carboxylic acid

Examples of carboxylic acid in the following topics:

• Carboxylic Acids

  • Carboxylic acids are a class of molecules which are characterized by the presence of one carboxyl group.
  • As proton donors, carboxylic acids are characterized as Brønsted-Lowry acids.
  • Salts and esters of carboxylic acids are called carboxylates.
  • Generally, in IUPAC nomenclature, carboxylic acids have an "-oic acid" suffix, although "-ic acid" is the suffix most commonly used.
  • Upon exposure to a base, the carboxylic acid is deprotonated and forms a carboxylate salt.

• 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.
  • Interestingly, the molecules of most natural fatty acids have an even number of carbon atoms.
  • The following formulas are examples of other naturally occurring carboxylic acids.

• 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 with aldehydes, the carboxyl group must be located at the end of a carbon chain.
  • The characteristic IUPAC suffix for a carboxyl group is "oic acid", and care must be taken not to confuse this systematic nomenclature with the similar common system.
  • Substituted carboxylic acids are named either by the IUPAC system or by common names.
  • Simple dicarboxylic acids having the general formula HO2C–(CH2)n–CO2H (where n = 0 to 5) are known by the common names: Oxalic (n=0), Malonic (n=1), Succinic (n=2), Glutaric (n=3), Adipic (n=4) and Pimelic (n=5) Acids.

• Reactions at the α-Carbon

  • In this section similar reactions of carboxylic acid derivatives will be examined.
  • Acid-catalyzed alpha-chlorination and bromination reactions proceed more slowly with carboxylic acids, esters and nitriles than with ketones.
  • 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.
  • To see a mechanism for the acyl halide-carboxylic acid exchange, view the second diagram below.

• 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.
  • In the table of fatty acids we see that the presence of a cis-double bond significantly lowers the melting point of a compound.
  • Thus, palmitoleic acid melts over 60º lower than palmitic acid, and similar decreases occur for the C18 and C20 compounds.
  • Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds.
  • A structural formula for the dimer of acetic acid is shown in the first diagram below.

• 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.
  • Acids with two or more carboxyl groups are called dicarboxylic, tricarboxylic, etc.
  • Salts and esters of carboxylic acids are called carboxylates.
  • Carboxylate ions are resonance stabilized, and this increased stability makes carboxylic acids more acidic than alcohols.

• Acidity of Carboxylic Acids

  • The pKa 's of some typical carboxylic acids are listed in the following table.
  • When we compare these values with those of comparable alcohols, such as ethanol (pKa = 16) and 2-methyl-2-propanol (pKa = 19), it is clear that carboxylic acids are stronger acids by over ten powers of ten!
  • Furthermore, electronegative substituents near the carboxyl group act to increase the acidity.
  • 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.

• Reductions & Oxidations of Carboxylic Acids

  • Sodium borohydride, NaBH4, does not reduce carboxylic acids; however, hydrogen gas is liberated and salts of the acid are formed.
  • 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.
  • Also, various iodide derivatives may be prepared directly from the corresponding carboxylic acids.

• Amino Acids

  • An amino acid contains an amino group, a carboxyl group, and an R group, and it combines with other amino acids to form polypeptide chains.
  • The name "amino acid" is derived from the amino group and carboxyl-acid-group in their basic structure.
  • When two amino acids are covalently attached by a peptide bond, the carboxyl group of one amino acid and the amino group of the incoming amino acid combine and release a molecule of water.
  • This end is called the N terminal, or the amino terminal, and the other end has a free carboxyl group, also known as the C or carboxyl terminal.
  • The carboxyl group of one amino acid is linked to the amino group of the incoming amino acid.