In this video, we will learn how to identify and name carboxylic acids and describe their physical properties.
Carboxylic acids are compounds that contain a carboxyl group. A carboxyl group, represented in a condensed formula by COOH, consists of a carbonyl group, a carbon atom double bonded to an oxygen atom, and a hydroxy group. The general formula of a molecule that contains only one carboxyl group is CnH2n+1COOH, where n represents the number of carbon atoms that are not a part of the carboxyl group. If a carboxylic acid contains three carbon atoms, the value of n will be two, as one of the three carbon atoms is part of the carboxyl group. Thus, this carboxylic acid can be represented by the formula C2H5COOH.
Carboxylic acids can be classified by the number of carboxyl groups they contain. Monocarboxylic acids contain one carboxyl group, dicarboxylic acids contain two carboxyl groups, and polycarboxylic acids contain two or more carboxyl groups. Dicarboxylic acids may be considered polycarboxylic acids by this definition. We can also classify carboxylic acids by the number of acidic protons they can donate in an acid–base reaction. Monocarboxylic acids may be considered monobasic. This means that the molecule is able to donate one proton in an acid–base reaction. Dicarboxylic acids may be considered dibasic as they are able to donate two protons in an acid–base reaction. And polycarboxylic acids may be considered polybasic as they are able to donate two or more protons in an acid–base reaction.
Now that we can recognize the structure of carboxylic acids, let’s take a look at the simplest carboxylic acids. These two carboxylic acids look similar. We can differentiate between them by assigning each a name following the rules established by the International Union of Pure and Applied Chemistry, or IUPAC for short. To name a carboxylic acid, we begin by naming the longest continuous chain of carbon atoms that contains the carboxyl carbon. This name consists of a prefix that indicates the number of carbon atoms in the continuous chain, followed by a term that indicates the type of carbon-carbon bonds. The first structure contains one carbon atom, so we name the chain methane, meth- indicating one carbon atom and -ane for alkane.
The second molecule contains two carbon atoms, so we name the chain ethane. Next, we drop the letter “e” from the end of the name and add the suffix -oic acid, indicating that the primary functional group of the molecule is a carboxylic acid. This gives us the IUPAC name methanoic acid for the first molecule and ethanoic acid for the second. If the carboxylic acid structure is more complex and includes alkenes, alkynes, or other substituents, we name any substituents and give position numbers for any alkenes or alkynes in the molecule. When assigning position numbers to the carbon atoms of a carboxylic acid, the carboxyl carbon will be given position number one. So, we can number the carbon chain in each of the example structures by starting with the carbon atom of the carboxyl group.
Following the naming rules, we name the longest continuous chain of carbon atoms in the first molecule butane. Then, we remove the letter “e” and replace it with -oic acid. We see there is a methyl group bonded to the third carbon atom of the chain. We add the substituent name and position number to complete the name of this carboxylic acid, 3-methylbutanoic acid. The second molecule contains an alkene, which begins with the second carbon atom of the chain. Its IUPAC name is but-2-enoic acid. When a carboxylic acid contains a ring structure, the IUPAC name is the name of the cyclic structure, followed by the term carboxylic acid. One notable exception to this rule is the carboxylic acid containing benzene, which is given the name benzoic acid.
While all carboxylic acids can be named following IUPAC rules, many also have common names or trivial names that are used just as often if not more often than the official IUPAC name. Now that we can recognize and name carboxylic acids, let’s take a look at how two specific carboxylic acids are prepared.
Ethanoic acid, also called acetic acid, is an important organic reagent for making plastics and is the main component of vinegar apart from water. It can be prepared in a number of ways. One way to produce ethanoic acid is oxidation of ethanol in the presence of a chromate salt, such as potassium dichromate in aqueous sulfuric acid. In this reaction, ethanol is first oxidized into ethanal and aldehyde. The capital O written in brackets is used to represent the oxygen of the oxidizing agent. The ethanol produced in this step of the reaction rapidly reacts with any additional oxidizing agent in the reaction vessel producing ethanoic acid. Shown here is the overall balanced reaction equation for the oxidation of ethanol into ethanoic acid.
The exact details of how this oxidation reaction works are beyond the scope of this video. But it’s worth noting that both steps of the reaction are occurring in the same initial reaction vessel. And any ethanol produced via the first step will be immediately converted into ethanoic acid. Another way to produce ethanoic acid is to first prepare ethanal via a process other than oxidation. Ethanal is commonly prepared by reacting ethyne, also called acetylene, with water in the presence of sulfuric acid and mercuric sulfate, a catalyst. This is a hydration reaction, a chemical reaction where water is added to a compound. Once the hydration reaction is complete, an oxidation reaction can be performed to oxidize the ethanal into ethanoic acid.
Other industrial methods for producing ethanoic acid include carbonylation, the reaction of methanol and carbon monoxide in the presence of a catalyst, and fermentation of ethanol in atmospheric oxygen using acetic acid bacteria. Now that we’ve looked at the many ways to produce ethanoic acid, let’s look at one of the primary ways to produce benzoic acid.
Benzoic acid is an aromatic carboxylic acid used in the production of plastics. It can be prepared by reacting toluene, also called methylbenzene, with atmospheric air at 400 degrees Celsius in the presence of vanadium pentoxide, which acts as a catalyst. Once a carboxylic acid has been prepared, we can test for its presence to confirm if the preparation was successful. One test we can perform is the litmus test. In this test, a few drops of the sample are placed on blue litmus paper. If the sample contains a carboxylic acid, the blue litmus paper will turn red as carboxylic acids are weak acids. Other functional groups like alcohols, amines, aldehydes, ketones, and esters are not acidic enough to cause this change.
Another test we can perform is the sodium bicarbonate test. Carboxylic acids rapidly react with sodium bicarbonate to produce a carboxylate salt, carbon dioxide gas, and water. If a sample contains a carboxylic acid and sodium bicarbonate is added, a reaction will occur and bubbles of carbon dioxide gas will be produced. Visible effervescence or bubbles when performing the sodium bicarbonate test is an indication that the sample contains a carboxylic acid.
A third test we can perform is the ester test. In this test, the sample is mixed with ethanol and a few drops of sulfuric acid. The mixture is then gently heated. If the sample contains a carboxylic acid, the carboxylic acid will react with the ethanol to produce an ester and water. Esters have a sweet, often floral, or fruity scent. If the resulting mixture of an ester test has a strong sweet smell, the original sample likely contained a carboxylic acid.
Now, let’s examine some of the properties of carboxylic acids. Carboxylic acids like acetic acid are weaker acids than mineral acids like sulfuric acid or hydrochloric acid. But carboxylic acids are stronger acids than comparable alcohols. To understand why acetic acid is a stronger acid than ethanol, we need to examine their conjugate base. Let’s look at the conjugate base of acetic acid acetate.
We would expect the carbon-oxygen double bond to be shorter than the carbon-oxygen single bond. However, both bonds are actually the same length. This is because the lone pair electrons of the charged oxygen atom are shared between the two oxygen atoms. We can represent this phenomenon, known as resonance effect, by using resonance structures or a resonance hybrid. While none of these pictures represent the true structure of acetate, resonance structures and resonance hybrids can help us to see that the charge is delocalized over two oxygen atoms.
Now, let’s have a look at the conjugate base of ethanol, ethoxide. Ethoxide does not exhibit resonance effect, so the charge is only associated with a single atom. As the charge can be delocalized over two atoms in acetate but is localized to a single atom in ethoxide, acetate will be more stable. As carboxylic acids have a more stable conjugate base than alcohols, they will lose their proton more readily and are therefore stronger acids.
While carboxylic acids and alcohols have different acid strengths, they both tend to be soluble in water to a similar degree. This is due to their ability to form strong hydrogen bonds with water molecules. In addition, the solubility of alcohols and carboxylic acids in water tends to decrease as the length of the carbon chain increases. This is because increasing the hydrocarbon chain increases the nonpolar characteristics of the molecule, causing the molecule to be less soluble in water.
As both alcohols and carboxylic acids can form strong hydrogen bonds, we might expect both species to have similar melting and boiling points. But carboxylic acids have higher melting and boiling points than comparable alcohols. To understand why this is the case, we must remember that other intermolecular forces, such as dispersion force, exist between the molecules.
Propan-1-ol and acetic acid have the same molecular mass and a similar number of electrons. So, the strength of the dispersion forces between the molecules should be similar. But two acetic acid molecules can form two hydrogen bonds with one another to produce a dimer. This in effect doubles the mass and number of electrons in the molecule. The more electrons a molecule has, the stronger the dispersion forces between molecules, and more energy will be required to overcome the intermolecular forces. Thus, because they can form dimers, carboxylic acids will have a higher melting and boiling point than alcohols.
We’ve learned a lot about carboxylic acids, but before we sum up this video, let’s take a look at some common carboxylic acid sources and applications. Formic acid, the simplest carboxylic acid, is found in the venom of some bees and ants and is used in the process of tanning leather. Acetic acid, a two-carbon carboxylic acid, is the main component of vinegar and is used in the production of polyvinyl acetate, the primary component of several types of glue. Lactic acid is found in sour milk and yogurt. It is also a product of anaerobic respiration. The buildup of lactic acid when exercising can cause a painful burning sensation in the muscles, signaling that the muscles need recovery.
Benzoic acid and salicylic acid are aromatic carboxylic acids. Benzoic acid is used in the production of plastics as a food preservative and as an antifungal treatment. Salicylic acid is the active ingredient in many acne medications. Citric acid, a polycarboxylic acid, is naturally found in citrus fruits and is used as a food additive in sodas, candies, and cheeses to add a sour taste or balance the pH. Two important biological carboxylic acids are ascorbic acid and amino acids. Ascorbic acid, commonly known as vitamin C, is required for several biological functions. It is naturally found in citrus fruits, red peppers, and other vegetables. Amino acids are molecules that contain an amine and a carboxylic acid. There are 20 amino acids that are the building blocks of proteins.
Now, let’s review what we’ve learned about carboxylic acids. Carboxylic acids are molecules that contain a carboxyl group. The IUPAC name of a carboxylic acid often ends in -oic acid. Common names are frequently used in place of IUPAC names for several compounds. Many carboxylic acids like acetic acid can be prepared via oxidation of a primary alcohol or aldehyde. We can test for the presence of a carboxylic acid using the litmus test, sodium bicarbonate test, or ester test. When compared to alcohols, carboxylic acids tend to be stronger acids as explained by the resonance effect, have similar solubilities as both alcohols and carboxylic acids can form hydrogen bonds with water, and have higher melting and boiling points due to increased dispersion forces.