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Chemistry LibreTexts

3.7: Pi Donation

  • Page ID
    195891
  • Some nucleophiles are added to carbonyls, lose a proton to drop a positive charge, and have a lone pair again. If the (former) carbonyl oxygen also has a lone pair, a potential lone-pair/lonepair repulsion problem exists. Partly for this reason, two heteroatoms bonded to one carbon often present an inherently unstable situation. These kinds of species often decompose readily via pi donation.

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    In pi donation, a lone pair on one heteroatom is donated to the carbon shared by both heteroatoms. As a result, the other heteroatom is pushed off the carbon. This event is helped if one of the heteroatoms is already protonated, so that it comes off as a neutral species.

    Exercise \(\PageIndex{1}\)

    Fill in any missing lone pairs, provide curved arrows to show pi donation, and show the resonance structures that result.

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    Answer

    CO13pt1.png

    Exercise \(\PageIndex{2}\)

    Show how each of the following species might break down into two molecules via pi donation.

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    Answer

    CO13pt2.png

    As a result of pi donation, neutral, protic nucleophiles often replace the carbonyl oxygen entirely. In effect, the nucleophile adds twice. The lone pair that is revealed after a deprotonation step adds again to the same carbon, pushing that carbonyl oxygen out of the molecule entirely. In the case of alcohol nucleophiles, a ketal or acetal results. This kind of molecule looks like two ethers that meet at one carbon. A ketal is a "masked" carbonyl; it still contains a carbon with two bonds to oxygen. However, a ketone is no longer an electrophile like a carbonyl compound.

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    Imines also result from pi donation. If an primary amine donates to a carbonyl, it can lose its first proton to reveal a lone pair. Once that lone pair donates, pushing off the carbonyl oxygen, a second proton can be dropped to allow the nitrogen to lose its positive charge. Imines are similar to carbonyls in that they contain a carbon-heteroatom double bond and so they are still good electrophiles.

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    Exercise \(\PageIndex{3}\)

    Fill in the missing reagents and products in the following reaction schemes.

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    Answer

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