carboxylation

Examples of carboxylation in the following topics:

• Carboxylic Acids

  • Carboxylic acids are a class of molecules which are characterized by the presence of one carboxyl group.
  • Salts and esters of carboxylic acids are called carboxylates.
  • Carboxylate ions are resonance-stabilized.
  • Upon exposure to a base, the carboxylic acid is deprotonated and forms a carboxylate salt.
  • Carboxylic acids are organic oxoacids characterized by the presence of at least one carboxyl group, which has the formula -C(=O)OH, usually written as -COOH or -CO2H.

• Preparation of Carboxylic Acids

  • The carbon atom of a carboxyl group has a high oxidation state.
  • Two other useful procedures for preparing carboxylic acids involve hydrolysis of nitriles and carboxylation of organometallic intermediates.
  • The hydrolysis may be either acid or base-catalyzed, but the latter give a carboxylate salt as the initial product.
  • The initial product is a salt of the carboxylic acid, which must then be released by treatment with strong aqueous acid.
  • An existing carboxylic acid may be elongated by one methylene group, using a homologation procedure called the Arndt-Eistert reaction.

• Nomenclature of Carboxylic Acids

  • The carboxyl functional group that characterizes the carboxylic acids is unusual in that it is composed of two functional groups described earlier in this text.
  • As may be seen in the formula below, the carboxyl group is made up of a hydroxyl group bonded to a carbonyl group.
  • As with aldehydes, the carboxyl group must be located at the end of a carbon chain.
  • Substituted carboxylic acids are named either by the IUPAC system or by common names.
  • In other cases, common names make use of the Greek letter notation for carbon atoms near the carboxyl group.

• Carboxylic Acid Natural Products

  • Carboxylic acids are widespread in nature, often combined with other functional groups.
  • Simple alkyl carboxylic acids, composed of four to ten carbon atoms, are liquids or low melting solids having very unpleasant odors.
  • As shown in the following table, these long-chain carboxylic acids are usually referred to by their common names, which in most cases reflect their sources.
  • The following formulas are examples of other naturally occurring carboxylic acids.

• Reductions & Oxidations of Carboxylic Acids

  • The carbon atom of a carboxyl group is in a relatively high oxidation state.
  • Diborane, B2H6, reduces the carboxyl group in a similar fashion.
  • Partial reduction of carboxylic acids directly to aldehydes is not possible, but such conversions have been achieved in two steps by way of certain carboxyl derivatives.
  • In the first, bromine replaces the carboxyl group, so both the carboxyl carbon atom and the remaining organic moiety are oxidized.
  • Lead tetraacetate will also oxidize mono-carboxylic acids in a manner similar to reaction #1.

• Oxoacids

  • Carboxylic acids are an important subclass of organic oxoacids, characterized by the presence of at least one carboxyl group.
  • Carboxylic acids are the most common type of organic acid.
  • Salts and esters of carboxylic acids are called carboxylates.
  • When a carboxyl group is deprotonated, its conjugate base, a carboxylate anion, forms.
  • Carboxylate ions are resonance stabilized, and this increased stability makes carboxylic acids more acidic than alcohols.

• Reactions at the α-Carbon

  • In this section similar reactions of carboxylic acid derivatives will be examined.
  • This may reflect the smaller equilibrium enol concentrations found in these carboxylic acid derivatives.
  • This difference may be used to facilitate the alpha-halogenation of carboxylic acids.
  • This simple modification works well because carboxylic acids and acyl chlorides exchange functionality as the reaction progresses.
  • To see a mechanism for the acyl halide-carboxylic acid exchange, view the second diagram below.

• Background and Properties

  • If these same functional groups are attached to an acyl group (RCO–) their properties are substantially changed, and they are designated as carboxylic acid derivatives.
  • Carboxylic acids have a hydroxyl group bonded to an acyl group, and their functional derivatives are prepared by replacement of the hydroxyl group with substituents, such as halo, alkoxyl, amino and acyloxy.
  • As noted earlier, the relatively high boiling point of carboxylic acids is due to extensive hydrogen bonded dimerization.

• Acidity of Carboxylic Acids

  • The pKa 's of some typical carboxylic acids are listed in the following table.
  • Furthermore, electronegative substituents near the carboxyl group act to increase the acidity.
  • Both the carboxyl group and the carboxylate anion are stabilized by resonance, but the stabilization of the anion is much greater than that of the neutral function, as shown in the first diagram below.
  • The resonance effect described here is undoubtedly the major contributor to the exceptional acidity of carboxylic acids.
  • In the case of carboxylic acids, if the electrophilic character of the carbonyl carbon is decreased the acidity of the carboxylic acid will also decrease.

• Physical Properties of Carboxylic Acids

  • The table at the beginning of this page gave the melting and boiling points for a homologous group of carboxylic acids having from one to ten carbon atoms.
  • Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds.