Benzene and extended organic reactions (HL)
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Flip to reveal answersHow is each carbon in benzene hybridised?
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All 12 Flashcards — Benzene and extended organic reactions (HL)
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Question
How is each carbon in benzene hybridised?
Answer
**sp²** — three σ bonds in a plane at **120°**, leaving one electron in a **p-orbital** perpendicular to the ring.
Question
What is the shape of a benzene molecule?
Answer
A **planar (flat) regular hexagon** of six carbons, each bonded to one hydrogen.
Question
What is the delocalised π system in benzene?
Answer
Two ring-shaped electron clouds **above and below** the ring, formed by sideways overlap of the six carbon p-orbitals.
Question
What are the relative lengths of the C–C bonds in benzene?
Answer
**All six are equal** (~140 pm) — **intermediate** between a single (~154 pm) and a double bond (~134 pm).
Question
State two pieces of evidence for delocalisation in benzene.
Answer
**(1)** All six C–C bonds are the **same length**. **(2)** The **enthalpy of hydrogenation** is **less negative** than expected for three isolated C=C bonds.
Question
Why is benzene's enthalpy of hydrogenation less negative than predicted?
Answer
Real benzene is **lower in energy (more stable)** than a Kekulé structure — the difference is the **delocalisation (resonance) stabilisation**.
Question
What is the difference between the Kekulé and delocalised models?
Answer
**Kekulé** = alternating single/double bonds (two bond lengths). **Delocalised** = a circle in the hexagon (all bonds equal) — the **accepted** model.
Question
Why does benzene resist addition reactions?
Answer
Addition would **break the delocalised π system** and lose its stability, so benzene avoids it.
Question
What type of reaction does benzene undergo instead of addition?
Answer
**Substitution** — a ring H is replaced (usually with a catalyst) and the **delocalised ring stays intact**.
Question
Which micro covers the benzene substitution mechanism?
Answer
Micro **6.4.3** (electrophilic substitution — the curly-arrow mechanism); 3.2.6 only sets it up qualitatively.
Question
How do you convert an alkene to an alcohol in two steps?
Answer
**(1)** add HX (e.g. HBr) → halogenoalkane; **(2)** warm with aqueous NaOH → alcohol (–X replaced by –OH).
Question
What colour change shows acidified dichromate oxidising an alcohol?
Answer
**Orange → green** (Cr_{2}O_{7}^{2-} is reduced to Cr^{3+}).
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Topic 3.2 hub
Functional groups: classification of organic compounds
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