9701 · 31.1
Halogen compounds flashcards
Revision flashcards for Cambridge 9701 Halogen compounds (syllabus 31.1). Flip, recall, then mark a real past-paper question.
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What is a halogenoalkane?
An alkane in which one or more hydrogen atoms have been replaced by a halogen atom (F, Cl, Br, I). General formula $C_nH_{2n+1}X$.
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What is a halogenoarene (or aryl halide)?
An aromatic compound (e.g., benzene) in which one or more hydrogen atoms on the ring have been directly replaced by a halogen atom.
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Define 'nucleophile'.
An electron-pair donor. It is a species that is attracted to an electron-deficient centre (a nucleus or positive charge). Examples include $OH^-$, $CN^-$, and $:NH_3$.
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What are the conditions for nucleophilic substitution of a halogenoalkane?
Warm aqueous solution of a nucleophile, e.g., warm aqueous sodium hydroxide, NaOH(aq).
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What are the conditions for elimination from a halogenoalkane?
Hot, ethanolic solution of a strong base, e.g., hot potassium hydroxide in ethanol, KOH(eth).
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Why are halogenoarenes much less reactive than halogenoalkanes towards nucleophilic substitution?
The p-orbital of the halogen atom's lone pair overlaps with the delocalised π-system of the benzene ring. This gives the C-X bond partial double bond character, making it stronger and harder to break.
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Rank the C-X bonds from strongest to weakest, and state the effect on reactivity.
Strength: C-F > C-Cl > C-Br > C-I. Weaker bonds are easier to break, so reactivity towards nucleophilic substitution is: Iodoalkane > Bromoalkane > Chloroalkane. Fluoroalkanes are very unreactive.
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What is the difference in mechanism between Sₙ1 and Sₙ2 reactions?
Sₙ1 is a two-step mechanism via a carbocation intermediate, favoured by tertiary halogenoalkanes. Sₙ2 is a single-step mechanism with a transition state, typical for primary halogenoalkanes.
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What were chlorofluorocarbons (CFCs) used for and why are they now banned?
Used as refrigerants, aerosol propellants, and solvents. They are now banned under the Montreal Protocol because they are very stable and catalyse the depletion of the ozone layer in the stratosphere.
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How can you experimentally distinguish between a chloro-, bromo-, and iodoalkane?
Warm each with aqueous silver nitrate (in ethanol as a common solvent). An iodide gives a yellow precipitate of AgI quickly. A bromide gives a cream precipitate of AgBr more slowly. A chloride gives a white precipitate of AgCl very slowly, often requiring heating.