Which carboxylic acids are more acidic

Carboxylic acids are more acidic than alcohols. But alcohols only reacts with Na. Reson is, carboxylate anion is more stable than alkoxide anion. Carboxylate anion can show resonance structures. Therefore carboxylate anion can spread it's negative charge by carbonyl group.

So that brings a stability to the carboxylate anion and equilibrium goes to the right side. Alkokside anion can't show resonance structures. Therefore stability of alkoxide is low. Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds.

A structural formula for the dimer of acetic acid is shown here. When the mouse pointer passes over the drawing, an electron cloud diagram will appear. The high boiling points of the amides and nitriles are due in large part to strong dipole attractions, supplemented in some cases by hydrogen bonding.

The pK a 's of some typical carboxylic acids are listed in the following table. Furthermore, electronegative substituents near the carboxyl group act to increase the acidity. Why should the presence of a carbonyl group adjacent to a hydroxyl group have such a profound effect on the acidity of the hydroxyl proton?

To answer this question we must return to the nature of acid-base equilibria and the definition of pK a , illustrated by the general equations given below. These relationships were described in an previous section of this text. We know that an equilibrium favors the thermodynamically more stable side, and that the magnitude of the equilibrium constant reflects the energy difference between the components of each side.

In an acid base equilibrium the equilibrium always favors the weaker acid and base these are the more stable components. Water is the standard base used for pK a measurements; consequently, anything that stabilizes the conjugate base A: — of an acid will necessarily make that acid H—A stronger and shift the equilibrium to the right.

Both the carboxyl group and the carboxylate anion are stabilized by resonance, but the stabilization of the anion is much greater than that of the neutral function, as shown in the following diagram. In the carboxylate anion the two contributing structures have equal weight in the hybrid, and the C—O bonds are of equal length between a double and a single bond. This stabilization leads to a markedly increased acidity, as illustrated by the energy diagram displayed by clicking the " Toggle Display " button.

Vinylagous Acids Compounds in which an enolic hydroxyl group is conjugated with a carbonyl group also show enhanced acidity. To see examples of such compounds Click Here. The resonance effect described here is undoubtedly the major contributor to the exceptional acidity of carboxylic acids. However, inductive effects also play a role. For example, alcohols have pK a 's of 16 or greater but their acidity is increased by electron withdrawing substituents on the alkyl group.

The acidic hydrogen is colored red in all examples. Water is less acidic than hydrogen peroxide because hydrogen is less electronegative than oxygen, and the covalent bond joining these atoms is polarized in the manner shown.

Alcohols are slightly less acidic than water, due to the poor electronegativity of carbon, but chloral hydrate, Cl 3 CCH OH 2 , and 2,2,2,-trifluoroethanol are significantly more acidic than water, due to inductive electron withdrawal by the electronegative halogens and the second oxygen in chloral hydrate. In the case of carboxylic acids, if the electrophilic character of the carbonyl carbon is decreased the acidity of the carboxylic acid will also decrease.

Similarly, an increase in its electrophilicity will increase the acidity of the acid. Acetic acid is ten times weaker an acid than formic acid first two entries in the second row , confirming the electron donating character of an alkyl group relative to hydrogen, as noted earlier in a discussion of carbocation stability. Electronegative substituents increase acidity by inductive electron withdrawal. Substituents also influence the acidity of benzoic acid derivatives, but resonance effects compete with inductive effects.

The methoxy group is electron donating and the nitro group is electron withdrawing last three entries in the table of pK a values. For additional information about substituent effects on the acidity of carboxylic acids Click Here. Preparation of Carboxylic Acids. The carbon atom of a carboxyl group has a high oxidation state.

It is not surprising, therefore, that many of the chemical reactions used for their preparation are oxidations.

Such reactions have been discussed in previous sections of this text, and the following diagram summarizes most of these.

To review the previous discussion of any of these reaction classes simply click on the number 1 to 4 or descriptive heading for the group. Two other useful procedures for preparing carboxylic acids involve hydrolysis of nitriles and carboxylation of organometallic intermediates. As shown in the following diagram, both methods begin with an organic halogen compound and the carboxyl group eventually replaces the halogen. Both methods require two steps, but are complementary in that the nitrile intermediate in the first procedure is generated by a S N 2 reaction, in which cyanide anion is a nucleophilic precursor of the carboxyl group.

The hydrolysis may be either acid or base-catalyzed, but the latter give a carboxylate salt as the initial product. In the second procedure the electrophilic halide is first transformed into a strongly nucleophilic metal derivative, and this adds to carbon dioxide an electrophile. The initial product is a salt of the carboxylic acid, which must then be released by treatment with strong aqueous acid.

An existing carboxylic acid may be elongated by one methylene group, using a homologation procedure called the Arndt-Eistert reaction. To learn about this useful method Click Here. Because of their enhanced acidity, carboxylic acids react with bases to form ionic salts, as shown in the following equations.

In the case of alkali metal hydroxides and simple amines or ammonia the resulting salts have pronounced ionic character and are usually soluble in water. Heavy metals such as silver, mercury and lead form salts having more covalent character 3rd example , and the water solubility is reduced, especially for acids composed of four or more carbon atoms.

Carboxylic acids and salts having alkyl chains longer than six carbons exhibit unusual behavior in water due to the presence of both hydrophilic CO 2 and hydrophobic alkyl regions in the same molecule.

Such molecules are termed amphiphilic Gk. 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. 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.

Inductive effects During the last 25 - 30 years, the resonance explanation for the enhanced acidity of carboxylic acids has been questioned. However, carboxylic acids are, in fact, less basic than simple ketones or aldehydes.

Inductive effect of a carbonyl group The pK a s of fluorinated ethanols suggests that the effect of a carbonyl group on the acidity of is about the same as that of a fluorinated-tert-butyl group.

Applications Looking at the resonance within the reactant can be used to assess other systems as well.