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. Lead tetraacetate will also oxidize mono-carboxylic acids in a manner similar to reaction 1. Three additional examples of the Hunsdiecker reaction and a proposed mechanism for the transformation will be shown above by clicking on the diagram.
Also, various iodide derivatives may be prepared directly from the corresponding carboxylic acids. A heavy metal carboxylate salt is transformed into an acyl hypohalide by the action of a halogen. The weak oxygen-halogen bond in this intermediate cleaves homolytically when heated or exposed to light, and the resulting carboxy radical decarboxylates to an alkyl or aryl radical.
A chain reaction then repeats these events. Since acyl hypohalites are a source of electrophilic halogen, this reaction takes a different course when double bonds and reactive benzene derivatives are present. In this respect remember the addition of hypohalous reagents to double bonds and the facile bromination of anisole. For a summary of the basic reactions of carboxylic acids Click Here. The following problems review many aspects of carboxylic acid chemistry. The first two questions concern nomenclature, including some carboxylic derivatives.
The third and fourth questions focus on the relative acidity of selected compounds. The fifth asks you to draw the product of a reaction selected from 48 possible combinations of carboxylic acids and reagents.
Derivatives of Carboxylic Acids. Return to Table of Contents. This page is the property of William Reusch. Comments, questions and errors should be sent to whreusch msu.
These pages are provided to the IOCD to assist in capacity building in chemical education. 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. For additional information about substituent effects on the acidity of carboxylic acids Click Here Preparation of Carboxylic Acids Preparation of Carboxylic Acids The carbon atom of a carboxyl group has a high oxidation state. H -substitution.
Carboxylic acids react with the more reactive metals to produce a salt and hydrogen. The reactions are just the same as with acids like hydrochloric acid, except they tend to be rather slower. For example, dilute ethanoic acid reacts with magnesium. The magnesium reacts to produce a colorless solution of magnesium ethanoate, and hydrogen is given off. If you use magnesium ribbon, the reaction is less vigorous than the same reaction with hydrochloric acid, but with magnesium powder, both are so fast that you probably wouldn't notice much difference.
These are simple neutralisation reactions and are just the same as any other reaction in which hydrogen ions from an acid react with hydroxide ions. They are most quickly and easily represented by the equation:.
If you mix dilute ethanoic acid with sodium hydroxide solution, for example, you simply get a colorless solution containing sodium ethanoate. The only sign that a change has happened is that the temperature of the mixture will have increased. This change could well be represented by the ionic equation above, but if you want it, the full equation for this particular reaction is:. In both of these cases, a salt is formed together with carbon dioxide and water. Both are most easily represented by ionic equations.
If you pour some dilute ethanoic acid onto some white sodium carbonate or sodium hydrogencarbonate crystals, there is an immediate fizzing as carbon dioxide is produced. You end up with a colorless solution of sodium ethanoate. There is very little obvious difference in the vigor of these reactions compared with the same reactions with dilute hydrochloric acid. However, you would notice the difference if you used a slower reaction - for example with calcium carbonate in the form of a marble chip.
With ethanoic acid, you would eventually produce a colorless solution of calcium ethanoate. As discussed, carboxylate ion, which is the conjugate base of a carboxylic acid, is stabilized with the help of two equivalent resonance structures.
At the same time, the negative charge is delocalized effectively between the two more electronegative oxygen atoms. Besides, in the case of phenols, a negative charge is delocalized with less electronegative carbon atoms in phenoxide ion and less effective over one oxygen atom. Thus, the carboxylate ion exhibits higher stability than that of phenoxide ion. Thus, carboxylic acids are more acidic than phenols. When carboxylic acids react with metals and the alkalis, they produce carboxylate ions, which only stabilize because of the resonance.
A simple way to understand carboxyl groups is by understanding that electrons withdrawal leads to the carboxyl group's increased acidity, whereas an electron donation leads to the decrease of the carboxyl group's acidity.
The carboxylic acid's acidity further depends on the substituent aryl or alkyl group's nature, which is attached to the carboxyl group. An electron-withdrawing group ensures the effective negative charge delocalization via inductive or resonance effect.
Therefore, the electron-withdrawing groups increase the stability of the conjugate base that is formed. 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.
0コメント