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Hemiacetal and Acetal Functional Groups

Hemiacetal and Acetal Functional Groups. Hemiacetal: alcohol and ether on same carbon Acetal: Two ethers on same carbon. Mechanism. Must be acid-catalyzed. Adding H + to carbonyl makes it more reactive with weak nucleophile, ROH.

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Hemiacetal and Acetal Functional Groups

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  1. Hemiacetal and Acetal Functional Groups • Hemiacetal: alcohol and ether on same carbon • Acetal: Two ethers on same carbon

  2. Mechanism • Must be acid-catalyzed. • Adding H+ to carbonyl makes it more reactive with weak nucleophile, ROH. • Hemiacetal forms first, then acid-catalyzed loss of water, then addition of second molecule of ROH forms acetal. • All steps are reversible. =>

  3. Hemiacetal Synthesis: aldehyde + alcohol • Alcohol + aldehyde --> hemiacetal • Ethanal + methanol • Alcohol oxygen becomes an ether • Carbon double bond oxygen becomes an alcohol

  4. => Mechanism for Hemiacetal

  5. Aldehyde and Ketone Reactions

  6. Synthesis of Acetal Functional Group • Acetal: alcohol plus hemiacetal (ether synthesis) • Acetal: Two ethers on same carbon

  7. =>

  8. Aldehyde and Ketone Reactions

  9. => Mechanism Hemiacetal to Acetal

  10. => Cyclic Acetals • Addition of a diol produces a cyclic acetal. • Sugars commonly exist as acetals or hemiacetals.

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