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Reactions of Carboxylic Acids

Reactions of Carboxylic Acids

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Carboxylic acids and esters may look like completely different types of molecules, but they're actually connected. Vinegars, like the balsamic vinegar you might put in a salad dressing, are about 5-8% acetic acid by volume. Acetic acid is a carboxylic acid that is sharp and stringent. On the other hand, the oil you mix with the vinegar in your salad dressing is an example of an ester, and it has a fruity, floral scent. However, it's simple to convert one into the other in a process called esterification. This is an example of a reaction of carboxylic acids.

Carboxylic acids are organic molecules that contain the -COOH functional group. In this article, we'll investigate the reactions of carboxylic acids in organic chemistry. First, we'll examine how they are formed. Next, we'll go over how they react as acids, including their behavior in water and their reactions with common bases. After that, we'll explore how carboxylic acids react with alcohols in esterification. Finally, we'll touch upon some other 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.

The general structure of a carboxylic acid
The general structure of a carboxylic acid

Producing carboxylic acids

In this article, we will explore three methods of making carboxylic acids: oxidation of alcohols, hydrolysis of nitriles, and hydrolysis of esters.

Let's begin with the oxidation of alcohols.

Producing carboxylic acids: Oxidation of alcohols

Have you ever left an open bottle of wine for too long and noticed it turned sour and acidic? This is because the alcohol in the wine oxidizes into a carboxylic acid.

In the lab, carboxylic acids can be made by heating a primary alcohol with an oxidizing agent like acidified potassium dichromate under reflux. This process releases water and the alcohol first turns into an aldehyde before forming a carboxylic acid. The oxidizing agent is represented by the symbol [O].

For example, when ethanol is reacted with acidified potassium dichromate, it produces ethanoic acid. During the reaction, we observe a colour change from orange to green as the alcohol oxidizes. To

Producing carboxylic acids: Hydrolysis of nitriles

Another method of producing carboxylic acids is through the hydrolysis of nitriles. Nitriles are organic compounds that contain the -C≡N functional group. Refluxing a nitrile with either a dilute acid or an alkali followed by a strong acid can hydrolyze it.

When a nitrile is refluxed with a dilute acid, a carboxylic acid and an ammonium salt are produced, with the acid acting as a catalyst. For example, when ethanenitrile reacts with hydrochloric acid, ethanoic acid and ammonium chloride are produced.

On the other hand, refluxing a nitrile with an alkali produces a carboxylate salt and ammonia. The addition of a strong acid then frees up the carboxylic acid. For instance, reacting propanenitrile with sodium hydroxide results in the production of sodium propanoate and ammonia. Adding hydrochloric acid to the reaction mixture then converts the sodium propanoate into propanoic acid and sodium chloride. 

Producing carboxylic acids: Hydrolysis of esters

Another method of producing carboxylic acids is through the hydrolysis of esters. Esters are organic molecules that contain the -COO- functional group. Similar to the hydrolysis of nitriles, this reaction is carried out under reflux using a dilute acid or alkali.

When an ester is hydrolyzed using a dilute acid under reflux, it produces a carboxylic acid and an alcohol. This reaction is reversible, so an excess of dilute acid is used to shift the equilibrium to the right. For example, when methyl ethanoate reacts with dilute hydrochloric acid, ethanoic acid and methanol are produced.

On the other hand, hydrolyzing an ester under reflux with a dilute alkali 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 obtained by adding a strong acid, such as hydrochloric acid. For instance, heating methyl ethanoate with dilute sodium hydroxide under reflux produces sodium ethanoate and methanol. Adding hydrochloric acid to the reaction mixture then converts the sodium ethanoate into ethanoic acid and sodium chloride.

Carboxylic acids are called acids because they are proton donors. In solution, they give up a hydrogen ion from their hydroxyl group and leave behind a negative carboxylate ion through a process known as ionization. However, carboxylic acids are weak acids, meaning they only partially ionize in solution, with some molecules remaining intact. The rate of ionization is the same as the rate of the reverse reaction, so the overall proportion of ionized molecules in the solution remains the same.

The reaction exists in a state of equilibrium
The reaction exists in a state of equilibrium

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: When a carboxylic acid ionises into a carboxylate ion, the charge delocalises.

‍The resulting ion is more stable
The resulting ion is more stable

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 can react with carbonates to form a carboxylate salt, water, and carbon dioxide. The salt is named after the acid it is produced from, using the suffix -oate. For instance, using propanoic acid produces a propanoate salt, while using methanoic acid produces a methanoate salt.

For example, when ethanoic acid reacts with sodium carbonate, it produces sodium ethanoate, carbon dioxide, and water.

This reaction is often used as a test for carboxylic acids. To perform the test, 2 cm3 of an unknown organic compound is transferred to a test tube using a pipette. Half a spatula’s worth of sodium carbonate, Na2CO3, is then added, and the reaction is observed for the formation of carbon dioxide gas bubbles. The presence of the gas indicates that the compound is an acid.

When drawing the salt or writing it using a structural formula, it is essential not to draw a bond between the metal ion and the carboxylate ion. This is because they are joined by an ionic bond, which is an electrostatic attraction between oppositely charged ions, rather than a covalent bond, which is a shared pair of electrons. For example, the salt sodium methanoate is drawn without a bond between the sodium ion and the carboxylate ion.

A carboxylate salt, sodium methanoate
A carboxylate salt, sodium methanoate

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

Carboxylic acids can also react with metals to produce carboxylate salts and hydrogen gas. For instance, reacting potassium with propanoic acid yields potassium propanoate and hydrogen gas. Potassium propanoate is commonly used as a stabilizer in processed foods.

When carboxylic acids react with ammonia, they produce an ammonium salt. For example, the reaction between ethanoic acid and ammonia produces a colourless solution of ammonium ethanoate with no other products.

Carboxylic acids can also undergo esterification reactions, where they react with alcohols to produce an ester and water. Esterification reactions are reversible, occurring in a state of dynamic equilibrium. Esters are organic molecules that contain the functional group -COO- and are used in a wide range of products, including soaps, shampoos, plastic packaging, and biodiesel.

The general structure of an ester. R comes from the carboxylic acid whilst R' comes from the alcohol used to form the ester
The general structure of an ester. R comes from the carboxylic acid whilst R' comes from the alcohol used to form the ester

Reactions of Carboxylic Acids: 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.

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

Reactions of Carboxylic Acids: 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

Reactions of Carboxylic Acids: 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.

Diethyl ether
Diethyl ether

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.

Reactions of Carboxylic Acids: Decarboxylation

Decarboxylation is a reaction where the -COOH group of a carboxylic acid is removed and replaced by a hydrogen atom, resulting in the production of an alkane and carbon dioxide. This reaction can be achieved 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.

Carboxylation, on the other hand, is the opposite of decarboxylation and is an essential step in photosynthesis. The enzyme RuBisCo captures carbon by combining carbon dioxide with RuBP to form 3-phosphoglycerate in a process called the Calvin cycle.

Overall, carboxylic acids are weak acids that partially dissociate in solution, forming carboxylate ions. They can be produced by oxidizing primary alcohols under reflux, hydrolyzing nitriles, or hydrolyzing esters. Carboxylic acids can react with bases to form carboxylate salts and with alcohols to produce esters through esterification. Additionally, carboxylic acids can be reduced, decarboxylated, and transformed into acid derivatives.

Reactions of Carboxylic Acids

Do carboxylic acids undergo addition reactions? 

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.

What happens when two carboxylic acids react? 

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

What is the chemical equation for carboxylic acid? 

Carboxylic acids can be represented by the general formula RCOOH.

How are carboxylic acids formed?

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

What is the name of the reactions of carboxylic acids and their derivatives? 

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

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