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Halogenoalkanes

AQA A Level Chemistry

QuestionAnswer
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