Chemistry Past Paper PDF 2019 Jan
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2019
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This document is a chemistry past paper focused on free radical substitution reactions of alkanes. It includes questions and mechanisms for different reactions involving chlorine, bromine, and other halogenations. The document covers topics like initiation, propagation, and termination steps in free radical reactions.
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## Chlorine and bromine, when mixed with alkanes undergo free radical substitution in presence of UV light through a chain reaction. ### Mechanism: Free radical substitution - **Initiation:** Cl-Cl → Cl . + Cl . - **Propagation:** - H-C-H + Cl . → H · + CH<sub>3</sub>Cl - Cl . + CH<sub>3</...
## Chlorine and bromine, when mixed with alkanes undergo free radical substitution in presence of UV light through a chain reaction. ### Mechanism: Free radical substitution - **Initiation:** Cl-Cl → Cl . + Cl . - **Propagation:** - H-C-H + Cl . → H · + CH<sub>3</sub>Cl - Cl . + CH<sub>3</sub> . → CH<sub>3</sub>Cl - **Termination:** - C . + C . is a free radical substitution - C . + radical → molecule - radical + radical → molecule ### Side reaction: - Free radical substitution isn't usually used because further substitution reactions take place, then the yield of target product reduces. **Q. Fill in the blanks:** a. H-C-H + Cl → HCl + CH<sub>3</sub> b. CH<sub>3</sub>Cl + Cl → CH<sub>2</sub>Cl<sub>2</sub> + Cl c. CH<sub>2</sub>Cl<sub>2</sub> + Cl → CHCl3 + Cl d. CHCl<sub>3</sub> + Cl → CCl<sub>4</sub> + Cl **2019 Jan** State why the main product CH<sub>3</sub>Cl in the free radical propagation step is higher than that formed in termination step. - **Mark 1:** In propagation step, radicals are regenerate. - **Mark 2:** The propagation keeps repeating to form a large number of CH<sub>3</sub>Cl (propagation continues). - **Mark 3:** In termination, two radicals give only one product and the number of radicals reduce. **Q. Give the mechanism of the reaction, arrows not needed:** C<sub>2</sub>H<sub>6</sub> + Br<sub>2</sub> → C<sub>2</sub>H<sub>5</sub>Br + HBr **Free radical substitution:** **Initiation:** Br<sub>2</sub> → 2Br . **Propagation:** - C<sub>2</sub>H<sub>6</sub> + Br . → HBr + C<sub>2</sub>H<sub>5</sub> . - C<sub>2</sub>H<sub>5</sub> . + Br<sub>2</sub> → C<sub>2</sub>H<sub>5</sub>Br + Br **Termination:** - C<sub>2</sub>H<sub>5</sub> . + Br . → C<sub>2</sub>H<sub>5</sub>Br - C<sub>2</sub>H<sub>5</sub> . + C<sub>2</sub>H<sub>5</sub> . → C<sub>4</sub>H<sub>10</sub> **Q. Give the mechanism for reaction:** [Image of a ring, not able to be converted to text] + Cl<sub>2</sub> → [Image of a ring, not able to be converted to text] + HCl **Initiation:** Cl<sub>2</sub> → 2Cl . **Propagation:** - [Image of a ring, not able to be converted to text] + Cl . → HCl + [Image of a ring, not able to be converted to text] **Termination:** [Image of a ring, not able to be converted to text] + Cl . → [Image of a ring, not able to be converted to text] + Cl **Q. Give the mechanism for [Image of a ring, not able to be converted to text] + Br<sub>2</sub> → [Image of a ring, not able to be converted to text] + Hbr** **Initiation:** Br<sub>2</sub> → 2Br . **Propagation:** - [Image of a ring, not able to be converted to text] + Br. → HBr + [Image of a ring, not able to be converted to text] **Termination:** - [Image of a ring, not able to be converted to text] + Br. → [Image of a ring, not able to be converted to text] - [Image of a ring, not able to be converted to text] + [Image of a ring, not able to be converted to text] → [Image of a ring, not able to be converted to text]
