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Dehydration of Alcohols to form Ethers

Dehydration of Alcohols to form Ethers. Simple, symmetrical ethers can be formed from the intermolecular acid-catalyzed dehydration of 1° (or methyl) alcohols (a “substitution reaction”) 2° and 3° alcohols can’t be used because they eliminate (intramolecular dehydration) to form alkenes.

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Dehydration of Alcohols to form Ethers

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  1. Dehydration of Alcohols to form Ethers • Simple, symmetrical ethers can be formed from the intermolecular acid-catalyzed dehydration of 1° (or methyl) alcohols (a “substitution reaction”) • 2° and 3° alcohols can’t be used because they eliminate (intramolecular dehydration) to form alkenes • Unsymmetrical ethers can’t be made this way because a mixture of products results:

  2. Mechanism of Formation of Ethers from Alcohols • First, an alcohol is protonated by H3O+ • Next, H2O is displaced by another alcohol (substitution) • Finally, a proton is removed by H2O to form the product

  3. Oxidation of Alcohols • Recall that oxidation is a loss of electrons and reduction is a gain of electrons • However, red-ox does not always involve ions • Oxidation can also be defined as a gain in bonds to oxygen • This is because O is more electronegative than all other elements (besides F), so it removes electron density from any element with which it forms a covalent bond • Primary alcohols can be oxidized to aldehydes and carboxylic acids and secondary alcohols can be oxidized to ketones

  4. Oxidation • Oxidation of a 1° alcohol gives an aldehyde or a carboxylic acid, depending on the experimental conditions • it is often possible to stop the oxidation at the aldehyde stage by distilling the mixture; the aldehyde usually has a lower boiling point than either the 1° alcohol or the carboxylic acid or by using a weaker oxidizing agent like chromic acid

  5. Oxidation • oxidation of a 2° alcohol gives a ketone • tertiary alcohols are resistant to oxidation

  6. Oxidation of Thiols • Oxidation can also be defined as a loss of bonds to hydrogen • This is because H is less electronegative than all other nonmetals (besides P which is the same), so adds electron density to any element with which it forms a covalent bond • Thiols can be oxidized to disulfides using I2 (or Br2) • In proteins, disulfide bonds between sulfur-containing amino acids (cysteines) are a major factor in preserving their shape

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