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MONOSACCHARIDESSimple sugars.Cannot be further hydrolyzed into simple
carbohydrates catagorized by the number of carbon
(typically 3-8) and whether an aldehyde or ketone.
MonosaccharidesAldoses (e.g., glucose) have an aldehyde group at one end.
Ketoses (e.g., fructose) have a keto group, usually at C2.
The asymmetric(chiral) carbon atom and D and L monosaccharidesAny carbon atom which is connected to four different groups will be chiral, and will have
twononsuperimposable mirror images; it is a
chiralcarbon or a center of chirality.
•When a molecule has more than one chiral carbon,each carbon can possibly be arranged in either theright-hand or left-hand form, thus if there are nchiral carbons, there are 2n possible stereoisomers.
Maximum number of possible stereoisomers = 2n
Isomerism in monosaccharidesCompounds that possess the same
chemical formula are called isomers.Glucose,fructose,mannose and
galactose all have the same formula,i.e C6H12O6 and therefore isomers of each other.
1.STERIOISOMERISMGeneral term,means two or more
monosaccharides having the same structure but differing from each other in that the configuration,i.e the arrangement of their atoms in space differ from each other.
2.ENANTIOMERS.Pair of stereoisomerism that are mirror
images of each other in regard to asymetric carbon atoms present in their molecules.
For example glucose occurs as D and L glucose isomers.
3.OPTICAL ISOMERISMThe enantiomeric monosaccharides by virtue of their
content of asymmetric carbon atom can rotate the plane-polarized light either to right or left.
The monosaccharides that rotate this light to right are called dextrorotatory(+type)
Those rotating this light to left are called levorotatory(-type).
Glucose is dextrorotatory(+),while fructose is levorotatory(-)
4.EPIMERSMonosaccharides(isomers) which differ in
configuration around one specific carbon atom other than the carbon atom of carbonyl group.
5.ANOMERSIsomers that differ in configuration around
the anomeric carbon i.e the carbon atom of the carbonyl group which is carbon no.1 in aldosed and carbon no.2 in ketoses.
These representations of the cyclic sugars are called Haworth projections.
H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
23
4
5
6
1 1
6
5
4
3 2
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose (linear form)
Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, & .
(OH below the ring) (OH above the ring).
H O
OH
H
OHH
OH
CH2OH
H
-D-glucose
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose
23
4
5
6
1 1
6
5
4
3 2
6.PYRANOSE-FURANOSE ISOMERISMThe ring structures of monosaccharides may
be similar to either pyran or furan and accordingly the monosaccharide is said to occur in pyranose or furanose form.
RECEMIC MIXTUREEqual amounts of dextro- and levorotatory
isomers in solution results no net rotation of plane of polarized light.Such a mixture is called racemic mixture.
MESOCOMPOUNDSDonot rotate the light inspite of asymmetric
carbon due to internal compensation.