glucose
HO
CH2OH
O
H
OH
OH
NH2
HO
CH2OH
O
H
OH
OH
NH
O
α-D-glucosamine
CH3
N-acetylα-D-glucosamine
18
Artificial Sweetners
Saccharin
O
C
NH
S
O
O
- discovered in 1879
- 300 times as sweet as sucrose
- leaves a bitter metallic taste
- banned in 1970 due to suspected
carcinogenic effects
Cyclomate
- 20 times as sweet as sucrose
- no bitter taste
- heat stable
- banned shortly after saccharin
NH
SO3H
19
Aspartame (NUTRASWEET®)
H
N
NH2
O
OH
C H O
O O CH
3
-
discovered in 1965, approved in 1981
160 times as sweet as sucrose
two amino acids (PHE and ASP)
unstable to heat
major problem is PKU (phenylketonuria)
(inability to metabolize phenylalanine)
20
Oxidation / Reduction Reactions of Sugars
e.g.
H
H
HO
H
H
C O
OH
H
OH
OH
CH2OH
Ag+ or Cu 2+
D-glucose
e.g.
H
H
HO
H
H
C O
OH
H
OH
OH
CH2OH
D-glucose
What is a reducing sugar?
H2, Pt catalyst
or NaBH 4
21
e.g.
CH2OH
O
HO
H
H
OH
H
OH
CH2OH
Ag+ or Cu 2+
D-fructose
e.g.
CH2OH
O
HO
H
H2, Pt catalyst
H
OH
or NaBH 4
H
OH
CH2OH
D-fructose