Question
Alcohols can also be used as ____ groups for other functional groups themselves.
Answer
Alcohols can also be used as protecting groups for other functional groups themselves.
Figure 5.11 (Above): Aldehydes and ketones can be reacted with two equivalents of an alcohol or a diol (dialcohol), forming acetals (primary carbons with two –OR groups and a hydrogen atom) or ketals (secondary carbons with two –OR groups). Carbonyls are very reactive with strong reducing agents like lithium aluminum hydride (LiAlH4). Acetals and ketals, on the other hand, do not react with LiAlH4. The acetal or ketal functionality thereby protects the aldehyde or ketone from reaction. After reducing other functionalities in the molecule, the acetal or ketal can be reverted back to a carbonyl with aqueous acid, a step called deprotection. These reactions are shown in Figure 5.11.
Figure Above: Consider even as simple a target as the dipeptide glycylalanine. Just heating glycine and
alanine to make the peptide bond by dehydration would result in a complex mixture of di-, tri-,
and higher peptides with random sequences. Because the two starting materials can form bonds
either to their own kind or to each other, there is no way to prevent random oligomerization.
Figure Above: To form peptide bonds selectively, the functional groups of the amino acids have to be
protected. The amino end is frequently blocked by a phenylmethoxycarbonyl group (abbreviated
carbobenzoxy or Cbz), introduced by reaction of an amino acid with phenylmethyl chloroformate (benzyl chloroformate).
Figure Above: The amino function is deprotected by hydrogenolysis, which initially furnishes the carbamic acid as a reactive intermediate. Decarboxylation occurs
instantly to restore the free amine.
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Alcohols have the general formula ____.
The functional group ____ is referred to as a hydroxyl group.
The functional group –OH is referred to as a ____ group.
Hydroxyl groups attached to aromatic rings are called ____.
Hydroxyl groups attached to ar____e called phenols.
____ attached to aromatic rings are called phenols.