Reactions of Carboxylic Acids

What are carboxylic acids?

The -COOH functional 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. Here it is shown with the carbonyl and hydroxyl groups highlighted. 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 (RCHO) before forming a carboxylic acid. The process also releases water:

$$RCH_2OH+2[O]\rightarrow RCOOH+H_2O$$

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

$$CH_3CH_2OH+2[O]\rightarrow CH_3COOH+H_2O$$

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}+\hspace{0.17em}2\left[\mathrm{O}\right]\to {\mathrm{CH}}_3\mathrm{COOH}+\hspace{0.17em}{\mathrm{H}}_2\mathrm{O}$

Reflux. commons.wikimedia.org

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:

$$RCN+H^++2H_2O\xrightarrow{H^+} RCOOH+{NH_4}^+$$

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

$$CH_3CN+HCl+2H_2O\xrightarrow{H^+} CH_3COOH+NH_4Cl$$

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

$$RCN+OH^-+H_2O\rightarrow RCOO^-+NH_3$$ $$RCOO^-+H^+\rightarrow RCOOH$$

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:

$$CH_3CH_2CN+NaOH+H_2O\rightarrow CH_3CH_2COONa+NH_3$$ $$CH_3CH_2COONa+HCl\rightarrow CH_3CH_2COOH+NaCl$$

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- ester linkage group. As with the hydrolysis of nitriles, this is carried out under reflux using a dilute acid or alkali.

Hydrolysing an ester under reflux, using a dilute acid as a catalyst, 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:

$$RCOOR'+H_2O\rightleftharpoons RCOOH+R'OH$$

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

$$CH_3COOCH_3+H_2O\rightleftharpoons CH_3COOH+CH_3OH$$

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:

$$RCOOR'+OH^-\rightarrow RCOO^-+R'OH$$ $$RCOO^-+H^+\rightarrow RCOOH$$

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:

$$CH_3COOR'+NaOH\rightarrow CH_3COONa+CH_3OH$$ $$CH_3COONa+HCl\rightarrow CH_3COOH+NaCl$$

Chemical reactions of carboxylic acids

It is now time to move on to the main focus of this article: the chemical reactions of carboxylic acids. Thanks to their carboxyl functional group (-COOH), which is made up of both the carbonyl (C=O) and hydroxyl (-OH) groups, carboxylic acids take part in multiple different types of reactions.

• Carboxylic acids behave as weak acids when mixed with water. This means that they partially ionise.
• As weak acids, carboxylic acids take part in all your typical acid-base reactions. For example, they react with carbonates, hydroxides, metals, and ammonia.
• In addition, carboxylic acids react with alcohols in esterification reactions. This produces an ester.
• Carboxylic acids can also be reduced, carboxylated, oxidised, and turned into acid derivatives.

Reactions of carboxylic acids with water

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

A proton is simply a positive hydrogen ion. Carboxylic acids are defined as acids because they give up a hydrogen ion from their hydroxyl group when they react with water. This leaves behind a negative carboxylate ion, and the process is known as ionisation.

However, carboxylic acids are only weak acids.

This means that within the solution there is a continuous equilibrium, in which some carboxylic acid 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. We represent the equilibrium using two half-headed arrows to show that the reaction is reversible:

$$RCOOH\rightleftharpoons RCOO^-+H^+$$

The ionisation of carboxylic acids. The reaction exists in a state of equilibrium. StudySmarter Originals

Delocalisation of carboxylic acids

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 of the molecule's oxygen atoms. This is called delocalisation. It occurs because it creates a more stable ion.

Delocalisation overrules the C=O double bond, making both of the carbon-oxygen bonds equal. Instead of one bond being a C-O single bond and the other being a C=O double bond, we can think of them both as one-and-a-half bonds. The delocalisation is represented using a dashed bond between the two oxygen atoms. So in reality, the equation for the ionisation of carboxylic acids should look like this:

When a carboxylic acid ionises into a carboxylate ion, the charge delocalises. The resulting ion is more stable.StudySmarter Originals

Carboxylic acids take part in all 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.

Reactions 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 react propanoic acid, you produce a propanoate salt; if you react methanoic acid, you produce a methanoate salt.

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

$$2CH_3COOH(aq)+Na_2CO_3(aq)\rightarrow 2CH_3COONa(aq)+H_2O(l)+CO_2(g)$$

In fact, the reaction between carboxylic acids and carbonates is so effective that it is commonly used as a test for carboxylic acids. You follow this method:

• 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.
• You can test the gas produced by bubbling it through limewater. Carbon dioxide turns clear limewater cloudy.

Testing for carbon dioxide. Carbon dioxide gas turns clear limewater cloudy.StudySmarter Originals

Reactions of carboxylic acids with hydroxides

Carboxylic acids react with metal hydroxides to produce a carbonate salt and water.

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

$$2COOH(aq)+Mg(OH)_2\rightarrow (CHOO)_2Mg(aq)+2H_2O(l)$$

Reactions 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.

$$2CH_3CH_2COOH(aq)+2K(s)\rightarrow 2CH_3CH_2COOK(aq)+H_2(g)$$

Reactions 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.

$$CH_3COOH(aq)+NH_3(aq)\rightarrow CH_3COONH_4(aq)$$

Ammonium ethanoate. commons.wikimedia.org

Reactions of carboxylic acids with alcohol

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.

In carboxylic acid esterification reactions, we combine a carboxylic acid (RCOOH) with an alcohol (R'OH) to produce an ester (RCOOR') and water (H₂O). We use an acid catalyst (typically sulphuric acid) and heat the solution. Here's the general equation:

$$RCOOH+R'OH\rightleftharpoons RCOOR'+H_2O$$

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

Note that esterification is a reversible reaction. This means that both the forward reaction and the backward reaction happen at the same time in a state of dynamic equilibrium.

Now, let's discover how you produce esters from carboxylic acids on both small- and large-scales.

Small-scale ester production

To make esters on a small-scale, use a water bath to gently heat 10 drops of a carboxylic acid in a test tube 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 form a layer on the surface of the water, whilst the unreacted acid and alcohol 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 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 (CH₃COOH) with butanol (C₄H₉OH) produces butyl ethanoate (CH₃COOC₄H₉), which smells like raspberry.

$$CH_3COOH+C_4H_9OH\rightleftharpoons CH_3COOC_4H_9+H_2O$$

The esterification reaction that produces butyl ethanoate. We've colour-coded the parts of the molecules so that you can see where the ester's name and structure come from.StudySmarter Originals

Large-scale ester production

Esters can also be produced on a large-scale. The method used depends on the type of ester you want to create.

To make short chain esters such as ethyl ethanoate (CH₃COOC₂H₅), heat ethanol and ethanoic acid with a strong, concentrated acid catalyst and distil off the product, which is 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 ester formed also shifts the equilibrium to the right, increasing the yield of the reaction.

If we want to make longer chain esters, we have to use reflux, like when we made 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.

Reactions of carboxylic acids and their derivatives

Carboxylic acids contain the hydroxyl group (-OH). This isn’t a good leaving group and means that carboxylic acids aren’t that reactive. However, we can make them more reactive 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, acyl chlorides swap the hydroxyl group in a carboxylic acid for a chlorine atom. To make acyl chlorides, we react carboxylic acids with phosphorus(V) chloride (PCl₅), phosphorus(III) chloride (PCl₃) or sulphur dichloride oxide (SOCl₂). The reaction with PCl₃ requires heat, whilst the reactions with PCl₅ and SOCl₂ don't require any special conditions. They all result in different products. For example:

$$CH_3COOH+PCl_5\rightarrow CH_3COCl+POCl_3+HCl$$

$$3CH_3COOH+PCl_3\rightarrow 3CH_3COCl+H_3PO_3$$

$$CH_3COOH+SOCl_2\rightarrow CH_3COCl+SO_2+HCl$$

Acid derivatives such as acyl chlorides frequently react in nucleophilic addition-elimination reactions. These reactions can form a range of products, such as esters, amides, and even carboxylic acids again!

Further reactions of carboxylic acids

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

• Reduction.
• Decarboxylation.
• Oxidation.

Reduction

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) Reduction initially 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 ((CH₃CH₂)₂O).

The overall reaction is as follows:

$$RCOOH+4[H]\rightarrow RCH_2OH+H_2O$$

For example, reducing ethanoic acid gives ethanol:

$$CH_3COOH+4[H]\rightarrow CH_3CH_2OH+H_2O$$

Decarboxylation

Carboxylic acids react in decarboxylation reactions. Here, 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. Here's the general equation:

$$RCOOH\rightarrow RH+CO_2$$

For example, decarboxylating ethanoic acid produces the alkane methane:

$$CH_3COOH\rightarrow CH_4+CO_2$$

Oxidation

Earlier on in this article, we explored how carboxylic acids are produced by oxidising alcohols (ROH). We first produce an aldehyde (RCHO) which is then oxidised further into a carboxylic acid (RCOOH). But for some particular carboxylic acids, the reaction doesn't stop there. Two carboxylic acids which can be oxidised further are methanoic acid (HCOOH) and ethanedioic acid ((COOH)₂).

Methanoic acid can be oxidised into carbon dioxide and water using either a mild oxidising agent, such as Fehling's solution or Tollens' reagent, or a stronger oxidising agent, such as acidified potassium manganate or acidified potassium dichromate. Here's the equation:

$$HCOOH+[O]\rightarrow CO_2+H_2O$$

Ethanedioic acid can also be oxidised into carbon dioxide and water. However, note two things:

• Ethanedioic acid is a dicarboxylic acid, meaning that it has two carboxyl functional groups. One mole of ethanedioic acid needs two moles of oxidising agent to oxidise it fully.
• Ethanedioic acid can only be oxidised by strong oxidising agents such as acidified potassium manganate or acidified potassium dichromate. Weak oxidising agents, like Fehling's solution or Tollens' reagent, have no effect.

Here's the equation:

$$(COOH)_2+2[O]\rightarrow 2CO_2+2H_2O$$