Nucleophilic substitution of halogenoalkanes
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Question
What is a nucleophile?
Answer
An **electron-pair donor** — it has a lone pair and is attracted to a δ+ (electron-poor) carbon. Examples: OH⁻, CN⁻, NH_{3}.
Question
Why is the carbon in a halogenoalkane δ+?
Answer
The C–halogen bond is **polar**: the more electronegative halogen pulls the bonding electrons, leaving carbon slightly positive (**δ+**).
Question
What does a curly arrow show?
Answer
The movement of a **pair of electrons** — the tail is at the electrons that move, the head is where the pair ends up.
Question
Describe the two curly arrows in nucleophilic substitution.
Answer
Arrow 1: the nucleophile's **lone pair → δ+ carbon** (new bond). Arrow 2: the **C–X bond → halogen**, which leaves as X⁻.
Question
What is the leaving group?
Answer
The atom/ion that departs **with the bonding pair** — here the **halide ion, X⁻** (e.g. Br⁻, Cl⁻).
Question
Product of a halogenoalkane + warm aqueous NaOH?
Answer
An **alcohol** (the –halogen is replaced by –OH), plus a halide ion.
Question
Conditions for OH⁻ substitution?
Answer
**Warm** (gentle heat) and **aqueous** sodium or potassium hydroxide.
Question
What is substitution?
Answer
A reaction in which **one group replaces another** on the carbon skeleton, which is otherwise unchanged.
Question
Nucleophile vs electrophile?
Answer
Nucleophile = electron-pair **donor** (attacks δ+); electrophile = electron-pair **acceptor** (attacks δ−). Opposites.
Question
Which C–halogen bond reacts fastest, and why?
Answer
**C–I** — it is the **weakest** bond, so it breaks most easily. C–F is strongest, so the fluoroalkane is slowest.
Question
Product with cyanide, CN⁻?
Answer
A **nitrile** (–CN) — and the chain gains one carbon atom.
Question
Product with ammonia, NH_{3}?
Answer
An **amine** (–NH_{2}), using excess ammonia.
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Topic 6.4 hub
Electron-pair sharing reactions
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