Organic Chemistry Of Alkanoates

 

WAEC-Style Practice Questions: Alkanoates and Detergents

Here are standard West African Senior School Certificate Examination (WASSCE) theory questions with detailed model answers, complete with structural diagrams formatted specifically for your educational blog.

1. Nomenclature and Structure

Question 1a:
(i) Give the IUPAC name of the compound with the structural formula $CH_3CH_2COOCH_2CH_3$.
(ii) State the functional group present in alkanoates and draw its open structural formula showing all bonds.

Answer:
(i) The IUPAC name of the compound is ethyl propanoate.
(ii) The functional group present in alkanoates is the ester group (alkoxycarbonyl group).
Its open structural formula is represented as:

O ║ — C — O —

Question 1b:
An organic compound X has the molecular formula $C_4H_8O_2$. If X is a sweet-smelling liquid formed by the reaction between an alkanol and an alkanoic acid:
(i) Identify the homologous series to which X belongs.
(ii) Draw two possible structural isomers of X and give their IUPAC names.

Answer:
(i) X belongs to the homologous series called alkanoates (or esters).
(ii) Two possible structural isomers of $C_4H_8O_2$ are:

* Ethyl ethanoate ($CH_3COOCH_2CH_3$):

H O H H │ ║ │ │ H — C — C — O — C — C — H │ │ │ H H H
* Methyl propanoate ($CH_3CH_2COOCH_3$):
H H O H │ │ ║ │ H — C — C — C — O — C — H │ │ │ H H H

2. Preparation (Esterification)

Question 2a:
(i) Write a balanced chemical equation for the laboratory preparation of ethyl ethanoate using ethanol and ethanoic acid.
(ii) State the role of the concentrated $H_2SO_4$ used in this reaction and explain why the reaction mixture is heated in a water bath rather than directly over a Bunsen burner flame.

Answer:
(i) The balanced chemical equation is:
$$CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$$
(ii) * Role of conc. $H_2SO_4$: It acts as a catalyst to speed up the reaction and as a dehydrating agent to remove water, shifting the equilibrium to the right to favor a higher yield of the ester.
* Heating method: A water bath is used because ethanol and ethyl ethanoate are highly volatile and flammable. Heating them directly over an open flame poses a severe fire hazard.

Question 2b:
Esterification is described as a reversible reaction.
(i) Briefly explain what is meant by a reversible reaction in this context.
(ii) State two physical properties of the ester product that can be used to confirm its formation in the laboratory.

Answer:
(i) A reversible reaction means that as ethanol and ethanoic acid react to form ethyl ethanoate and water (forward reaction), the products can also react together to reform the starting alkanol and alkanoic acid (backward reaction) until chemical equilibrium is established.
(ii) * It has a characteristic pleasant, fruity smell.
* It forms a distinct, immiscible oily layer that floats on water (due to being less dense than water).

3. Saponification (Soap Making)

Question 3a:
(i) Define the term saponification.
(ii) Write a structural equation representing the production of soap from concentrated sodium hydroxide and a glyceryl trialkanoate.

Answer:
(i) Saponification is the alkaline hydrolysis of fats or oils (glyceryl trialkanoates) using a strong alkali (such as $NaOH$ or $KOH$) to produce propane-1,2,3-triol (glycerol) and the sodium or potassium salts of long-chain fatty acids (soap).
(ii) The structural equation is represented as:

CH2 — O — CO — R CH2 — OH │ │ CH — O — CO — R + 3NaOH ───────> CH — OH + 3R — COO⁻Na⁺ │ │ CH2 — O — CO — R CH2 — OH (Glyceryl Trialkanoate) (Sodium (Glycerol) (Sodium Soap) [Fat / Oil] Hydroxide)

Note: "R" represents the long-chain alkyl group (e.g., $-C_{17}H_{35}$ for stearic acid).

Question 3b:
During the industrial preparation of soap, a concentrated solution of sodium chloride ($NaCl$) is added to the reaction mixture.
(i) Explain the purpose of adding sodium chloride to the mixture.
(ii) State one major difference between the type of soap produced when sodium hydroxide ($NaOH$) is used compared to when potassium hydroxide ($KOH$) is used.

Answer:
(i) Sodium chloride is added to decrease the solubility of the soap, causing it to precipitate out or separate from the glycerol and excess water. This process is known as "salting out".
(ii) $NaOH$ produces hard soap (used commonly as laundry or bar soap), whereas $KOH$ produces soft soap (used as liquid soaps, shaving creams, or toilet soaps).

4. Detergents

Question 4a:
Detergents are generally classified into soapy (saponifiable) and soapless (non-saponifiable) detergents.
(i) Distinguish clearly between saponifiable and non-saponifiable detergents based on their chemical raw materials.
(ii) State one environmental disadvantage of using certain non-saponifiable detergents over soapy detergents.

Answer:
(i) Saponifiable (soapy) detergents are manufactured from natural fats and vegetable oils derived from plants and animals. Non-saponifiable (soapless) detergents are synthetic compounds manufactured from petroleum fractions (hydrocarbons) and coal derivatives.
(ii) Many synthetic (non-saponifiable) detergents contain branched hydrocarbon chains that are non-biodegradable. They persist in water bodies, causing persistent foam that cuts off oxygen supply to aquatic life.

Question 4b:
(i) Explain why soapless detergents wash effectively in hard water while ordinary soaps form scum.
(ii) Name the specific ions responsible for the hardness of water and write a chemical formula for the scum formed when soap reacts with hard water.

Answer:
(i) Soapless detergents form calcium and magnesium salts that are soluble in water, allowing them to lather easily. Ordinary soap reacts with the ions in hard water to form insoluble calcium and magnesium salts, which precipitate out as scum before lather can form.
(ii) * The ions responsible are Calcium ions ($Ca^{2+}$) and Magnesium ions ($Mg^{2+}$).
* Formula for scum (using sodium palmitate soap as an example): $(C_{15}H_{31}COO)_2Ca$ or $(C_{17}H_{35}COO)_2Ca$.

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