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Vinegar, be it the malt vinegar you shake over your chips or that balsamic vinegar you stir into a salad dressing, is generally 5-8% acetic acid by volume. It has a sharp, astringent taste and a low pH. Acetic acid is scientifically known as ethanoic acid and is one of the most common carboxylic acids. It is quite simple to make. Leave a bottle of apple cider out in the sun and before too long, naturally occurring Acetobacter bacteria start turning the ethanol present into acetic acid. But what actually is a carboxylic acid?
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Sign up for freeAll carboxylic acids are ____ acids.
Which molecule has a higher boiling point? Justify your answer.
Name the two functional groups present in carboxylic acids.
What suffix is used to name carboxylic acids according to IUPAC nomenclature?
Which molecule has a higher boiling point? Justify your answer.
As chain length increases, the solubility of carboxylic acids _____.
Carboxylic acids can be formed from secondary alcohols. True or false?
Oxidising a primary alcohol gives:
Which molecule has a higher boiling point? Justify your answer.
As chain length increases, the solubility of carboxylic acids _____.
Carboxylic acids can be formed from secondary alcohols. True or false?
Oxidising a primary alcohol gives:
All carboxylic acids are ____ acids.
Which molecule has a higher boiling point? Justify your answer.
Name the two functional groups present in carboxylic acids.
What suffix is used to name carboxylic acids according to IUPAC nomenclature?
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Send FeedbackCarboxylic acids are organic molecules with the carboxyl functional group, -COOH.
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 combination of the hydroxyl and carbonyl functional groups gives carboxylic acids the general formula RCOOH.
The general structure of a carboxylic acid is shown with the carbonyl group circled in blue and the hydroxyl group circled in red. StudySmarter Originals
Look at the general structure of a carboxylic acid, shown above. We know that 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: two form a C=O double bond with the oxygen atom and one bond with the hydroxyl group, -OH. This means that the carbon atom has just one electron remaining that it can use to form a bond. This means that it can only bond to one other R group, be it a long complex chain or just a simple hydrogen atom. Regardless of the R group, this arrangement means that the carboxylic acid functional group must always be at the end of a hydrocarbon chain.
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.
| Common name | IUPAC name | Number of carbon atoms |
| Formic acid | Methanoic acid | 1 |
| Acetic acid | Ethanoic acid | 2 |
| Propionic acid | Propanoic acid | 3 |
| Butyric acid | Butanoic acid | 4 |
| Valeric acid | Pentanoic acid | 5 |
| Caproic acid | Hexanoic acid | 6 |
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
The amino acid tryptophan.commons.wikimedia.org
By looking at the common names of many 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.
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.
These tables should give you a quick reminder of the different root names and prefixes used to name molecules.
| Length of carbon chain | Root name |
| 1 | -meth- |
| 2 | -eth- |
| 3 | -prop- |
| 4 | -but- |
| Functional group present | Prefix |
| -Cl | chloro- |
| -Br | bromo- |
| -I | iodo- |
| -OH | hydroxy- |
| -NH2 | amino- |
Let's look at an example.
An unknown carboxylic acid. StudySmarter Originals
This molecule's 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.
2-chloropropanoic acid, labelled. StudySmarter Originals
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.
Note that we've drawn the hydroxyl group in full; the reason for this will become clear in just a second.
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.
| Element | Electronegativity |
| H | 2.20 |
| C | 2.55 |
| N | 3.04 |
| O | 3.44 |
| F | 3.98 |
| Cl | 3.16 |
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.
Carboxylic acid hydrogen bonding. StudySmarter Originals
Check out Intermolecular Forces for a more in-depth explanation of hydrogen bonds.
Hydrogen bonds are relatively strong. They influence many of the properties of carboxylic acids.
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
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.
Carboxylic acids, as their name suggests, are acids.
An acid is a proton donor.
To be more specific, carboxylic acids are weak acids.
A weak acid is an acid that only partially dissociates in solution. In contrast, strong acids fully dissociate in solution.
Head over to Acids and Bases for more about strong and 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.
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.
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.
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
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.
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.
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
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.
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
We use [O] to represent an oxidising agent.
Likewise, oxidising butanol (CH3CH2CH2CH2OH) gives butanoic acid (CH3CH2CH2COOH):
CH3CH2CH2CH2OH + 2[O] → CH3CH2CH2COOH + H2O
The alcohol used must be a primary alcohol. Oxidising a secondary alcohol produces a ketone whilst tertiary alcohols cannot be oxidised at all. This is because oxidising a tertiary alcohol would involve breaking a strong C-C bond. It just isn't energetically favourable to do that, so no reaction occurs.
Check out Oxidation of Alcohols for a more detailed look at oxidation reactions.
You can make vinegar out of any sort of alcohol. For example, oxidising beer produces a rich and intense malt vinegar, whilst oxidising white wine produces a fruity wine vinegar. To make it yourself, first dilute your chosen alcohol to 10% abv in a large container. Mix in a source of Acetobacter, such as a live vinegar, i.e., one containing a living culture of bacteria. Cover the container with a fine muslin cloth and leave in a warm, dark place for a couple of months, tasting every week or so to see how it is getting along. Before too long, you'll have a unique, flavourful vinegar on your hands!
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:
Find out more about these reactions in Nitriles, Reactions of Esters, and Acylation respectively. However, we also provide additional information about them in 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.
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:
2CH3COOH(aq) + Na2CO3(aq) → 2CH3COONa(aq) + CO2(g) + H2O(l)Sign up for free to gain access to all our flashcards.
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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