Carboxylic Acids and Their Derivatives Help
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.
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