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Carbohydrates
o They are polyhydroxy aldehydes and ketones .
o Carbohydrates are compounds of C, H, and O.
Major Categories of Carbohydrates
1. Monosaccharide
2. Disaccharide
3. Polysaccharide
Monosaccharide
o The simplest form of sugars.
o Cannot be changed to simpler sugars upon
hydrolysis.
o Classified according to:
1. The number of carbon atoms present in the
molecule.
2. Whether they contain an aldehyde or ketone.
Monosaccharide
# of Carbons
Class
3
triose
4
tetrose
5
pentose
6
hexose
Monosaccharide
o The prefixes ‘aldo-’ and ‘keto-’ are used
depending on whether the monosaccharide
has an aldehyde or ketone group.
Monosaccharide
o Monosaccharide are called reducing sugars
because they act as reducing agents and are
easily oxidized to carboxylic acids.
Glucose
o White crystalline solid that is soluble in most
organic liquids
o Is an aldohexoses
o Found along with fructose in many fruite
juices
o Most important monosaccharide
o Found in the blood stream and in tissue fluids
o Requires no digestion
Glucose
Forms of Glucose
o α and β forms:
- When the OH on C number1 is downward the
form is α.
- When the OH on C number 1 is upward the
form is β.
o D and L forms:
CH 2 OH
H
C
OH
C
CH 2OH
O
H
H
H
OH
H
C
C
H
OH
α –D glucose
C
C
OH
OH
Mirror
O
C
H
C
H
C
H
OH
C
H
OH
OH
β-L glucose
Fructose
o Is a ketohexose
o Occurs naturally in juices and honey
o The most soluble and sweetest of all sugars
α- D Fructose
β-D Fructose
Disaccharide
Maltose
hydrolysis
Glucose + Glucose
Sucrose
Lactose
hydrolysis
Glucose + Fructose
Glucose + Galactose
hydrolysis
o Sucrose is soluble in water, maltose is fairly
soluble, and lactose is slightly soluble.
o They are too large to pass through cell
membrane.
o Formed by the combination of tow
monosaccharide’s.
o Bond between tow monosaccharide’s is called
glycosidic linkage.
o Maltose
o Present in germinating grain
o Produced by hydrolysis of starch
o Reducing sugar
o Sucrose:
o Is the ordinary table
sugar used in home.
o Non-reducing sugar.
Polysaccharide
o Comparison of polysaccharide with mono and
disaccharide
Property
Mono and Di
Poly
Molecular mass
Low
Very high
Taste
Sweet
Tasteless
Solubility in water Soluble
Insoluble
Size of particle
Mono only pass through the
membrane
Do not pass
Reducing
property
Reducing except for sucrose
Non
reducing
Polysaccharide
o Starch is the storage form of glucose in plants.
o Glycogen is the storage form of glucose in
animals where it is found in liver and muscles.
o Cellulose is the primary structural component
of plants.
Chemical properties
1. Molisch test
- General test for carbohydrates
- Shows positive result for all carbohydrates
To the left negative result, to the right positive result
2. Fehling’s test
- Fehling is copper (II) complex ion
- When reducing agent reacts with oxidizing
agent (Cu2+) a red-orange ppt of copper(I)
oxide is formed
R
CHO + Cu2+
deep blue
color
R COOH + Cu2O + H2O
red-orange
ppt
3. Barfoed’s test
- To distinguish mono from disaccharide on the
bases of different rate of reaction.
- Mono reacts much faster.
- +ve result  prick red
4. Seliwanoff’s test
- To distinguish ketoses (fructose) from
aldoses (glucose) on the bases of different
rates of reation
- Ketohexoses reacts much faster.
- +ve resultred
5. Iodine test
- To distinguish starch from other
polysaccharide.
- +ve result  blue coloration