Reactions of Carboxylic Acids

Carboxylic acids and esters, at first glance, seem to be completely different types of molecules. For example, vinegars, such as the balsamic vinegar you might mix in a salad dressing, are about 5-8% acetic acid by volume. Acetic acid is a carboxylic acid with the IUPAC name ethanoic acid, and is sharp and stringent. But the oil you combine with the vinegar in your salad dressing? That’s an example of an ester - compounds with fruity, floral aromas. However, it is easy to go from one to the other in a process called esterification. Esterification is an example of a reaction of carboxylic acids.

Reactions of Carboxylic Acids Reactions of Carboxylic Acids

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Contents
Table of contents
    • This article is about the reactions of carboxylic acids in organic chemistry.
    • Firstly, we will look at the production of carboxylic acids.
    • We’ll then look at how they react as acids, including their reaction with water, carbonates, hydroxides, metals, and ammonia.
    • We’ll then explore reacting carboxylic acids with alcohols in esterification.
    • Subsequently, we'll learn about reactions of carboxylic acids and their derivatives.
    • Finally, we’ll touch upon further reactions of carboxylic acids, such as reduction, decarboxylation, and oxidation.

    What are carboxylic acids?

    Carboxylic acids are organic molecules containing the carboxyl functional group, -COOH.

    The -COOH functional group is in turn made up of two other functional groups - the carbonyl group, C=O, and the hydroxyl group, -OH.

    Reactions of Carboxylic Acids general structure functional groups StudySmarterThe general structure of a carboxylic acid. Here it is shown with the carbonyl and hydroxyl groups highlighted. StudySmarter Originals

    If this is your first time coming across this family of organic molecules, we'd recommend reading Carboxylic Acids before you go any further, to understand the basics of their chemistry.

    Producing carboxylic acids

    There are a few different ways of making carboxylic acids. We'll look at three of them in this article:

    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:

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

    We use [O] in chemical equations to represent the oxidising agent.

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

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

    CH3CH2OH +2[O] CH3COOH +H2O

    Reactions of Carboxylic Acids Reflux StudySmarterReflux. commons.wikimedia.org

    You might carry out the oxidation of alcohols in class. If you do, observe the reaction. What colour change do you notice? The potassium dichromate should turn from orange to green as the alcohol is oxidised.

    To explore this reaction in more depth, check out Oxidation of Alcohols.

    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:

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

    You'll compare these two reactions in the article Reactions of Esters.

    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.

    An acid is a proton donor.

    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.

    Weak acids are acids that only partially ionise in solution.

    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:

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

    Reactions of Carboxylic Acids dissociation ionisation StudySmarterThe ionisation of carboxylic acids. The reaction exists in a state of equilibrium. StudySmarter Originals

    Check out Weak Acids and Bases to find out more about how weak acids and bases differ from their strong acid/base relatives. In addition, you might want to visit Chemical Equilibrium for an introduction to the wonderful world of equilibria and reversible reactions.

    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:

    Reactions of Carboxylic Acids dissociation ionisation charge delocalisation StudySmarterWhen 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.

    Reactions of Carboxylic Acids test carboxylic acids limewater cloudy co2 carbon dioxide StudySmarterTesting for carbon dioxide. Carbon dioxide gas turns clear limewater cloudy.StudySmarter Originals

    When you draw the salt or write it out using a structural formula, make sure you don’t draw a bond between the metal ion and the carboxylate ion. This is because they are joined by an ionic bond, not a covalent bond. An ionic bond is the electrostatic attraction between oppositely charged ions, whereas a covalent bond is a shared pair of electrons.

    For example, we draw the salt sodium methanoate as shown below:

    Reactions of Carboxylic Acids sodium methanoate StudySmarterA carboxylate salt: sodium methanoate. StudySmarter Originals

    For more information on the different types of chemical bonding, check out the articles Covalent and Dative Bonding and Ionic Bonding.

    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.

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

    Carboxylate ions have a charge of -1. This means that when carboxylic acids react with a group 2 metal base, you need two moles of the acid for every mole of the base. You can see this in the example above.

    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.

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

    Reactions of Carboxylic Acids ammonium ethanoate StudySmarterAmmonium 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.

    Esterification is a type of reaction that produces an ester: an organic molecule that contains the ester linkage group (-COO-).

    Esters are used in a variety of different products, from soaps and shampoos to plastic packaging and biodiesel.

    If you want an overview of these molecules, including their properties and their nomenclature, feel free to visit the article Esters.

    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:

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

    Reactions of Carboxylic Acids General structure of an ester StudySmarterThe 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.

    1. You've seen the reversible reaction before, earlier on in this article. Hydrolysing esters breaks them down into alcohols and carboxylic acids, and so is one way of producing carboxylic acids.
    2. For more on reversible reactions, take a brief look at the article Chemical 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.

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

    Reactions of Carboxylic Acids esterification reaction butyl ethanoate  StudySmarterThe 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.

    You can learn how to influence the yield of equilibrium reactions in Le Chatelier's Principle and Application of Le Chatelier's Principle.

    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!

    There are additional types of acid derivatives beyond acyl chlorides, such as acid anhydrides. You can learn more about these molecules in Acid Derivatives, Once you've read that article, explore Acylation to see how acid derivatives react.

    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:

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

    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.

    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:

    $$CH_3COOH\rightarrow CH_4+CO_2$$

    Notice how we started with ethanoic acid, which has a carbon chain with two carbon atoms, and ended up with methane, which has a carbon chain with just one carbon atom. Decarboxylation shortens the carbon chain - the spare carbon atom is released as CO2.

    The opposite to decarboxylation is carboxylation and is an important step in photosynthesis. The enzyme RuBisCo captures carbon by combining carbon dioxide with RuBP to form 3-phosphoglycerate as part of a process called the Calvin cycle. Check out Light-independent Reaction for more information.

    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:

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

    All the processes above show the characteristic colour changes that you associate with successful oxidation reactions involving these reagents:

    • Blue Fehling's solution forms a brick-red precipitate.
    • Colourless Tollens' reagent forms a silver mirror deposit.
    • Purple acidified potassium manganate decolourises.
    • Orange acidified potassium dichromate turns green.

    Reactions of Carboxylic Acids diagram showing the colour changes of common oxidising agents StudySmarterColour changes of common oxidising agents.StudySmarter Originals

    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:

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

    Reactions of Carboxylic Acids - Key takeaways

    • Carboxylic acids are organic molecules containing the carboxyl functional group, -COOH. This is made up of the carbonyl group, C=O, and the hydroxyl group, -OH.
    • Carboxylic acids are weak acids, meaning they only partially dissociate in solution. When they dissociate, they lose a proton to form a negative carboxylate ion. The charge delocalises across the molecule and makes both C-O bonds equal.
    • Carboxylic acids are formed by oxidising primary alcohols under reflux, hydrolysing nitriles or hydrolysing esters.
    • Carboxylic acids react with bases to form carboxylate salts.
    • Carboxylic acids react with alcohols in the presence of a strong, concentrated acid catalyst to produce esters. This is known as esterification.
    • Carboxylic acids can also be reduced, decarboxylated and turned into acid derivatives. In addition, certain carboxylic acids can be oxidised.
    Frequently Asked Questions about 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.

    Test your knowledge with multiple choice flashcards

    The carboxyl functional group is made up of two other functional groups. Which groups are these?

    Name the salt produced when sodium hydroxide reacts with ethanoic acid.

    Name the salt produced when magnesium reacts with butanoic acid.

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