Experiment 7: Carbohydrates

Question 1

What are Carbohydrates?

Answer 1

Carbohydrates are biological sugar molecules. They are composed of carbon, hydrogen & oxygen. They are also known as polyhydroxy aldehydes or polyhydroxy ketones.

Question 2

The four classifications of carbohydrates are:

Answer 2

1. Monosaccharide
2. Disaccharide
3. Oligosaccharides
4. Polysaccharides

Question 3

• They are simple sugars; basic constituents of all carbohydrates
• Contain only 1 saccharide group
• Can’t be hydrolyzed into simpler forms

Answer 3

Monosaccharides or C6H12O6

Question 4

Most important monosaccharides:

Answer 4

• Glucose or dextrose or corn sugar
• Fructose or levulose or fruit sugar
• Galactose

Question 5

What are disaccharides made of?

Answer 5

Two monosaccharide molecules minus a molecule of water

C6H12O6 + C6H12O6 = C12H24O12 - H2O = C12H22O11

Question 6

What happens to disaccharides when they are hydrolyzed?

Answer 6

When hydrolyzed, disaccharides yield 2 molecules of monosaccharides, either they are the same or different

Question 7

The most common disaccharides, and what they are built from:

Answer 7

• Sucrose or cane or beet sugar: Glucose + Fructose
• Lactose or milk sugar: Glucose + Galactose
• Maltose or malt sugar: Glucose + Glucose

Question 8

Classification of carbohydrate that when hydrolyzed, yields 3-6 monosaccharide units

Answer 8

Oligosaccharides

Question 9

• Made up of several monosaccharide groups; most complex classification of all the carbohydrates
• They are white, crystalline, tasteless
• Have high molecular weight
• Mostly insoluble and non-fermentable

Answer 9

Polysaccharides or C6H10O5

• Starch
• Cellulose
• Dextrin
• Glyocogen
• Inulin

Question 10

What do polysaccharides yield when hydrolyzed?

Answer 10

They yield more than 6 monosaccharides units

Question 11

A.1. It is the general test for carbohydrates

Answer 11

Molisch Test

Question 12

A.2. Molisch Test process:

Answer 12

(1) Water, (2) Glucose, (3) Sucrose, (4) Starch + Molisch reagent or C10H8OH + conc. H2SO4

Question 13

A.3. What is the purpose of conc. H2SO4?

Answer 13

It is used as a hydrolyzing & dehydrating agent of monosaccharides in carbohydrates

Question 14

A.4. Results of Molish Test

Answer 14

• With water: No reaction—no carbohydrates
• With Glucose: Purple ring forms at junction—positive reaction
• With Sucrose: Purple ring forms at junction—positive reaction
• With Starch: Slight purple ring forms at junction—positive reaction

Question 15

A.5. How does purple ring form in Molisch Test?

Answer 15

1. Carbohydrates are detected because they are hydrolyzed in the solution, being broken down into monosaccharides
2. Monosaccharides are then dehydrated by conc. H2SO4 which form furfural derivatives
3. Furfural derivatives interact with the a-napthol in Molisch reagent, causing a purple ring to form

Question 16

B.1. It is the general test for carbonyl group and reducing sugar

Answer 16

Phenylhydrazine Test

Question 17

B.2. Phenylhydrazine Test process:

Answer 17

Glucose + H2NNHC6H5 or phenylhydrazine

1. Shake.
2. Boil.

Question 18

B.3. What forms in Phenylhydrazine test?

Answer 18

Yellow precipitate crystals called Osazone crystals

Question 19

B.4. What is the purpose of Phenylhydrazine?

Answer 19

It is a derivatizing agent

Question 20

B.5. How is osazone crystals formed?

Answer 20

1. The H2NNHC6H5 reacts with the carbonyl group in Glucose to form glucose phenylhydrazone.
2. The glucose phenylhydrazone further reacts into osazone.

Question 21

C.1. It is a test for the action of alkalines and presence of reducing sugars

Answer 21

Moore’s Test

Question 22

C.2. Moore’s Test process:

Answer 22

(1) Glucose, (2) Sucrose + 25% NaOH

1. Boil

Question 23

C.3. What kind of substance is NaOH? What was its function in Moore’s Test?

Answer 23

It is alkaline, or basic, used for polymerization

Question 24

C.4. Results of Moore’s Test:

Answer 24

• With Glucose: Brown solution forms, emitting a caramel scent; positive reaction
• With Sucrose: No reaction

Question 25

C.5. In Moore’s Test did positive reaction show in Glucose, but not in Sucrose?

Answer 25

The color change and formation of resinous caramel is caused by the **polymerization of the free aldehydes** from the **reducing sugar**, which is **Glucose** **Sucrose is not a reducing sugar**, thus **doesn’t have free aldehydes**; therefore doesn’t react

Question 26

D.1. What are **Reducing Sugars**?

Answer 26

These are sugars that contain **reducing properties** because of the presence of their **free aldehyde or ketone group**.

Question 27

D.2. Which classifications of sugars are reducing and non-reducing?

Answer 27

**All monosaccharides** are reducing sugars. **Lactose and maltose** are reducing disaccharides. **Sucrose and polysaccharides** are non-reducing sugars.

Question 28

D.3. What do reducing sugars reduce?

Answer 28

They reduce **alkaline solutions of copper and silver**, which are converted to **cuprous oxide**

Question 29

D.4. Benedict's Test process:

Answer 29

(1) **Glucose**, (2) **Lactose**, (3) **Sucrose**, (4) **Starch** + **Benedict's reagent** 1. Boil, then cool

Question 30

D.4.2. What is Benedict's reagent composed of?

Answer 30

- Copper sulfate - Sodium carbonate - Sodium citrate

Question 31

D.5. Results of Benedict's Test:

Answer 31

- With **Glucose & Lactose**: Blue to **red precipitates**; **reducing sugars detected** - With **Sucrose & Starch**: No reaction; **no reducing sugars detected**

Question 32

D.6. How are reducing sugars detected in Benedict's Test?

Answer 32

1. Benedict's solution contains **copper**. 2. In the presence of reducing sugars, the copper in the reagent is **reduced** to **red cuprous oxide / Copper (I) ions**.

Question 33

E.1. What is **Hydrolysis**?

Answer 33

It is the **addition of water** to **disaccharides and polysaccharides** to ***break them apart*** into their component **monosaccharides**.

Question 34

E.2. Sucrose Hydrolysis process:

Answer 34

**Sucrose** + **HCl**, then boil Add **NaOH** to make it alkaline Test half with **Benedict's reagent**, the other with **Seliwanoff's reagent**

Question 35

E.3. What kind of substance is HCl?

Answer 35

It is a **non-oxidizing acid**

Question 36

E.4. What happens when HCl is added to Sucrose?

Answer 36

HCl **hydrolyzes** Sucrose, breaking it down into separate **Glucose & Fructose** molecules

Question 37

E.5. What happens when the **alkaline hydrolyzed sucrose** was added with **Benedict's reagent**?

Answer 37

A **dark orange-yellow color** formed. This is because the freed Glucose molecules–which are **reducing sugars**-reduced the copper in the reagent to **cuprous oxide**

Question 38

E.6. It is a **specific test for ketoses**

Answer 38

Seliwanoff's Test

Question 39

E.6. What are **ketoses**?

Answer 39

Ketoses are sugars with the presence of a **ketone group**

Question 40

E.7. What is Seliwanoff's reagent made of?

Answer 40

C6H6O2 (Resorcinol) & HCl

Question 41

E.8. What happens when the **alkaline hydrolyzed sucrose** was added with **Seliwanoff's reagent**?

Answer 41

It turned **mahogany red** because **Fructose**, a **ketose**, was detected

Question 42

F.1. Starch Hydrolysis process:

Answer 42

**Starch** + **HCl**, then boil Add **I2 drops** on test plate, then add **starch solution drop** on each I2 drop until **no color change is detected**

Question 43

F.2. Upon addition of **HCl**, what is starch **hydrolyzed** into?

Answer 43

Multiple glucose forms

Question 44

F.3. The point at which starch no longer gives the blue color and is **colorless** is called _____

Answer 44

**Achromatic point**; *starch no longer detected*

Question 45

F.4. Stages of Starch Hydrolysis:

Answer 45

1. Starch stage / **dark blue** 2. Soluble Starch / **dark blue** 3. Amylodextrin / **purple** 4. Erythrodextrin / **red** 5. Achrodextrin / **colorless**; the **achromatic point** 6. Maltose / **colorless** 7. Glucose / **colorless** *As starch is catabolized and converted to sugars, there will be **less and less starch** to react with the iodine*

Question 46

F.5. During the hydrolysis of starch, there is a decrease in _____

Answer 46

Molecular Weight

Question 47

F.6. What happens when the neutralized starch is added with **NaOH** and **Benedict's reagent**?

Answer 47

**Red-orange precipitate forms** This is because Glucose, a **reducing sugar**, reduces the copper in Benedict's reagent to **copper oxide**