Carboxylic Acids and Their Derivatives Help (page 2)
Carboxylic acids (RCO2H or ArCO2H) have the structure shown below. Some have names derived from acetic acid; e.g., (CH3)3CCO2H and C6H5CH2CO2H, are trimethylacetic acid and phenylacetic acid, respectively. Occasionally they are named as carboxylic acids, e.g., the compound here is cyclohexanecarboxylic acid.
For IUPAC names, replace the -e of the corresponding alkane with -oic acid: thus, CH3CH2CO2H is propanoic acid. The carbons are numbered; the carbon of CO2H is numbered 1.
Carboxylic Acid Derivatives
The common types of acid derivatives are given in the table, with conventions of nomenclature that involve changes of the name of the corresponding carboxylic acid.
Preparation of Carboxylic Acids
Oxidation of 1° Alcohols, Aldehydes, and Arenes. Carboxylic acids can be prepared by oxidizing primary alcohols or aldehydes with CrO3 in H2SO4 (Jones oxidation). Aromatic carboxylic acids can be made by side chain oxidation of substituted benzenes using KMnO4.
Oxidative Cleavage of Alkenes and Alkynes.
Carboxylic acids can be prepared by oxidative cleavage of alkenes or alkynes using KMnO4 in acid.
Grignard Reagent and CO2. Addition of a Grignard reagent to CO2 followed by acid workup leads to carboxylic acids.
Hydrolysis of Acid Derivatives and Nitriles. Hydrolysis of carboxylic acid derivatives (acid chlorides, esters, amides, anhydrides, and nitriles) using either acidic or basic water produces a carboxylic acid.
Acidity of Carboxylic Acids
The H of CO2H is acidic because RCO2– is delocalized over both oxygens and is more stable and a weaker base than RO–, whose charge is localized on only one oxygen.
RCO2H forms carboxylate salts with bases; when R is a long alkyl chain, these salts are called soaps.
The influence of substituents on acidity is best understood in terms of the conjugate base, RCO2–, and can be summarized as follows. Electron-withdrawing groups stabilize the carboxylate anion, strengthening the acid. Electron-donating groups destabilize RCO2– and weaken the acid.
Like all halogens, Cl is electronegative, electron-withdrawing, and acid-strengthening. Since F is more electronegative than Cl, it is a better withdrawing group and a better acid strengthener.
Inductive effects diminish as the number of C's between Cl and the O's increases. ClCH2CO2H is a stronger acid than ClCH2CH2CO2H, since the chlorine is closer to the negative charge on the anion that it stabilizes. Two Cl's are more electron-withdrawing than one Cl, so Cl2CHCO2H is a stronger acid than ClCH2CO2H.
Reactions of Carboxylic Acids
Acid Chloride Formation. Carboxylic acids give acid chlorides when they are treated with thionyl chloride.
Reaction with SOCl2 is particularly useful because the two gaseous products SO2 and HCl are readily separated from RCOCl.
Ester Formation. Carboxylic acids react with alcohols to give esters. An acid catalyst is required.
In this reaction, the oxygen of the C?O is protonated, which increases the electrophilicity of the carbonyl carbon and renders it more easily attacked in the slow step by the weakly nucleophilic R'OH. The tetrahedral intermediate undergoes a sequence of fast deprotonations and protonations, the end result being the loss of H+ and H2O and the formation of the ester.
Polyfunctional Carboxylic Acids
Dicarboxylic Acids. [HO2C(CH2)n CO2H] The chemistry of dicarboxylic acids depends on the value of n. For n = 1, decarboxylations can occur upon heating the diacid. When n = 2 or 3, the diacid forms cyclic anhydrides when heated. Longer-chain ??α, ω-dicarboxylic acids usually undergo intermolecular dehydration on heating to form long-chain polymeric anhydrides.
Hydroxyacids: Lactones. Reactions of hydroxycarboxylic acids, HO(CH2)nCO2H, also depend on value of n. In acid solutions, γ-hydroxycarboxylic acid (n = 3) and δ-hydroxycarboxylic acid (n = 4) form cyclic esters (lactones) with five-membered and six-membered rings, respectively. Intramolecular nucleophilic displacements, such as those in lactone formation, have faster reaction rates than intermolecular SN2 reactions because the latter require two species to collide.
Reactions of Acid Derivatives
The more reactive derivatives are readily converted to the less reactive ones. Because acetic anhydride reacts less violently, it is often used instead of the more reactive acetyl chloride to make derivatives of acetic acid. Since other anhydrides are not readily available, the acid chlorides are used to make acid derivatives. The reactivity order is acid chlorides > anhydrides > ester > amide.
Nucleophilic substitution of acyl compounds takes place readily if the incoming group (Nu:– or Nu:) is a stronger base than the leaving group (G:–) or if the final product is a resonance-stabilized RCO2–.
The reactions of acid derivatives generally involve nucleophilic attack at the carbonyl carbon. Nucleophilic substitutions of RCOG, such as RCOCl, occur in two steps. The first step (addition) resembles nucleophilic addition to ketones and aldehydes and the second step (elimination) is loss of G, in this case, Cl–.
Reactions of this type can be carried out either in acid or in base. For example, in hydrolysis, the protonation of carbonyl O makes C more electrophilic and hence more reactive toward weakly nucleophilic H2O. Strongly basic –OH readily attacks the carbonyl C. Unlike acid hydrolysis, this reaction is irreversible, because –OH removes H+ from RCO2H to form resonance-stabilized RCO2–.
Acid Chlorides. Acid chlorides are used in Friedel-Craft acylations of benzene rings with AlCl3 catalyst, discussed in Chapter 7. They are also readily converted to other acid derivatives by reaction with the appropriate nucleophile.
Acid Anhydrides. Heating dicarboxylic acids, HO2(CH2)nCO2H (n = 2 or 3), forms cyclic anhydrides by intramolecular dehydration. Intermolecular dehydration of carboxylic acids is used to prepare acetic anhydride, but other anhydrides are not readily formed using this method. Although they are less reactive than acid chlorides, anhydrides resemble acid halides in their reactions. Acid anhydrides can also be used to acylate aromatic rings in electrophilic substitutions.
Esters. Esters react more slowly than acid chlorides or anhydrides. They can be used to prepare amides by reaction with an amine. Reduction of esters with LiAlH4 gives alcohols, as discussed in Chapter 9.
Fats and Oils. Fats and oils are mixtures of esters of glycerol, HOCH2CHOHCH2OH, with acyl groups from carboxylic acids, usually with long carbon chains. These triacylglycerols, also called triglycerides, are types of lipids because they are naturally occurring and soluble only in nonpolar solvents. The acyl groups may be identical, or they may be different.
Amides. Unsubstituted amides may be prepared by careful partial hydrolysis of nitriles. Amides are slowly hydrolyzed under either acidic or basic conditions.
Imides. The hydrogen on N of the imides is acidic because the negative charge on N of the conjugate base is delocalized to each O of the two C=O groups, thereby stabilizing the anion.
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