Pre-Test Carbohydrates EXP 6

Question 1

what is carbohydrates molecular formula?

Answer 1

Cn(H20)n

Question 2

what are monosaccharides?

Answer 2

simple carbohydrates

Question 3

what are aldoses?

Answer 3

monosaccharide that is polyhydroxy aldehydes with a general structure of A is also known as aldoses

Question 4

what are ketoses?

Answer 4

monosaccharide that is polyhydroxy ketones with a general structure of B is also known as ketoses

Question 5

examples of monosaccharides give 3

Answer 5

glucose, fructose, ribose

Question 6

examples of disaccharides give 3

Answer 6

maltose, sucrose, lactose

Question 7

how many carbons does monosaccharides consist of?

Answer 7

3-6 carbons

Question 8

if there are 3-6 carbons in monosaccharide, how do you classify each one of them?

Answer 8

3C = trioses 4C = tetroses
5C = pentoses 6C = hexoses

Question 9

what are disaccharides?

Answer 9

when two monosaccharides combine with the loss of one molecule water is when disaccharides are formed

Question 10

what is a glycosidic bond?

Answer 10

its the bond that connects 2 monosaccharide unit in a disaccharide

Question 11

in a figure chart, how do you differentiate ribose, glucose and fructose?

Answer 11

ribose has only 3 chain of (hydroxide, carbon and hydrogen) while glucose has 4 chain of (hydroxide, carbon and hydrogen) and fructose has a double bond Carbon and Oxygen with only 3 chains of (hydroxide, carbon and hydrogen)

Question 12

what is a polysaccharide?

Answer 12

many monosaccharides linked together

Question 12

examples of polysaccharides give 4

Answer 12

starch, pectin, glycogen and cellulose

Question 13

what general test is for the presence of carbohydrates?

Answer 13

Molisch Test

Question 13

how does the Molisch test work? explain in details

Answer 13

Molisch test is involved adding the Molisch’s reagent to the analyte and an addition of a few drops of concentrated sulfuric acid which (H2SO4) to the mixture. The formation of purple or purplish-red ring confirms the presence of carbohydrates in the analyte.

Question 14

who was credited with the discovery of the Molisch’s test?

Answer 14

Czech-Austrian botanist Han Molisch

Question 14

TRUE OR FALSE: EXPLAIN
A positive reaction for Molisch’s test is given by almost all carbohydrates

Answer 14

FALSE because there are exceptions like tetroses and trioses which are monosaccharides that only have 4 carbons and 3 carbons and generally would not produce a positive reaction

Question 15

what is a reducing sugar?

Answer 15

it is a carbohydrate that is oxidized by a weak oxidizing agent in basic aqueous solutions

Question 16

what is a non-reducing sugar

Answer 16

it is a carbohydrate that has not been oxidized

Question 17

TRUE OR FALSE: EXPLAIN
all aldoses are the true reducing sugars because they contain the aldehyde functional group

Answer 17

TRUE since aldoses contain the aldehyde functional group they are all reducing sugars

Question 18

TRUE OR FALSE: EXPLAIN
ketoses contain ketones and not aldehydes however they also still show reducing sugar properties

Answer 18

TRUE because in basic aqueous solutions ketoses undergo a process called keto-enol tautomerization that converts ketoses into a mixture of aldoses thus we can say that monosaccharides are all reducing sugars

Question 18

TRUE OR FALSE: EXPLAIN
Disaccharides are all reducing sugars

Answer 18

FALSE because disaccharides may be reducing sugars or not. Only disaccharides that are reducing are those that contain free anomeric carbon because the free anomeric carbon has the capacity to open and expose the aldehyde group

Question 18

TRUE OR FALSE: EXPLAIN
All polysaccharides are reducing

Answer 18

FALSE polysaccharides are non-reducing sugars typically because it consists of many monosaccharides linked up together

Question 18

How do you test a carbohydrate if it is reducing or not? Explain in details.

Answer 18

There are many weak oxidizing agents to test if a carbohydrate is reducing or not such as the Benedict’s reagent, Fehling’s reagent, Tollen’s reagent, Nylander’s reagent, and picric acid with sodium carbonate solution.

Question 19

knowing all of the weak oxidizing agents to determine whether the carbohydrate is either a reducing sugar or not, what do they all have in common?

Answer 19

they are all in basic conditions which means their pH levels are always greater than 7

Question 19

what is a picric acid test?

Answer 19

is a test when a saturated picric acid solution is used as an oxidizing agent. the environment is made alkaline by 10% Na2CO3. A formation of mahogany red colored solution which is due to the reduction of picric acid to picramic acid indicates a positive result

Question 20

what is the benedict’s test?

Answer 20

it is a test based on cupric ion (Cu2+) that will oxidize aliphatic aldehydes such as aldoses including a-hydroxyaldehydes. A formation of a brick red precipitate of cuprous oxide (Cu2O) indicates a positive test. However, the color of the precipitate can vary from red to orange to green depending on the amount of sugar present in the solution. Hence, why the benedict’s test can be used to quantitatively estimate the amount of sugar in a solution. Benedict’s test can also be used to detect glucose in the urine for example.

Question 20

What is Moore’s Test

Answer 20

reducing sugars have free carbonyl groups in their structure. At basic environment, this carbonyl group can undergo aldol condensation. For reducing sugars, the product is caramel with a brown color and characteristic odor. The odor intensifies when carefully acidified with diluted sulfuric acid.

Question 21

What is Seliwanoff’s Test for Ketoses?

Answer 21

Seliwanoff’s test is used to distinguish aldoses from ketoses. Seliwanoff’s reagent dehydrates ketoses more rapidly and the derivatives undergo condensation reaction with resorcinol that gives a cherry red solution after 2 minutes which indicates a positive test. Seliwanoff’s test can be keto-containing carbohydrates

Question 21

What is Iodine Test for Polysaccharides?

Answer 21

Iodine test is a very specific test for polysaccharides because polysaccharides have highly coiled structures. The color depends on the length of the linear chain available for complex formation. Therefore, different colors are produced for different polysaccharides.

Question 21

What is Bial’s Test for Pentoses?

Answer 21

Bial’s test is used to distinguish pentoses from hexoses. The distinction is based on the color that develops in the presence of the reagent. If a pentose is present it will produce bluish-green solution. If a hexose is present it will produce a
muddy-brown, yellow or gray solution.

Question 21

what is barfoed’s test for monosaccharides?

Answer 21

Barfoed’s test depends on the reducing properties of sugars. The oxidizing reagent used here is like benedict’s test which is cupric ion (Cu2+). A positive test for monosaccharides is indicated by the formation of the brick red precipitate of Cu2O within 2-3 minutes.