Halogenoalkanes (AQA A Level Chemistry): Flashcards
Exam code: 7405
1/210Still learning
Know0
Define bond enthalpy in the context of halogenoalkane reactivity.
Was this flashcard helpful?
Cards in this collection (21)
Define bond enthalpy in the context of halogenoalkane reactivity.
Bond enthalpy is the energy required to break one mole of a specific covalent bond in gaseous molecules. A lower bond enthalpy means the bond breaks more easily, so the halogenoalkane is more reactive.
True or False?
Iodoalkanes are more reactive than fluoroalkanes in nucleophilic substitution reactions.
True.
The C–I bond has the lowest bond enthalpy of the carbon–halogen bonds, so it breaks most easily. The C–F bond has the highest bond enthalpy and rarely undergoes substitution.
The order of C–X bond strength from weakest to strongest is: C–I < C–.......... < C–.......... < C–F.
The order of C–X bond strength from weakest to strongest is: C–I < C–Br < C–Cl < C–F.
How can aqueous silver nitrate be used to compare the reactivity of halogenoalkanes?
Halogenoalkanes are added to ethanol and acidified silver nitrate solution. The rate at which a precipitate forms indicates reactivity. Iodoalkanes give a pale yellow precipitate fastest; chloroalkanes give a white precipitate slowest.
When a bromoalkane reacts with aqueous silver nitrate, a .......... precipitate of silver .......... forms.
When a bromoalkane reacts with aqueous silver nitrate, a cream precipitate of silver bromide forms.
True or False?
Fluoroalkanes react readily with nucleophiles at room temperature.
False.
Fluoroalkanes have a very strong C–F bond with a high bond enthalpy, so they do not undergo nucleophilic substitution reactions under normal conditions.
What is nucleophilic substitution in the context of halogenoalkanes?
Nucleophilic substitution is a reaction in which a nucleophile replaces the halogen in a halogenoalkane by attacking the partially positive carbon atom attached to the electronegative halogen.
Define nucleophile.
Nucleophile is a species that is attracted to areas of positive charge and donates a lone pair of electrons to form a new covalent bond. Examples include OH-, CN- and NH3.
Halogenoalkanes undergo nucleophilic substitution with aqueous NaOH to form ........... The C–X bond breaks ...........
Halogenoalkanes undergo nucleophilic substitution with aqueous NaOH to form alcohols. The C–X bond breaks heterolytically.
True or False?
Reacting a halogenoalkane with ethanolic potassium cyanide adds a carbon atom to the chain.
True.
The CN- nucleophile adds a carbon atom when it replaces the halogen, increasing the chain length by one. The product is a nitrile.
What conditions are required to produce a primary amine from a halogenoalkane?
Heat the halogenoalkane with excess ethanolic ammonia (NH3 dissolved in ethanol) under pressure. The NH3 acts as the nucleophile and replaces the halogen.
In elimination reactions, a halogenoalkane is heated with .......... sodium hydroxide, producing an .......... as the organic product.
In elimination reactions, a halogenoalkane is heated with ethanolic sodium hydroxide, producing an alkene as the organic product.
True or False?
Heating a halogenoalkane with warm aqueous NaOH gives an alkene as the main product.
False.
Warm aqueous NaOH favours nucleophilic substitution to give an alcohol. Ethanolic NaOH and heat are needed for elimination to produce an alkene.
Define elimination reaction in the context of halogenoalkanes.
Elimination reaction is a reaction in which a small molecule (a hydrogen halide) is removed from a halogenoalkane, forming a C=C double bond. The organic product is an alkene.
What is a CFC (chlorofluorocarbon)?
CFC (chlorofluorocarbon) is a halogenoalkane in which all hydrogen atoms are replaced by chlorine and fluorine atoms. CFCs are chemically inert, non-flammable and non-toxic, and were widely used as refrigerants and aerosol propellants.
True or False?
In the upper atmosphere, it is the C–F bond in a CFC that breaks to generate chlorine radicals.
False.
The C–Cl bond breaks because it has a lower bond enthalpy than C–F. UV radiation causes homolytic fission of C–Cl to produce chlorine radicals.
Chlorine radicals destroy ozone in two propagation steps. In the first step, Cl• + O3 → .......... + O2.
Chlorine radicals destroy ozone in two propagation steps. In the first step, Cl• + O3 → ClO• + O2.
Why can a single chlorine radical destroy many thousands of ozone molecules?
The chlorine radical is regenerated at the end of each two-step cycle, so it acts as a catalyst. It can repeatedly react with ozone without being permanently consumed.
HFCs were developed as a safer alternative to CFCs because they contain no .......... atoms and so do not damage the .......... layer.
HFCs were developed as a safer alternative to CFCs because they contain no chlorine atoms and so do not damage the ozone layer.
True or False?
The Montreal Protocol (1987) banned the use of CFCs as aerosol propellants and refrigerants.
True.
International scientific evidence of ozone depletion led to legislation under the Montreal Protocol that restricted and banned CFC use globally.
What is a hydrofluorocarbon (HFC)?
Hydrofluorocarbon (HFC) is a compound containing only carbon, hydrogen and fluorine atoms. Like CFCs, HFCs are chemically inert, but they do not deplete the ozone layer as they contain no chlorine.
Sign up to unlock flashcards
or