Question

An organic compound "X" with formula \(\mathrm{C_2H_6O}\) is used to blend with petrol in order to reduce the dependence on fossil fuels and lower down the harmful emissions from the cars. It is also used as a solvent in medicines such as tincture iodine, cough syrups and many tonics.
(i) Draw the electron dot structure of "X".
(ii) Write the name and formula of the next homologue of "X".
(iii) Write the reaction of "X" with acetic acid to form a sweet smelling substance.

Hint:

Identify X from its formula and uses. Homologues differ by one CH2 unit. Alcohol plus carboxylic acid gives an ester.

Answer 1

Step 1: Identify compound X:
The formula \(\mathrm{C_2H_6O}\) fits ethanol, \(\mathrm{C_2H_5OH}\). Ethanol is blended with petrol (it burns cleanly and cuts the use of fossil fuel) and it is the solvent in tincture iodine and cough syrups.
So X is ethanol.

Step 2: Part (i): Electron dot structure:
Count the valence electrons: 2 carbon atoms give \(2 \times 4 = 8\), 6 hydrogen atoms give 6 and the oxygen atom gives 6. The total is 20 electrons.
Ethanol has 8 covalent bonds: 5 C-H bonds, 1 C-C bond, 1 C-O bond and 1 O-H bond. These use \(8 \times 2 = 16\) electrons.
The other \(20 - 16 = 4\) electrons stay on oxygen as two lone pairs. Each carbon now has 8 electrons around it and each hydrogen has 2.
\[ \begin{array}{ccccccccc} & & \text{H} & & \text{H} & & & & \\ & & \cdot\cdot & & \cdot\cdot & & \cdot\cdot & & \\ \text{H} & \colon & \text{C} & \colon & \text{C} & \colon & \text{O} & \colon & \text{H} \\ & & \cdot\cdot & & \cdot\cdot & & \cdot\cdot & & \\ & & \text{H} & & \text{H} & & & & \end{array} \]

Step 3: Part (ii): Next homologue:
A homologous series is a family of compounds in which each member differs from the next by one \(\mathrm{CH_2}\) unit.
Adding one \(\mathrm{CH_2}\) to \(\mathrm{C_2H_5OH}\) gives \(\mathrm{C_3H_7OH}\). Its name is propanol (propan-1-ol).

Step 4: Part (iii): Reaction with acetic acid:
An alcohol and a carboxylic acid react to form an ester. This is called esterification. A few drops of conc. \(\mathrm{H_2SO_4}\) are used as a catalyst.
Here ethanol reacts with ethanoic (acetic) acid to give ethyl ethanoate, which has a sweet fruity smell, and water.
\[ \mathrm{CH_3COOH + C_2H_5OH \xrightarrow{\text{conc. }H_2SO_4} CH_3COOC_2H_5 + H_2O} \]

Final Answer:
(i) the dot structure of ethanol is drawn above; (ii) the next homologue is propanol, \(\mathrm{C_3H_7OH}\); (iii) ethanol and acetic acid give ethyl ethanoate (ester) and water. \[ \boxed{X=\text{ethanol};\ \text{next homologue}=\text{propanol}\ (C_3H_7OH);\ \mathrm{CH_3COOH+C_2H_5OH\rightarrow CH_3COOC_2H_5+H_2O}} \]