Hybridisation, conjugation and delocalisation in organic molecules · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Hybrids set shape, sharing sets character

Science · Chemistry · ages 18-19
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  1. Why does a conjugated region often favour a coplanar arrangement?

    • Adjacent p orbitals can overlap more effectively
    • All atoms must become sp3 hybridised before they overlap
    • Each single bond must become a separate local pi bond

  2. Each carbon in ethene has three sigma bonds and no lone pair. What is its hybridisation?

    • sp3
    • sp
    • sp2

  3. What is the ideal bond angle at a tetrahedral sp3 carbon?

    • 90 degrees
    • 109.5 degrees
    • 180 degrees

  4. Two valid contributors to the same anion have full octets and equal charge separation. All other factors are comparable. One places negative charge on oxygen, the other on carbon. Which usually contributes more?

    • Negative charge on oxygen
    • Negative charge on carbon
    • Both forms contribute equally

  5. Why is rotation about an amide C-N bond restricted?

    • Its sigma bond must break during every small rotation
    • Delocalisation adds partial C-N double-bond character
    • Nitrogen cannot move whenever any part of the molecule rotates

  6. Each ethyne carbon uses two sp hybrids for its sigma bonds. What local geometry does this predict?

    • Bent near 109.5 degrees
    • Planar near 120 degrees
    • Linear at 180 degrees

  7. A carboxylate ion has two equivalent C-O bonds. Both are shorter than comparable C-O single bonds and longer than comparable C=O double bonds. Which model fits this evidence?

    • Pi electrons are shared across both C-O bonds
    • One C-O bond stays single and the other stays double
    • The ion switches between fixed single and double bonds

  8. A proposed resonance contributor moves a hydrogen nucleus from one atom to another. Why is it not a resonance contributor?

    • Any nucleus may move if the total charge stays the same
    • Only electron placement may change between contributors
    • Hydrogen nuclei may move if every atom retains a full octet
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Hybrids set shape, sharing sets character W1-mt_Anv0MSQ4hq-s1

  1. Adjacent p orbitals can overlap more effectively · Suitable alignment improves overlap between neighbouring p orbitals. The whole molecule need not be planar.
  2. sp2 · Three sigma bonds and no lone pair give an sp2 description, leaving one p orbital for the pi bond.
  3. 109.5 degrees · The ideal tetrahedral angle is about 109.5 degrees. Actual angles can differ in particular molecules.
  4. Negative charge on oxygen · With other factors comparable, oxygen's greater electronegativity favours the contributor with negative charge on oxygen.
  5. Delocalisation adds partial C-N double-bond character · Nitrogen-carbonyl overlap gives the C-N bond partial double-bond character. Rotation reduces this overlap and costs energy.
  6. Linear at 180 degrees · The two sp hybrids point in opposite directions, predicting a linear H-C-C arrangement.
  7. Pi electrons are shared across both C-O bonds · Equivalent intermediate lengths support delocalisation across O-C-O, rather than fixed unequal bonds or switching between contributors.
  8. Only electron placement may change between contributors · Resonance contributors retain the same nuclear framework. Changing hydrogen's attachment describes a different change.
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