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LECTURE 14
Carbonhydrates. Monosacchrides.
Structure. Isomery.
Reactionary ability of the
monosaccharides.
Ass. Medvid I.I.
ass. Burmas N.I.
Outline:
1. Carbohydrates.
2. Classification of carbohydrates.
3. Chirality:
handedness
in
molecules.
Stereoisomers. Fisher Projections.
4. Classification of monosaccharides.
5. Reactions of monosaccharides.
6. Biologically important monosaccharides.
1. Carbohydrates.
Carbohydrates are the most abundant class of
bioorganic molecules on planet Earth. Although their
abundance in the human body is relatively low,
carbohydrates constitute about 75% by mass of dry
plant materials. Green (chlorophyll-containing) plants
produce carbohydrates via photosynthesis. In this
process, carbon dioxide from the air and water from the
soil are the reactants, and sunlight absorbed by
chlorophyll is the energy source. Plants have two main
uses for the carbohydrates they produce. In the form of
cellulose, carbohydrates serve as structural elements,
and in the form of starch, they provide energy reserves
for the plants. Dietary intake of plant materials is the
major carbohydrate source for humans and animals. The
average human diet should ideally be about two-thirds
carbohydrate by mass.
Carbohydrates have the following functions in
human organism:
1. Carbohydrate oxidation provides energy.
2. Carbohydrate storage, in the form of glycogen,
provides а short- term energy reserve.
3. Carbohydrates supply carbon atoms for the
synthesis of other biochemical substances
(proteins, lipids, and nucleic acids).
4. Carbohydrates form part of the structural
framework of DNA and RNA molecules.
5. Carbohydrate "markers" on cell surfaces play
key roles in cell -cell recognition processes.
Classification of carbohydrates.
Most simple carbohydrates have empirical
formulas that fit the general formula СnН2nОn.
An early observation by scientists that this
general formula can also be written as
Сn(Н2О)n is the basis for the term carbohydrate
- that is, "hydrate of carbon." It is now known
that this hydrate viewpoint is not correct, but the
term carbohydrate still persists. Today the term
is used to refer to an entire family of
compounds, only some of which have the
formula СnН2nОn.
Examples of carbohydrates
IUPAC nomenclature doesn't use for carbohydrates, trivial
nomenclature uses for them.
Carbohydrates are polyhydroxy aldehydes,
polyhydroxy ketones, or compounds that yield such
substances upon hydrolysis. The carbohydrate glucose
is а polyhydroxy aldehyde, and the carbohydrate
fructose is а polyhydroxy ketone. А striking structural
feature of carbohydrates is the large number of
functional groups present. In glucose and fructose there
is а functional group attached to each carbon atom.
Carbohydrates are classified on the basis of molecular size
as monosaccharides, oligosaccharides, and
polysaccharides.
Monosaccharides are carbohydrates that contain a
single polyhydroxy aldehyde or polyhydroxy ketone
unit. Monosaccharides cannot be broken down into
simpler units by hydrolysis reactions. Both glucose and
fructose are monosaccharides. Naturally occurring
monosaccharides have4rom three to seven carbon
atoms; five- and six-carbon species are especially
common. Pure monosaccharides are water-soluble,
white, crystalline solids.
Oligosaccharides are carbohydrates that contain from two to ten
monosaccharide units. Disaccharides are the most common type of
oligosaccharide. Disaccharides are carbohydrates composed of two
monosaccharide units covalently bonded to each other. Like
monosaccharides, disaccharides are crystalline, water-soluble
substances. Sucrose (table sugar) and lactose (milk sugar) are
disaccharides. Within the human body, oligosaccharides are often
found associated with proteins and lipids in complexes that have
both structural and regulatory functions. Free oligosaccharides,
other than disaccharides, are seldom encountered in biological
systems. Complete hydrolysis of an oligosaccharide produces
monosaccharides. Upon hydrolysis, а disaccharide produces two
monosaccharides, а trisaccharide three monosaccharides, а
hexasaccharide six monosaccharides, and so on.
Polysaccharides are carbohydrates made up of many monosaccharide
units. Polysaccharides, which are polymers, often consist of tens of
thousands of monosaccharide units. Both cellulose and starch are
polysaccharides. We encounter these two substances everywhere.
The paper on which this book is printed is mainly cellulose, as are
the cotton in our clothes and the wood in our houses. Starch is а
component of many types of foods including bread, pasta, potatoes,
rice, corn, beans, and peas.
3. Chirality: handedness in molecules.
Stereoisomers. Fischer Projections.
Before considering structures for and reactions of
specific carbohydrates, we will consider handedness, а
biologically important structural property exhibited by
most carbohydrates. Most carbohydrate molecules can
exist in two forms - а left-handed form and а righthanded form. Significantly, these different forms often
elicit different responses within the human body.
Mirror Images. The concept of mirror images is
the key to understanding molecular handedness. All
objects, including all molecules, have mirror images.
The mirror image of an object is the object’ reflection
in а mirror. For example: human hands.
A person’s left and right hands are not
superinposable upon each other.
Chirality. Objects that cannot be superimposed upon their
mirror image are said to be chiral objects. А chiral object
is an object that is not identical to its mirror image. Your
hands and feet are chiral objects, as are gloves and shoes.
Objects that can be superimposed upon their mirror
images are achiral. An achiral object is identical to its
mirror image. Achiral objects include tube socks, solidcolored ties and Т-shirts.
Molecules, like larger objects, can be chiral or achiral. А
simple example of а chiral molecule is the trisubstituted
methane bromochloroiodomethane.
 The simplest example of а chiral
carbohydrate is the three-carbon
molecule glyceraldehyde.
Trying to superimpose the mirror image of а
molecule on that molecule visually, is one way to
determine molecular chirality. Another method,
which is much easier to apply, makes use of the
observation that generally, whenever а carbon atom
in а molecule is bonded to four different groups, the
molecule as а whole is chiral. Any organic molecule
containing а single carbon atom with four different
groups attached to it exhibits chirality. Such а carbon
atom is called а chiral center. А chiral center is an
atom in а molecule that has four different groups
tetrahedrally bonded to it. Chiral centers within
molecules are often denoted by а small asterisk. Note
the chiral centers in the following molecules.
2-butanol
1-chloto-1-iodoethane
3-methylhexane
 Organic molecules, especially
carbohydrates, may contain more than one
chiral center. For example, the following
carbohydrate has two chiral centers.
 Stereoisomers are isomers whose atoms are connected in the
same way but differ in their arrangement in space. The two
nonsuperimposable mirror-image forms of а chiral molecule are
stereoisomers.
 There are two major causes of stereoisomerism: (1) the presence
of а chiral center in а molecule, and (2) the presence of
"structural rigidity" in а molecule. Structural rigidity is caused by
restricted rotation about chemical bonds. It is the basis for cis trans stereoisomerism, а phenomenon found in some substituted
cycloalkanes and some alkenes.
 For example, aldopentose has 3 asymmetric carbon atoms and
can forms 8 stereoisomers (2 in 3 power = 8). Aldohesose has 4
asymmetric carbon atoms and can forms 16 stereoisomers (2 in 4
power = 16).
 For representation of stereoisomers use formulas of Fisher. All
isomers of monosaccharides deviated into D- and Lstereoisomers.
Handedness (D and L configuration) is determined by
the configuration at the highest-numbered chiral
center (for pentoses – C4, for hexoses – C5).
Majority of monosaccharides refer to D-row.
Stereoisomers can be subdivided into two types: enantiomers and
diastereomers.
Enantiomers are stereoisomers whose molecules are
nonsuperimposable mirror images of each other. Left- and righthanded forms of а molecule with а single chiral center are
enantiomers.
Diastereomers are stereoisomers whose molecules are not
mirror images of each other. Cis - trans isomers (of both the
alkene and the cycloalkane types) are diastereomers. We will see
additional examples of carbohydrate diastereomers in the next
section. Stereoisomers that are not enantiomers are diastereomers;
they must be one or the other.
Enantiomers
Diastereomers
 Fisher Projections. Drawing three-dimensional
representations of chiral molecules, can be both timeconsuming and awkward. Fischer projections represent
а method for giving molecular chirality specifications in
two dimensions. А Fisher projection is а twodimensional notation showing the spatial arrangement
of groups about chiral centers in molecules. In а Fischer
projection, а chiral center is represented as the
intersection of vertical and horizontal lines. The atom at
the chiral center, which is almost always carbon, is not
explicitly shown.
 The tetrahedral arrangement of the four groups attached
to the atom at the chiral center is governed by the
following conventions: (1) Vertical lines from the chiral
center represent bonds to groups directed into the
printed page. (2) Horizontal lines from the chiral center
represent bonds to groups directed out of the printed
page.
Fischer projection
 Our immediate concern is Fisher projections for
monosaccharides. Such projections have the monosaccharide
carbon chain positioned vertically with the carbonyl group
(aldehyde or ketone) at or near the top.
The smallest monosaccharide that has а chiral center is the
compound glyceraldehydes (2,3-dihydroxypropanal). The
structural formula and Fischer projections for the two
enantiomers of glyceraldehyde are:
D-glyceraldehyde
L-glyceraldehyde
The handedness (right and left) of these two enantiomers is
specified by using the designations D and L. The enantiomer
with the chiral center - ОН group on the right in the Fischer
projection is by definition the right-handed isomer (Dglyceraldehyde), and the enantiomer with the chiral center - ОН
group on the left in the Fisher projection is by definition the lefthanded isomer (L-glyceraldehyde).
We now consider Fisher projections for the compound 2,3,4trihydroxybutanal, а monosaccharide with four carbons and two
chiral centers.There are four stereoisomers for this compound two pairs of enantiomers:
A
B
First enantiomeric pair
C
D
Second enantiomeric pair
In the first enantiomeric pair, both chiral center - ОН groups
are on the same side of the Fischer projection, and in the second
enantiomeric pair, the chiral center - ОН groups are on opposite
sides of the Fisher projection. These are the only - ОН group
arrangements possible.
The D, L system used to designate the handedness of
glyceraldehyde enantiomers is extended to monosaccharides with
more than one chiral center in the following manner. The carbon
chain is numbered, starting at the carbonyl group end of the
molecule, and the highest-numbered chiral center is used to
determine D or L configuration.
A
D- isomer
B
L-isomer
C
D-isomer
D
L- isomer
The D, L nomenclature gives the configuration (handedness)
only at the highest-numbered chiral center. The configuration at
other chiral centers in а molecule is accounted for by assigning а
different common name to each pair of D, L enantiomers. In our
present example, compounds А and В (the first enantiomeric pair)
are D-erythrose and L-erythrose; compounds С and D (the second
enantiomeric pair) are D-threose and L-threose.
А and С are diastereomers, stereoisomers that are not mirror
images of each other. Other diastereomeric pairs in our example
are А and D, В and С, and В and D. These four pairs are epimers.
Epimers are diastereomers that differ only in the configuration at
one chiral center. In general, а compound that has n chiral centers
may exist in а maximum of 2n stereoisomeric forms. For
example, when three chiral centers are present, at most eight
stereoisomers (23 = 8) are possible (four pairs of enantiomers).
Stereoisomers.
1. Isomers in which the atoms have the same connectivity but differ
in spatial arrangement.
2. Stereoisomerism results either from the presence of а chrial
center or from structural rigidity caused by restricted rotation
about chemical bonds.
Enantiomers.
1. Stereoisomers that are nonsuperimposable mirror
images of each other.
2.Handedness (D and L configuration) is determined by
the configuration at the highest-numbered chiral center.
3. Enantiomers rotate plane-polarized light in different
directions. (+) Enantiomers are dextrorotatory
(clockwise), and (-) enantiomers are levorotatory
(counterclockwise).
Diastereomers.
1. Stereoisomers that are not mirror images of each other.
2. Epimers are diastereomers whose configurations differ
only at one chiral center.
Properties of Enantiomers.
Structural isomers differ in most chemical
and physical properties. For example, structural
isomers have different boiling points and
melting points. Diastereomers also differ in most
chemical and physical properties. They also have
different boiling points and freezing points. In
contrast, nearly all the properties of а pair of
enantiomers are the same; for example, they
have identical boiling points and freezing points.
Enantiomers exhibit property differences in only
two areas: their interaction with plane-polarized
light and their interaction with other chiral
substances.
Interactions between chiral compounds.
А left-handed baseball player (chiral) and а right-handed baseball player
(chiral) can use the same baseball bat (achiral) or wear the same baseball hat
(achiral). However, left- and right-handed baseball players (chiral) cannot use
the same baseball glove (chiral). This nonchemical example illustrates that the
chirality of an object becomes important when the object interacts with another
chiral object. Applying this generalization to molecules, we find that the two
members of an enantiomeric pair, because of their differing chirality, interact
differently with other chiral molecules. We find that:
1. А pair of enantiomers have the same solubility in an achiral solvent, such as
ethanol, but differing solubilities in а chiral solvent, such as в-2-butanol.
2. The rate and extent of reaction of enantiomers with another reactant are the
same if the reactant is achiral but differ if the reactant is chiral. The different
reactions that different enantiomers undergo are further considered in the
paragraph that follows.
3. Enantiomers have identical boiling points, freezing points, and densities,
because such properties depend on the strength of intermolecular forces, and
intermolecular force strength does not depend on chirality. Intermolecular
force strength is the same for both forms of а chiral molecule, because both
forms have identical sets of functional groups
The two enantiomeric forms of а chiral molecule
often generate different responses from the human
body. Sometimes both enantiomers are biologically
active, each form giving a different response;
sometimes both give the same response, but one
isomer’s response is many times greater than that of the
other; and sometimes only one of the two forms is
biologically active, the other form giving no response.
For example, the body’s response to the D isomer of the
hormone epinephrine is 20 times greater than its
response to the L isomer. Epinephrine exerts its effect
by binding to specialized receptors. It binds to the
receptor site by means of а three-point contact, Depinephrine makes а perfect three-point contact with the
receptor surface, but the biologically weaker Lepinephrine can make only а two-point contact.
Because of the poorer fit, the binding of the isomer is
weaker, and less physiological response is observed.
4. Classification of monosaccharides.
Now that we have considered molecular chirality and its
consequences, we return to the subject of carbohydrates by
considering further details about monosaccharides, the simplest
carbohydrates. Although there is no limit to the number of carbon
atoms that can be present in а monosaccharide, only
monosaccharides with three to seven carbon atoms are
commonly found in nature. А three-carbon monosaccharide is
called а triose, and those that contain four, five, and six carbon
atoms are called tetroses, pentoses, and hexoses, respectively.
Monosaccharides are classified as aldoses or ketoses on the
basis of type of carbonyl group present. Aldoses are
monosaccharides that contain an aldehyde group. Ketoses are
monosaccharides that contain а ketone group.
Monosaccharides are often classified by both their number
of carbon atoms and their functional group. А six-carbon
monosaccharide with an aldehyde functional group is an
aldohexose; а five-carbon monosaccharide with а ketone
functional group is а ketopentose.
Monosaccharides are also often called sugars. Hexoses are
six-carbon sugars, pentoses five-carbon sugars, and so on. The
word sugar is associated with "sweetness," and most (but not all)
monosaccharides have а sweet taste. The designation sugar is
also applied to disaccharides, many of which also have а sweet
taste. Thus sugar is а general designation for either а
monosaccharide or а disaccharide. The simplest aldose and
ketose are the trioses glyceraldehyde and dihydroxyacetone.
glyceraldehydes
dihydroxycaetone
The D and L designations specify the configuration at the
highest-numbered chiral center in а monosaccharide. The
configurations about other chiral centers are accounted for by
assigning а different common name to each pair of D and L
enantiomers. This naming system, for simple aldoses, is given
The L forms are mirror images of the molecules shown.
А major difference between glyceraldehyde and dihydroxyacetone is that the
latter does not possess а chiral carbon atom. Thus, D and L forms are not
possible for dihydroxy acetone. This reduces by half (compared with aldoses)
the number of stereoisomers possible for ketotetroses, ketopentoses, and
ketohexoses. An aldohexose has four chiral carbon atoms, but а ketohexose
has only three. atoins.
Structure of monosaccharides
 Aldehyde react with alcohols with formation
of hemiacetals:
Monosaccharides as polyhydroxyaldehydes or polyhydroxyketons also form cyclic
hemiacetals by intermolecular interaction between carbonyl and spatially
approximate alcohol group. These interaction cases formation of five- and
sixmember cycles. Sixmember cycle called pyranose and fivemember cycle –
furanose.
Pyranose cycle forms after interaction of oxo-group with hydroxy-group at C5
carbon atom of aldohexoses or C6 carbon atom of ketohexoses.
Furanose cycle forms after interaction of oxo-group with hydroxy-group at C4
carbon atom of aldohexoses or C5 carbon atom of ketohexoses.
Shown higher formulas of monosaccharides name formulas of CollyTollens.
Cyclic forms of monosaccharides.
So far in this chapter, the structures of monosaccharides
have been depicted as open-chain polyhydroxy aldehydes or
ketones. However, experimental evidence indicates that for
monosaccharides containing five or more carbon atoms, such
open-chain structures are actually in equilibrium with two cyclic
structures, and the cyclic structures are the dominant forms at
equilibrium. The cyclic forms of monosaccharides result from the
ability of their carbonyl group to react intramolecularly with а
hydroxyl group. The result is а cyclic hemiacetal or cyclic
hemiketal. Such an intramolecular cyclization reaction for Dglucose is shown:
 Structure 2 is а rearrangement of the projection formula for Dglucose in which the carbon atoms have locations similar to those
found for carbon atoms in а six-membered ring. All hydroxyl
groups drawn to the right in the original projection formula
appear below the ring. Those to the left in the projection formula
appear above the ring. Structure 3 is obtained by rotating the
groups attached to carbon-5 in а counterclockwise direction so
that they are in the positions where it is easiest to visualize
intramolecular hemiacetal formation. The intramolecular reaction
occurs between the hydroxyl group on carbon-5 and the carbonyl
group (carbon-1). The - ОН group adds across the carbon oxygen double bond, producing а heterocyclic ring that contains
five carbon atoms and one oxygen atom. Addition across the
carbon - oxygen double bond with its accompanying ring
formation produces а chiral center at carbon-l, so two
stereoisomers are possible. These two forms differ in the
orientation of the - ОН group on the hemiacetal carbon atom
(carbon-1). In -D-glucose, the - ОН group is on the opposite
side of the ring from the CH2OH group attached to carbon-5. In
-D-glucose, the СН2ОН group on carbon-5 and the - ОН group
on carbon-1 are on the same side of the ring
In an aqueous solution of β-glucose, а dynamic equilibrium exists among
the , , and open-chain forms, and there is continual interconversion
among them. For example, a freshly mixed solution of pure -D-glucose
slowly converts to а mixture of both - and -D-glucose by an opening and
а closing of the cyclic structure. When equilibrium is established, 63 % of
the molecules are -D-glucose, 37 % are -D-glucose, and less than 0.01 %
are in the open-chain form. Intramolecular cyclic hemiacetal formation and
the equilibrium between forms associated with it is not restricted to glucose.
All aldoses with five or more carbon atoms establish similar equilibria, but
with different percentages of the alpha, beta, and open-chain forms.
Fructose and other ketoses with а sufficient number of carbon atoms also
cyclize; here, cyclic hemiketal formation occurs. Galactose, like glucose,
forms а six-membered ring, but both D-fructose and D-ribose form а fivemembered ring.
D-fructose
D-ribose
D-Fructose cyclization involves carbon-2 (the keto group) and carbon-5,
which results in two CH2OH groups being outside the ring (carbons 1 and
6). D-Ribose cyclization involves carbon-1 (the aldehyde group) and
carbon-4.
Haworth projection formulas.
The structural representations of the cyclic forms of
monosaccharides found in the previous section are examples of
Haworth projection formulas. А Haworth projection is а of а
carbohydrate. In а Haworth projection, the hemiacetal ring
system is viewed "edge on" with the oxygen ring atom at the
upper right (six-membered ring) or at the top (five-membered
ring).
The D or L form of а monosaccharide is determined by the position of
the terminal СН2ОН group on the highest-numbered ring carbon atom. In the
в form, this group is positioned above the ring. In the ь form, which is not
usually encountered in biological systems, the terminal CH2OH group is
positioned below the ring.
 or  configuration is determined by the position of the ОН group on carbon-1 relative to the CH2OH group that
determines D or L series.
-D-Monosaccharide
-D-Monosaccharide
-L-Monosaccharide
Where  or  configuration does not matter, the -ОН group
on carbon-1 is placed in a horizontal position, and а wavy line is
used as the bond that connects it to the ring.
The specific identity of а monosaccharide is
determined by the positioning of the other: - ОН groups
in the Haworth projection. Any - ОН group at а chiral
center that is to the right in а Fisher projection formula
points down in the Haworth projection. Any group to
the left in а Fisher projection points up in the Haworth
projection. The following is a matchup between
Haworth projection and Fisher projection.
-form
-form
Transition from Colly-Tollens formulas to Haworth
formulas
Tautomery
In crystal state monosaccharides have cyclic structure. In water solution
monosaccharides have five tautomeric forms – open form, α- and βpyranose, α- and β-furanose.
These kind of tautomery called cyclo-oxo-tautomery
Conformation of monosaccharides
 Furanose forms of monosaccharides have plain structure,
for pyranoses more profitable is form of armchair.
 There are two kinds of armchair conformation of pyranose
forms. More stable is conformation with maximal number of
substitutions (OH- and CH2OH-groups) in equatorial
locations.
For aldohexose more profitable is
form of armchair
Transition from Haworth formulas to armchair configuration:
Methods of getting of monosaccharides
1. In green plants by photosynthesis:
2. Disaccharides, oligosaccharides, and polysaccharides can be
broken down to monosaccharide subunits by hydrolysis.
3. Disintegration by Ruff. By these method we can get shorter
carbon chain based on oxidation of monosaccharides.
4. Cyanhydrinic synthesis. By these method we
can get bigger carbon chain:
5. Reactions of monosaccharides.
1.The reduction of D-fructose forms D-mannitol and Dglucitol, the C-2 epimer of D-mannitol. D- lucitol—also called
sorbitol—is about 60% as sweet as sucrose. It is found in plums,
pears, cherries, and berries and is used as a sugar substitute in the
manufacture of candy.
1.Reduction. The carbonyl group present in а
monosaccharide (either an aldose or а ketose) can be reduced to а
hydroxyl group, using hydrogen as the reducing agent. For aldoses and
ketoses, the product of the reduction is the corresponding polyhydroxy
alcohol, which is sometimes called а sugar alcohol. For example, the
reduction D-glucose gives D-glucitol.
D-Glucitol is also known by the common name D-sorbitol.
Hexahydroxy alcohols such as D-sorbitol have properties similar
to those of the trihydroxy alcohol glycerol. These alcohols are
used as moisturizing agents in foods and cosmetics because of
their affinity for water. D-Sorbitol is also used as а sweetening
agent in chewing gum; bacteria that cause tooth decay cannot use
polyalcohols as food sources, as they can glucose and many other
monosaccharides.
2. Oxidation. Monosaccharide oxidation can yield three different types of
oxidation products. The oxidizing agent used determines the product. Weak
oxidizing agents, such as Tollens, Fehling's, and Benedict's solutions, oxidize
the carbonyl group end of а monosaccharide to give an -onic acid. Oxidation
of the aldehyde end of glucose produces gluconic acid, and oxidation of the
aldehyde end of galactose produces galactonic acid. The structures involved in
the glucose reaction are:
Because monosaccharides act as reducing agents in such reactions, they are called
reducing sugars. With Tollens solution, glucose reduces Ag+ ion to Ag, and
with Benedict's and Feling's solutions, glucose reduces Cu2+ ion to Cu+ ion.
А reducing sugar is a carbohydrate that gives a positive test with Tollens,
Benedict's and Feling's solutions. All monosaccharides are reducing sugars.
Tollens, Feling's, and Benedict's solutions can be used to test for glucose
in urine, а symptom of diabetes. For example, using Benedict's solution, we
observe that if no glucose is present in the urine (а normal condition), the
Benedict's solution remains blue. The presence of glucose is indicated by the
formation of а red precipitate. Testing for the presence of glucose in urine is
such а common laboratory procedure that much effort has been put into the
development of easy-to-use test methods. Strong oxidizing agents can oxidize
both ends of а monosaccharide at the same time (the carbonyl group and the
terminal primary alcohol group) to produce а dicarboxylic acid. Such
polyhydroxy dicarboxylic acids are known as -aric acids. For glucose, such an
oxidation produces glucaric acid.
At selective oxidation primary alcohol group in aldoses
molecules form uronic acids:
Although it is difficult to do in the laboratory, in biological
systems enzymes can oxidize the primary alcohol end of an
aldose such as glucose, without oxidation of the aldehyde group,
to produce а -uronic acid. For glucose, such an oxidation
produces D-glucuronic acid.
D-Glucose
D-Glucitol
D-Glucitol is also known by the common name D-sorbitol.
Hexahydroxy alcohols such as D-sorbitol have properties similar
to those of the trihydroxy alcohol glycerol. These alcohols are
used as moisturizing agents in foods and cosmetics because of
their affinity for water. D-Sorbitol is also used as а sweetening
agent in chewing gum; bacteria that cause tooth decay cannot use
polyalcohols as food sources, as they can glucose and many other
monosaccharides.
3. Transformation of monosaccharides by action of
alkali (epimerization)
4. Ozazone formation.
The tendency of monosaccharides to form syrups that do not
crystallize made the purification and isolation of
monosaccharides difficult. Emil Fisher found that when
phenylhydrazine is added to an aldose or a ketose, a yellow
crystalline solid that is insoluble in water is formed. He called
this derivative an ozazone (“ose” for sugar; “azone” for
hydrazone). Ozazones are easily isolated and purified and were
once used extensively to identify monosaccharides
Mechanism of reaction of ozazone formation
By the action of conc. HCl or heating wjth
benzaldehyde forms ozones (ketoaldehydes).
At the reduction of ozones forms ketoses.
5. Mutarotation. - and -forms of
monosaccharides are readily interconverted when dissolved in
water. This spontaneous process, called mutarotation, results in
an equilibrium mixture of - and -furanose forms and - and pyranose forms. The open chain that is formed during
muterotation can participate in oxidation-reduction reactions
6. Interaction with hydroxylamine
 Aldoses with hydroxylamine forms oxymes.
This method use for obtaining lower aldoses from
higher aldoses
7. Intermolecular dehydratation
These reaction allow to distinguish hexoses from
pentoses. Furfurol gives red color with aniline at the
presence of HCl, 5-hydroxymethylfurfurol forms red color
with resorcine (Selivanov reaction)
8.Glycoside Formation.
As you known, that hemiacetals and
hemiketals can react with alcohols in acid solution to produce acetals and
ketals. Because the cyclic forms of monosaccharides are hemiacetals and
hemiketals, they react with alcohols to form acetals and ketals, as is illustrated
for the reaction of -D-glucose with methyl alcohol.
-D-glucose
Methyl -D-glucopyranoside
The general name for monosaccharide acetals and ketals is glycoside. А glycoside
is an acetal or а ketal fragment in а cyclic monosaccharide. More specifically,
а glycoside produced from glucose is а glucoside, from galactose а
galactoside, and so on. Glycosides, like the hemiacetals and hemiketals from
which they are formed, can exist in both  and  forms. Glycosides are named
by listing the alkyl or aryl group attached to the oxygen, followed by the name
of the monosaccharide involved, with the suffix –ide appended to it.
Methyl--Dglucopyranoside
Methyl--Dglucopyranoside
The acetal (or ketal) of a sugar is called a glycoside, and the bond
between the anomeric carbon and the alkoxy oxygen is called a glycosidic
bond.
Similar to the reaction of a monosaccharide with an alcohol
is the reaction of a monosaccharide with an amine in the presence
of a trace amount of acid. The product of the reaction is an Nglycoside. An N-glycoside has a nitrogen in place of the oxygen
at the glycosidic linkage. An S-glycoside has a sulfur in place of
the oxygen at the glycosidic linkage.
 Ancarbohydrate part of the glycoside
molecule called aglicone.
Glycosides very easy hydrolyses in acid medium:
O-, N- and S-glycosides
9.Phosphate ester formation. The hydroxyl groups of а
monosaccharide can react with inorganic oxyacids to form
inorganic esters. Phosphate esters, formed from phosphoric acid
and various monosaccharides, are commonly encountered in
biological systems. For example, specific enzymes in the human
body catalyze the esterification of the carbonyl group (carbon-1)
and the primary alcohol group (carbon-6) in glucose to produce
the compounds glucose 1-phosphate and glucose б-phosphate,
respectively
-D-Glucose-1-phosphate
-D-Glucose-6-phosphate
These phosphate esters of glucose are stable in aqueous solution
and play important roles in the metabolism of carbohydrates
10. Amino Sugar Formation. Amino sugars of
glucose, mannose, and galactose are common in nature. Such
sugars are produced by replacing the hydroxyl group on carbon-2
on the monosaccharide with an amino group. Amino sugars and
their N-acetyl derivatives are important building blocks of
polysaccharides found in cartilage.
-D-Glucosamine
-D-Glalactosamine
N-acety1-D-glucosanune
The N-acetyl derivatives of D-glucosamine and Dgalactosamine are present in the biochemical markers on
red blood cells, which distinguish the various blood
types.
11. Acylation and alkylation of monosaccharides.
The OH-groups of monosaccharides show the chemistry
typical of alcohols. For example, they react with acetyl chloride
or acetic anhydride to form esters.
The OH-groups also react with dimethylsulfate/BaO or methyl
iodide/silver oxide to form ethers. The OH group is a relatively poor nucleophile, so
silver oxide or BaO is used to increase the leaving tendency of the iodide ion in the
SN2 reaction.
9. Isomerization. Monosaccharides undergo several types
of isomerization, for example, after several hours an alkaline
solution of D-glucose will also contain D- mannose and Dfructose. Both isomerizations involve an intramolecular shift of
a hydrogen atom and a charge in the location of a double bond
The intermediate that is formed during this process is called an endiol.
The reversible transformation of glucose to fructose is an example of an
aldose-ketose interconversation. Because there is a change in the conversion
of glucose to mannose is referred to as an epimerization. Several enzymecatalyzed reactions involving endiols occur in carbohydrate metabolism.
Representatives of monosaccarides
Representatives of dezoxy- and
aminosaccharides
Derivatives of monosacchrides
 Ascorbinic acid – strong reduction agent.
Synthesis of ascorbinic acid
Biologically important monosaccharides.
 Of the many monosaccharides, the most important in the human body are the
D-forms of glucose, galactose, fructose, and ribose. Glucose and galactose are
aldohexoses, fructose is а ketohexose, and ribose is an aldopentose. All four of
these monosaccharides are water-soluble, white, crystalline solids.
D-Glucose. Of all monosaccharides,
D-Glucose
о-glucose is the most abundant in
nature and the most important from а
nutritional standpoint. Its Fischer
projection.
Ripe fruits, particularly ripe grapes (20% - 30% glucose by mass), are а good
source of glucose, which is often referred to as grape sugar. Two other names for о-glucose
are dextrose and blood sugar. The name dextrose draws attention to the fact that the
optically active D-glucose, in aqueous solution, rotates plane-polarized light to the right.
The term blood sugar draws attention to the fact that blood contains dissolved glucose. The
concentration of glucose in human blood is fairly constant; it is in the range of 70 - 100 mg
per 100 mL of blood. Cells use this glucose as а primary energy source.
А 5% (m/v) glucose solution is often used in hospitals as an intravenous source of
nourishment for patients who cannot take food by mouth. The body can use it as an energy
source without digesting it.
 D-Galactose. А comparison of the Fischer projections for Dgalactose and D-glucose shows that these two compounds differ
only in the configuration of the - ОН group and - Н group on
carbon-4.
D-Galactose
D-Glucose
D-Galactose and D-glucose are epimers. D-Galactose is seldom
encountered as а free monosaccharide. It is, however, а component of
numerous important biochemical substances. In the human body, galactose is
synthesized from glucose in the mammary glands for use in lactose (milk
sugar), а disaccharide consisting of а glucose unit and а galactose unit. DGalactose is sometimes called brain sugar because it is а component of
glycoproteins (protein-carbohydrate compounds) found in brain and nerve
tissue. D-Galactose is also present in the chemical markers that distinguish
various types of blood - А, В, АВ, and O.
D-Fructose is the most important ketohexose. It is also
known as levulose and fruit sugar. Aqueous solutions of naturally
occurring D-fructose rotate plane-polarized light to the left hence
the name levulose. The sweetest-tasting of all sugars, D-fructose
is found in many fruits and is present in honey in equal amounts
with glucose. It is sometimes used as a dietary sugar, not because
it has fewer calories per gram than other sugars but because less
is needed for the same amount of sweetness. From the third to the
sixth carbon, the structure of D-fructose is identical to that of Dglucose. Differences at carbons 1 and 2 are related to the
presence of а ketone group in fructose and an aldehyde group in
glucose.
D-Fructose
D-Glucose
D-Ribose.
The three monosaccharides previously discussed in this
section have all been hexoses. D-Ribose is а pentose. If carbon-3 and its
accompanying - Н and - ОН groups were eliminated from the structure of Dglucose, the remaining structure would be that of D-ribose
D-Glucose
D-Ribose
D-Ribose is а component of а variety of complex molecules, including
ribonucleic acids (RNAs) and energy-rich compounds such as adenosine
triphosphate (ATP). The compound 2-deoxy-D-ribose is also important in
nucleic acid chemistry. This monosaccharide is а component of DNA
molecules. The prefix deoxy- means "minus an oxygen"; the structures of
ribose and 2-deoxyribose differ in that the latter compound lacks an oxygen
atom at carbon-2.
D-Ribose
2-Deoxy-D-ribose
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