Halogenoalkanes (AQA A Level Chemistry): Flashcards
Exam code: 7405
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Define bond enthalpy in the context of halogenoalkane reactivity.
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Define bond enthalpy in the context of halogenoalkane reactivity.
A non-polar molecule is a molecule with no overall dipole moment, either because all bonds are non-polar or because the bond dipoles cancel due to the symmetric arrangement of bonds.
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?
A polar molecule is a molecule with a permanent dipole moment, resulting from an uneven distribution of electron density caused by differences in electronegativity between bonded atoms and an asymmetric shape.
Define nucleophile.
A permanent dipole is the separation of charge in a polar molecule caused by a difference in electronegativity between bonded atoms, which creates a partial positive charge (δ+) on one atom and a partial negative charge (δ–) on the other.
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.
An enthalpy profile diagram is a diagram showing the relative enthalpy levels of reactants and products and the activation energy barrier for a reaction. The y-axis represents enthalpy and the x-axis represents the progress of the reaction.
What is a CFC (chlorofluorocarbon)?
A dipole–dipole force is an intermolecular force of attraction between the positive end of one polar molecule and the negative end of an adjacent polar molecule.
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)?
A hydrogen bond is a strong intermolecular force of attraction between a hydrogen atom bonded to a highly electronegative atom (F, O or N) and the lone pair on an electronegative atom (F, O or N) of a neighbouring molecule.
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