Structure and Naming of Carboxylic Acids

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Structure and Naming of Carboxylic Acids

• Carboxylic acids contain a carbonyl attached to a
  hydroxyl group this is called a carboxyl group
• Parent name ends in -oic acid
• Find longest chain containing the carboxyl group
  carbon
• Number Cs starting at carboxyl group carbon
• Locate and number substituents and give full name
• The smallest carboxylic acids are usually named
  by their common names
• The carboxyl group takes precedence over all
  other groups

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Nomenclature

• in these examples, the common name is given in
  parentheses
• an -OH substituent is indicated by the prefix
  hydroxy- an -NH2 substituent by the prefix
  amino-

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Nomenclature

• to name a dicarboxylic acid, add the suffix
  -dioic acid to the name of the parent alkane that
  contains both carboxyl groups
• the numbers of the carboxyl carbons are not
  indicated because they can be only at the ends of
  the chain

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Physical Properties of Carboxylic Acids

• Carboxylic acids are very polar due to both the
  carbonyl group and the hydroxyl group
• Carboxylic acids can H-bond with each other, and
  in fact exist primarily of dimers (two molecules
  held together by H-bonding)
• Because of the above properties, carboxylic acids
  have high boiling points (higher than
  corresponding alcohols)
• Those with less than 5 carbons are soluble in
  water
• - those with more than 5 Cs can be soluble when
  ionized

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carboxylic acids are more soluble in water than
are alcohols, ethers, aldehydes, and ketones of
comparable molecular weight
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Acidity and Salts of Carboxylic Acids

• Carboxylic acids are weak acids (partially ionize
  in water)
• They are stronger acids than alcohols, or even
  phenols, due to the high stability of their
  conjugate bases (resonance)
• Carboxylic acids are neutralized by bases to form
  salts
• Salts of carboxylic acids are useful because they
  are solids at room temperature, and most are
  soluble in water

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Acid-Base Equilibria

• Carboxylic acids like acetic acid is a weak acid,
  and the position of its equilibrium lies very far
  to the left

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Acid Ionization Constants

• when a weak acid, HA, dissolves in water
• the equilibrium constant, Keq, for this
  ionization is
• because water is the solvent and its
  concentration changes very little when we add HA
  to it, we treat H2O as a constant

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Acid Ionization Constants

• because the acid ionization constants for weak
  acids are numbers with negative exponents, we
  commonly express acid strengths as pKa where

• the weaker the acid, the smaller its Ka, but the
  larger its pKa
• values of Ka for most unsubstituted aliphatic and
  aromatic carboxylic acids fall within the range
  10-4 to 10-5 (pKa 4.0 - 5.0)

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Acidity of RCOOH

• substituents of high electronegativity,
  especially -OH, -Cl, and -NH3, near the carboxyl
  group increase the acidity of carboxylic acids
• both dichloroacetic acid and trichloroacetic acid
  are stronger acids than H3PO4 (pKa 2.1)

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Preparation of Carboxylic Acids

• Carboxylic acids can be prepared by oxidation of
  primary alcohols or aldehydes
• Primary alcohols form acids when treated with
  Jones reagent (CrO3/H3O), as well most other
  oxidizing agents Aldehydes can be oxidized to
  carboxylic acids with most oxidizing agents, such
  as Tollensreagent (AgNO3/NH3)
• - alcohols do not react with Tollens

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Reaction With Bases

• All carboxylic acids, whether soluble or
  insoluble in water, react with NaOH, KOH, and
  other strong bases to form water-soluble salts

• they also form water-soluble salts with ammonia
  and amines

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Preparation of Esters from Carboxylic Acids

• A carboxylic acid can be reacted with an alcohol
  to form an ester using an acid catalyst and heat
  (called Fischer esterification)
• - esters have an alkoxy group attached to the
  carbonyl
• There are other types of esterification that we
  wont study
• Fischer esterification is a type of
  intermolecular dehyration
• Because this is a reversible reaction, an excess
  of either the acid or the alcohol is normally
  used to shift the equilibrium towards products
  (sometimes H2O is removed as it forms)

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Mechanism of Fischer Esterification

• First, the carbonyl oxygen is protonated
• Next, the alcohol oxygen attacks the carbonyl
  carbon
• A proton is then transferred from the alkoxy to a
  hydroxyl
• Finally, the proton is removed from the carbonyl
  oxygen, and water is expelled

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Naming Esters

• Parent name ends in -oate
• First name the alkyl group attached to the oxygen
• Follow by the carboxylic acid name converted to
  end in -oate
• For IUPAC use the IUPAC name for both alkyl group
  and acid
• Many small esters are also know by their common
  names

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Physical Properties of Esters

• Because esters cant H-bond with themselves, they
  have lower boiling points than carboxylic acids
  and alcohols
• However, they are fairly polar, and have higher
  boiling points than ethers or hydrocarbons
  (similar to aldehydes and ketones)
• Esters with less than 5 carbons are somewhat
  soluble in water
• - solubility is between that of ethers and
  aldehydes or ketones
• Esters are not as flammable as ethers or
  hydrocarbons, but are similar to alcohols and
  ketones
• Most esters have a pleasant smell and are often
  used as food additives to simulate or enhance
  natural flavors

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Hydrolysis of Esters

• Esters can be hydrolyzed by heating with water
  and an acid catalyst (the reverse of Fischer
  esterification)
• - the hydrolysis is favored by adding an excess
  of water
• Esters can also be hydrolyzed by heating with
  aqueous base (called saponification, this is how
  soap is made)
• - saponification produces a salt of the
  carboxylic acid

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Mechanism of Base Hydrolysis of an Ester

• First, the hydroxyl group attacks the carbonyl
  carbon
• Next, the alkoxide ion is eliminated as the
  carbonyl reforms
• Finally, the alkoxide removes the proton from the
  acid and the resulting carboxylate ion forms a
  salt with the metal ion

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Anhydrides

• The functional group of an anhydride is two
  carbonyl groups bonded to the same oxygen
• the anhydride may be symmetrical (from two
  identical acyl groups), or mixed (from two
  different acyl groups)
• to name an anhydride, drop the word "acid" from
  the name of the carboxylic acid from which the
  anhydride is derived and add the word "anhydride"

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Hydrolysis of Anhydrides

• carboxylic anhydrides, particularly the
  low-molecular- weight ones, react readily with
  water to give two carboxylic acids

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Phosphoric Anhydrides

• the functional group of a phosphoric anhydride is
  two phosphoryl (PO) groups bonded to the same
  oxygen atom

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Phosphoric Esters

• phosphoric acid forms mono-, di-, and
  triphosphoric esters
• in more complex phosphoric esters, it is common
  to name the organic molecule and then indicate
  the presence of the phosphoric ester by either
  the word "phosphate" or the prefix phospho-
• dihydroxyacetone phosphate and pyridoxal
  phosphate are shown as they are ionized at pH
  7.4, the pH of blood plasma