Halogenoalkanes Flashcards

Active recall practice for nucleophilic substitution, elimination reactions, and ozone depletion mechanisms.

Properties and Reactivity: read all 4 questions and answers

Why is the carbon-halogen bond polar?
Halogens are more electronegative than carbon, drawing electron density towards themselves and creating a partial negative charge (delta-) on the halogen and a partial positive charge (delta+) on the carbon.
Explain the trend in reactivity of halogenoalkanes down Group 7.
Reactivity increases down the group. The C-X bond enthalpy decreases (bonds get weaker because atomic radius increases and overlap of orbitals is poorer), making the bond easier to break, which overrides the polarity effect.
Which carbon-halogen bond is the strongest?
The C-F bond, because fluorine is the smallest halogen atom, leading to a short, very strong bond with high bond enthalpy.
Which carbon-halogen bond is the weakest?
The C-I bond, because iodine has a large atomic radius, leading to a long, weak bond with low bond enthalpy.

Nucleophilic Substitution Mechanisms: read all 8 questions and answers

What is nucleophilic substitution?
A reaction where a nucleophile replaces a halogen atom in a halogenoalkane.
Name three common nucleophiles that react with halogenoalkanes.
Hydroxide ion (OH⁻), ammonia (NH₃), and cyanide ion (CN⁻).
State the reagent and conditions for substitution with hydroxide ions.
Aqueous sodium or potassium hydroxide (NaOH/KOH), heated under reflux.
Write the organic product formed when bromoethane reacts with aqueous KOH.
Ethanol (CH₃CH₂OH).
State the reagent and conditions for reaction with cyanide ions.
Ethanolic potassium cyanide (KCN), heated under reflux.
What is the key synthetic benefit of reacting a halogenoalkane with cyanide ions?
It extends the carbon chain by one carbon atom (forming a nitrile).
State the reagent and conditions for reaction with ammonia.
Excess ethanolic ammonia (NH₃), heated under pressure in a sealed tube.
Why is excess ammonia used in the reaction with halogenoalkanes?
To minimise further substitution of the amine product, ensuring a higher yield of primary amine.

Elimination & Ozone Depletion: read all 7 questions and answers

What is an elimination reaction of a halogenoalkane?
A reaction where a halogenoalkane loses a hydrogen halide molecule to form an alkene.
State the reagent and conditions to favour elimination over substitution.
Ethanolic sodium or potassium hydroxide (NaOH/KOH) and heating to high temperatures.
What role does the hydroxide ion (OH⁻) play in an elimination reaction?
It acts as a base (accepting a proton/H⁺ from a carbon atom adjacent to the C-X carbon).
What are CFCs?
Chlorofluorocarbons - organic compounds containing chlorine, fluorine, and carbon atoms, formerly used as refrigerants and propellants.
Write the initiation step for the decomposition of ozone by a CFC molecule (e.g. CF₂Cl₂).
\(\text{CF}_2\text{Cl}_2 \rightarrow {}^\bullet\text{CF}_2\text{Cl} + \text{Cl}^\bullet\) (homolytic fission of the weaker C-Cl bond by UV light).
Write the two propagation steps for ozone depletion by chlorine radicals.
1) \(\text{Cl}^\bullet + \text{O}_3 \rightarrow {}^\bullet\text{ClO} + \text{O}_2\); 2) \({}^\bullet\text{ClO} + \text{O}_3 \rightarrow \text{Cl}^\bullet + 2\text{O}_2\) (overall: 2O₃ → 3O₂).
Why are chlorine radicals described as catalysts in ozone depletion?
They are regenerated in the second propagation step and can go on to decompose thousands of other ozone molecules.