True or False a) Glucose is regarded as a ketohexose. b) d-Galactose differs fro
ID: 515429 • Letter: T
Question
True or False a) Glucose is regarded as a ketohexose. b) d-Galactose differs from d-glucose in the orientation of the hydroxyl group on the C-4 carbon. c) Fructose, a ketohexose, is regarded as a reducing sugar since the ketone group can rearrange to a free aldehyde group that tests positively to the Benedict's Test. d) Sucrose, disaccharide composed of a glucose unit and a fructose unit, is a reducing sugar that gives a positive test to Benedict's Test. e) The Barfoed Test is similar to the Benedict test, except it is carried out in acidic solution; thus, it can distinguish between reducing monosaccharides and reducing disaccharides. f) The iodine test gives a positive signal of a colored complex with monosaccharides. Show the Fischer projection of d-glucose and d-galactose.Explanation / Answer
a) False: because glucose is a sugar (aldohexose) containing aldehyde -CHO functional group whereas ketohexose contains -C(O)CH3 group.
b) True: If it was D- galactose and D- glucose ( D and L are relative configuration w.r.t to glyceraldehyde whereas d and l are dextro and leavo rotatory configuration
c) True: Benedict's test detects the presence of reducing sugars (the presence of aldehydes, alpha-hydroxy-ketones) A reducing sugar is any sugar that can act as a reducing agent because it has a free aldehyde group or a free ketone group. Since fructose is a reducing sugar (tautomerization), it gives a positive test
d) False: Sucrose is a disaccharide containing both glucose and fructose unit and joined by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Therefore it is not a reducing sugar, it will not give positive test.
e) True
f) False: iodine test is for polysaccharide viz. starch
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