Answer the questions that follow with reference to hydrocarbons A, B and C below - Leaving Cert Chemistry - Question 8 - 2014

To prepare compound A (ethyne) in the laboratory, a typical setup can be represented as:

    water dropping on to tap funnel containing water  
    ┌──────────── ┐
    │  CaC₂      │
    └────────────┘
          ↓
    delivery tubing
          ↓
      collection flask  

In the labelled diagram:

• Calcium carbide (CaC₂) is placed in a flask.
• Water drips onto calcium carbide to produce ethyne gas, which is collected over water.

To distinguish between benzene (compound B) and ethyne (compound C), we can use bromine water.

1. Add a few drops of bromine water to each compound in separate test tubes.
2. If benzene is present, there will be no color change, as benzene does not react with bromine water.
3. If ethyne is present, the orange-brown color of bromine will disappear, indicating that a reaction has occurred, resulting in the formation of colorless dibromoethane.

Therefore, the disappearance of color when testing with bromine water is a key indicator to distinguish between the two.

The reaction follows a free radical substitution mechanism, characterized by the following steps:

1. Initiation: Chlorine molecules (Cl₂) are broken down by ultraviolet light into free radicals: $$$$

ightarrow 2Cl^*$$

2. Propagation: The free chlorine radical (Cl*) reacts with ethyne (C₂H₄) to replace a hydrogen atom: $$$$

ightarrow C_2H_3Cl + H^$$ The hydrogen radical (H) can further react with another chlorine molecule:

ightarrow HCl + Cl^*$$ This step forms a chloroethane product while regenerating the chlorine radical. 3. **Termination:** The reaction concludes when free radicals combine to form stable products: $$Cl^* + Cl^* ightarrow Cl_2$$ Such steps illustrate the mechanism of the reaction clearly, showing the involvement of free radicals.

The presence of butane (C₄H₁₀) in the product mixture can indicate that multiple reactions may have occurred due to the radical mechanism. This occurs as:

1. The radical substitution process provides sufficient energy for further reactions, allowing the formation of larger hydrocarbons from the simpler ethyne.
2. If butane is detected, it suggests that combinations of radicals have taken place, leading to the coupling of smaller hydrocarbon chains into larger ones.

In conclusion, the formation of butane is consistent with the free radical substitution mechanism, as it demonstrates that the radicals formed during the reaction have recombined, validating the proposed pathway.