Introduction to Organic Chemistry Bettelheim, Brown, Campbell and Farrell Chapter 10.
Carbohydrates Bettelheim, Brown, Campbell and Farrell Chapter 20.
-
Upload
augustus-russell -
Category
Documents
-
view
351 -
download
42
Transcript of Carbohydrates Bettelheim, Brown, Campbell and Farrell Chapter 20.
Carbohydrates
Bettelheim, Brown, Campbell and Farrell
Chapter 20
Carbohydrates• Carbohydrate:Carbohydrate: polyhydroxyaldehyde or
polyhydroxyketone, or a substance that can be hydrolyzed to form these compounds
• Monosaccharide:Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate (simple sugar)
Monosaccharide:Monosaccharide: – Monosaccharides have the general formula
CCnnHH2n2nOOnn, where nn varies from 3 to 8
– aldose:aldose: a monosaccharide containing an aldehyde group
– ketose:ketose: a monosaccharide containing a ketone group
Monosaccharides• Monosaccharides are classified by their
number of carbon atoms
• “ose” ending for sugars
Hexose
Heptose
Octose
TrioseTetrose
Pentose
FormulaName
C3H6 O3C4H8 O4
C5H1 0O5
C6H1 2O6
C7H1 4O7C8H1 6O8
Monosaccharides• There are only two trioses
– Often simply call these trioses – Tells the number of carbons
Dihydroxyacetone (a ketotriose)
Glyceraldehyde (an aldotriose)
CHO
CHOH
CH2OH
CH2OH
C=O
CH2OH
Monosaccharides
• Glyceraldehyde, the simplest aldose, contains a stereocenter and exists as a pair of enantiomers
CHO
CH OH
CH2OH
CHO
C
CH2OH
HHO
Monosaccharides• Fischer projection:Fischer projection: a two dimensional
representation for showing the configuration of tetrahedral stereocenters– Horizontal lines represent bonds projecting
forward – Vertical lines represent bonds projecting to the
rear
CHO
CH OH
CH2OH
H OHCHO
CH2OH
convert to a Fischerprojection
D,L Monosaccharides• In 1891, Emil Fischer made the arbitrary
assignments of D- and L- to the enantiomers of glyceraldehyde
– D-monosaccharide:D-monosaccharide: the -OH on its penultimate (next to last) carbon is on the right
– L-monosaccharide:L-monosaccharide: the -OH on its penultimate (next to last) carbon is on the left
L-GlyceraldehydeD-Glyceraldehyde
CHOCHO
H OH
CH2OH CH2OH
HHO
[]25 = +13.5°D
[]25 = -13.5°D
D,L Monosaccharides– the most common D-tetroses and D-pentoses
– the three common D-hexoses
CH2OH
CHO
OHOHH
H
CH2OH
CHO
OHHHO
HH
CH2OH
CHO
OHOHH
OHH
CH2OH
CHO
OHHH
HOHH
D-Erythrose D-Threose D-Ribose 2-Deoxy-D-ribose
CHO
HOHH
HOOHH
CH2OHOHH
CHO
HOHH
HOHHO
CH2OHOHH
CH2OH
HHOC=O
OHH
CH2OHOHH
D-FructoseD-Glucose D-Galactose
Review: Addition of Alcohols to Carbonyls
• Addition of an alcohol to the carbonyl group of an aldehyde or ketone forms a hemiacetalhemiacetal (a half-acetal)– Functional group of a hemiacetal is a carbon
bonded to one -OH group and one -OR group– H of the alcohol adds to the carbonyl oxygen
and -OR adds to the carbonyl carbon
Addition of Alcohols to Carbonyl– Hemiacetals are generally unstable and are only
minor components of most equilibrium mixtures– Major exception: When both the carbonyl group
and the hydroxyl group are in the same molecule and can form a cyclic hemiacetal with a 5- or 6-member ring
– Cyclic hemiacetals predominate
H
O
O-HC
O O
H
H
O O-H
H
4-Hydroxypentanal A cyclic hemiacetal
123
45
1345
redraw to show the -OH and -CHO close
to each other2
Cyclic Structure• Aldehydes and ketones react with alcohols
to form hemiacetalshemiacetals– cyclic hemiacetals form readily when the
hydroxyl and carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring
O-HH
O
CO O
H
H
O O-H
H4-Hydroxypentanal
A cyclic hemiacetal
14
14
redraw to show -OH and -CHO
close to each other
Haworth Projections– D-Glucose forms these cyclic hemiacetals
CHO
OH
H
OH
H
HO
H
H OH
CH2OH
HH OH
HHO
HOH
OH
H
CH2OHO
C
H OH
HHO
HOH
H
CH2OHOH
O
H
OHH OH
HHO
HH
OH
H
CH2OHO
D-Glucose
-D-Glucopyranose (-D-Glucose)
()
()
-D-Glucopyranose (-D-Glucose)
+
anomericcarbon
5
5
1
1
redraw to show the -OH on carbon-5 close to thealdehyde on carbon-1
Carbohydrate Rings• -OH group can react with C=O group to
give hemiacetal
• Fischer Fischer ring Haworth
Haworth Projections– Cyclic hemiacetal is represented as a planar
ring, lying roughly perpendicular to the plane of the paper
– Groups bonded to the carbons of the ring then lie either above or below the plane of the ring
– New carbon stereocenter created in forming the cyclic structure is called an anomeric carbonanomeric carbon
– Stereoisomers that differ in configuration only at the anomeric carbon are called anomersanomers
– Anomeric carbon of an aldose is C-1; that of the most common ketoses is C-2
Haworth Projections• In the terminology of carbohydrate
chemistry, form: -OH on the anomeric carbon is on the
same side of the ring as the terminal -CH2OH (up)
form: -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH (down)
– Pyranose: Pyranose: 6-member ring sugar containing O – Furanose: Furanose: 5-member ring sugar containing O
PyranFuranOO
Haworth Projections– aldopentoses also form cyclic hemiacetals– the most prevalent forms of D-ribose and
other pentoses in the biological world are furanoses
OH ()
H
HOH OH
H HOHOCH2
H
OH ()
HOH H
H HOHOCH2
-D-Ribofuranose(-D-Ribose)
-2-Deoxy-D-ribofuranose(-2-Deoxy-D-ribose)
Haworth Projections– D-fructose also forms a five-membered cyclic
hemiacetal
HO
HOCH2 OH
HHO
CH2OH
OHH
H
C=O
CH2OH
HOH
CH2OH
OHH
HO HOH
HOHOCH2
HO HCH2OH
OH
D-Fructose
1
2
5
5
5
1
2
2
()
-D-Fructofuranose(-D-Fructose)
-D-Fructofuranose(-D-Fructose)
()
1
Chair Conformations
• For pyranoses, the six-membered ring is more accurately represented as a chair chair conformationconformation
OCH2OH
HOHO
OHOH()
CHOH
HO
CH2OHOHHO
OHO
OH()HO
HO
CH2OHO
(-D-Glucose)
(-D-Glucose)
-D-Glucopyranose
-D-Glucopyranose
D-Glucose
anomericcarbon
Chair Conformations
– in both a Haworth projection and a chair conformation, the orientations of groups on carbons 1- 5 of -D-glucopyranose are up, down, up, down, and up
OCH2OH
HOHO
OHOH()H
H OH
HHO
HOH()
OH
H
CH2OHO
-D-Glucopyranose(chair conformation)
-D-Glucopyranose(Haworth projection)
123
4
5
6
1
23
4
5
6
Mutarotation• Mutarotation: Mutarotation: - and -anomers can have the
ring open and then form the other anomer.
• A change in specific optical rotation accompanies the equilibration of in aqueous solution– example: when either -D-glucose or -D-glucose
is dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms
Fig 19.2, p.472
Mutarotation
[]D25 = + 18.7°
-D-Glucopyranose-D-Glucopyranose[]D
25 = +112°
OHOH
HOHO
CH2OHO HO OH
OC
CH2OH
HO
HOH
OCH2OH
HO
HOOH
HO
Open-chain form
Mutarotation
• Rings can open to form open chain compound. • New ring can be either - or -anomer.• Equilibrium among - and - forms (and open chain)
will be established. • Change in specific optical rotation accompanies this
equilibration – If either -D-glucose or -D-glucose is dissolved in water,
specific rotation changes to an equilibrium value of +52.7°– 64% beta and 36% alpha forms at equilibrium– Very little will exist in open chain form
Fig. 17.6 Another representation of ring
opening and closure
Fig 19.2, p.472
Mutarotation
• Shown in chair conformation
[]D25 = + 18.7°
-D-Glucopyranose-D-Glucopyranose[]D
25 = +112°
OHOH
HOHO
CH2OHO HO OH
OC
CH2OH
HO
HOH
OCH2OH
HO
HOOH
HO
Open-chain form
Physical Properties
• Monosaccharides – Colorless crystalline solids– Very soluble in water– Only slightly soluble in ethanol– Sweet taste
Sweetness relative to Sucrose
Carbohydrate
fructose
glucose
galactose
sucrose (table sugar)
lactose (milk sugar)
honey
SweetnessRelative to Sucrose
1.741.000.970.74
0.320.16
Artificial Sweetener
SweetnessRelative to Sucrose
maltose 0.33
saccharin 450acesulfame-K 200aspartame 180
Formation of Glycosides• Reaction of a hemiacetal with alcohol
gives an acetal
HH OH
HHO
HOH
OH
H
CH2OHO
CH3OHH+
-H2O
OCH2OH
H
OH
OCH3H
HOH
OHH
H
OCH2OH
H
OH
HH
HOH
OHH
OCH3
(-D-Glucose)-D-Glucopyranose
Methyl -D-glucopyranoside(Methyl -D-glucoside)
anomeric carbon
+
+
Methyl -D-glucopyranoside(Methyl -D-glucoside)
glycosidicbond
Hemiacetal
Acetals
Formation of Glycosides– Cyclic acetal derived from a monosaccharide is
called a glycosideglycoside– Bond from the anomeric carbon to the -OR group
is called a glycosidic bondglycosidic bond– Mutarotation is not possible in a glycoside
• Ring can’t open up• No equilibrium with the open-chain form
– Can be hydrolyzed in acidic solution to form alcohol and a monosaccharide
Oxidation and Reduction of simple sugars
• Aldehyde and ketone groups can be reduced to form alcohols– Requires reducing agents such as
NaBH4 or H2 with a transition metal catalyst
• Aldehyde group can be oxidized to produce a carboxylic acid
Reduction to Alditols
• The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2 in the presence of a transition metal catalyst– the reduction product is called an alditolalditol
OHOH
HOHO
CH2OHO
CHOOHHHHOOHH
CH2OHOHH
NaBH4
CH2OHOHHHHOOHH
CH2OHOHH
D-Glucitol(D-Sorbitol)
D-Glucose-D-Glucopyranose
Reduction to Alditols– sorbitol is found in the plant world in many berries
and in cherries, plums, pears, apples, seaweed, and algae
– it is about 60 percent as sweet as sucrose (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics
– Other common alditols
CH2OH
CH2OH
OHHOHH
CH2OH
CH2OH
OHHHHOOHH
CH2OHHHOHHOOHH
CH2OHOHH
D-Mannitol XylitolErythritol
Oxidation to Aldonic Acids– the aldehyde group of an aldose is oxidized under
basic conditions to a carboxylate anion– the oxidation product is called an aldonic acidaldonic acid– any carbohydrate that reacts with an oxidizing
agent to form an aldonic acid is classified as a reducing sugarreducing sugar (it reduces the oxidizing agent)
OCH2OH
HOHO
OHOH
COHHHHOOHH
CH2OHOHH
O HC
OHHHHOOHH
CH2OHOHH
O O-
oxidizingagent
D-GluconateD-Glucose-D-Glucopyranose(-D-Glucose)
basicsolution
Common Disaccharides• Sucrose (table sugar)
– sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets
– sucrose is a nonreducing sugar (ring can’t open)
HOOH
OH
CH2OH
O
OH
HOO
CH2OH
HOCH2
OHO
HO
O
OH
CH2OH
OH
HOO
CH2OH
HOCH2
1
1
2
1
2
1
a unit of -D-glucopyranose
a unit of -D-fructofuranose
Fig. 17.10
Common Disaccharides• Lactose
– Principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk
– it consists of D-galactose bonded by a -1,4-glycosidic bond to carbon 4 of D-glucose
– lactose is a reducing sugar
O
OH
HOOH
O
CH2OH
O
HOOH
OH
CH2OHOOH O
OH
OH
CH2OH
O OH
OH
OH
CH2OH
1
1
4
4
-1,4-glycosidic bond
Common Disaccharides
• Maltose– present in malt, the juice from sprouted barley
and other cereal grains – maltose consists of two glucose units joined
by an -1,4-glycosidic bond– maltose is a reducing sugar
OHO
HOOH
OOHO OH
OH
CH2OH
CH2OHO
OH
O
OHHO
O OH
HO
OH
CH2OH
HOCH2 1
4
-1,4-glycosidicbond
1 4
Polysaccharides
• Polysaccharide:Polysaccharide: Polymer containing many monosaccharide units
• Starch:Starch: a polymer of D-glucose– Two forms: amylose and amylopectin– Amylose is composed of unbranched chains of up to
4000 D-glucose units joined by -1,4 bonds– Amylopectin contains chains up to 10,000 D-glucose
units also joined by -1,4-glycosidic bonds• Branched compound• New chains of 24 to 30 units attached by -1,6 linkages
Polysaccharides
• GlycogenGlycogen is the energy-reserve carbohydrate for animals– Glycogen is a branched polysaccharide of
approximately 106 glucose units joined by -1,4- and -1,6-glycosidic bonds
– Total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle
Amylose
Branched
Polysaccharides• CelluloseCellulose is a chain of D-glucose units joined
by -1,4-glycosidic bonds– Average chain has approximately 2200 glucose
units per molecule– cellulose chain is much stiffer than starch
• Chains line up side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds
– Arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength
– Cellulose is insoluble in water
Polysaccharides• Cellulose (cont’d)
– humans and other animals cannot digest cellulose– Can only digest starch and glycogen to get glucose– Many bacteria and microorganisms can digest
cellulose– Termites have such bacteria in their intestines and
can use wood as their principal food– Ruminants (cud-chewing animals) and horses can
also digest grasses and hay
Cellulose
Blood Groups
• Several sugar groups attached to red blood cell (RBC) surface
• Difference in one sugar accounts for Types A, B, AB and O blood.
Sugars attached to RBC
(α 1,4) β (1,3)
X –-------- D-Galactose-----------N-Acetyl-D- RBC
| glucosamine
| (α 1,2)
|
L-fucose
Blood Groups -- Type A
• N-Acetyl-D-Galactosamine on RBC surface
Blood Groups -- Type B
• galactose on RBC surface
Blood Groups -- Type O
• Neither N-Acetyl-D-galactosamine nor Galactose attached to rest of sugar groups on RBC surface