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Reactions of Carboxylic Acids

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Chemistry

Carboxylic acids and esters, at first glance, seem to be completely different types of molecules. For example, vinegars, such as the balsamic vinegar you might mix in a salad dressing, are about 5-8% acetic acid by volume. Acetic acid is a carboxylic acid with the IUPAC name ethanoic acid, and is sharp and stringent. But the oil you combine with the vinegar in your salad dressing? That’s an example of an ester - compounds with fruity, floral aromas. However, it is easy to go from one to the other in a process called esterification. This is an example of a reaction of carboxylic acids.

Carboxylic acids are organic molecules containing the -COOH functional group.

  • This article is about the reactions of carboxylic acids in organic chemistry.
  • First of all, we will look at how they are formed.
  • We’ll then look at how they react as acids, including their behaviour in water and their reactions with common bases.
  • We’ll then explore reacting carboxylic acids with alcohols in esterification.
  • Finally, we’ll touch upon some further reactions, such as reduction, decarboxylation and the formation of acid derivatives.

What are carboxylic acids?

As mentioned above, carboxylic acids are molecules that contain the carboxyl functional group, -COOH. This group is in turn made up of two other functional groups - the carbonyl group, C=O, and the hydroxyl group, -OH.

Reactions of carboxylic acids general structure StudySmarterThe general structure of a carboxylic acid. The carbonyl group, C=O, is circled in red whilst the hydroxyl group, -OH, is circled in blue. Anna Brewer, StudySmarter Originals

Producing carboxylic acids

There are a few different ways of making carboxylic acids. We'll look at three of them in this article:

  • Oxidation of alcohols
  • Hydrolysis of nitriles
  • Hydrolysis of esters

First up: oxidation of alcohols.

Oxidation of alcohols

If you leave an open bottle of wine for a long enough period of time, it’ll turn sour and acidic. This is because the alcohol within oxidises into a carboxylic acid.

To make carboxylic acids in the lab, heat a primary alcohol with an oxidising agent such as acidified potassium dichromate under reflux. The alcohol will first turn into an aldehyde before forming a carboxylic acid. The process also releases water:

We use [O] to represent the oxidising agent.

For example, reacting ethanol with acidified potassium dichromate produces ethanoic acid:

Reflux. commons.wikimedia.org

Observe the reaction. What colour change do you notice? The potassium dichromate should turn from orange to green as the alcohol is oxidised.

To explore this reaction in more depth, check out “Oxidation of alcohols”

Hydrolysis of nitriles

Another way of producing carboxylic acids is by hydrolysing nitriles. Nitriles are organic compounds with the -C≡N functional group. We hydrolyse them using reflux with either a dilute acid, or an alkali followed by a strong acid.

Refluxing a nitrile with a dilute acid produces a carboxylic acid and an ammonium salt. The acid is a catalyst in the reaction:

For example, the reaction between ethanenitrile and hydrochloric acid produces ethanoic acid and ammonium chloride:

Refluxing a nitrile with an alkali produces a carboxylate salt and ammonia. Adding a strong acid frees up the carboxylic acid:

For example, reacting propanenitrile with sodium hydroxide produces sodium propanoate and ammonia. Adding hydrochloric acid turns the sodium propanoate into propanoic acid and sodium chloride:

Hydrolysis of esters

The final method of producing carboxylic acids that we'll look at today is the hydrolysis of esters. Esters are organic molecules with the -COO- functional group. As with the hydrolysis of nitriles, this is carried out under reflux using a dilute acid or alkali.

Hydrolysing an ester using a dilute acid under reflux produces a carboxylic acid and an alcohol. The reaction is reversible, so we use an excess of the dilute acid to shift the equilibrium to the right:

For example, reacting methyl ethanoate with dilute hydrochloric acid produces ethanoic acid and methanol:

Hydrolysing an ester under reflux with a dilute alkali forms a slightly different product. This reaction produces a carboxylate salt and an alcohol. Note that this reaction goes to completion, but doesn't directly produce a carboxylic acid. Instead, the carboxylic acid can be freed by adding a strong acid, such as hydrochloric acid:

For example, heating methyl ethanoate with dilute sodium hydroxide under reflux produces sodium ethanoate and methanol. Adding hydrochloric acid produces ethanoic acid and sodium chloride:

You'll compare these two reactions in the article "Reactions of Esters".

Reactions of carboxylic acids as acids

You may have wondered why we call carboxylic acids, well, carboxylic acids. So far they haven’t done anything remotely acid-like! In fact, carboxylic acids are indeed acids.

An acid is a proton donor.

A proton is simply a positive hydrogen ion. In this case, carboxylic acids are defined as acids because in solution, they give up a hydrogen ion from their hydroxyl group. This leaves behind a negative carboxylate ion. This process is known as ionisation.

However, carboxylic acids are actually only weak acids.

Weak acids are acids that only partially ionise in solution.

This means that within the solution there is a continuous equilibrium, in which some molecules are ionised and some remain intact. The rate of ionisation is the same as the rate of the reverse reaction, so the overall proportion of ionised molecules in the solution remains the same.

Reactions of carboxylic acids dissociation ionisation StudySmarter

The dissociation of carboxylic acids. The reaction exists in a state of equilibrium.
Anna Brewer, StudySmarter Originals

To be completely honest, the equation above doesn’t show the whole picture. When carboxylic acids ionise into a carboxylate group and hydrogen ion, the negative charge in the carboxylate group spreads out across both oxygen atoms in the molecule. This is called delocalisation and creates a more stable ion. It overrules the C=O double bond, making both of the bonds joining oxygen to carbon equal. We represent the delocalisation using a dashed bond between the two oxygen atoms. So in reality, the equation should look like this:

Reactions of carboxylic acids dissociation ionisation charge delocalistion StudySmarterWhen a carboxylic acid ionises into a carboxylate ion, the charge delocalises. The resulting ion is more stable. Anna Brewer, StudySmarter Originals

Carboxylic acids take part in all of the usual reactions of acids. However, they react a lot more slowly than, say, hydrochloric acid, because they are weak acids and only partially ionise in solution. Let’s explore some of these reactions next.

Reaction of carboxylic acids with carbonates

Carboxylic acids react with carbonates to produce a carboxylate salt, water and carbon dioxide. We name the salt after the acid it is produced from, using the suffix -oate. If you use propanoic acid, you produce a propanoate salt; if you use methanoic acid you produce a methanoate salt.

For example, ethanoic acid reacts with sodium carbonate to produce sodium ethanoate, carbon dioxide and water.

In fact, the reaction between carboxylic acids and carbonates is commonly used as a test for carboxylic acids:

  • Use a pipette to transfer 2 cm3 of an unknown organic compound to a test tube.
  • Add in half a spatula’s worth of sodium carbonate, Na2CO3, and observe.
  • If you see bubbles of carbon dioxide gas fizzing up towards the surface, you know that your compound is an acid.

When you draw the salt or write it out using a structural formula, make sure you don’t draw a bond between the metal ion and the carboxylate ion. This is because they are joined by an ionic bond, not a covalent bond. An ionic bond is electrostatic attraction between oppositely charged ions whereas a covalent bond is a shared pair of electrons.

For example, we draw the salt sodium methanoate as shown below:

Reactions of carboxylic acids sodium methanoate StudySmarterA carboxylate salt, sodium methanoate. Anna Brewer, StudySmarter Originals

For more information on the different types of bonding, check out the articles "Covalent and dative bonding" and "Ionic bonding".

Reaction of carboxylic acids with hydroxides

Carboxylic acids neutralise metal hydroxides to produce a salt and water.

For example, magnesium hydroxide reacts with methanoic acid to produce magnesium methanoate and water.

Carboxylate ions have a charge of -1. This means that when carboxylic acids react with a base containing a group 2 metal, you need two moles of the acid for every mole of the base. You can see this in the example above.

Reaction of carboxylic acids with metals

Reacting a metal with a carboxylic acid again produces a carboxylate salt, this time alongside hydrogen.

For example, potassium reacts with propanoic acid to produce potassium propanoate and hydrogen gas. Potassium propanoate is also known as potassium propionate and is a common stabiliser in processed foods.

Reaction of carboxylic acids with ammonia

Carboxylic acids react with ammonia to produce an ammonium salt. Note that there isn’t any other product here. The reaction between ethanoic acid and ammonia is given below - it produces a colourless solution of ammonium ethanoate.

Reactions of carboxylic acids ammonium ethanoate StudySmarterAmmonium ethanoate. commons.wikimedia.org

Esterification

We’ve explored how carboxylic acids act as acids by donating a proton in solution. But they can also take part in other reactions, including one known as esterification.

Esterification is a type of reaction that produces an ester.

Esters are organic molecules that contain the functional group -COO-.

Esters are used in a variety of different products, from soaps and shampoos to plastic packaging and biodiesel.

If you want a quick recap of esters, their properties and their nomenclature, feel free to revisit the article "Esters".

In this case, we react a carboxylic acid with an alcohol to produce an ester and water. This is a reversible reaction, meaning both the forward reaction and the backward reaction happen at the same time in a state of dynamic equilibrium.

For more on reversible reactions, take a quick look at the article "Equilibria".

The general structure of an ester. R comes from the carboxylic acid whilst R' comes from the alcohol used to form the ester. Anna Brewer, StudySmarter Originals

Small scale ester production

To make esters in a test tube, use a water bath to gently heat 10 drops of a carboxylic acid with 10 drops of an alcohol and 2 drops of an acid catalyst, such as sulfuric acid. You wouldn’t do this directly over an open flame because the organic liquids used are highly flammable.

Because this reaction is reversible, you’ll only produce a tiny amount of the ester. To smell it, pour the solution into a beaker of water. Longer chain esters are insoluble and so will form a layer on the surface of the water, whilst the unreacted acid and alcohol will readily dissolve. If you waft the air over the top of the beaker, you should be able to smell the ester. Whilst short chain esters such as methyl ethanoate, commonly known as methyl acetate, smell like solvents or glue, longer chain esters smell fruity and aromatic.

We name esters using names based off of the alcohols and carboxylic acids they are produced from. The name derived from the alcohol comes first, followed by the name derived from the carboxylic acid. All esters end in the suffix -oate.

Let’s have a go at writing an equation. For example, reacting ethanoic acid with butanol produces butyl ethanoate, which smells like raspberry.

Reactions of carboxylic acids butyl ethanoate StudySmarterButyl ethanoate. Anna Brewer, StudySmarter Originals

Butyl ethanoate. In this ester, R comes from ethanoic acid and is circled in blue, whilst R' comes from butanol and is circled in red. Anna Brewer, StudySmarter Originals

Large scale ester production

Large scale ester production depends on the type of ester you want to create.

To make short chain esters such as ethyl ethanoate, CH3COOCH2CH3, heat ethanol and ethanoic acid with a strong, concentrated acid catalyst and distill off the product, the ester. The ester has the lowest boiling point out of all the substances involved because it cannot form hydrogen bonds with itself, unlike alcohols and carboxylic acids. Distilling off the product also shifts our equilibrium to the right, increasing the yield of the reaction.

However, if we want to make longer chain esters we have to use reflux, as when making carboxylic acids earlier in this article. Reflux involves heating a reaction mixture in a sealed container. This means that any volatile components that evaporate condense and fall back into the reaction mixture, preventing them from evaporating off before they can react. The products can then be separated by fractional distillation.

Other reactions of carboxylic acids

There are a few other reactions involving carboxylic acids that you might want to know about. These include:

  • Conversion into acid derivatives such as acyl chlorides and acid anhydrides.
  • Reduction.
  • Decarboxylation

Conversion into acid derivatives

Carboxylic acids contain the hydroxyl group, -OH. This isn’t a very good leaving group and means that carboxylic acids aren’t that reactive. However, we can turn them into acid derivatives by swapping the -OH group with another more reactive functional group, such as -Cl. These new molecules are called acid derivatives.

One type of acid derivative is acyl chlorides. As in the example above, these swap the hydroxyl group on a carboxylic acid for a chlorine atom. To make them, we react carboxylic acids with phosphorus(V) chloride or phosphorus(III) chloride.

You’ll learn more about acid derivatives in “Acylation

Reduction of carboxylic acids

Remember how oxidising a primary alcohol produces a carboxylic acid? Well, we can reverse the reaction and go the other way instead - reducing a carboxylic acid forms a primary alcohol. This reaction uses a reducing agent such as lithium aluminium hydride, LiAlH4. This produces an aluminium salt, but if you treat it with dilute sulfuric acid, it’ll turn into an alcohol. The reaction is carried out at room temperature in a solution of dry diethyl ether.

Reactions of carboxylic acids diethyl ether StudySmarterDiethyl ether. commons.wikimedia.org

The overall reaction is as follows:

For example, reducing ethanoic acid gives ethanol:

You might know another common reducing agent - sodium tetrahydridoborate. This is often used to reduce aldehydes and ketones. However, it isn’t a strong enough reducing agent to reduce carboxylic acids and so we can’t use it here.

Decarboxylation

In a decarboxylation reaction, the -COOH group of a carboxylic acid is removed and replaced by a hydrogen atom. This produces an alkane and carbon dioxide. It is done by heating the carboxylic acid with soda lime, which is a mixture of sodium hydroxide, calcium oxide and calcium hydroxide.

For example, decarboxylating ethanoic acid produces methane gas:

Notice how we started with ethanoic acid, which has a carbon chain with two carbon atoms, and ended up with methane, which has a carbon chain with just one carbon atom. Decarboxylation shortens the carbon chain - the spare carbon atom is released as CO2.

The opposite to decarboxylation is carboxylation and is an important step in photosynthesis. The enzyme RuBisCo captures carbon by combining carbon dioxide with RuBP to form 3-phosphoglycerate as part of a process called the Calvin cycle. Check out “Light-independent reaction” for more information.

Reactions of carboxylic acids - Key takeaways

  • Carboxylic acids are organic molecules containing the carboxyl functional group, -COOH. This is made up of the carbonyl group, C=O, and the hydroxyl group, -OH.
  • Carboxylic acids are weak acids, meaning they only partially dissociate in solution. When they dissociate, they lose a proton to form a negative carboxylate ion. The charge delocalises across the molecule and makes both C-O bonds equal.
  • Carboxylic acids are formed by oxidising primary alcohols under reflux, hydrolysing nitriles or hydrolysing esters.
  • Carboxylic acids react with bases to form carboxylate salts.
  • Carboxylic acids react with alcohols in the presence of a strong, concentrated acid catalyst to produce esters. This is known as esterification.
  • Carboxylic acids can also be reduced, decarboxylated and turned into acid derivatives.

Reactions of Carboxylic Acids

Carboxylic acids don't tend to undergo addition reactions. Instead, they take part in many substitution reactions such as esterification and conversion into acid derivatives.

Reacting two carboxylic acids together produces an acid anhydride. This is a dehydration reaction that removes a molecule of water.

Carboxylic acids can be represented by the general formula RCOOH.

Carboxylic acids are formed by the oxidation of primary alcohols, the hydrolysis of nitriles or the hydrolysis of esters.

Carboxylic acids undergo many different types of reactions, from neutralisation reactions to substitution reactions. Carboxylic acid derivatives often undergo addition-elimination reactions.

Final Reactions of Carboxylic Acids Quiz

Question

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

Show answer

Answer

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

Show question

Question

Name the functional group found in carboxylic acids.


Show answer

Answer

The carboxyl group.

Show question

Question

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


Show answer

Answer

Hydroxyl group

Show question

Question

Why are carboxylic acids described as weak acids?


Show answer

Answer

They only partially dissociate in aqueous solution.

Show question

Question

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


Show answer

Answer

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

Show question

Question

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


Show answer

Answer

Sodium ethanoate

Show question

Question

Name the salt produced when magnesium reacts with butanoic acid.


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Answer

Magnesium butanoate

Show question

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.


Show answer

Answer

Methyl ethanoate

Show question

Question

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


Show answer

Answer

Ethanol

Show question

Question

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


Show answer

Answer

The partially positively charged carbon.

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Question

Decarboxylating a carboxylic acid gives _____ and _____.

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Answer

An alkane

Show question

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.

Show question

Question

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

Show answer

Answer

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

Show question

Question

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

Show answer

Answer

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

Show question

Question

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

Show answer

Answer

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

Show question

Question

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

Show answer

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.

Show question

Question

Which of the following are ways of making carboxylic acids?

Show answer

Answer

Hydrolysing esters

Show question

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