Carboxylic Acids

Carboxylic acid definition

Carboxylic acid functional group

The definition above tells us that carboxylic acids all contain the carboxyl functional group, -COOH. This group is made up of two other functional groups:

• The hydroxyl group found in alcohols, -OH,
• The carbonyl group found in aldehyde and ketones, C=O.

Carboxylic acid general formula

The combination of the hydroxyl and carbonyl functional groups gives carboxylic acids the general formula RCOOH.

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.

Other examples of carboxylic acids include all Amino 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.

The amino acid glycine.commons.wikimedia.org

Carboxylic acid nomenclature

Carboxylic acids are named using standard IUPAC nomenclature (check out Organic Nomenclature if this is your first look at naming organic molecules). The methodical IUPAC system makes naming carboxylic acids pretty simple, really. Let's take a quick look at some of the rules.

• Carboxylic acids have the suffix -oic acid.
• We use the standard root names to show the length of the molecule.
• We show additional functional groups and side chains using prefixes and numbers to indicate their position on the carbon chain, counting the carbon atom in the -COOH functional group as carbon 1.

These tables should give you a quick reminder of the different root names and prefixes used to name molecules.

Let's look at an example.

Properties of carboxylic acids

Take a closer look at the -COOH group. As we know, it contains not only the carbonyl functional group, C=O, but also the hydroxyl functional group, -OH. Let's draw these both out.

The general structure of a carboxylic acid. StudySmarter Originals

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

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 in the -COOH group 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.

You can see the partial charges in the diagram below, as well as the oxygen atoms' lone pairs of electrons.

Carboxylic acid partial charges. StudySmarter Originals

In fact, the O-H bond in carboxylic acids is so polar, due to the different electronegativities of oxygen and hydrogen, that carboxylic acids can form hydrogen bonds.

• 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.

Carboxylic acid hydrogen bonding. StudySmarter Originals

Hydrogen bonds are relatively strong. They influence many of the properties of carboxylic acids.

Melting and boiling points

Carboxylic acids have higher melting and boiling points than similar alkanes and aldehydes. As we now know, this is because carboxylic acids form hydrogen bonds between molecules. In contrast, the strongest intermolecular forces between aldehydes are permanent dipole-dipole forces, whilst the strongest forces between alkanes are van der Waal forces. Hydrogen bonds are much stronger than both permanent dipole-dipole forces and van der Waal forces, and so require more energy to overcome.

Additionally, carboxylic acids have higher melting points than similar alcohols, despite alcohols also forming hydrogen bonds. This is because two carboxylic acids can form hydrogen bonds in a certain way to produce a molecule called a dimer. We can consider a dimer as two carboxylic acid molecules joined together to form one larger molecule. This means that it experiences double-strength van der Waals forces. On the other hand, alcohols don't form these dimers.

Two ethanoic acid molecules create a dimer by hydrogen bonding with each other. StudySmarter Originals

Solubility

Carboxylic acids can also form hydrogen bonds with water. This makes shorter chain carboxylic acids soluble in aqueous solutions. However, long chain molecules are insoluble because their non-polar hydrocarbon chains get in the way of 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 of carboxylic acids

Carboxylic acids, as their name suggests, are acids.

To be more specific, carboxylic acids are weak acids.

In solution, carboxylic acids form an equilibrium, where some of the molecules dissociate into a positive hydrogen ion and a negative carboxylate ion, and some remain intact.

RCOOH ⇌ RCOO^- + H⁺

Because carboxylic acids are so weak, the equilibrium lies well to the left. This means that only a few of the molecules dissociate. And because carboxylic acids are acids, they have a pH below 7. They take part in many typical acid-base reactions, which we'll introduce you to later.

Relative acidity of carboxylic acids, alcohols and phenol

Carboxylic acids are weak acids because their hydroxyl group (-OH) gives up a proton (which is just a hydrogen ion) in solution. You might consequently wonder why other molecules that have the same hydroxyl functional group, such as alcohols (ROH) and phenols (C₆H₅OH), aren't acidic. To understand this, we need to consider two factors:

• The strength of the O-H bond.

• The stability of the negative ion formed.

Bond strength

The O-H bond in carboxylic acids is much weaker than the O-H bond in alcohols and phenol. This is all thanks to the carboxylic acid's other functional group, the carbonyl group (C=O). The carbonyl group is electron-withdrawing, meaning that it attracts the shared pair of electrons in the O-H bond over towards itself, weakening the O-H bond. A weaker O-H bond means that it is easier for carboxylic acids to lose hydrogen as an H⁺ ion, and therefore gives them a greater acidity.

However, alcohols and phenol lack an electron-withdrawing group, and so their O-H bonds are just as strong as ever.

Ion stability

Let’s now think about the ion formed when carboxylic acids, alcohols and phenol act as acids by losing a proton (a hydrogen ion, H⁺). The more stable this ion, the less readily it joins back up with a hydrogen ion, and the greater the acidity of the original molecule.

When carboxylic acids lose a proton, they form negative carboxylate ions, RCOO^-. The negative charge delocalises across both carbon-oxygen bonds. Instead of having one C-O single bond and one C=O double bond, the carboxylate ion has two identical carbon-oxygen bonds, which are each equivalent in strength to a one-and-a-half bond. Delocalisation is great for the ion - it stabilises the molecule, and makes oxygen’s electrons much less available for joining back up with a hydrogen ion.

However, alcohols and phenols don't form such a stable negative ion. When alcohols ionise, they form the alkoxide ion, RO^-. This is a very unstable ion. Firstly, the R group tends to be a hydrocarbon chain, which is electron-donating and so increases the oxygen’s electron density. Secondly, the negative charge can’t delocalise and so is concentrated on the oxygen atom. All in all, this makes for a reactive ion that can’t wait to join back up with a hydrogen ion to form an alcohol again.

When phenols ionise, they form the phenoxide ion, C₆H₅O^-. Like with the carboxylate ion, the negative charge delocalises; in this case, it delocalises across the enitre benzene ring. Once again, delocalisation makes the ion more stable, and so phenol is a stronger acid than alcohols. But the delocalisation in phenoxide ions is weaker than the delocalisation in carboxylate ions because it is spread over less electronegative carbon atoms. This means that oxygen in phenoxide ions still keeps most of its negative charge and is more attractive to H⁺ ions than oxygen in carboxylate ions. All in all, phenol is a stronger acid than alcohols, but a weaker acid than carboxylic acids.

The stability of the resulting ion formed plays a role in the acidity of carboxylic acids, alcohols and phenol. StudySmarter Originals

Relative acidity of different carboxylic acids

Acidity also varies between different carboxylic molecules. We'll explore the trends in acidity in carboxylic acids with varying chain lengths and different numbers of chlorine substituents.

Chain length

Increasing the length of the carboxylic acid's hydrocarbon R group, by adding additional -CH₂- groups, decreases the strength of the acid. The longer the hydrocarbon chain, the weaker the acid. This is because alkyl groups are electron-donating. They push electrons away from themselves and increase the strength of the O-H bond. This makes it harder for the -COOH group to give up a hydrogen ion. It also increases the charge density of the resulting carboxylate ion's -COO^- group, making it easier for the ion to bond to H⁺ again.

Chlorine substitutents

Swapping some of the hydrogen atoms in the carboxylic acid's R group for electron-withdrawing groups, such as electronegative chlorine atoms, increases the strength of the acid. The more chlorine substituents, the stronger the acid. This is because electron-withdrawing groups like chlorine atoms pull electrons away from the -COOH group, weakening the O-H bond and making it easier for the carboxylic acid to lose a hydrogen ion. These groups also decrease the charge density of the resulting carboxylate's -COO^- group, making it harder for the ion to bond to H⁺ again.

The effect of chain length and chlorine substituents on the relative acidity of carboxylic acids. StudySmarter Originals

Carboxylic acid production

At the start of this article, we mentioned how if you leave cider out in the sun, it eventually turns into vinegar. Cider is an alcohol. In this reaction, it is oxidised into first an aldehyde and then a carboxylic acid. Oxidation is one way of producing carboxylic acids.

Oxidation

In the lab, we typically produce carboxylic acids through oxidation by heating a primary alcohol under reflux with an oxidising agent such as acidified potassium dichromate (K₂Cr₂O₇) . Reflux prevents the aldehyde first formed from evaporating off, and allows it to react further into a carboxylic acid.

Equipment setup for reflux, StudySmarter Originals

For example, reacting ethanol (CH₃CH₂OH) with acidified potassium dichromate produces first ethanal (CH₃CHO), and then ethanoic acid (CH₃COOH) :

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Likewise, oxidising butanol (CH₃CH₂CH₂CH₂OH) gives butanoic acid (CH₃CH₂CH₂COOH):

CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O

Other methods

Oxidation isn't the only way of producing carboxylic acids. You're likely to come across a few other methods during your organic chemistry journey. These include:

• Hydrolysis of nitriles using either a dilute acid, or a dilute alkali followed by acidification.
• Hydrolysis of esters using either a dilute acid, or a dilute alkali followed by acidification.
• Electrophilic addition-elimination reaction of acyl chlorides with water.
• Electrophilic addition-elimination reaction of acid anhydrides with water.

Reactions of carboxylic acids

Carboxylic acids react in multiple ways, thanks to their polar -COOH group. Some examples include:

• Nucleophilic substitution, when a nucleophile attacks the partially positively charged carbon atom. You should remember that a nucleophile is an electron pair donor with a lone pair of electrons and negative or partially negative charge. This can form a whole range of products known as acid derivatives, such as acyl chlorides and acid anhydrides.

• Esterification, another type of nucleophilic substitution reaction, where the nucleophile is an alcohol. This forms an ester.

• Addition reactions across the C=O bond.

• Neutralisation reactions, in which the molecule acts as an acid and a hydrogen ion is lost from the -OH group. This process forms a salt.

You can see many of these in more detail in Reactions of Carboxylic Acids.

Testing for carboxylic acids

To test for carboxylic acids, we rely on their behaviour as an acid. Carboxylic acids react with carbonates to form a salt, water, and carbon dioxide gas, whilst most other organic molecules won't react at all. Gas bubbling up through the test tube is a tell-tale sign of a reaction.

For example, reacting ethanoic acid with sodium carbonate forms sodium ethanoate, water, and carbon dioxide: