9701 · 29.3
Shapes of aromatic organic molecules; σ and π bonds flashcards
Revision flashcards for Cambridge 9701 Shapes of aromatic organic molecules; σ and π bonds (syllabus 29.3). Flip, recall, then mark a real past-paper question.
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What is the hybridisation of the carbon atoms in a benzene molecule?
sp² hybridisation. Each carbon uses three sp² hybrid orbitals to form σ bonds and has one unhybridised p-orbital.
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What are the C-C-C and H-C-C bond angles in benzene?
Both are 120°. This is characteristic of the trigonal planar geometry around each sp² hybridised carbon atom.
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Describe the shape of a benzene molecule.
Benzene is a planar, regular hexagonal molecule.
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How are the σ bonds in benzene formed?
Through head-on overlap of orbitals: sp²-sp² overlap between adjacent carbon atoms, and sp²-1s overlap between carbon and hydrogen atoms.
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How is the delocalised π system in benzene formed?
By the sideways overlap of the six unhybridised p-orbitals (one from each carbon) above and below the plane of the ring, forming a continuous electron cloud.
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How many electrons are in benzene's delocalised π system?
There are 6 π electrons, one contributed from each of the six carbon atoms.
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How do C-C bond lengths in benzene provide evidence for delocalisation?
All C-C bond lengths are identical (0.139 nm), which is intermediate between a typical C-C single bond (0.154 nm) and C=C double bond (0.134 nm). This disproves the Kekulé model of alternating single and double bonds.
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What is delocalisation energy?
The additional stability of a molecule due to electron delocalisation. It is the difference in energy between the actual molecule and a hypothetical structure with localised bonds (e.g., benzene vs. cyclohexa-1,3,5-triene).
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Why does benzene typically undergo electrophilic substitution rather than electrophilic addition?
Substitution reactions preserve the very stable delocalised π system. Addition reactions would destroy this system, which would require overcoming the large delocalisation energy.
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What is a common error when drawing the intermediate in the electrophilic substitution of benzene?
Drawing the positive charge delocalised over all six carbons. The positive charge is spread over five carbons; the carbon bonded to the new electrophile is sp³ hybridised and cannot participate in the delocalised system.
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What is the bond order of the carbon-carbon bonds in benzene?
The bond order is 1.5, representing an average of a single and a double bond, consistent with its intermediate bond length and strength.
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Compare the reactivity of benzene with cyclohexene.
Benzene is less reactive than cyclohexene. Cyclohexene readily undergoes electrophilic addition, decolourising bromine water at room temperature. Benzene requires a halogen carrier catalyst for substitution and does not react with bromine water.