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Structure and Naming of Carboxylic Acids

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 C’s starting at carboxyl group carbon

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Structure and Naming of Carboxylic Acids

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  1. 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 C’s 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

  2. 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-

  3. 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

  4. 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 C’s can be soluble when ionized

  5. carboxylic acids are more soluble in water than are alcohols, ethers, aldehydes, and ketones of comparable molecular weight

  6. 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

  7. Acid-Base Equilibria • Carboxylic acids like acetic acid is a weak acid, and the position of its equilibrium lies very far to the left

  8. 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

  9. 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)

  10. 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)

  11. 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 Tollens’reagent (AgNO3/NH3) - alcohols do not react with Tollens

  12. 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

  13. 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 won’t 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)

  14. 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

  15. 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

  16. Physical Properties of Esters • Because esters can’t 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

  17. 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

  18. 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

  19. 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"

  20. Hydrolysis of Anhydrides • carboxylic anhydrides, particularly the low-molecular- weight ones, react readily with water to give two carboxylic acids

  21. Phosphoric Anhydrides • the functional group of a phosphoric anhydride is two phosphoryl (P=O) groups bonded to the same oxygen atom

  22. 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

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