carboxylic acid

(noun)

Any of a class of organic compounds containing a carboxyl functional group—a carbon with one double bond to an oxygen and a single bond to another oxygen, which is in turn bonded to a hydrogen.

Related Terms

  • aldehyde
  • alkyl
  • alcohol
  • oxyacid
  • alkane
  • oxoacid
  • leaving group
  • nucleophile

(noun)

Any of a class of organic compounds containing a carboxyl functional group—a carbon with a double bond to an oxygen and a single bond to another oxygen, which is in turn bonded to a hydrogen.

Related Terms

  • aldehyde
  • alkyl
  • alcohol
  • oxyacid
  • alkane
  • oxoacid
  • leaving group
  • nucleophile

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.
  • Fatty Acids

    • Acid or base-catalyzed hydrolysis yields the component fatty acid, some examples of which are given in the following table, together with the alcohol component of the lipid.
    • These long-chain carboxylic acids are generally referred to by their common names, which in most cases reflect their sources.
    • Natural fatty acids may be saturated or unsaturated, and as the following data indicate, the saturated acids have higher melting points than unsaturated acids of corresponding size.
    • The trans-double bond isomer of oleic acid, known as elaidic acid, has a linear shape and a melting point of 45 ºC (32 ºC higher than its cis isomer).
    • Because of their enhanced acidity, carboxylic acids react with bases to form ionic salts, as shown in the following equations.
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