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Halogenoalkanes
AQA A Level Chemistry
| Question | Answer |
|---|---|
| What is a halogenoalkane? | A saturated hydrocarbon with a halogen bonded to at least one carbon |
| What explains halogenoalkanes increased reactivity relative to alkanes? | The polar carbon-halogen bond |
| Why do halogenoalkanes contain polar bonds? | Due to the difference in electronegativity between carbon and halogens |
| What kind of reactions do halogenoalkanes undergo? | Nucleophilic substitution |
| What is a nucleophile | A substance with a lone pair of electrons |
| Which three nucleophiles do halogenoalkanes react with? | OH⁻, CN⁻ and NH₃ |
| What reagents and conditions are necessary for hydroxyl substitutions of halogenoalkanes? | Reflux in aqueous solution of NaOH |
| What reagents and conditions are necessary for nitrile (cyanide) substitution of halogenoalkanes? | Aqueous, alcoholic KCN |
| What is the importance of nucleophilic substitution with cyanide ions? | Increases the length of the carbon chain |
| What reagents and conditions are necessary for ammonia substitution of halogenoalkanes? | Reflux in aqueous, alcoholic excess ammonia |
| Why is excess ammonia required in its nucleophilic substitution with a halogenoalkane? | To prevent further substitutions |
| Draw a nucleophilic substitution mechanism for a given halogenoalkane reacting with a given nucelophile | Draw answer |
| How is the rate of reaction dependent on the enthalpy of the C-X bond? | The greater the enthalpy of the C-X bond, the greater its strength, the slower the reaction |
| What is the trend in C-X reaction rates | Increases down the group from C-F to C-I |
| What is an elimination reaction? | A reaction where a halogenoalkane becomes an alkene |
| Which elements are "eliminated" in an elimination reaction | H and X |
| What conditions favour elimination reactions? | Secondary or tertiary halogenoalkane, hot ethanolic KOH |
| Draw the mechanism for the elimination of a given halogenoalkane with OH⁻ | Draw answer |
| What is ozone? | O₃ |
| Where is ozone found and why is it beneficial? | Upper atmosphere, absorbs harmful UV radiation |
| What is a CFC? | A chlorofluorocarbon: a compound containing C-Cl and C-F bonds |
| What were CFCs used for? | Refrigerants and solvents |
| What happens to CFCs when exposed to UV? | C-Cl bond breaks and generates Cl∙ |
| How do chlorine radicals interact with ozone? | Catalyse its decomposition into O₂ thus forming a "hole" in the ozone layer |
| Why are CFCs now banned for use? | Due to research results from different groups within the scientific community |
| Give two equations showing the decomposition of ozone with chlorine radicals | Cl∙ + O₃ → ClO∙ + O₂ , ClO∙ + O₃ → 2O₂ + Cl∙ |
| What is the overall equation for the decomposition of ozone? | 2O₃ → 3O₂ |
Created by:
JBearfield
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