You will love this comprehensive resource if you want to learn about carboxylic acids and derivatives.

But first of all, let’s recall what carboxylic acids actually are.

They are organic compounds that contain a carboxyl functional group (COOH). This general formula shows this:

R-COOH

Here, R refers to an alkyl, alkenyl, aryl, or another group. This takes us straight to our first topic.

Production of benzoic acid:

Note that benzoic acid is the simplest benzene-based carboxylic acid.

In simple words, a carboxyl (COOH) group is attached to benzene. Now the question is, how is benzene converted into benzoic acid?

Alkylbenzenes undergo oxidation to form benzoic acid (C6H5COOH). Here are the details:

  • First of all, alkylbenzene is heated under reflux with hot alkaline KMnO4.
production of benzoic acid

Due to this, a brown precipitate of MnO2 is formed.

  • Then, this mixture is acidified with dilute acid.

This is how benzoic acid is formed. This takes us straight to the next topic.

Reactions of carboxylic acids:

You should know that carboxylic acids produce acyl chlorides when they react with PCl3, PCl5 or SOCl2.

Here are the details:

Recall that acyl chlorides (acid chlorides) are reactive derivatives of carboxylic acids. This simply means that they are formed (derived) from carboxylic acids.

Acyl chlorides are represented as:

R-COCl

Here, R is a side chain.

We can produce acyl chlorides in three ways. Let’s take a look at each of them.

Note that these reactions involve the replacement of OH (from carboxylic acid) with a Cl atom.

1. Liquid sulfur dichloride oxide (SOCl2):

In this reaction, sulfur dichloride oxide (thionyl chloride) reacts with a carboxylic acid to produce an acyl chloride, sulfur dioxide (SO2) and HCl gas.

Here’s an example.

CH3COOH + SOCl2 → CH3COCl + SO2 + HCl

The condition for this reaction is room temperature. Now, let’s move on to the second method.

2. Liquid phosphorus(III) chloride (PCl3) and heat:

In this reaction, a carboxylic acid reacts with PCl3 to form an acyl chloride and phosphorous acid (H3PO3).

Note that heating is required for this reaction to occur. Here’s an example.

CH3CH2COOH + PCl3 → CH3CH2COCl + H3PO3

This takes us to the last method of preparing acyl chloride.

3. Solid phosphorus(V) chloride (PCl5)

Note that room temperature is the condition for this reaction to occur.

In this reaction, phosphorus(V) chloride reacts with a carboxylic acid to produce an acyl chloride, POCl3 and HCl.

reactions of carboxylic acids

Here’s an example.

CH3COOH + PCl5 → CH3COCl + POCl3 + HCl

So, these are the three methods to prepare acyl chlorides from carboxylic acids.

Now the question is, why do we need acyl chlorides?

Remember that acyl chlorides are more reactive than carboxylic acids or carboxylic esters. Now, let’s talk about the oxidation of carboxylic acids.

Oxidation:

Generally, carboxylic acids are not oxidised.

However, some carboxylic acids can get further oxidised to carbon dioxide and water. These are:

  • Methanoic acid (HCOOH)
  • Ethanedioic acid (HOOCCOOH)

Let’s take a look at each of them in detail.

Oxidation of methanoic acid:

Methanoic acid is oxidised by mild oxidising agents like Fehling’s solution or Tollens’ reagent.

If you look at its structure, you will notice an aldehyde group. This is the hydrogen attached to the carbon-oxygen double bond.

We have already studied that aldehydes are oxidised by Fehling’s solution or Tollens’ reagent. Here are the equations:

(with Fehling’s solution):

HCOOH + 4OH + 2Cu2+→ CO2 + 3H2O + Cu2O

Note that Fehling’s solution contains Cu2+ ions (blue). After the reaction, we get a red precipitate of copper(I) oxide.

(with Tollens’ reagent):

HCOOH + 2OH + 2Ag(NH3)2+ → CO2 + 2H2O + 2Ag + 4NH3

In the above reaction, Ag+ ions are reduced to silver metal (Ag). In short, we get a silver mirror.

Apart from these, we can also use stronger oxidising agents such as:

  • Acidified potassium dichromate(VI) solution (K2Cr2O7): Orange solution changes to green.
  • Acidified potassium manganate(VII) solution (KMnO4): The purple solution becomes colourless.

Summary: Methanoic acid is oxidised to carbon dioxide and water.

oxidation of methanoic and ethanedioic acid

Now, let’s talk about the oxidation of ethanedioic acid.

Oxidation of ethanedioic acid:

Ethanedioic acid is oxidised to carbon dioxide.

It is done by warm potassium manganate(VII) solution (KMnO4) acidified with dilute sulfuric acid.

The balanced ionic equation for this redox reaction is:

5(COOH)2 + 2MnO4 + 6H+ → 10CO2 + 2Mn2+ + 8H2O

You should know that the manganese ions (Mn2+) catalyse the reaction. This is known as autocatalysis (product catalyses the reaction).

Now, let’s move on to the next topic.

The acidity of carboxylic acids:

Relative acidities of carboxylic acids, phenols and alcohols:

Here’s a fact.

Carboxylic acids are weak acids. But, they are stronger acids than phenols or alcohols.

Before talking about this in detail, note that an acid ionises by releasing a proton (H+ ion). The two factors that determine the ionisation of acid are:

  • The strength of the bond that is breaking.
  • The stability of the ions being formed.

Let’s talk about these two aspects to find out why carboxylic acids are relatively stronger.

  1. In a carboxylic acid, the H from the OH is lost to form a carboxylate ion.

This is because this OH bond is weakened by the carbonyl group. So, now we have COO instead of COOH.

This delocalisation spreads out the negative charge throughout the carboxylate ion. This reduces the charge density, making it less likely to bond with an H+ ion.

Summary: A stronger acid ionises (release H+ ion) more rapidly. As delocalisation makes the carboxylate ion more stable, it is more likely to form.

relative acidities of carboxylic acids, phenols and alcohols

Now, let’s talk about phenols.

Phenol:

The question is, why is phenol less acidic than carboxylic acid?

To recap, phenols have an OH group attached directly to a benzene ring. When this oxygen-hydrogen bond breaks, we get a phenoxide ion (C6H5O).

Let’s study this ion in detail.

  1. If we compare the phenoxide ion to the carboxylate ion, it is less effectively delocalised.

Therefore, the carboxylate ion is more stable than the phenoxide ion. Remember, the delocalisation of the charge makes the ion more stable and less likely to gain a proton.

Pretty simple, isn’t it?

Alcohols:

Alcohols are the weakest acids.

When the hydrogen-oxygen bond breaks, a hydrogen ion is released. Now, we have RO_ instead of ROH. Here, R is an alkyl group.

  1. This alkyl group is an electron-donating group that donates electron density to the oxygen atom.

This makes it less stable. As a result, alcohol is likely to be reformed by gaining an H+ ion. This is the reason why they are hardly acidic.

Now, let’s move on to the next topic.

Relative acidities of chlorine-substituted carboxylic acids:

Here’s the concept.

The acidity of carboxylic acids is also dependent on the substituent alkyl or aryl group. Let me explain.

Chlorine atoms are an example of electron-withdrawing groups.

Note that the electron-withdrawing group bonded to the carbon atom next to the -COOH group makes the acid stronger.

To better understand this, take a look at the table below:

AcidKa at 25°C / mol dm-3
Ethanoic acid (CH3COOH)1.7 x 10-5
Chloroethanoic acid (CH2ClCOOH)1.3 x 10-3
Dichloroethanoic acid (CHCl2COOH)5.0 x 10-2
Trichloroethanoic acid (CCl3COOH)2.3 x 10-1
The larger the value of Ka, the stronger the acid.

What does this show?

Important Note: The larger the value of Ka, the stronger the acid. But, the lower the value of pKa (index to express the acidity of weak acids), the stronger the acid.

As the number of chlorine atoms increases, the acid becomes stronger. This is the reason why CCl3COOH is the strongest acid.

Here’s why.

Trichloroethanoic acid (CCl3COOH) has three electronegative chlorine atoms. Therefore, these three electron-withdrawing groups decrease in strength of the O-H bond.

These electron-withdrawing groups also help stabilise the anion formed. So, it is less likely to gain the H+ ion.

This explains why trichloroethanoic acid (CCl3COOH) is the strongest acid. This takes us straight to the next topic.

Production of esters:

We know that carboxylic acids and alcohols react to form an ester.

The OH from the carboxylic acid and the H from the alcohol form a water molecule. Here’s an example:

CH3COOH + CH3CH2CH2OH → CH3CO2CH2CH2CH3 + H2O

As you can see in the reaction above, ethanoic acid reacts with propanol to form propyl ethanoate. Note that the first part of the name (ester) comes from the alcohol.

But, there’s a problem.

The reaction of alcohols and carboxylic acids is slow and hardly goes to completion.

Therefore, acyl chlorides are used instead of carboxylic acids. They are more reactive than carboxylic acids. So, the formation of esters occurs more rapidly.

Here’s an example.

Ethanoyl chloride + Ethanol → Ethyl ethanoate + Hydrogen chloride

Instead of alcohol, we can also use phenols.

But for this reaction to occur, we need a base (NaOH) to convert phenol into a phenoxide ion. Remember that this phenoxide ion acts as a strong nucleophile to attack the acyl chloride.

With this, let’s move on straight to the next topic.

Acyl chlorides:

As we discussed earlier, acyl chlorides are reactive derivatives of carboxylic acids.

Recall that acyl chlorides are produced when carboxylic acids react with PCl3, PCl5 or SOCl2. Now, let’s take a look at some reactions of acyl chlorides.

Hydrolysis:

Before talking about this reaction, note that acyl chlorides undergo addition-elimination reactions.

As the word suggests, there is an addition of a small molecule across the C=O. This is followed by the elimination of a small molecule.

I will explain the mechanism later in this article. But first, let’s talk about hydrolysis.

  • Change in the functional group: Acyl chloride Carboxylic acid
  • Reagent: Water
  • Condition: Room temperature

In the hydrolysis of acyl chlorides, the addition of water produces carboxylic acid and HCl (steamy acidic gas). For example, the reaction of ethanoyl chloride and cold water produces ethanoic acid and HCl gas.

Now, let’s take a look at the addition-elimination mechanism:

  1. First of all, the nucleophile attacks the positive carbon in the acyl chloride.

This allows the nucleophile to form a bond.

Note: Nucleophiles are negative species (ions or molecules) that are attracted to a region of positive charge.

In this case, water acts as a nucleophile.

hydrolysis of carboxylic acids and derivatives

With this, the first step of “addition” is complete (due to the lone pair on the oxygen atom).

  1. Now, the elimination stage is divided into two parts.

First of all, the carbon-oxygen double bond is formed again. So, the electrons on the carbon-chlorine bond are repelled.

This pushes the electrons on the chlorine atom, forming a chloride ion. This chlorine and hydrogen from the nucleophile (water) make HCl gas.

Pretty simple, isn’t it?

Reaction with alcohols:

Here’s what you need to know:

Change in the functional group: Acyl chloride Ester

Reagent: Alcohol

Condition: Room temperature

You should know that esters are formed when acyl chlorides react with alcohol. Here is an example.

Ethanoyl chloride reacts with cold ethanol to form ethyl ethanoate (ester) and HCl gas. To understand this reaction, let’s take a look at the addition-elimination mechanism.

Addition:

At this stage, the nucleophile attacks the positive carbon atom.

We call this addition as the carbon-oxygen double bond in the acyl chloride is broken. Plus, the nucleophile also forms a bond.

carboxylic acids and derivatives

Now, the second stage of this reaction occurs:

Elimination:

  1. First of all, the carbon-oxygen double bond reforms.

Recall that this bond was broken in the first step. During this stage, a chloride ion is pushed off as well.

  1. Now, a hydrogen ion is also removed.

This results in the formation of ethyl ethanoate and hydrogen chloride. With this, let’s move on to the next reaction.

Reaction with phenol:

Here is what you need to know.

Change in the functional group: Acyl chloride Ester

Reagent: Phenol

Condition: Base (NaOH)

Recall that phenols are aromatic compounds with an alcohol group (-OH) bonded to a benzene ring.

If I talk about their reaction with acyl chlorides, an aromatic ester is formed.

This reaction requires a base (NaOH) and heat. Now you might be wondering, why is this so?

Note that this base converts phenol into a phenoxide ion. This phenoxide ion is a better nucleophile than phenol. So, the reaction occurs more rapidly.

With this, let’s move on to the next reaction.

Reaction with ammonia:

Here is the overview of this reaction:

  • Change in the functional group: Acyl chloride → Amide
  • Reagent: Ammonia
  • Condition: Room temperature

Now, let’s take a look at an example.

When ethanoyl chloride reacts with a cold concentrated solution of ammonia, a white solid product is formed.

This product is a mixture of ethanamide (an amide) and ammonium chloride. Let’s take a look at the addition-elimination mechanism:

Addition:

First of all, there is a nucleophilic attack on the positive carbon atom. This is done by the lone pair on the nitrogen atom in ammonia.

We call this addition as the carbon-oxygen double bond in the acyl chloride is broken. This takes us to the next stage.

Elimination:

Now, the carbon-oxygen double bond reforms. The chloride ion is then pushed off.

carboxylic acids and derivatives

Over here, a hydrogen ion is also removed that forms HCl gas. This HCl gas reacts with excess ammonia to form ammonium chloride.

Pretty simple, isn’t it?

Reaction with primary amine:

Here’s the overview of this reaction:

Change in the functional group: Acyl chloride Secondary amide

Reagent: Primary amine

Condition: Room temperature

Before talking about this reaction, let’s discuss the difference between amines and amides.

Note: The main difference is that amines do not have a carbonyl group (C=O) attached to the nitrogen atom. However, amides have a carbonyl group attached to a nitrogen atom.

Here is an example.

Propanoyl chloride reacts with methylamine to form methyl propanamide and HCl. In the same way, secondary amines will also react at room temperature to produce amide and HCl.

This takes us straight to the next topic.

The relative ease of hydrolysis:

I have a question for you.

Which one of the following organic compounds is more easily hydrolysed?

  1. Acyl chlorides
  1. Alkyl chlorides
  1. Halogenoarenes (aryl chlorides)

Recall that hydrolysis is a reaction in which one or more chemical bonds are broken. So, here’s what you need to know.

Acyl chlorides are easily hydrolysed by water (a weak nucleophile).

This is because acyl carbon is strongly electrophilic (electron deficient) in nature due to the presence of oxygen and chlorine (highly electronegative).

Now, let’s talk about alkyl chlorides.

Alkyl chlorides are less reactive than acyl chlorides. So, they are hydrolysed by aqueous NaOH (strong nucleophile) upon heating under reflux (but not with water).

Here’s why.

The C-Cl bond in alkyl chlorides is less polar. Hence, they are not attacked by weak nucleophiles such as water.

Moving on, aryl chlorides are the least reactive.

They are not hydrolysed by water and aqueous OH- ions. This is because of the greater strength of the C-Cl bond.

As we have discussed this concept earlier, the lone pair of electrons on the Cl atom is partially delocalised on the benzene ring. In short, the C-Cl bond is stronger.

This strength of the C-Cl bond makes it less reactive.

Wrapping Up:

So, there you have it.

Our topic about carboxylic acids and derivatives is covered. To recap, we studied the reactions of carboxylic acids and derivatives in terms of the nucleophilic addition-elimination mechanism.

If you want to ace this topic in your exam, do practise past paper questions.

Thank you for reading and staying with me till the end. Stay tuned for more.

Further reading:

Hydrocarbons (Arenes) | A2 Organic Chemistry Notes

Hydroxy Compounds (Phenol) | A2 Organic Chemistry Notes

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