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Carboxylic Acids

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Chemistry

Vinegars, be it a malt vinegar you shake over your chips or that balsamic vinegar you stir into a salad dressing, are generally 5-8 percent acetic acid by volume. They have 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 will start turning the ethanol present into acetic acid.

But what actually is a carboxylic acid?

Carboxylic acids are organic molecules with the carboxyl functional group , .

  • This article is an introduction to carboxylic acids in organic chemistry.
  • We'll begin by looking at carboxylic acids and their nomenclature before moving on to explore their properties.
  • You'll have a chance to practice naming and drawing different carboxylic acids, and you'll be able to 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 .

Carboxylic acids general structure ethanoic acid StudySmarterLeft: The general structure of a carboxylic acid, shown with the carbonyl group circled in red, and the hydroxyl group circled in blue. Right: Ethanoic acid. Anna Brewer, StudySmarter Originals

Look at ethanoic 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 double bond with the oxygen atom and one joins to the hydroxyl group, . This means that the carbon atom has just one electron remaining to form a bond with. It could either bond with an R group or perhaps a hydrogen atom, but it can't be attached to more than one other organic group. Therefore the carboxylic acid functional group must always be at the end of a hydrocarbon chain.

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.

Common nameIUPAC nameNumber of carbon atoms
Formic acidMethanoic acid1
Acetic acidEthanoic acid2
Propionic acidPropanoic acid3
Butyric acidButanoic acid4
Valeric acidPentanoic acid5
Caproic acidHexanoic acid6

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.

Carboxylic acids glycine StudySmarter The amino acid glycine. commons.wikimedia.org

Carboxylic acids tryptophan StudySmarter The amino acid tryptophan. commons.wikimedia.org

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 pretty confident at naming organic molecules. If you aren't, check out Organic Compounds. But naming carboxylic acids is 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 count the carbon that is part of the -COOH functional group as carbon 1 in the carbon chain.
  • Additional functional groups and side chains are shown using prefixes and numbers to indicate their position on the carbon chain.

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

Length of carbon chainRoot name
1-meth-
2-eth-
3-prop-
4 -but-
PrefixFunctional group present
chloro-
bromo
iodo-
hydroxy
amino

Let's look at some examples. You already know ethanoic acid, shown earlier. Its carbon chain is two atoms long and it has no other functional groups - pretty simple to name, right? However, this next molecule is a little more complicated.

Carboxylic Acids 2-chloropropanoic acid StudySmarterCan you name this molecule? Anna Brewer, StudySmarter Originals

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.

Carboxylic acids 2-chloropropanoic acid StudySmarter2-chloropropanoic acid. The carbon atoms are numbered to help you name the molecule. Anna Brewer, StudySmarter Originals

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.

Carboxylic acids reflux StudySmarter Equipment set up for reflux. commons.wikimedia.org

For example, reacting ethanol with acidified potassium dichromate produces first ethanal, and then ethanoic acid:

We use [O] to represent an oxidising agent.

Likewise, oxidising butanol gives butanoic acid:

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 oxidation would involve breaking a strong CC bond. It just isn't energetically favourable to do that, so no reaction occurs.

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 percent 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!

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.

Carboxylic acids COOH functional group StudySmarterThe -COOH functional group. Anna Brewer, 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.

Carboxylic acids electronegativity table StudySmarter A table showing electronegativity values of some common elements. Anna Brewer, StudySmarter Originals

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.

Carboxylic acids COOH polarity StudySmarter The -COOH group is polar. Note how partial charges are indicated using the delta symbol, δ. Anna Brewer, StudySmarter Originals

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.

Carboxylic acids hydrogen bond StudySmarter A hydrogen bond between two carboxylic acid molecules. Anna Brewer, StudySmarter Originals

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. As we now know, this is because carboxylic acids form hydrogen bonds between molecules which are much stronger than the intermolecular forces found in alkanes and aldehydes.

Carboxylic acids also 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. Because a dimer is essentially two carboxylic acid molecules joined together to form one larger molecule, it experiences much stronger van der Waals forces.

Carboxylic acids intermolecular forces dimer melting boiling point StudySmarter Two ethanoic acid molecules create a dimer by hydrogen bonding with each other. This doubles the strength of the van der Waals forces between molecules. Anna Brewer, StudySmarter Originals

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, 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.

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.

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, as explored below.

Reactions of carboxylic acids

Carboxylic acids react in multiple ways, thanks to their polar 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 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:

Carboxylic Acids - Key takeaways

  • Carboxylic acids have the general formula and contain both the carbonyl and hydroxyl functional groups.
  • We name carboxylic acids using the suffix -oic acid.
  • Carboxylic acids are polar molecules. Because they contain a hydrogen atom bonded to an oxygen atom, they also experience hydrogen bonding.
  • Carboxylic acids have higher melting and boiling points than similar alkanes, aldehydes, and alcohols due to the nature of their hydrogen bonds.
  • Carboxylic acids can react in multiple ways, including as an acid, in addition reactions, and in reactions involving nucleophiles.

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.

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. 

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.

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. 

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

Final Carboxylic Acids Quiz

Question

Name the two functional groups present in carboxylic acids.

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Answer

The carbonyl group

Show question

Question

What suffix is used to name carboxylic acids according to IUPAC nomenclature?

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Answer

-oic acid

Show question

Question

Which molecule has a higher boiling point? Justify your answer.

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Answer

Propane (Mr = 44)

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Question

Which molecule has a higher boiling point? Justify your answer.

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Answer

Hexanoic acid (Mr = 116)

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Question

Explain why carboxylic acids have higher boiling points than similar mass aldehydes.


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Answer

  • Aldehydes cannot form hydrogen bonds with each other as they don’t contain a hydrogen atom bonded to an oxygen atom. Their strongest intermolecular forces are permanent dipole-dipole forces.
  • Carboxylic acids do form hydrogen bonds between molecules as they contain an O-H bond. 
  • Hydrogen bonds are stronger than permanent dipole-dipole forces and require more energy to overcome.

Show question

Question

Explain why short chain carboxylic acids are soluble in water.


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Answer

They can form hydrogen bonds with water molecules.

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Question

As chain length increases, the solubility of carboxylic acids _____.


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Answer

Decreases

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Question

Name some reaction types carboxylic acids can take part in.


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Answer

  • Neutralisation
  • Addition reactions
  • Esterification
  • Nucleophilic substitution

Show question

Question

Carboxylic acids can be formed from secondary alcohols. True or false? 

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Answer

False

Show question

Question

Oxidising a primary alcohol gives:

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Answer

An aldehyde

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Question

All carboxylic acids are ____ acids.

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Answer

Weak

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Question

Name the first three carboxylic acids with the shortest carbon chain lengths.

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Answer

  • Methanoic acid.
  • Ethanoic acid.
  • Propanoic acid.

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Question

Name the functional group found in carboxylic acids.


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Answer

The carboxyl group.

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Question

The carboxyl functional group is made up of two other functional groups. Which groups are these?


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Answer

Hydroxyl group

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Question

Why are carboxylic acids described as weak acids?


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Answer

They only partially dissociate in aqueous solution.

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Question

Give three reactions that carboxylic acids partake in that are typical of acids.


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Answer

  • Neutralisation reactions with carbonates and hydroxides.
  • Reaction with ammonia.
  • Reaction with metals.

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Question

Name the salt produced when sodium hydroxide reacts with ethanoic acid.


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Answer

Sodium ethanoate

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Question

Name the salt produced when magnesium reacts with butanoic acid.


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Answer

Magnesium butanoate

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Question

Write a word equation for the reaction between a carboxylic acid and one other reagent to form an ester and one other product.


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Answer

Carboxylic acid + alcohol ↔ ester + water

Show question

Question

What conditions are required for esterification?

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Answer

  • Heat
  • Strong concentrated acid catalyst.

Show question

Question

Name the ester produced when ethanoic acid reacts with methanol with a strong acid catalyst.


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Answer

Methyl ethanoate

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Question

Name the alcohol and carboxylic acid used to produce ethyl propanoate.


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Answer

Ethanol

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Question

Carboxylic acids can be attacked by nucleophiles. Which part of the molecule will the nucleophile attack? 


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Answer

The partially positively charged carbon.

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Question

Decarboxylating a carboxylic acid gives _____ and _____.

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Answer

An alkane

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Question

Reducing a carboxylic acid gives _____.

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Answer

A primary alcohol and water.

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Question

How do you make acid derivatives?

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Answer

React carboxylic acids with phosphorus(V) chloride or phosphorus(III) chloride.

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Question

Describe how you produce carboxylic acids from nitriles using a dilute acid.

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Answer

Heat under reflux using a dilute acid catalyst. This produces a carboxylic acid and an ammonium salt.

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Question

Describe how you produce carboxylic acids from nitriles using a dilute alkali.

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Answer

Heat under reflux with a dilute alkali. This produces a carboxylate salt and ammonia. Adding a strong acid frees up the carboxylic acid.

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Question

Describe how you produce carboxylic acids from esters using a dilute acid.

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Answer

Heat under reflux with an excess of a dilute acid catalyst. This produces a carboxylic acid and an alcohol. 

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Question

Describe how you produce carboxylic acids from esters using a dilute alkali.

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Answer

Heat under reflux with a dilute alkali. This produces a carboxylate salt and an alcohol. Adding a strong acid turns the carboxylate salt into a carboxylic acid.

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Question

Which of the following are ways of making carboxylic acids?

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Answer

Hydrolysing esters

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