Lesson Explainer: Reactions of Carboxylic Acids Chemistry • Third Year of Secondary School

In this explainer, we will learn how to describe various reactions of carboxylic acids and predict what products are formed.

We will see demonstrations of the acidity of carboxylic acids through their reactions with metals, metal oxides, metal hydroxides, carbonates, and hydrogen carbonates. We will also see how to make esters from them, by reacting them with alcohols, and how to turn them into alcohols themselves through reactions with reducing agents.

A carboxylic acid is a type of chemical. Carboxylic acids contain the carboxyl functional group:

The following are the reactions we will be looking at.

Carboxylic acids are weak acids. Ethanoic acid, the active ingredient in vinegar, is a carboxylic acid:

Carboxylic acids can react with metals that are more reactive than hydrogen. We use the reactivity series as a guide; metals above hydrogen in the reactivity series should react with carboxylic acids.

This is the general equation for a reactive metal reacting with an acid:

For the reaction of a reactive metal with a carboxylic acid, the salt produced is a metal carboxylate salt.

For example,

We can confirm the gas produced from this reaction is hydrogen by doing the pop test, applying a lit splint to the end of a test tube of the gas. If the gas is hydrogen, it will ignite, producing a characteristic pop.

Acids can also react with metal oxides according to the general equation

When a metal oxide reacts with a carboxylic acid, a metal carboxylate salt is formed.

For example,

Being acidic, carboxylic acids will react with metal hydroxides in an acid–base reaction to produce salt and water. The salt produced is a metal carboxylate salt.

The reaction of sodium hydroxide with propanoic acid is

Carbonates are substances that contain carbonate ions ().

Many metal carbonates react with acids:

When a metal carbonate reacts with a carboxylic acid, the salt formed is a metal carboxylate.

For example, one of the most well-known carbonates is calcium carbonate (the primary component of limestone).

Here is the reaction of calcium carbonate with aqueous ethanoic acid:

Hydrogen carbonates are substances that contain hydrogen carbonate ions ().

Many metal hydrogen carbonates react with acids:

The reaction of a metal hydrogen carbonate with a carboxylic acid produces the same products as the reaction of a metal carbonate with a carboxylic acid; however, the proportions of the reactants and products will be different.

When a metal hydrogen carbonate reacts with a carboxylic acid, the salt formed is a metal carboxylate.

For example, this is the reaction of sodium hydrogen carbonate with aqueous ethanoic acid:

The reaction with carbonates or hydrogen carbonates is a way of testing for the presence of carboxylic acids. This test is known as the acidity test. The production of carbon dioxide, often observed as effervescence, can be used to help identify a carboxylic acid.

Ester groups are similar to carboxylic acids, except they have an alkyl group where the hydrogen is.

Esterification of a carboxylic acid is often achieved by reacting the carboxylic acid with an alcohol in the presence of an acid catalyst. This specific type of esterification is known as Fischer esterification. There are many other types of esterification reactions, but here the focus will be on Fischer esterification.

Reaction: Acid-Catalyzed Esterification of a Carboxylic Acid with an Alcohol

This reaction is reversible, so excess alcohol or carboxylic acid may be used to increase the equilibrium yield. You may have noticed that this reaction is the exact reverse of the acid-catalyzed hydrolysis of an ester. By changing the conditions, we can shift the position of equilibrium toward one side or the other, giving us more or less of the ester.

If we wanted to synthesize ethyl ethanoate, we could heat a mixture of ethanol and ethanoic acid together, with some concentrated sulfuric acid. We could improve the yield by distilling off the ethyl ethanoate as it is produced; ethyl ethanoate has a boiling point of , which is lower than that of ethanol (), ethanoic acid (), water (), and sulfuric acid (). The distillate would still contain a high proportion of ethanol without further purification:

The reaction of propanoic acid with ethanol is

Again, the products of this reaction are an ester and water. The ester produced is called ethyl propanoate.

Fischer esterification reactions also occur between cyclic carboxylic acids and alcohols. Consider the reaction between cyclohexanecarboxylic acid and methanol. Even though the carboxylic acid is attached to a cyclic hydrocarbon, the reaction still proceeds in the usual way to give the corresponding ester and water. The reaction is shown below:

Esterification also provides another way to test for the presence of carboxylic acids. In the esterification test, ethanol is added to a sample along with a few drops of sulfuric acid. The resulting mixture is then gently heated. If the sample contains a carboxylic acid, an esterification reaction occurs and an ester is formed. As many esters have a sweet, often floral or fruity, odor, then if the mixture has a sweet smell upon heating, the original sample likely contained a carboxylic acid.

The last type of reaction we will look at here is reduction. A carboxyl group can be considered to have a carbon atom in a +3 oxidation state. A carboxyl group can be reduced to form an aldehyde and reduced again to form a primary alcohol ( alcohol).

This diagram shows the oxidation state of the central carbon in a carboxylic acid, an aldehyde, and a primary alcohol:

This reduction can be achieved using various reducing agents, such as

• lithium aluminum hydride ()
• hydrogen, with hydrogenation catalysts such as copper chromite.

There are methods to stop the reduction at the aldehyde stage; however, we will only be looking at the full reduction of carboxylic acids here.

The general reaction is

The following is the overall reaction.

Reaction: Complete Reduction of a Carboxylic Acid to a Primary Alcohol

Let’s look at an example, using hydrogen () as the reducing agent and copper chromite as the catalyst:

Copper chromite () can form , which is the active catalyst. It allows the hydrogen to reduce the carboxylic acid at a reasonable rate. Copper chromate () can also be used.

We can also consider the reduction of propanoic acid using , in the presence of sulfuric acid. The full reaction is more complicated, but we can show the simplified reaction:

Again, the reduction produces the corresponding alcohol, in this case propanol.

The by-products of the reduction of carboxylic acids will depend on the reducing agent used.

Carboxyl groups that are joined to aromatic rings behave slightly differently than those that are not.

Let’s compare benzoic acid with cyclohexanecarboxylic acid. Here are their structures:

Benzoic acid is the more reactive of the two:

• It is more acidic, so it will react faster with metals, metal oxides, metal hydroxides, carbonates, and hydrogen carbonates.
• It is easier to esterify, so it will react more quickly and give higher yields when reacted with alcohols.

For example, benzoic acid can be easily converted to methyl benzoate:

However, when trying to hydrogenate benzoic acid to phenylmethanol, there is a risk that the aromatic ring is hydrogenated, so specialist catalysts are needed.

Example 3: Determining the Nature of the Reactants and Products of the Reaction of Propanoic Acid in the Presence of CuCrO₄

In an experiment, a sample of propanoic acid is heated in the presence of at . The reaction scheme is

1. What name does product B have?
   1. Butan-1-ol
   2. Ethanol
   3. Butan-2-ol
   4. Propan-1-ol
   5. Propan-2-ol
2. What other reactant is needed in the process? How many moles are needed?

Answer

Part 1

Our reactant is propanoic acid (a carboxylic acid). Copper chromate () can be used as a catalyst in the reduction of carboxylic acids.

When a carboxylic acid is reduced, it can be reduced partially (to form an aldehyde) or completely (to form a primary alcohol).

Under these conditions (catalyst + high temperature), we expect the reduction to go to completion.

Therefore, the unknown product is propan-1-ol, and the answer is D.

Part 2

The reduction reaction displayed above has water as one of the products. The chemical formula of propanoic acid is and the chemical formula of propan-1-ol is .

Let’s write out the whole reaction:

The equation is currently balanced in terms of carbon and oxygen, but not in terms of hydrogen. There are 6 hydrogens on the left and 10 on the right.

This means that A must provide the remaining hydrogens. Since hydrogen would likely be present as hydrogen molecules (), we would need two hydrogen molecules per molecule of propanoic acid to perform the reduction:

A reduction performed using hydrogen is a hydrogenation.

Key Points

• Carboxylic acids are weak acids that can react with metals, metal oxides, metal hydroxides, metal carbonates, and metal hydrogen carbonates:
• They can react with alcohols to produce esters:
  Acid-catalyzed esterification, also known as Fischer esterification, of a carboxylic acid with an alcohol is as follows:

  

• The reaction with carbonates (the acidity test) and Fischer esterification can be used as ways to test for carboxylic acid.
• They can be reduced to form aldehydes and primary alcohols:

  

  

• Aromatic carboxylic acids are more acidic than aliphatic carboxylic acids, so they react faster with metals, metal oxides, metal hydroxides, carbonates, and hydrogen carbonates, and they are easier to esterify.