25.3 Formation and Reactions of Carboxylic Acids

Learning Objectives

By the end of this section, you will be able to:

  • Describe the preparation of carboxylic acids.
  • Examine chemical reactions of and with carboxylic acids.

Organic functional groups can be converted into other functional groups through reactions.  A map of some of the more common reactions to convert functional groups can be found in Section 19.6 – General Reactions of Carbon in Infographic 19.6a.

Preparation of Carboxylic Acids

Oxidation

Carboxylic acids are the most polar organic compounds because both functional groups are polar. The hydroxyl (-OH) group is similar to that in alcohols while the carbonyl group (C=O) has similarities to aldehydes and ketones. We prepare carboxylic acids by the oxidation of aldehydes or alcohols whose –OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol (Figure 25.3a.).

A chemical diagram with two arrows is shown. On the left, an alcohol, indicated with a C atom to which an R group is bonded to the left, H atoms are bonded above and below, and in red, a single bonded O atom with an H atom bonded to the right is shown. Following the first reaction arrow, an aldehyde is shown. This structure is represented with an R group bonded to a red C atom to which an H atom is bonded above and to the right, and an O atom is double bonded below and to the right. Appearing to the right of the second arrow, is a carboxylic acid comprised of an R group bonded to a C atom to which, in red, an O atom is single bonded with an H atom bonded to its right side. A red O is double bonded below and to the right. All O atoms have two pairs of electron dots.
Figure 25.3a. Diagram representing the addition of a double bond to oxygen and then a hydroxyl group to an alcohol to create a carboxylic acid. (credit: Chemistry (OpenStax), CC BY).

For example, in the presence of an oxidizing agent, ethanol is oxidized to acetaldehyde, which is then oxidized to acetic acid (Figure 25.3b.). This process also occurs in the liver, where enzymes catalyze the oxidation of ethanol to acetic acid using dehydrogenase. Acetic acid can be further oxidized to carbon dioxide and water.

A chemical diagram with two arrows is shown. On the left, ethanol, indicated with a C atom to which an R group is bonded to the left, H atoms are bonded above and below, and in red, a single bonded O atom with an H atom bonded to the right is shown. Following the first reaction arrow, acetylaldehyde is shown. This structure is represented with an R group bonded to a red C atom to which an H atom is bonded above and to the right, and an O atom is double bonded below and to the right. Appearing to the right of the second arrow, is acetic acid comprised of an R group bonded to a C atom to which, in red, an O atom is single bonded with an H atom bonded to its right side. A red O is double bonded below and to the right. All O atoms have two pairs of electron dots.
Figure 25.3b. Similar to figure 25.2a. we now see specific examples of an alcohol, aldehyde and carboxylic acid represented. (credit: Intro to Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

Hydrolysis of Nitriles

Nitriles are organic compounds in which a cyano group (carbon triple bonded to a nitrogen) is attached to a carbon. In Chapter 24, hydrogen cyanide was added to an aldehyde or ketone to form a cyanohydrin.  The cyanohydrin contains a nitrile (-C≡N – where ≡ is triple bond).  Another method to form a nitrile is shown in Figure 25.3c. Here a primary or secondary alkyl halide will react with sodium cyanide in a substitution reaction to form the alkyl nitrile and sodium halide (Morsch et al, n.d.).

The reaction show displays the formation of nitrile through the substitution reaction of alkyl bromide with sodium cyanide. This produces two products; the nitrile and sodium bromide.
Figure 25.3c. Formation of nitrile through substitution reaction of alkyl bromide with sodium cyanide. (credit: Organic Chem (Morsch et al.), CC BY-SA 4.0).

Nitriles can undergo hydrolysis reactions in the presence of an acidic or basic aqueous solution to form carboxylic acids. In the case of acid catalysts, the nitrile becomes pronated (the addition of a proton or hydrogen cation to an atom forming a conjugate acid). In the case of basic catalysts, the hydroxide anion is capable of direct addition to the carbon-nitrogen triple bond. The examples below outline the reactions taking place during hydrolysis of nitriles. Figure 25.3d. shows the basic reaction for nitriles in an acidic catalyst.

The reaction of a nitrile with heat and an acid catalyst will form the corresponding carboxylic acid.
Figure 25.3d. Hydrolysis reaction of a nitrile with an acidic catalyst forming a carboxylic acid (credit: Organic Chem (Morsch et al.), CC BY-SA 4.0).

Figure 25.3e. below is a specific example of acidic hydrolysis using cyclopentanecarbonitrile.

If cyclopentanecarbonitrile reacts with heat and an acidic catalyst, cyclopentanecarboxylic acid is formed.
Figure 25.3e. Hydrolysis reaction of a cyclopentanecarbonitrile with an acidic catalyst forming a cyclopentanecarboxylic acid (credit: Organic Chem (Morsch et al.), CC BY-SA 4.0).

Figure 25.3f., shows the basic hydrolysis reaction of nitriles with an alkaline catalyst.

Hydrolysis of a nitrile with a basic catalyst and heat can also lead to production of a carboxylic acid.
Figure 25.3f. Hydrolysis reaction of a nitrile with a basic catalyst forming a carboxylic acid (credit: Organic Chem (Morsch et al.), CC BY-SA 4.0).

Figure 25.3g. below is a specific example of basic hydrolysis using Butane nitrile.

When butane nitrile reacts with a basic catalyst such as sodium hydroxide, it produces butanoic acid.
Figure 25.3g. Hydrolysis reaction of butane nitrile with a basic catalyst forming a butanoic acid (credit: Organic Chem (Morsch et al.), CC BY-SA 4.0).

Reactions of Carboxylic Acids

Carboxylic acids are weak acids, meaning they are not 100% ionized in water. Generally, only about 1% of the molecules of a carboxylic acid dissolved in water are ionized at any given time. The remaining molecules are undissociated in solution. They are however considered to be more acidic than most other organic compounds. When carboxylic acids dissociate in water a hydrogen ion is transferred to a water molecule and a carboxylate ion and hydronium ion (H3O+) are formed (Figure 25.3h.). Refer back to Infographic 19.6a showing reactions of organic molecules.

CH3COOH + H2O → CH3COO– + H3O+

      Carboxylic Acid + water → Carboxylate Ion + Hydronium Ion

Figure 25.3h. Reaction of a carboxylic acid with water to produce a carboxylate ion and hydronium ion.

Acid-Base Reactions of Carboxylic Acids

Because of the acidic properties of carboxylic acids, they are able to react with bases to form ionic salts. Alkali metal hydroxides and simple amines result in salts with pronounced ionic character that are usually soluble in water. An example of this can be seen in Figure 25.3i. below.

A chemical reaction is shown. On the left, a structure of propionic acid is indicated. This structure includes a 2 carbon hydrocarbon group on the left end in black. Above, below, and to the left, H atoms are bonded. This group is bonded to a red group comprised of a C atom to which an O atom is double bonded above. To the right of the red C atom, an O atom is connected with a single bond. To the right of the O atom, an H atom is bonded. To the right of this structure appears a plus and N a O H. Following the reaction arrow, the propionate ion is shown. This structure is in brackets. Appearing inside the brackets, is a 2 carbon hydrocarbon group on the left end. Above, below, and to the left, H atoms are bonded. To the right of this group, a group in red is attached comprised of a C atom to which an O atom is double bonded above and a second O atom is single bonded to the right. Outside the brackets appears a superscript minus symbol. This is followed by a plus sign, N a superscript plus another plus sign and H subscript 2 O. The singly bonded O atom in the propionate ion structure has 3 pairs of electron dots. All other O atoms have two pairs of electron dots.
Figure 25.3i. Reaction of a carboxylic acid with a strong base (sodium hydroxide) forming a carboxylate ion, a sodium ion and water. (credit: Chemistry (OpenStax), CC BY).

Heavy metals such as silver, mercury and lead form salts with more covalent characteristics which reduces water solubility, particularly for acids composed of four or more carbon atoms in the chain (Figure 25.3j).

RCO2H + AgOH → RCO2δ(-) Agδ(+) +   H2O

Figure 25.3j. Reaction of a heavy metal base with a carboxylic acid.

Esterification

Another reaction which takes place with carboxylic acids is esterification. This reaction type is commonly used to convert carboxylic acids to their ester derivatives. In order to produce an ester from an alcohol and a carboxylic acid, we must heat them in the presence of an acid catalyst such as sulfuric acid (Figure 25.3k. and Figure 25.3l.). This reaction will produce a fragrant ester and water. The reaction is reversible and will reach equilibrium with approximately equivalent amounts of reactants and products. Using excess amounts of alcohol and continuously removing a product, can drive the reaction towards the product side as per LeChatelier’s principle.

The reaction here shows esterification in which a carboxylic acid and an alcohol combine to produce an easter and water.
Figure 25.3k. General esterification reaction with carboxylic acid and alcohol combining to form ester with byproduct of water. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).
A chemical reaction is shown. On the left, a C H subscript 3 group bonded to a red C atom. The C atom forms a double bond with an O atom which is also in red. The C atom is also bonded to an O atom which is bonded to an H atom, also in red. A plus sign is shown, which is followed by H O C H subscript 2 C H subscript 3. The H O group is in red. Following a reaction arrow, a C H subscript 3 group is shown which is bonded to a red C atom with a double bonded O atom and a single bonded O. To the right of this single bonded O atom, a C H subscript 2 C H subscript 3 group is attached and shown in black. This structure is followed by a plus sign and H subscript 2 O. The O atoms in the first structure on the left and the structure following the reaction arrow have two pairs of electron dots.
Figure 25.3l. Reaction of acetic acid and an alcohol to produce an ester and water. (credit: Chemistry (OpenStax), CC BY).

Example 25.3a

Preparation of an ester via esterification uses a carboxylic acid and alcohol, heated in the presence of an acid catalyst (Figure 25.3m.). This reaction is reversible and will reach equilibrium with approximately equal amounts of reactants and products.

Write the esterification of acetic acid with 1-butanol.

Solution:

The solution shows the reaction of acetic acid with 1-butanol to form butyl acetate and a byproduct of water.
Figure 25.3m. Esterification of acetic acid and 1-butanol forming butyl acetate and water (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Amide Formation

Similar to esterification, carboxylic acids will react with ammonia to form a primary amide (Figure 25.3n.). When a carboxylic acid reacts with primary or secondary amines, secondary or tertiary amides are produced, respectively (Figure 25.3o.). Tertiary amines do not form amides when reacted with carboxylic acids.

Reaction of acetic acid and ammonia to form a primary amide, acetamide and water.
Figure 25.3n. Primary amide formation from reaction of carboxylic acid with ammonia. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).
The reaction of propanoic acid with a primary amine to form a secondary amide and water.
Figure 25.3o. Formation of secondary amide from reaction of carboxylic acid with primary amine. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Exercise 25.3a

Write the product that results from each of the following reactions.

  1. methanol + 2-methylbutanoic acid
  2. ethanol + pentanoic acid
  3. propanoic acid + ammonia

Check Your Answers:[1]

Exercise and image source: Exercise 25.3a questions and answers are created by Samantha Sullivan Sauer, using images from Biovia Draw, licensed under CC BY-NC 4.0

Acid Chloride Formation and Reactions

Carboxylic acids react with thionyl chloride (SOCl2) to form acid chlorides (also known as acyl chlorides) (Figure 25.3p.). During the reaction the hydroxyl group of the carboxylic acid is converted to a chlorosulfite intermediate making it a better leaving group. The chloride anion produced during the reaction acts a nucleophile. Acyl chlorides are extremely reactive, resulting in the chlorine being replaced with something else.

A carboxylic acid reactions with thionyl chloride catalyst to produce acid chloride, hydrochloric acid and sulphur dioxide.
Figure 25.3p. General reaction of carboxylic acid with thionyl chloride to produce acid chloride (credit: Supplemental Modules (Organic Chemistry, CC BY-NC-SA 4.0).

An acid chloride will react with a carboxylic acid to form an acid anhydride (Figure 25.3q.), with water to form a carboxylic acid (Figure 25.3r.), with an alcohol to form an ester (Figure 25.3s.) and with ammonia or an amine to form an amide (Figure 25.3t.).

 

An acid chloride reacting with a carboxylic acid to produce an acid anhydride and hydrochloric acid.
Figure 25.3q. Formation of an acid anhydride from substitution of an acid chloride with a carboxylic acid (credit: UIS: CHE 269 (Morsch and Andrews) , CC BY-NC-SA 4.0).
An acid chloride reacting with water to produce a carboxylic acid and hydrochloric acid.
Figure 25.3r. Formation of a carboxylic acid from substitution of an acid chloride with water (credit: UIS: CHE 269 (Morsch and Andrews) , CC BY-NC-SA 4.0).
An acid chloride and an alcohol reacting to produce an ester and hydrochloric acid.
Figure 25.3s. Formation of an ester from substitution of an acid chloride with an alcohol (credit: UIS: CHE 269 (Morsch and Andrews) , CC BY-NC-SA 4.0).
An acid chloride and an amine reacting to produce an amide
Figure 25.3t. Formation of an amide from substitution of an acid chloride with an amine or ammonia (credit: UIS: CHE 269 (Morsch and Andrews) , CC BY-NC-SA 4.0).

 

Example 25.3b

Complete each reaction.

(a) A benzene ring with a carbon double bonded to an oxygen and single bonded to a chlorine reacts with ethanoic acid to produce what?

(b) A 3 carbon chain with a double bonded oxygen and chlorine bonded to carbon 1 reacts with water to produce what?

(c) A benzene ring with a carbon double bonded to an oxygen and single bonded to a chlorine reacts with ethanol to produce what?

(d) A 2 carbon chain with a double bonded oxygen and a chlorine bonded to the first carbon, reacts with 2 ammonia to produce what?

(e) A 2 carbon chain with a double bonded oxygen and a chlorine bonded to the first carbon, reacts with 2 NH2CH2CH3 to produce what?

Solutions:

(a) Answer to a).

(b) Answer to b).

(c) Answer to c).

(d) Answer to d).

(e) Answer to e).

Example source and images source: UIS: CHE 269 (Morsch and Andrews), CC BY-NC-SA 4.0).

For more advanced understanding of Carboxylic acid structure and reactions check out the videos below.

Watch Crash Course – Organic Chemistry #30 on YouTube (11 min)

Video source: Crash Course – Organic Chemistry. (2021, June 23). Carboxylic Acids: Crash Course Organic Chemistry #30 [Video]. YouTube.

Watch Crash Course – Organic Chemistry #31 on YouTube (12 min)

Video source: Crash Course – Organic Chemistry. (2021, July 28). Carboxylic Acid Derivatives and Hydrolysis Reactions: Crash Course Organic Chemistry #31 [Video]. YouTube.

Attribution & References

Except where otherwise noted, this page is written and adapted by Caryn Fahey and Samantha Sullivan Sauer from

References cited in-text

Farmer, S., Kennepohl, D., Morsch, L., & Reusch, W. (n.d.). Chemistry of Nitriles. In Organic Chemistry (Morsch et al.). CC BY-SA 4.0.

  1. Shows the line diagram equations for the solutions to the exercise question. a) methanol plus 2-methylbutanoic acid forms methyl 2-methylbutanoate plus water. b) ethanol plus pentanoic acid forms ethyl pentanoate plus water. c) propanoic acid plus ammonia forms propanamide and water.

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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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