Unit 11: Carboxylic Acids & Alkanoates
Exploring the chemistry of the carboxyl group, from acidic reactions to the formation of esters.
11.7 Introduction to Carboxylic Acids
Carboxylic acids (or alkanoic acids) are a homologous series of organic compounds containing the carboxyl (-COOH) functional group. This group consists of a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom.
They are named using the suffix -anoic acid (e.g., ethanoic acid). The carboxyl carbon is always carbon-1, so a number is not needed to locate it.
Like alcohols, carboxylic acids can form strong hydrogen bonds with each other and with water. In fact, they can form dimers, where two molecules are held together by two hydrogen bonds. This leads to:
- Very high boiling points, even higher than alcohols of similar molar mass.
- Good solubility in water for short-chain carboxylic acids.
Solved Examples:
- What is the functional group of a carboxylic
acid?
Solution: The carboxyl group (-COOH). - What is the IUPAC name for the carboxylic acid with four carbon
atoms?
Solution: Butanoic acid. - Why do carboxylic acids have higher boiling points than alcohols of similar
size?
Solution: They can form stronger hydrogen bonds, often in pairs (dimers), which requires more energy to overcome. - What is the common name for ethanoic
acid?
Solution: Acetic acid (the main component of vinegar). - Draw the structure of propanoic acid.
Solution: $CH_3CH_2COOH$. - What is a dicarboxylic acid? Give an
example.
Solution: A molecule with two carboxyl groups. An example is ethanedioic acid, $HOOC-COOH$. - How are carboxylic acids typically formed in the
lab?
Solution: By the strong oxidation of primary alcohols. - Is benzoic acid (a benzene ring attached to -COOH) likely to be soluble in
water?
Solution: No, its solubility is low because the large non-polar benzene ring disrupts water's hydrogen bonding network. - What gives vinegar its characteristic sour taste and sharp
smell?
Solution: Ethanoic acid. - What is the general formula for aliphatic carboxylic
acids?
Solution: $C_nH_{2n}O_2$.
11.8 Acidic Properties & Reactions
Carboxylic acids are weak acids. They partially dissociate in water to
establish an equilibrium, releasing a hydrogen ion ($H^+$) and forming a carboxylate ion
(e.g., ethanoate).
Equation: $CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq)$
Because they are acids, they undergo typical acid reactions (neutralisation):
- Reaction with Bases: They react with strong bases like sodium hydroxide
to form a salt and water.
$CH_3COOH(aq) + NaOH(aq) \rightarrow CH_3COONa(aq) + H_2O(l)$ - Reaction with Carbonates: They react with carbonates and
hydrogencarbonates to produce a salt, water, and carbon dioxide gas (effervescence).
This is a useful test for a carboxylic acid.
$2CH_3COOH(aq) + Na_2CO_3(s) \rightarrow 2CH_3COONa(aq) + H_2O(l) + CO_2(g)$ - Reaction with Reactive Metals: They react with metals more reactive
than hydrogen to produce a salt and hydrogen gas.
$2CH_3COOH(aq) + Mg(s) \rightarrow (CH_3COO)_2Mg(aq) + H_2(g)$
Solved Examples:
- Are carboxylic acids strong or weak acids?
Explain.
Solution: Weak acids, because they only partially dissociate in water. - What is the simple laboratory test for a carboxylic
acid?
Solution: Add a carbonate or hydrogencarbonate (like sodium bicarbonate). If fizzing (effervescence) occurs, an acid is present. - Write an equation for the reaction between propanoic acid and potassium
hydroxide.
Solution: $CH_3CH_2COOH + KOH \rightarrow CH_3CH_2COOK + H_2O$. - What are the products when butanoic acid reacts with magnesium
carbonate?
Solution: Magnesium butanoate, water, and carbon dioxide. - What is the name of the anion formed when ethanoic acid
dissociates?
Solution: The ethanoate ion ($CH_3COO^-$). - Would a solution of ethanoic acid have a high or low
pH?
Solution: A low pH (e.g., 2-3), but not as low as a strong acid of the same concentration. - What gas is produced when zinc metal is added to propanoic
acid?
Solution: Hydrogen gas. - Why is the reaction with sodium carbonate a good test to distinguish a
carboxylic acid from a phenol (which is also weakly
acidic)?
Solution: Carboxylic acids are strong enough to react with carbonates and produce $CO_2$, while phenols are generally not. - Write the dissociation equation for methanoic acid in
water.
Solution: $HCOOH(aq) \rightleftharpoons HCOO^-(aq) + H^+(aq)$. - What would you observe if you added calcium carbonate to
vinegar?
Solution: Fizzing (effervescence) as carbon dioxide is produced.
11.9 Carboxylate Salts
The salt formed from the neutralisation of a carboxylic acid is called a carboxylate salt or an alkanoate salt.
They are named with the cation first, followed by the name of the acid with the "-oic acid"
ending changed to -oate.
Example: The reaction of ethanoic acid with sodium hydroxide produces
sodium ethanoate ($CH_3COONa$).
Carboxylate salts are ionic compounds and are typically soluble in water. Since they are
salts of a weak acid and a strong base, their solutions are weakly alkaline
(basic) due to salt hydrolysis. The carboxylate anion reacts with water to
produce hydroxide ions.
Hydrolysis Equation: $CH_3COO^-(aq) + H_2O(l) \rightleftharpoons
CH_3COOH(aq) + OH^-(aq)$
Solved Examples:
- What is the name of the salt produced from propanoic acid and potassium
hydroxide?
Solution: Potassium propanoate. - Write the formula for sodium
methanoate.
Solution: $HCOONa$. - Will a solution of sodium ethanoate be acidic, basic, or neutral? Explain
why.
Solution: Basic (alkaline). The ethanoate ion is the conjugate base of a weak acid and will hydrolyse water to produce $OH^-$ ions. - What is the name of the salt
$(CH_3CH_2COO)_2Ca$?
Solution: Calcium propanoate. - What type of compounds are carboxylate
salts?
Solution: They are ionic compounds. - Write the equation for the hydrolysis of the methanoate
ion.
Solution: $HCOO^-(aq) + H_2O(l) \rightleftharpoons HCOOH(aq) + OH^-(aq)$. - How can you convert a carboxylate salt back into its parent carboxylic
acid?
Solution: By adding a strong acid (like HCl), which will protonate the carboxylate ion. - What is the pH of a solution of potassium butanoate expected to
be?
Solution: Greater than 7 (alkaline). - Are carboxylate salts typically soluble in
water?
Solution: Yes, especially those with Group 1 cations. - What is the general name for a compound derived from a carboxylic
acid?
Solution: An alkanoate. This includes both salts and esters.
11.10 Esterification (Preparation of Esters)
Esters are a class of organic compounds formed from the reaction between a carboxylic acid and an alcohol. This reaction is called esterification.
Esterification is a reversible condensation reaction, where a molecule of
water is eliminated. It is typically catalysed by a few drops of concentrated sulphuric acid
and requires gentle heating.
General Equation: Carboxylic Acid + Alcohol $\rightleftharpoons$ Ester
+ Water
Example: $CH_3COOH + CH_3CH_2OH \rightleftharpoons CH_3COOCH_2CH_3 +
H_2O$
Esters have characteristic sweet, fruity smells and are responsible for the natural fragrances of many fruits and flowers. They are widely used in the food industry as flavourings and in the perfume industry.
Naming Esters:
An ester's name has two parts.
- The first part comes from the alcohol, with the "-anol" ending changed to -yl (e.g., ethanol → ethyl).
- The second part comes from the carboxylic acid, with the "-oic acid" ending changed to -oate (e.g., ethanoic acid → ethanoate).
Solved Examples:
- What two types of compounds react to form an
ester?
Solution: A carboxylic acid and an alcohol. - What is the name of the reaction that forms an
ester?
Solution: Esterification. - What are the conditions for
esterification?
Solution: Gentle heating with a strong acid catalyst, such as concentrated sulphuric acid. - Name the ester formed from methanol and propanoic
acid.
Solution: Methyl propanoate. - Name the alcohol and carboxylic acid needed to make propyl
ethanoate.
Solution: Propan-1-ol and ethanoic acid. - What small molecule is eliminated during
esterification?
Solution: Water. - What are the characteristic smells of
esters?
Solution: Sweet and fruity. - Write the equation for the formation of methyl
propanoate.
Solution: $CH_3CH_2COOH + CH_3OH \rightleftharpoons CH_3CH_2COOCH_3 + H_2O$. - What is a condensation reaction?
Solution: A reaction where two molecules join together with the elimination of a small molecule, like water. - What is the laboratory test for an alcohol, based on this
reaction?
Solution: Warm the unknown substance with a carboxylic acid (like ethanoic acid) and a few drops of concentrated sulphuric acid. A sweet, fruity smell indicates the presence of an alcohol.
11.11 Ester Hydrolysis
Ester hydrolysis is the reverse of esterification. It is the process of splitting an ester back into its parent carboxylic acid and alcohol by reacting it with water. The reaction can be catalysed by either acid or alkali, with different outcomes.
1. Acid Hydrolysis
Heating an ester with a dilute acid (like $HCl$ or $H_2SO_4$) catalyses the hydrolysis. This
is a reversible reaction and reaches an equilibrium.
Equation: $CH_3COOCH_2CH_3 + H_2O \stackrel{H^+}{\rightleftharpoons}
CH_3COOH + CH_3CH_2OH$
2. Alkaline Hydrolysis (Saponification)
Heating an ester with a strong alkali (like $NaOH$) also splits the ester. This reaction is
irreversible because the carboxylic acid produced immediately reacts with
the alkali to form a carboxylate salt. The reaction goes to completion, giving a better
yield of the products.
Equation: $CH_3COOCH_2CH_3 + NaOH \rightarrow CH_3COONa + CH_3CH_2OH$
This process is called saponification, which means "soap-making," as it
is the reaction used to make soap from fats and oils (which are natural esters).
Solved Examples:
- What is ester hydrolysis?
Solution: The breakdown of an ester into a carboxylic acid and an alcohol by reaction with water. - What are the two types of ester
hydrolysis?
Solution: Acid hydrolysis and alkaline hydrolysis. - What are the products when ethyl propanoate is hydrolysed with dilute
acid?
Solution: Propanoic acid and ethanol. - Why is acid hydrolysis a reversible
reaction?
Solution: Because it is the exact reverse of esterification, which is also a reversible equilibrium reaction. - What is alkaline hydrolysis of an ester also known
as?
Solution: Saponification. - What are the products when methyl ethanoate is heated with sodium hydroxide
solution?
Solution: Sodium ethanoate and methanol. - Why is alkaline hydrolysis an irreversible
reaction?
Solution: Because the carboxylic acid product is immediately neutralised by the alkali to form a salt, preventing the reverse reaction from occurring. - Which method gives a better yield of the alcohol from an ester: acid or
alkaline hydrolysis?
Solution: Alkaline hydrolysis, because the reaction goes to completion. - Fats are natural esters. What are the products of their
saponification?
Solution: Soap (the sodium salt of a fatty acid) and glycerol (an alcohol). - How could you get the carboxylic acid back from the salt produced during
alkaline hydrolysis?
Solution: By adding a strong acid (like HCl) to the salt solution.