Nucleophilic Addition and the Tetrahedral Intermediate · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Attacking the flat polar carbon

Science · Chemistry · ages 20-21
Name ______________________   Date ____________

  1. A nucleophile attacks an aldehyde and the pi bond lifts onto oxygen. What formed?

    • A tetrahedral alkoxide intermediate
    • A flat carbocation
    • A free radical pair

  2. Why does a nucleophile attack the carbonyl carbon?

    • Because oxygen pushes electrons onto the carbon
    • Because the carbon carries a partial positive charge
    • Because the carbon already holds a full negative charge

  3. Aldehydes and ketones give alcohols because they carry no leaving group.

    Circle one:   True   False

  4. What is the steric reason aldehydes beat ketones?

    • Aldehydes are too crowded for any attack
    • Ketones hold their carbon outside the molecule
    • The second alkyl group on ketones blocks the approach path

  5. What is the electronic reason aldehydes beat ketones?

    • Aldehydes carry two donating alkyl groups
    • Each alkyl group feeds density in, so two donors invite less
    • Alkyl groups pull electrons away from the carbon

  6. How do acid and base catalysis each speed the same addition?

    • Acid protonates the oxygen while base strengthens the nucleophile
    • Acid removes the nucleophile while base removes the carbonyl
    • Both work by heating the mixture directly

  7. A nucleophile attacks a substrate carrying a good leaving group on the carbonyl carbon. What follows?

    • The intermediate takes a proton and stops as an alcohol
    • The intermediate collapses, reforms the carbonyl, and ejects the group
    • The intermediate splits into two free radicals

  8. Cyanide adds to an unsymmetrical ketone. What is the stereochemical outcome at the new centre?

    • Only one mirror form, since attack comes from above
    • Both mirror forms, since either face is equally open
    • No stereocentre forms at all
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Answer key

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Attacking the flat polar carbon W1-mt_N9Eydek3s7-s1

  1. A tetrahedral alkoxide intermediate · Four single bonds around carbon mark the tetrahedral intermediate.
  2. Because the carbon carries a partial positive charge · Oxygen pulls shared electrons away, leaving carbon inviting.
  3. True · With nothing to eject, the intermediate takes a proton instead.
  4. The second alkyl group on ketones blocks the approach path · Bulk near the carbon narrows the path the nucleophile must take.
  5. Each alkyl group feeds density in, so two donors invite less · More donation means a less positive carbon and a slower attack.
  6. Acid protonates the oxygen while base strengthens the nucleophile · Each catalyst wakes a different partner of the same attack.
  7. The intermediate collapses, reforms the carbonyl, and ejects the group · A group able to leave steers the fork toward substitution.
  8. Both mirror forms, since either face is equally open · The flat carbonyl offers two equal faces to the attacker.
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