Organic Chemistry of Alkanoic Acids

 

Alkanoic Acids

Alkanoic Acids (Carboxylic Acids): These are homologous series of organic compounds containing the carboxyl functional group (—COOH) attached to an alkyl group or a hydrogen atom. They have the general molecular formula CnH2n+1COOH (where n = 0, 1, 2...).

Alkanoates (Esters): These are organic compounds formed when the hydroxyl (—OH) group of an alkanoic acid is replaced by an alkoxy (—OR) group from an alkanol. They contain the functional group (—COOR) and are characterized by sweet, fruity smells.

Esterification: This is the reversible reaction between an alkanoic acid and an alkanol in the presence of a mineral acid catalyst (such as concentrated H2SO4) to yield an alkanoate (ester) and water.

Saponification: This is the alkaline hydrolysis of fats and oils (long-chain alkanoates) using a strong base like NaOH or KOH to produce propane-1,2,3-triol (glycerol) and the sodium or potassium salt of the fatty acid (soap).

WAEC Theory Questions and Solutions

QUESTION 1

(a) Give the IUPAC name and draw the structural formula of the third member of the alkanoic acid series.
(b) Explain why alkanoic acids generally have higher boiling points than alkanols of comparable molecular mass.

SOLUTION

(a) The third member of the alkanoic acid series contains three carbon atoms. Its IUPAC name is Propanoic acid.

Its structural formula is displayed below:

H H O
| | //
H — C — C — C — O — H
| |
H H

(b) Alkanoic acids have higher boiling points than alkanols because alkanoic acid molecules can form stronger, dimeric intermolecular hydrogen bonds. Each pair of alkanoic acid molecules is held together by two hydrogen bonds, whereas alkanol molecules are held together by only single hydrogen bonds per molecule. More thermal energy is required to break these dimeric bonds in acids.

QUESTION 2

Describe, with the aid of a balanced chemical equation and essential conditions, how a pure sample of ethanoic acid can be prepared in the laboratory by the oxidation of ethanol.

SOLUTION

In the laboratory, ethanoic acid is prepared by heating ethanol under reflux with an excess of an oxidizing agent, such as acidified potassium heptaoxodichromate(VI) (K2Cr2O7/H2SO4) or acidified potassium tetraoxomanganate(VII) (KMnO4/H2SO4).

The reaction occurs in two stages: ethanol is first oxidized to ethanal, which is then further oxidized completely to ethanoic acid. The color changes from orange to green (if Cr2O72− is used). The pure acid is subsequently separated by fractional distillation.

Balanced Equation:
CH3CH2OH + 2[O] → CH3COOH + H2O

QUESTION 3

State what would be observed and write a balanced molecular equation for the reaction of ethanoic acid with:
(a) Sodium trioxocarbonate(IV) pellets.
(b) Aqueous Sodium hydroxide.

SOLUTION

(a) Reaction with Sodium trioxocarbonate(IV) (Na2CO3):
Observation: Rapid effervescence occurs with the evolution of a colorless, odorless gas (CO2) that turns lime water milky.
Equation: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2↑

(b) Reaction with Aqueous Sodium hydroxide (NaOH):
Observation: A neutralization reaction occurs. No visible change is seen immediately as both reactants are colorless, but heat is liberated (exothermic), and a clear solution of sodium ethanoate is formed.
Equation: CH3COOH + NaOH → CH3COONa + H2O

QUESTION 4

(a) Outline the process of esterification between ethanoic acid and ethanol, stating the role of the catalyst.
(b) Give the IUPAC name and structural formula of the ester produced.

SOLUTION

(a) When ethanoic acid is heated with ethanol in the presence of a few drops of concentrated tetraoxosulphate(VI) acid (H2SO4), a reversible reaction occurs to produce an ester and water. The concentrated H2SO4 acts both as a catalyst to speed up the reaction rate and as a dehydrating agent to shift the equilibrium position to the right, favoring a higher yield of the ester.

Equation: CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O

(b) The IUPAC name of the ester produced is Ethyl ethanoate.

Its structural formula is displayed below:

H O H H
| // | |
H — C — C — O — C — C — H
| | |
H H H

QUESTION 5

An organic compound 'Y' with a pleasant fruity smell has the molecular formula C4H8O2. When heated with dilute NaOH solution, it yields an alcohol 'A' and a salt 'S'. 'A' gives a positive iodoform test.
(i) Identify 'Y', 'A', and 'S'.
(ii) Write a balanced chemical equation for the alkaline hydrolysis of 'Y'.

SOLUTION

(i) Identification of compounds:

  • Compound 'Y': Ethyl ethanoate (CH3COOCH2CH3). The pleasant smell indicate it is an ester.
  • Alcohol 'A': Ethanol (CH3CH2OH). Ethanol is a primary alcohol that gives a positive iodoform test due to its CH3CH(OH)- structure.
  • Salt 'S': Sodium ethanoate (CH3COONa).

(ii) The balanced equation for the alkaline hydrolysis (saponification) of 'Y' is:

CH3COOCH2CH3 + NaOH → CH3COONa + CH3CH2OH

QUESTION 6

(a) Contrast the chemical behavior of alkanoic acids and alkanols using their reactions with metallic sodium and litmus paper.
(b) Give one commercial use of alkanoic acids and two uses of alkanoates.

SOLUTION

(a) Contrast Table:

Test/Reactant Alkanoic Acid (e.g., Ethanoic acid) Alkanol (e.g., Ethanol)
Litmus Paper Turns blue litmus paper red (acidic). No effect on blue or red litmus paper (neutral).
Sodium Metal Reacts vigorously to liberate Hydrogen gas and form a soluble salt. Reacts steadily/moderately to liberate Hydrogen gas and form an alkoxide.

(b) Commercial Uses:

Alkanoic acids (e.g., Ethanoic acid): Used in the manufacture of cellulose acetate (for photographic films and textiles), as a preservative in the food industry (vinegar), and in the production of dyes and pharmaceuticals.

Alkanoates (Esters): Used extensively as artificial flavorings and perfumes in food and cosmetics because of their pleasant fruity odors. They are also widely used as industrial solvents for paints, nail polishes, and adhesives.

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