HS Chemistry - Carbonyl Functional Groups

Carboxylic Acids

Overview of The Page

This page will cover:

• What are carboxylic acids?
• How are carboxylic acids named?
• What reactions do carboxylic acids participate in?
• How can the presence of carboxylic acids in a compound be tested for?

Carbonyl functional groups are functional groups that contain a C=O bond $i.e. there is at least one instance of a Carbon atom double-bonded to an Oxygen atom$. This does not mean, however, that all C=O bonds are carbonyl bonds. CO₂, for example, has the structural formula O=C=O, but the C=O bonds are not considered carbonyl because it is not an organic compound. Carboxylic acids are a type of carbonyl functional group, and this subpage will look at them.

Carboxylic acids are not too different from aldehydes. In aldehydes, the C=O is bonded to one Hydrogen atom and one other non-Hydrogen atom $i.e. it is at the end of the organic molecule's main chain$. In carboxylic acids, the C=O is bonded to one hydroxyl group and one other non-Hydrogen atom. Therefore, carboxylic acid groups are also found at the end of the organic molecule's main chain.

The formula for the carboxylic acid functional group is RCOOH, where R represents the rest of the organic molecule. To form the name for a carboxylic acid, we use the same naming convention as we do for other organic molecules. The suffix ending for carboxylic acids is "-oic acid", but the way it's added is different. Instead of removing the entire suffix, we take the suffix of the hydrocarbon and add "-oic acid" to the end. Thus, if we have the following molecule:

Where there is both an alkene functional group and an carboxylic acid group, we take the alkene suffix ("-ene") and add "-oic acid" to the end after removing the "e" at the end (giving us "-enoic acid"). Thus, the molecule shown above is 2-butenoic acid, or but-2-enoic acid.

Following that same logic, this:

Is pentanoic acid. Although it is preferable to keep all the Carbon atoms in the main chain in a single line if they are single-\bonded to each other $and not double\-bonded to each other like in alkenes$:

As you can see, the bond angles for the carboxylic acid functional group are still retained, but the carboxylic acid functional group has been rotated when drawing the molecule's displayed formula so that the single-bonded Carbon atoms in the main chain are all on the same line. It is not necessary to keep the bond angles, however, and it is acceptable to draw the bonds at perpendicular angles to each other, like this:

It is important to note, however, that this does not change the bond angles between the atoms - it merely makes it easier to draw the molecule.

It should also be noted that an unintended limitation of our diagram above is that it may seem like the Oxygen atom is closer to the Carbon atom than the other Hydrogen atoms are to their Carbon atoms. That is not what the diagram is trying to depict.

The skeletal formula of pentanoic acid would be:

You may be wondering why we didn't number the organic molecule pentanoic acid as pentan-1-oic acid. After all, we usually number the position of the functional group to indicate where it is in the organic molecule. However, since carboxylic acids must be present at the end of the main chain of an organic molecule, it is not necessary to note their position in the organic molecule with a number. Additionally, when numbering an organic molecule, numbering starts from the end with the carboxylic acids functional group.

This raises an important question. What if we have a displayed formula of an organic molecule that shows the carboxylic acid functional group as a side chain?

In this organic molecule, not only is the carboxylic acid functional group shown as a side chain, but the longest chain of Carbon atoms is the chain that doesn't include the carboxylic acid group:

However, whenever there is a type of carbonyl group present in an organic compound, it takes precedence over the Carbon-Carbon double bonds and the Carbon-Carbon single bonds. Additionally, carboxylic acids have the highest precedence of any functional group.

What does it mean for the carboxylic acid functional group to take precedence? It means:

1. The main chain of the organic compound is the longest Carbon chain that includes the carboxylic acid.

2. When we number the Carbon atoms in the main chain to determine positions, the numbering must start at the carboxylic acid. The Carbon atom that is part of the aldehyde functional group must be labeled #1.

3. Even if the organic compound includes an alcohol or alkene or some other functional group, the compound will be classified as a carboxylic acid.

Thus, the main chain for the organic molecule shown above is:

In this case, since Carbon-Carbon single bonds are rotatable, the molecule can be re-drawn as:

Now its clear that the propyl group is the side chain, and we can number and name the molecule accordingly. The main chain is numbered in blue, and the side chain is numbered in red:

The organic molecule is 2-propylpentanoic acid.

Even if the side chain included an alkene functional group, like in the example below:

The main chain would still be the longest Carbon chain that includes the carboxylic acid, because the carboxylic functional group takes precedence over the alkene functional group $as well as all other functional groups$.

However, if there are multiple possible main chains that include carboxylic acid groups, then there are a few criteria for determining which chain should be used as the main chain:

1. If there is a chain that includes two carboxylic acid groups (on both ends), it will be used as the main chain over a chain that only has one carboxylic acid group.

2. If there are no possible main chains with two carboxylic acid groups, then the chain with one carboxylic acid group that has the most Carbon atoms will be used as the main chain.

When there are two carboxylic acid functional groups present in a molecule, the molecule is called a dicarboxylic acid. Dicarboxylic acids can be treated as carboxylic acids, except that when naming them, the suffix is "-dioic acid" rather than "-oic acid". Naming an organic compound with more than two carboxylic acid functional groups is beyond the scope of this page.

There are also organic molecules with both hydroxyl $OH$ groups and carboxylic acid groups. In these organic molecules, the carboxylic acid group takes precedence over the hydroxyl group, and the molecule is classified as an carboxylic acid. Thus, the suffix is "-oic acid" $or "\-dioic acid" if it is a dicarboxylic acid$. The hydroxyl group is added to the prefix of the organic molecule with the prefix "n-hydroxy-", where n is a number noting the hydroxyl group's position. Again, since carboxylic acid take precedence over hydroxyl groups, the numbering starts from the end with the carboxylic acid group.

Thus, to name the following organic molecule:

We would number the main chain:

And the molecule would be named 6-hydroxyhexanoic acid.

Since the "hydroxy" part is a prefix, it follows the same rules as other prefixes - namely, that we have to indicate how many hydroxyl groups there are, and their positions, with the same rules that we have for other prefixes.

Thus, the following molecule:

Would be named 5,6-dihydroxyhexanoic acid. Organic molecules with both halogenoalkane and carboxylic acid groups are also treated the same way $and so are organic molecules with all three\!$. So are organic molecules with ketone and carboxylic acid groups.

Longer suffixes are used when a carboxylic acid group is attached to a cycloalkane or arene. When naming carboxylic acids attached to a cycloalkane or arene, the suffix "-carboxylic acid" is used, with a prefix like "di" or "tri" appended to the beginning of the suffix to indicate the number of carboxylic acid groups present. The numbering starts from the carboxylic acid group $as it has the highest precedence$.

Thus, the following molecule:

Would be named 1,2-cyclobutanedicarboxylic acid.

Carboxylic acids can also be formed from, and participate in, certain reactions.

Carboxylic Acid Reactions

• Forming Carboxylic Acids Through Oxidation:

  • Carboxylic acids can be formed by oxidizing an aldehyde using hot, concentrated acidified potassium permanganate $KMnO~4~$ solution or hot, concentrated acidified potassium dichromate $K~2~Cr~2~O~7~$ solution. There are two ways to attain the aldehyde if it is not present at hand $covered in [Aldehydes](2-Aldehydes.md)$.
  • Unlike when forming an aldehyde through oxidation, when carboxylic acids are formed through oxidation, the reaction is carried out in a reflux vessel. That way, we ensure that the alkene/primary alcohol/aldehyde remains in the vessel and keeps getting oxidized until a carboxylic acid is formed. If we start off with an alkene/primary alcohol and first oxidize that into an aldehyde before attaining the carboxylic acid, the aldehyde is not distilled off into a separate vessel when it is formed, but is kept in the same vessel to allow it to be oxidized into a carboxylic acid.
  • In these reactions, if KMnO₄ is used as the oxidizing agent, the KMnO₄ solution will lose its purple color and become colorless. If K₂Cr₂O₇ is used as the oxidizing agent, the K₂Cr₂O₇ solution will turn from an orange color to a green color.

  • When writing the oxidation reaction, we usually just show the Oxygen atoms from the oxidizing agent, rather than the whole oxidizing agent. We represent those Oxygen atoms as [O]. For example, the oxidation of 1-butanol, a primary alcohol, to the aldehyde butanal can be represented as:

    • CH₃CH₂CH₂CH₂OH + [O] → CH₃CH₂CH₂CHO + H₂O

  • And the oxidation of butanal to butanoic acid can be represented as:

    • CH₃CH₂CH₂CHO + [O] → CH₃CH₂CH₂COOH

  • Thus, the oxidation of 1-butanol, a primary alcohol, to butanoic acid can be represented as:

    • CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O

  • Here, the primary alcohol 1-butanol is oxidized to yield butanoic acid and water.

• Forming Carboxylic Acids Through Hydrolysis:

  • There are two types of substances that yield a carboxylic acid after undergoing hydrolysis (breakdown of the compound by water):

    1. Nitriles
    1. Esters

  • A carboxylic acid can be formed when a nitrile undergoes hydrolysis with a slightly acidic or basic solution under reflux. The water molecule and dissociated Hydrogen ion in the solution react with the nitrile to form a carboxylic acid and an ammonium ion. If, for example, we had ethanenitrile $CH~3~CN$, the full reaction would be:

    • CH₃CN + 2H₂O + H⁺ → CH₃COOH + NH₄⁺
    • Here, ethane nitrile undergoes hydrolysis to yield ethanoic acid.

  • The reaction can also work with other nitriles that contain other functional groups as well. For example, if we used 4-bromobutanenitrile $CH~2~BrCH~2~CH~2~CN$:

    

    The full reaction would be:

    • CH₂BrCH₂CH₂CN + 2H₂O + H⁺ → CH₂BrCH₂CH₂COOH + NH₄⁺

    

    The only requirement is that the organic molecule reactant is a nitrile. Most of the molecule is left unchanged; only the functional group at the end of the molecule has changed, from a nitrile group to a carboxylic acid group.

    Esters can also undergo hydrolysis with a slightly acidic solution under reflux to produce a carboxylic acid. This is covered more in Esters, but essentially the water molecule reacts with the ester to yield an alcohol and a carboxylic acid. H+ ions act as catalysts for the reaction, which is why the solution must be slightly acidic. This is usually accomplished by adding an acid.

  • Reducing Carboxylic Acids:

    • Given that carboxylic acids can be formed through oxidation, they can also be reduced back when reacted with a reducing agent. Reducing a carboxylic acid will yield a primary alcohol. The reaction depends on the reducing agent used:

    • If lithium tetrahydridoaluminate $LiAlH~4~, also known as lithium aluminum hydride$ is the reducing agent, the LiAlH₄ must be dissolved in a dry ether at room temperature before being added to the carboxylic acid. The product will be a primary alcohol.

    1. Sodium borohydride $NaBH~4~$ will not reduce carboxylic acids, as it is too weak of a reducing agent to do so.

    2. When writing the reduction reaction, we usually just show the Hydrogen atoms from the reducing agent, rather than the whole reducing agent. We represent those Hydrogen atoms as [H]. For example, the reduction of butanoic acid can be represented as:

       • CH₃CH₂CH₂COOH + 4[H] → CH₃CH₂CH₂CH₂OH + H₂O

       Here, butanoic acid is reduced to yield water and the primary alcohol 1-butanol.

  • Esterification:

    • Esterification is the process of forming an ester molecule from a carboxylic acid and an alcohol. This is covered more in Esters.

  • Additionally, since carboxylic acids are a type of acid, they can participate in the same reactions that normal acids can. They can react with a base in a neutralization reaction, with carbonates to form H₂O, CO₂, and a carboxylate salt, and with reactive metals to form a carboxylate salt and H₂ gas.

Testing for Carboxylic Acids

There is no specific test for carboxylic acids in the way that there are tests for aldehydes and ketones. However, since the carboxylic acid group is a weak acid, it will react with a base. Thus, if an organic compound that is known to have a carbonyl $C\=O$ reacts with a base, it is a carboxylic acid.

When the carboxylic acid reacts with the base, a neutralization reaction occurs. it will produce an ionic compound with a carboxylate $COO^\-^$ anion. This ionic compound is a carboxylate salt. Carboxylate salts follow the same naming pattern as carboxylic acids, except that the "-oate" suffix is used instead of the "-oic acid" suffix, and the cation's name must come first. For example, the reaction of pentanoic acid with potassium hydroxide will produce potassium pentanoate and water:

CH₃CH₂CH₂CH₂COOH + KOH → CH₃CH₂CH₂CH₂COO^-K⁺ + H₂O

In this reaction as well, most of the organic molecule is left unchanged. The Hydrogen atom from the carboxylic acid group in pentanoic acid substitutes for the Potassium atom from the KOH, forming sodium pentanoate and water.