post lab discussion: exp. 5 Flashcards
what does carbohydrates consists of and the empirical formula
consists :
hydrates of carbon
empirical formula:
Cx(H2O)y
- they are polyhydroxyaldehydes (CHO) - glucose / polyhydroxyketones (C=O) - fructose
we can find many hydroxy groups in them
examples of carbohydrate
monosaccharide:
B-D- glucose
disaccharide:
lactose
polysaccharide:
glycogen
– general tests –
molisch test:
iodine test:
– specific test –
benedict’s test:
barfoed’s test:
seliwanoff’s test:
bial’s orcinol test:
– general tests –
molisch test: for all carbs
iodine test: certain polysaccharides (starch & glycogen)
– specific test –
benedict’s test: reducing sugar
barfoed’s test: monosaccharides vs disaccharides (but specifically used for mono but can also for di)
seliwanoff’s test: ketoses
bial’s orcinol test: pentoses
- molisch test
- iodine test
- benedict’s test
- barfoed’s test
- seliwanoff’’s test
- bial’s orcinol test
reagent:
positive result:
- molisch test
reagent: a-naphthol w sulfuric acid
positive result: purple ring at the interphase - iodine test
reagent: iodine soln (its color brownish)
positive result: deep blue mixture - benedict’s test
reagent: copper (II) sulfate in citrate and carbonate
positive result: red-orange ppt (brick red ppt)
*benedict’s reagent is vibrant blue - the copper (II) is giving that color
- barfoed’s test
reagent: copper (II) acetate in glacial acetic acid - glacial acetic accid amoy suka
*its color is vibrant blue bce of the copper (II)
positive result: red- orange ppt - need to time the reaction:
fast - mono
slow - di - seliwanoff’’s test
reagent: resorcinol in conc. hydrochloric acid
positive result: deep red soln.
bial’s orcinol test: pentoses
reagent: orcinol, ferric (III) chloride & conc. hydrochloric acid
positive result: blue soln
which reagent is highly concentrated and shld always be handled under the fume hood
a-naphthol w sulfuric acid
*sulfuric acid is a strong acid and highly conc
why is the barfoed’s and benedict’s test have similar positive result
as the copper (II) metal oxidized to become copper (I)
structure of the starch iodine- complex
(refer to ppt)
the rod is the iodine
starch is at the right side of the rod
starch will enter the rod
(parang isaw manok) = dark blue soln
what are reducing sugar
contain a free aldehyde/ ketone grp
make them capable to act as a reducing agent
hemiacetal ()
contains hydroxy group (-OH) and alkoxy group (-OR) in the same carbon
why is monosaccharides with a hemiacetal are also “reducing sugar”
as their open chain form contains an aldehyde (or alpha-hydroxy ketone)
hemiacetal: H-C-OH
OH will turn to C=O
why benedict’s reagent is needed to be alkaline in nature?
alkaline in nature causes isomeric transformation of ketoses to aldoses, resulting all monosaccharides and some disacccharides to test positive for benedict’s
(structure at ppt)
how to know if it’s hemiacetal
ppt
aside from benedict’s, what other tests can be used to detect reducing sugars
fehling’s test
it is similar to benedict’s test - it uses Cu^2+ as well [copper (II) can oxidized the sample, cos redox is happening]
REDUCTION HAPPENS
Cu^2+ (blue) —–> Cu+ (red)
that is why its called reducing sugar as it reduces the counterpart
aside from benedict’s what other tests can be used to detect reducing sugars
ppt
how come iodine test only works for starch? does it also produce positive result with glycogen
starch forms helical coils
iodine atoms fits into the helicase to form starch-iodine complex
yes as glycogen has similar structure of starch
(glycogen has many branches than amylose or amylopectin) hence can put more iodine
