Electrophilic Addition Revisited: Bridged Ions and Stereospecificity · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Bridges over the double bond

Science · Chemistry · ages 19-21
Name ______________________   Date ____________

  1. What intermediate forms when bromine adds to a double bond?

    • An open carbocation
    • A free carbon radical
    • A three membered bromonium ring with shared charge

  2. Why does the proton add to the end that gives the stabler cation?

    • Stabler cations form faster and fix the regiochemistry after
    • Protons always add to the more substituted carbon
    • Cations never form during addition

  3. Additions through a bromonium ion give anti addition.

    Circle one:   True   False

  4. Why can a bromonium intermediate never rearrange?

    • Rearrangements are forbidden in all chemistry
    • No open cation exists to shift, since charge stays shared in the ring
    • Bromine atoms are too heavy to move

  5. Cyclohexene is treated with bromine water. Where do the groups land?

    • Bromine on the more substituted carbon, hydroxyl on the less
    • Bromine on the less substituted carbon, hydroxyl on the more, trans
    • Both groups on the same carbon

  6. HBr adds to an alkene in the presence of peroxides and bromine lands on the less substituted carbon. Why?

    • A bromine radical adds first through the stablest carbon radical
    • The carbocation rearranged before capture
    • Peroxides block all addition

  7. A student predicts syn addition for bromine water on cyclohexene. What is the error?

    • The bridge blocks one face, so opening must come from the back
    • Syn addition always wins for rings
    • Water never acts as a nucleophile

  8. An addition product shows a shifted skeleton. A student blames a bromonium bridge. Why is that wrong?

    • Bridges always rearrange
    • Rearranged skeletons fingerprint open cations, which bridges forbid
    • Skeletons never shift in any addition
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Answer key

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

Bridges over the double bond W1-mt_sbsBxFJyZy-s1

  1. A three membered bromonium ring with shared charge · The second bromine stays bound and bridges both carbons.
  2. Stabler cations form faster and fix the regiochemistry after · One choice of cation decides where everything lands.
  3. True · The ring opens from the back, so groups finish trans.
  4. No open cation exists to shift, since charge stays shared in the ring · Shifts need an open cation, and the bridge forbids one.
  5. Bromine on the less substituted carbon, hydroxyl on the more, trans · Water opens the more substituted end from the back.
  6. A bromine radical adds first through the stablest carbon radical · Radical stability, not cation stability, rules here.
  7. The bridge blocks one face, so opening must come from the back · Back opening forces the groups trans.
  8. Rearranged skeletons fingerprint open cations, which bridges forbid · Only an open cation can shift before capture.
Worksheet · LightMySky