What makes carboxylic acid acidic

There is no need to invoke the properties of the anion. Looking at the resonance within the reactant can be used to assess other systems as well. For example, amides are well-recognized to have extensive resonance interaction in the neutral, to the extent that the nitrogen atom adopts a planar geometry and sp 2 hybridization. Yet, the acidity difference between acetamide and ethyl amine is greater than the acidity difference between acetic acid and ethanol.

Therefore, there is no indication that resonance has any role in enhancing the acidity of amides. In contrast, acetone is case where there is no resonance delocalization in the reactant. This is the component that can be attributed to the inductive effect of the carbonyl.

The rest of the enhancement can be attributed to resonance stabilization. The rest of the enhancement can be attributed to resonance stabilization of the anion. In particular, the anion is highly stabilized by the ability to distribute negative charge on to the carbonyl oxygen.

Vitamin C is a relatively strong acid, with a pK a of 4. The aliphatic protons are not very acidic, so ignore them. There are two ways to explain the acidity of Vitamin C. The standard answer for determining the acidity of Vitamin C is to compare the anions obtained by deprotonation. When deprotonation occurs at the 4-position, there is an extra, good resonance structure that can be drawn, and so the charge is delocalized.

It works, and gives the correct prediction for which proton is most acidic. We can also answer the question using the explanation similar to that used for carboxylic acids in this article. Upon polarizing the carbonyl, it is possible to draw resonance delocalizaiton in the neutral acid. By examiniation of the structure on the right, it is immediately apparent that the most acidic position in the molecule is at position 4, where there is extensive positive charge character at the proton.

Siggel, M. Paul G. Wenthold Purdue University. Origins of the Enhanced Acidity of Carboxylic Acids Resonance The common explanation for why carboxylic acids are more acidic than other molecules such as alcohols is that resonance delocalization of charge stabilizes the conjugate base anion relative to the reactant acid. Depending on the nature of the hydrophilic portion these compounds may form monolayers on the water surface or sphere-like clusters, called micelles, in solution.

This reaction class could be termed electrophilic substitution at oxygen , and is defined as follows E is an electrophile. Some examples of this substitution are provided in equations 1 through 4.

If E is a strong electrophile, as in the first equation, it will attack the nucleophilic oxygen of the carboxylic acid directly, giving a positively charged intermediate which then loses a proton. If E is a weak electrophile, such as an alkyl halide, it is necessary to convert the carboxylic acid to the more nucleophilic carboxylate anion to facilitate the substitution. This is the procedure used in reactions 2 and 3. Equation 4 illustrates the use of the reagent diazomethane CH 2 N 2 for the preparation of methyl esters.

This toxic and explosive gas is always used as an ether solution bright yellow in color. The reaction is easily followed by the evolution of nitrogen gas and the disappearance of the reagent's color. This reaction is believed to proceed by the rapid bonding of a strong electrophile to a carboxylate anion.

Alkynes may also serve as electrophiles in substitution reactions of this kind, as illustrated by the synthesis of vinyl acetate from acetylene. Intramolecular carboxyl group additions to alkenes generate cyclic esters known as lactones. Five-membered gamma and six-membered delta lactones are most commonly formed. Electrophilic species such as acids or halogens are necessary initiators of lactonizations. Examples of these reactions will be displayed by clicking the " Other Examples " button.

Reactions in which the hydroxyl group of a carboxylic acid is replaced by another nucleophilic group are important for preparing functional derivatives of carboxylic acids. The alcohols provide a useful reference chemistry against which this class of transformations may be evaluated. In general, the hydroxyl group proved to be a poor leaving group, and virtually all alcohol reactions in which it was lost involved a prior conversion of —OH to a better leaving group.

This has proven to be true for the carboxylic acids as well. Four examples of these hydroxyl substitution reactions are presented by the following equations. In each example, the new bond to the carbonyl group is colored magenta and the nucleophilic atom that has replaced the hydroxyl oxygen is colored green. The hydroxyl moiety is often lost as water, but in reaction 1 the hydrogen is lost as HCl and the oxygen as SO 2.

This reaction parallels a similar transformation of alcohols to alkyl chlorides, although its mechanism is different. Reaction 4 is called esterification , since it is commonly used to convert carboxylic acids to their ester derivatives. Esters may be prepared in many different ways; indeed, equations 1 and 4 in the previous diagram illustrate the formation of tert-butyl and methyl esters respectively. The acid-catalyzed formation of ethyl acetate from acetic acid and ethanol shown here is reversible, with an equilibrium constant near 2.

The reaction can be forced to completion by removing the water as it is formed. This type of esterification is often referred to as Fischer esterification. As expected, the reverse reaction, acid-catalyzed ester hydrolysis , can be carried out by adding excess water.

A thoughtful examination of this reaction 4 leads one to question why it is classified as a hydroxyl substitution rather than a hydrogen substitution. The following equations, in which the hydroxyl oxygen atom of the carboxylic acid is colored red and that of the alcohol is colored blue, illustrate this distinction note that the starting compounds are in the center.

In order to classify this reaction correctly and establish a plausible mechanism, the oxygen atom of the alcohol was isotopically labeled as 18 O colored blue in our equation. Since this oxygen is found in the ester product and not the water, the hydroxyl group of the acid must have been replaced in the substitution.

A mechanism for this general esterification reaction will be displayed on clicking the " Esterification Mechanism " button; also, once the mechanism diagram is displayed, a reaction coordinate for it can be seen by clicking the head of the green " energy diagram " arrow.

Addition-elimination mechanisms of this kind proceed by way of tetrahedral intermediates such as A and B in the mechanism diagram and are common in acyl substitution reactions. Acid catalysis is necessary to increase the electrophilic character of the carboxyl carbon atom, so it will bond more rapidly to the nucleophilic oxygen of the alcohol. Base catalysis is not useful because base converts the acid to its carboxylate anion conjugate base, a species in which the electrophilic character of the carbon is reduced.

Since a tetrahedral intermediate occupies more space than a planar carbonyl group, we would expect the rate of this reaction to be retarded when bulky reactants are used. To test this prediction the esterification of acetic acid was compared with that of 2,2-dimethylpropanoic acid, CH 3 3 CO 2 H. Here the relatively small methyl group of acetic acid is replaced by a larger tert-butyl group, and the bulkier acid reacted fifty times slower than acetic acid.

Increasing the bulk of the alcohol reactant results in a similar rate reduction. The carbon atom of a carboxyl group is in a relatively high oxidation state. One third of the hydride is lost as hydrogen gas, and the initial product consists of metal salts which must be hydrolyzed to generate the alcohol.

These reductions take place by the addition of hydride to the carbonyl carbon, in the same manner noted earlier for aldehydes and ketones.

The resulting salt of a carbonyl hydrate then breaks down to an aldehyde that undergoes further reduction. Diborane, B 2 H 6 , reduces the carboxyl group in a similar fashion. Sodium borohydride, NaBH 4 , 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.

These will be described later. Because it is already in a high oxidation state, further oxidation removes the carboxyl carbon as carbon dioxide. Depending on the reaction conditions, the oxidation state of the remaining organic structure may be higher, lower or unchanged.

The following reactions are all examples of decarboxylation loss of CO 2. In the first, bromine replaces the carboxyl group, so both the carboxyl carbon atom and the remaining organic moiety are oxidized. Silver salts have also been used to initiate this transformation, which is known as the Hunsdiecker reaction. The second reaction is an interesting bis-decarboxylation, in which the atoms of the organic residue retain their original oxidation states.

Whereas, the electron-donating groups destabilize the conjugate base that is formed and thus decrease the acidity of the carboxylic acid. The general trend of acidic strength of carboxylic acid or the order of acidity of carboxylic acids can be represented as follows. We can also call it the order of acidic strength of carboxylic acids.

Commonly, carboxylic acids are identified using their trivial names. They often contain the suffix -ic acid. There also exist the recommended IUPAC names; in this system, carboxylic acids contain an -oic acid suffix. For the nomenclature of complex molecules that contains a carboxylic acid, carboxyl is the considered position as one of the parent chain even if there exist other substituents, like 3-chloro propanoic acid. In an alternate way, it is named either as a "carboxylic acid" or "carboxy" substituent on another parent structure, like 2-carboxy furan.

As an example, the conjugate base of acetic acid is given as acetate. Carbonic acid, which takes place in bicarbonate buffer systems in nature, is not classed as one of the carboxylic acids generally, despite that it contains a moiety that is like a COOH group. Carboxylic acids can be used in the production of pharmaceuticals, polymers, food additives, and solvents. The important industrial carboxylic acids are given as follows. Acetic acid a component of vinegar, a precursor to coatings and solvents ,.

Adipic acid polymers ,. Maleic acid polymers ,.