Organic Chemistry Of Carbohydrates Proteins. Fats and Oils

 

WASSCE / WAEC Chemistry Theory Questions & Answers

Topics Covered: Carbohydrates, Proteins, Fats and Oils

SECTION A: CARBOHYDRATES

Question 1

(a) Name the three main classes of carbohydrates and give one example of each.

(b) Describe briefly how a sample of starch can be converted to glucose in the laboratory, stating the conditions and the test to confirm the presence of glucose.

Solution / Marking Scheme:

(a) Classes of Carbohydrates:

• Monosaccharides: Glucose / Fructose / Galactose
• Disaccharides: Sucrose / Maltose / Lactose
• Polysaccharides: Starch / Cellulose / Glycogen

(b) Laboratory Conversion of Starch to Glucose (Hydrolysis):

• Mix the starch sample with dilute hydrochloric acid ($\text{HCl}$) or tetraoxosulphate(VI) acid ($\text{H}_2\text{SO}_4$) in a flask.
• Boil/heat the mixture for about 10 to 15 minutes to allow complete hydrolysis to take place.

$(\text{C}_6\text{H}_{10}\text{O}_5)_n + n\text{H}_2\text{O} \xrightarrow{\text{dil. } \text{H}^+ / \text{heat}} n\text{C}_6\text{H}_{12}\text{O}_6$
(Starch)                                                  (Glucose)

Confirmation Test:

• Neutralize the acid in the solution with a few drops of sodium hydroxide ($\text{NaOH}$).
• Add Fehling's solution (or Benedict's solution) and warm gently.
• Observation: A brick-red precipitate confirms the presence of glucose (a reducing sugar).

Question 2

A student was provided with two unlabelled reagent bottles containing Starch solution and Glucose solution respectively. Describe a simple chemical test that can be used to distinguish between the two solutions, stating the observation for each.

Solution / Marking Scheme:

Test: Iodine Test

• Add a few drops of Iodine solution to a portion of each solution in separate test tubes.

Observations:

• Starch solution: A dark-blue / blue-black color develops.
• Glucose solution: The solution remains yellowish-brown (or no characteristic color change occurs).

Alternative Test: Fehling's / Benedict's test. Heating glucose with Fehling's solution gives a brick-red precipitate, while starch gives no reaction.

SECTION B: PROTEINS

Question 3

(a) What are the monomeric units of proteins? Draw their general structural formula.

(b) Explain clearly how two of these monomers combine to form a dimer, highlighting the functional group created in the process.

Solution / Marking Scheme:

(a) Monomer Units: Amino acids.

General Structural Formula:

    H
    |
H₂N—C—COOH
    |
    R

(Where $-\text{NH}_2$ is the amino group, $-\text{COOH}$ is the carboxyl group, and $\text{R}$ is the variable alkyl side chain).

(b) Combination (Condensation Reaction):

Two amino acids link together via a condensation reaction where the amino ($-\text{NH}_2$) group of one amino acid reacts with the carboxyl ($-\text{COOH}$) group of another, with the elimination of a molecule of water ($\text{H}_2\text{O}$).

The link formed between them is called a peptide bond / amide linkage ($-\text{CO}-\text{NH}-$).

R₁-CH(NH₂)-COOH + R₂-CH(NH₂)-COOH → R₁-CH(NH₂)-CO-NH-CH(R₂)-COOH + H₂O

Question 4

(a) Define the term denaturation of proteins.

(b) State two factors that can cause denaturation.

(c) Name the specific chemical test used to detect the presence of proteins and state the expected observation.

Solution / Marking Scheme:

(a) Denaturation: This is the process whereby a protein loses its native 3D secondary, tertiary, or quaternary structure (and consequently its biological activity) due to external stress, without the breaking of its primary peptide bonds.

(b) Factors that cause denaturation (Any two):

• High temperature / Heat
• Extreme pH changes (Strong acids or bases)
• Heavy metal ions (e.g., Lead, Mercury)
• Organic solvents (e.g., Alcohol, Acetone)
• Mechanical agitation

(c) Chemical Test for Protein:

• Test: Biuret Test. Add sodium hydroxide ($\text{NaOH}$) solution followed by a few drops of dilute copper(II) tetraoxosulphate(VI) ($\text{CuSO}_4$) solution to the sample.
• Observation: A violet / purple color develops, indicating the presence of peptide bonds.

SECTION C: FATS AND OILS

Question 5

(a) State the structural difference between a fat and an oil.

(b) Write a word equation for the alkaline hydrolysis of a lipid (triglyceride) using sodium hydroxide. What is the commercial name given to this process?

Solution / Marking Scheme:

(a) Structural Difference:

• Fats: Contain predominantly saturated fatty acid chains (single bonds only between carbon atoms), which makes them solid at room temperature.
• Oils: Contain predominantly unsaturated fatty acid chains (one or more carbon-carbon double bonds), which makes them liquid at room temperature.

(b) Alkaline Hydrolysis Equation:

Fats/Oils (Triglyceride) + Sodium Hydroxide → Propane-1,2,3-triol (Glycerol) + Sodium Soap

Commercial Name: Saponification.

Question 6

Vegetable oils are often converted to margarine in the food industry.

(a) Name the chemical process involved in this conversion.

(b) State the necessary catalyst and conditions required for the process.

(c) Explain why the physical state of the oil changes during this process.

Solution / Marking Scheme:

(a) Chemical Process: Hydrogenation (or Hardening of oils).

(b) Catalyst and Conditions:

• Catalyst: Finely divided Nickel ($\text{Ni}$) (or Platinum / Palladium).
• Conditions: Temperature of about 150°C - 180°C and high pressure.

(c) Explanation of Physical State Change:

Vegetable oils are liquids because they contain unsaturated carbon chains with double bonds ($\text{C}=\text{C}$), which prevent close packing of molecules. During hydrogenation, hydrogen atoms are added across these double bonds, converting them into single bonds ($\text{C}-\text{C}$).

This transforms the unsaturated oil into a saturated fat, increasing its melting point and turning it into a solid or semi-solid (margarine) at room temperature.