In this explainer, we will learn how to identify and name carboxylic acids and describe their physical properties.
Carboxylic acids are organic molecules that contain at least one carbon atom that is both double bonded to an oxygen atom () and single bonded to a hydroxy group (). This statement could be reworded to say that carboxylic acids are molecules that contain at least one carboxyl functional group (). The general structure of a carboxylic acid is shown below.
A carboxylic acid is said to be monobasic if it contains a single carboxyl group. The general formula for a monobasic carboxylic acid is . Common monobasic carboxylic acids include formic acid, acetic acid, and butyric acid.
Dibasic carboxylic acids contain two carboxyl functional groups. Oxalic acid is a relatively common dibasic carboxylic acid that is found in green leafy plants like spinach. Succinic acid is another commonly occurring dibasic carboxylic acid that is found in plant and animal tissues.
Citric acid is a six-carbon carboxylic acid that is found in citrus fruits. It can be called a polybasic carboxylic acid because it contains three carboxyl groups. All polybasic acid molecules contain at least two carboxyl groups and each polybasic molecule can release at least two protons () into solution. Some polybasic carboxylic acids will release just two protons into solution and others will release several protons into solution.
Example 1: Determining Which Listed Option Is Not a Monobasic Carboxylic Acid
Which of the following is not an example of a monobasic carboxylic acid?
- Benzoic Acid
- Oxalic Acid
- Butyric Acid
- Acetic Acid
- Formic Acid
Upon analyzing the chemical structures of benzoic acid, butyric acid, acetic acid, and formic acid, we will find they all contain only one carboxyl group and are, therefore, monobasic carboxylic acids. Oxalic acid is the only carboxylic acid listed that contains more than one carboxyl group in its chemical structure. Therefore, the correct answer choice is B.
Up until this point, we have only used common names to describe and reference carboxylic acids. Common names tend to describe how carboxylic acids were originally discovered and prepared. Formic acid was originally derived from ants (Formica) and palmitic acid was originally derived from palm oil.
Carboxylic acids can also be named according to a more systematic naming system. They can be assigned a name using the International Union of Pure and Applied Chemistry (IUPAC) classification system.
Recall that naming alkanes using the IUPAC system consists of first determining the longest carbon chain within the molecule. This longest carbon chain domain is sometimes called the parent chain. The substituents are then assigned a number that shows which part of the parent chain they are bonded with.
The same principle applies to naming carboxylic acids using the IUPAC nomenclature, except the terminal “e” of the alkane name will be replaced with “oic acid” and the numbering of the longest carbon chain always consists of labeling the carbonyl carbon () as the C-1 carbon.
For example, the IUPAC name for the two-carbon alkane is called ethane, so the IUPAC name for the two-carbon carboxylic acid is ethanoic acid. The structure and numbering scheme for ethane and ethanoic acid are shown below.
The table below shows the structure and name of some common carboxylic acids. You will notice that the stem terms indicate how many carbon atoms are contained within the parent-chain domain. The ethan- stem term indicates that the molecule contains two main-chain carbon atoms, and the propan- and butan- stem terms indicate that molecules contain three or four main-chain carbon atoms. All of the molecule names are terminated with the -oic acid suffix to show that there is a carboxyl functional group at the C-1 carbon atom.
It is important to realize here that the propan- and butan- stem terms also indicate that the parent chain contains single carbon–carbon bonds. We have to use slightly different stem terms if we want to indicate that the parent-chain domain contains at least one carbon–carbon double bond. We have to use stem terms that are taken from alkene molecule names such as the propen- or buten- stem terms. The table shows how the propen- stem is used with the -oic acid suffix to show that the parent chain of a carboxylic acid molecule contains three main-chain carbon atoms and one carbon–carbon double bond.
When a carboxylic acid contains a saturated or unsaturated ring, the IUPAC name often contains the “carboxylic acid” term after the IUPAC name of the cyclic compound. This statement can be understood by examining the system in the following table. The system contains a benzene ring that has a single carboxyl group substituent. The systematic IUPAC name for this molecule is benzenecarboxylic acid. The benzene- stem term indicates that the molecule contains an unsaturated benzene ring and the -carboxylic acid suffix indicates that the ring contains a single carboxyl functional group. This molecule tends, however, to be called benzoic acid more than it is called benzenecarboxylic acid. Benzoic acid is its preferred IUPAC name, and preferred IUPAC names are used more often than systematic IUPAC names.
When naming branched carboxylic acids or those containing other substituents, the position of the substituent is indicated by a number that corresponds to the carbon atom along the longest carbon atom chain the substituent resides on.
Example 2: Identifying the Structure of the 3-Methylbutanoic Acid Molecule
Consider the following molecules:
Which of these molecules has the name 3-methylbutanoic acid?
The 3-methylbutanoic acid name contains the butan- stem and the -oic acid suffix. The butan- stem indicates that the longest carbon chain contains four carbon atoms. This rules out molecules B, C, E, and F because they do not contain parent-chain domains that are four carbon atoms long.
The 3-methyl- prefix indicates that the molecule has a methyl group () at the C-3 carbon atom. The C-3 carbon atom will be the carbon atom that is separated from the group by one methylene () group. This statement rules out system D.
Option (A) is the only molecule that contains a carboxyl group, a four-carbon parent-chain domain, and a methyl group at the C-3 carbon atom. We can use these statements to determine that option (A) is the correct answer for this question.
The previous paragraphs discussed how the benzoic acid term is usually used for what should rightly be called benzenecarboxylic acid. There are other aromatic carboxylic acids that have common names that do not match their systematic IUPAC names. The following figure shows the common names of three other aromatic carboxylic acids of special status.
Carboxylic acids can be prepared through the oxidation of either a primary alcohol or an aldehyde. The oxidation is often carried out using any of the following solid chromate salts in the presence of aqueous sulfuric acid (): potassium chromate (), potassium dichromate (), sodium chromate (), or sodium dichromate ().
Utilizing one of the chromate salts and aqueous acid generates chromic acid () in solution, which is the real oxidizing agent. The generic two-step reaction of preparing a carboxylic acid from its corresponding primary alcohol is shown below.
Reaction: Preparation of Carboxylic Acids
Chemists can make acetic acid from ethanol. They can oxidize ethanol to make acetaldehyde and then they can oxidize the acetaldehyde to make acetic acid. Acetic acid can also be produced through a different two-step reaction process. Chemists can first make acetaldehyde through the catalytic hydration of acetylene. They can then oxidize the resulting acetaldehyde molecules to make a desirable acetic acid product. This information is summarized in the following figure.
Some Egyptian chemists use special bacteria to oxidize ethanol molecules and turn them into acetic acid. This process is sometimes called acetification. The rate of acetification reactions depends on the gaseous concentration of oxygen molecules.
The preparation of benzoic acid consists of oxidizing toluene (methylbenzene) in the presence of a vanadium catalyst, like .
The following equation shows a specific example of this generic reaction process. The equation shows how a vanadium pentoxide catalyst can be used to transform toluene into benzoic acid.
We can test for the presence of carboxylic acids using different reagents. We could use litmus paper to test for the presence of a carboxylic acid because carboxylic acids can turn blue litmus paper red. We can also use hydrogen carbonates, or other types of carbonates, to determine if a carboxylic acid is present in a solution. Carboxylic acids produce water, a salt, and carbon dioxide when they react with hydrogen carbonate or other carbonate compounds. Below is an example of a reagent test being performed on acetic acid.
Now that we have discussed how to identify, name, and prepare carboxylic acids, let’s discuss the properties of this class of organic compounds as it pertains to acidity, boiling point, and solubility.
Recall that the strength of an acid is directly related to the stability of its conjugate base. The greater the stability of the conjugate base, the stronger the acid, and vice versa.
Carboxylic acids tend to be more acidic than corresponding alcohols because carboxylate ions are generally more stable than alkoxide ions. The following figure shows the structure of the ethanoate ion that is formed when an ethanoic acid molecule loses a single hydrogen ion.
You will notice that the negative charge is effectively shared between two oxygen atoms. The electrons are delocalized and they are spread out over the negatively charged carboxylate () group. The ethanoate ion is highly stable because it shows resonance and its negative charge is distributed across its two carbon–oxygen bonds.
The next figure shows the structure of the comparative ethoxide ion that is formed when an ethanol molecule loses a single hydrogen ion.
The structure is unstable, and it does not show resonance. The negative charge is effectively locked onto a single oxygen atom because it cannot delocalize across adjacent carbon–hydrogen bonds. It can generally be assumed that any carboxylic acid is more acidic than its corresponding alcohol because the carboxylic acid can form a carboxylate ion and carboxylate ions show resonance. It is imperative to state here, however, that carboxylic acids have higher pH values than mineral acids. Mineral acids are strong acids and they can completely dissociate in solution. Carboxylic acids are weak acids and they only partially dissociate in solution.
It is also important to mention that aromatic carboxylic acids tend to be more acidic than aliphatic carboxylic acids. Aromatic carboxylic acids have relatively high acidic strength because their group is attached to an hybridized carbon atom ring system. The unsaturated ring stabilizes carboxylate ions more effectively than alkyl groups.
Example 3: Understanding Acidity Differences between Alcohols and Carboxylic Acids
Which of the following correctly compares carboxylic acids with alcohols?
- Alcohols are more acidic than carboxylic acids because alcohols can form dimers between their molecules.
- Carboxylic acids are more acidic than alcohols because carboxylate anions are stabilized by resonance, while alkoxides are not.
- Carboxylic acids are more acidic than alcohols because carboxylic acids can form dimers between their molecules.
- Alcohols are more acidic than carboxylic acids because alkoxides are stabilized by resonance, while carboxylate anions are not.
Options A and C suggest that substances are more acidic if they have molecules that form dimers. These statements are wrong. Substances tend to have higher boiling points if they have molecules that can form dimers. Substances are not any more acidic if they have molecules that can form dimers. We can use this line of reasoning to discount options A and C.
Options B and D suggest that the acidity of an organic molecule is linked with the stability of its conjugate base. Option B suggests that carboxylic acids have relatively low pH values because carboxylate anions are stabilized by resonance. Option D suggests that alcohols have relatively low pH values because alkoxide anions are stabilized by resonance. Both statements correctly suggest that organic compounds are stronger acids if they show resonance but option D incorrectly states that alkoxides show resonance and carboxylate ions do not. This is factually incorrect. Carboxylate anions always show resonance because they contain at least one negatively charged carboxylate group (). Alkoxides do not tend to show resonance because they have the structure, where is the organic group. We can use all of these statements to determine that option B is the correct answer for this question.
Intermolecular forces are attractive or repulsive interactions between neighboring molecules or nonbonded atoms. Intermolecular forces are the bridging interactions between molecules, and they determine how soluble one substance is in another. The following figure shows the bridging interactions between water molecules and either ethanol or ethanoic acid molecules.
You will notice that ethanol and ethanoic acid both contain the same number of carbon atoms and they both have a single functional group. Ethanol and ethanoic acid molecules can both form hydrogen bonds with adjacent water molecules.
The ethanoic acid and ethanol molecules can both dissolve in water because they can form hydrogen bonds with polar water molecules. It is important to realize, however, that not all carboxylic acid and alcohol molecules are soluble in water. Small carboxylic acids and alcohol molecules tend to be soluble in water but large carboxylic acids and alcohol molecules tend to be insoluble in water. Carboxylic acids and alcohols tend to be soluble in water if they contain no more than a few carbon atoms.
The following table compares the physical properties of alcohols with corresponding carboxylic acids. The table shows that carboxylic acids and alcohols are insoluble in water if they contain a high number of carbon atoms. It also shows that water solubility values are similar for carboxylic acid and alcohol molecules if they have the same number of carbon atoms
|Formula||Common Name||IUPAC Name||Boiling Point ()||Water Solubility (g /100 g of Water)|
|Formic acid||Methanoic acid||101||Soluble|
|Acetic acid||Ethanoic acid||118||Soluble|
|Propionic acid||Propanoic acid||141||Soluble|
|Butyric acid||Butanoic acid||164||Soluble|
|Valeric acid||Pentanoic acid||185||5.0|
|Caproic acid||Hexanoic acid||205||1.1|
One might assume that corresponding alcohols and carboxylic acids will have similar boiling points because they can both form hydrogen bonds.
However, when two carboxylic acid molecules experience a hydrogen bond between the hydrogen of one carboxylic acid molecule and the carbonyl oxygen of another carboxylic acid molecule, this aligns the other sections of the two molecules to form a second hydrogen bond between the two of them. This creates a dimer, shown in the image below.
The formation of dimers among carboxylic acid molecules strengthens the London dispersion forces within the solution and, therefore, increases the boiling points of carboxylic acids compared to their corresponding alcohol. That is why acetic acid’s boiling point () is much higher than ethanol’s boiling point ().
The boiling point of any one carboxylic acid molecule depends on its size. Large carboxylic acid molecules tend to have high boiling points because they are long and they generate relatively strong London dispersion forces. Small carboxylic acid molecules tend to have lower boiling points because they are small and they generate weaker London dispersion forces.
Example 4: Understanding How the Size of a Carboxylic Acid Affects Its Boiling Point
Which of the following is the correct order of boiling points of carboxylic acids from lowest to highest?
- Ethanoic acid, methanoic acid, propanoic acid, butanoic acid
- Butanoic acid, propanoic acid, ethanoic acid, methanoic acid
- Methanoic acid, propanoic acid, butanoic acid, ethanoic acid
- Methanoic acid, ethanoic acid, butanoic acid, propanoic acid
- Methanoic acid, ethanoic acid, propanoic acid, butanoic acid
The boiling point of any one carboxylic acid molecule depends on its size. Large carboxylic acid molecules tend to have high boiling points because they are long and they generate relatively strong London dispersion forces. Small carboxylic acid molecules tend to have low boiling points because they are small and they generate weaker London dispersion forces.
The correct answer must be the one that orders the carboxylic acids in terms of increasing size and molecular mass. The correct answer must start with methanoic acid and end with butanoic acid. It must also have ethanoic acid listed after methanoic acid and propanoic acid listed before butanoic acid. We can use these statements to determine that option E is the correct answer for this question.
Example 5: Identifying the Properties of Carboxylic Acids
Which of the following is not correct about carboxylic acids?
- They can be detected using sodium carbonate.
- They can be prepared by the oxidation of the corresponding alcohols.
- They have a lower boiling point than alcohols that have the same number of carbon atoms.
- They are more acidic than alcohols that have the same number of carbon atoms.
- They are less acidic than mineral acids.
Carboxylic acids tend to be more acidic than similarly sized alcohol molecules, but they tend to be less acidic than mineral acids. This statement can be used to determine that options D and E are not the correct answer for this question.
Choices A and B are true statements as well and they cannot be the correct answer for this question. Sodium carbonate can be used to detect carboxylic acids and carboxylic acids can be prepared by oxidizing primary alcohols.
Option C is the statement about carboxylic acids that is false. Due to dimerization occurring between identical carboxylic acid molecules, carboxylic acid molecules do not have a lower boiling point but rather have a higher boiling point than their corresponding alcohol molecule. We can state with certainty that option C is the correct answer for this question.
Carboxylic acids have lots of interesting properties and they tend to be used all over the world. Chemical companies use carboxylic acids to make desirable substances, and animals sometimes use carboxylic acids to help them survive and deal with predators. Carboxylic acids are also produced as a waste product during different biological processes.
Red ants use formic acid to stave off predators and to protect their territory. They bite the unwelcome animals with their mandibles and then they use their stinger to inject the animals with formic acid. Chemical companies use formic acid to make commercial liquids and solids. Formic acid is frequently used to make dyes and plastics. It is also used to make perfumes, drugs, and even insecticides.
Acetic acid is a corrosive and pungent acid that is used to make both edible and nonedible substances. Its application depends on its concentration. Pure acetic acid is commonly known as glacial acetic acid. Glacial acetic acid is used as a chemical feedstock to make dyes, insecticides, and synthetic silk products. Acetic acid tends to be called vinegar when it has a concentration that is less than 4% and it is mixed with a large amount of water. Vinegar is used to make sour-tasting food items and tart vinaigrettes.
Lactic acid has the chemical formula and a molar mass of 90.08 g/mol. The structure of lactic acid is shown in the following figure. Lactic acid is found in sour milk because it is produced by some bacteria as they ferment lactose sugars. Lactic acid is also generated in the human body during vigorous exercise.
Ascorbic acid is colloquially known as vitamin C. Ascorbic acid does not contain a typical carboxyl functional group. Its double-bonded oxygen () and hydroxy groups () are not bonded to the same carbon atom. The double-bonded oxygen is bonded to one carbon atom and the hydroxy group is bonded to the adjacent carbon atom. Vitamin C is found in citrus fruits and in most vegetables. The inadequate consumption of vitamin C can cause gingivitis and other related health problems like scurvy.
Benzoic acid is the simplest carboxylic acid molecule that contains a benzene ring. Benzoic acid is found in different types of plants and it is used to produce plastics, dyes, cosmetics, and insect repellants. The closely related sodium benzoate compound is frequently added to food products because it can prevent the growth of fungi.
Salicylic acid is another carboxylic acid molecule that contains a benzene ring. The following figure compares the structure of salicylic acid with the structure of benzoic acid. Salicylic acid is an active ingredient in different over-the-counter acne medications, and its ester derivative is better known as aspirin. The compound is frequently used to make cosmetics because it can make human skin softer and more flexible.
Citric acid is a weak acid that is found in most types of citrus fruits. The molecule has the chemical formula and it can constitute up to 8% of the dry weight of lemons and limes. Citric acid is known to prevent the growth of bacteria in food. Citric acid is sometimes added to frozen fruits to help them retain their color and taste.
Amino acids are biological compounds that contain a carboxyl group on one end and an amino group () on the other end. The importance of amino acids cannot be overstated. They are used to make proteins, and proteins make life possible. Glycine is the simplest amino acid. It has the chemical formula and its structure is shown in the following figure. The figure also shows the general structure of an amino acid because there are twenty different types of amino acids that are found in natural proteins.
All of the carboxylic acid compounds mentioned above, except citric acid, can be classed as being monobasic carboxylic acids because they all contain one carboxyl functional group.
Let’s summarize what has been learned in this explainer.
- A carboxylic acid molecule contains one or more carboxyl groups ( or ).
- Carboxylic acid molecules can have at least one common name and another entirely different IUPAC name.
- Carboxylic acids can be made by oxidizing a primary alcohol or an aldehyde.
- Carboxylic acids tend to be more acidic than similarly sized alcohols and less acidic than mineral acids.
- The conjugate base of a carboxylic acid is stabilized by resonance.
- Carboxylic acids can form tightly interlinked dimers and this helps us to understand why carboxylic acids usually have higher boiling points than corresponding alcohols.
- Long carboxylic acids tend to have higher boiling points than short carboxylic acids because they generate stronger London dispersion forces.
- Short-chain carboxylic acids tend to be more soluble in water than longer carboxylic acids.