Vinegars are a common kitchen ingredient used to add flavor to food. They can be a malt vinegar for your chips or a balsamic vinegar for your salad dressing. Did you know that vinegars are made up of 5-8 percent acetic acid? Acetic acid is a type of carboxylic acid that gives vinegar its sharp taste and low pH. Carboxylic acids are organic molecules with a carboxyl functional group. They are quite simple to make, just leave a bottle of apple cider out in the sun and naturally occurring Acetobacter bacteria will start turning the ethanol present into acetic acid. In this article, we'll explore more about carboxylic acids and their properties, such as their nomenclature and bonding. You'll have a chance to practice naming and drawing different carboxylic acids, and apply your knowledge of bonding to explain some of the properties of these molecules.

What is the carboxyl group?

The carboxyl group is made up of two other functional groups: the hydroxyl group found in alcohols, , and the carbonyl group found in aldehyde and ketones, . This gives carboxylic acids the general formula. Left: The general structure of a carboxylic acid, shown with the carbonyl group circled in red, and the hydroxyl group circled in blue. Right:

Let's take a closer look at ethanoic acid. As we know, a carbon atom can only form four covalent bonds because it has just four outer shell electrons. The carboxyl functional group takes up three of these electrons, leaving just one electron remaining to form a bond with another atom. This means that the carbon atom in the carboxyl group can only bond with one other organic group, such as an R group or a hydrogen atom. As a result, the carboxylic acid functional group must always be at the end of a hydrocarbon chain. In ethanoic acid, two of the carbon's outer shell electrons form a double bond with an oxygen atom, and the remaining electron joins with the hydroxyl group. This gives ethanoic acid its unique properties and structure.

Examples of carboxylic acids

Carboxylic acids range from simple molecules like methanoic acid, which has just one carbon atom, to complex molecules that are tens of carbon atoms long. Below, you'll find a table giving both the common and IUPAC names of some of the smaller carboxylic acids. All amino acids are carboxylic acids, from the smallest amino acid, glycine, to the largest, tryptophan. Fatty acids are carboxylic acids as well. You might have heard of omega 3 and omega 6, two essential nutrients. They're both fatty acids; therefore, they are carboxylic acids.

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By looking at the common names of carboxylic acids, you can take a guess as to where they come from. The Latin word capra means goat, so caproic acid is found in goat fat. Myristic acid, a carboxylic acid with 14 carbon atoms, comes from nutmeg - an aromatic spice in the family Myristica.

Naming carboxylic acids

By now, you should be familiar with naming organic molecules. If you need a refresher, check out our article on Organic Compounds. Naming carboxylic acids is also straightforward. They are given the suffix -oic acid and the carbon that is part of the -COOH functional group is counted as carbon 1 in the carbon chain. Functional groups and side chains are shown using prefixes and numbers to indicate their position on the chain. The tables below provide a quick reference for root names and prefixes used in naming molecules. Let's look at some examples. Ethanoic acid is a simple molecule with a two-carbon chain and no other functional groups. However, the next molecule we'll look at is a bit more complex.

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Its carbon chain is three atoms long, so we know it takes the root name   -prop-. It also contains a chlorine atom. We therefore need to use the prefix chloro-. Remember that we count the carbon atom that is part of the carboxyl group as carbon 1, so in this case, the chlorine atom is attached to carbon 2. We call this molecule 2-chloropropanoic acid.

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Making carboxylic acids

At the start of this article, we mentioned how, if you leave cider out in the sun, it will eventually turn into vinegar. Cider is an alcohol. In this reaction, it is oxidised into first an aldehyde and then a carboxylic acid. In the lab, we achieve this by heating a primary alcohol under reflux with an oxidising agent such as acidified potassium dichromate. Reflux prevents the aldehyde first formed from evaporating off before it can react further into a carboxylic acid.

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An example of a reaction that produces carboxylic acids is the oxidation of primary alcohols. Reacting ethanol with acidified potassium dichromate produces ethanal, which can be further oxidized to ethanoic acid. Oxidizing butanol gives butanoic acid. The use of a primary alcohol is essential, as secondary alcohols produce ketones when oxidized, and tertiary alcohols cannot be oxidized at all due to the energetically unfavorable nature of breaking a strong CC bond.

Interestingly, vinegar can be made from any type of alcohol through a process of oxidation. Diluting the alcohol to 10% abv and mixing it with Acetobacter, such as a live vinegar containing a culture of bacteria, can produce a unique and flavorful vinegar after a few months of fermentation. The resulting vinegar's flavor will depend on the type of alcohol used, such as malt vinegar from beer or wine vinegar from white wine.

Properties of carboxylic acids

Take a closer look at the group. As we know, it contains both the carbonyl functional group,   , and the hydroxyl functional group,   . Let's draw them out.

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If we look at a table of electronegativities, we can see that oxygen is a lot more electronegative than both carbon and hydrogen.

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What does that mean? Well, electronegativity is an atom's ability to attract a shared or bonding pair of electrons towards itself. In this case, both of the oxygen atoms pull on the electrons they use to bond to the other carbon and hydrogen atoms, tugging the electrons closer to themselves. This makes the two oxygen atoms partially negatively charged and leaves the carbon and hydrogen atoms partially positively charged. The bonds are now polar. We label them using the delta symbol, δ.

In fact, oxygen and hydrogen have such different electronegativities that carboxylic acids can form hydrogen bonds. Let's quickly recap them.

In an OH bond, the oxygen atom attracts the shared pair of electrons towards itself quite strongly. This leaves the hydrogen atom with a partial positive charge. Because the hydrogen atom is so small, the charge is densely concentrated. The hydrogen atom is attracted to one of the lone pairs of electrons on an oxygen atom belonging to a neighbouring molecule. This is a hydrogen bond. They are relatively strong, and these influence the properties of carboxylic acids.

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Check out Intermolecular Forces for a more in-depth explanation about hydrogen bonds.

Melting and boiling points

Carboxylic acids have higher melting and boiling points than similar alkanes and aldehydes due to the presence of hydrogen bonds between molecules. These hydrogen bonds are much stronger than the intermolecular forces found in alkanes and aldehydes, resulting in higher melting and boiling points for carboxylic acids.

Interestingly, carboxylic acids also have higher melting points than similar alcohols, despite both forming hydrogen bonds. This is because carboxylic acids can form dimers, which are molecules made up of two carboxylic acid molecules joined together. These dimers experience much stronger van der Waals forces than individual carboxylic acid molecules, resulting in higher melting points for carboxylic acids.

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Solubility

Carboxylic acids can also form hydrogen bonds with water. This makes shorter chain carboxylic acids soluble in aqueous solutions. However, longer chain molecules are insoluble because their non-polar hydrocarbon chains get in the way of the hydrogen bonding, breaking the bonds up. Imagine using a magnet to pick up iron filings. If you put something in between the magnet and the filings, such as a block of wood, you won't be able to pick as many up - the strength of the attraction has decreased.

Acidity

Carboxylic acids are weak acids, meaning that they only partially dissociate in solution. This is because the equilibrium between the dissociated hydrogen ion and the carboxylate ion lies well to the left due to the weak nature of the acid. As a result, carboxylic acids have a pH below 7 and take part in typical acid-base reactions.

As acids, carboxylic acids can donate protons to bases. For example, they can react with strong bases like sodium hydroxide to form salts and water. They can also react with weaker bases like ammonia to form amides. In addition to acid-base reactions, carboxylic acids can undergo other types of reactions. For instance, they can undergo esterification reactions with alcohols to form esters. These reactions are important in the production of fragrances, flavors, and plastics.

Reactions of carboxylic acids

Carboxylic acids react in multiple ways, thanks to their polar   group. Some examples include: You can see many of these in more detail in Reactions of Carboxylic Acids.

Testing for carboxylic acids

Testing for carboxylic acids involves their characteristic reaction with carbonates to form a salt, water, and carbon dioxide gas. This reaction is a result of the acidic nature of carboxylic acids, and most other organic molecules won't react at all. The reaction can be observed by the bubbling up of carbon dioxide gas in the test tube.

Overall, carboxylic acids have a unique set of properties that distinguish them from other organic molecules. They are polar due to the presence of both a carbonyl and hydroxyl functional group, and they experience hydrogen bonding, resulting in higher melting and boiling points. They can react in various ways, including as an acid, in addition reactions, and in reactions involving nucleophiles.

Carboxylic Acids

What are carboxylic acids?

Carboxylic acids are organic molecules containing the carboxyl functional group, -COOH. This consists of the hydroxyl group, -OH, and the carbonyl group, C=O.

Why are carboxylic acids weak?

Carboxylic acids are weak acids because they only partially dissociate in solution. They form an equilibrium, where some of the molecules ionise into positive hydrogen ions and negative carboxylate ions, and some remain intact. 

How are carboxylic acids formed?

Carboxylic acids are formed by oxidising primary alcohols. To do this, heat a primary alcohol under reflux with an oxidising agent such as acidified potassium dichromate. The alcohol will first oxidise into an aldehyde before turning into a carboxylic acid.

What are some carboxylic acids in daily life?

All amino acids, the building blocks of proteins, are carboxylic acids. Another example is ethanoic acid, found in all types of vinegar.  Citric acid is also a carboxylic acid. 

How do you make an ester from an alcohol and a carboxylic acid?

To make an ester, you can react a carboxylic acid and an alcohol together in an esterification reaction, using a strong acid catalyst.

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