Study the reaction scheme and answer the questions that follow - Leaving Cert Chemistry - Question 8 - 2013

Conversion X involves an addition reaction, specifically a hydrogenation, where hydrogen is added across a double bond, converting ethene (C₂H₄) to ethane (C₂H₆). This reaction can be represented as:

$C₂H₄ + H₂ \rightarrow C₂H₆$

In ethene, which has a double bond, the geometry around the carbon atoms is planar due to sp² hybridization, resulting in bond angles of approximately 120°. In contrast, ethane, formed after conversion X, has single C–C bonds with tetrahedral geometry, leading to bond angles of approximately 109.5°. This transition from planar to tetrahedral structure indicates the change in hybridization from sp² in ethene to sp³ in ethane.

(i) The reagent required for conversion Y is bromine (Br₂). (ii) The reagent for conversion Z is hydrogen chloride (HCl). (iii) The reagent for conversion W is chlorine (Cl₂).

The mechanism of reaction W involves the electrophilic addition of chlorine (Cl₂) to ethene. It begins with the formation of a cyclic chloronium ion as the electrophile attacks the double bond:

1. The double bond of ethene attacks a Cl atom, leading to the formation of a cyclic chloronium ion.
2. The other Cl atom, acting as a nucleophile, attacks the more stable carbon atom adjacent to the cyclic ion, effectively opening the ring.
3. Finally, we obtain 1,2-dichloroethane as the product. This step is characterized by the addition of chlorine across the double bond, converting it into a single bond.

The structure of A is 1,2-dichloroethane, represented as follows:

       Cl   H
        |   |
    H -- C -- C -- H
        |   |
       H   Cl

The name of compound A is 1,2-dichloroethane.

The structure of two repeating units of PVC (polyvinyl chloride) is shown below:

    H   Cl        H   Cl
    |   |        |   |
H -- C -- C -- H -- C -- C -- H
    |   |        |   |
    H   H        H   H

Each repeating unit consists of the vinyl chloride unit, where the Cl represents the chlorine substituent.