Carboxylic acids are organic compounds characterised by the presence of the carboxyl functional group, \( -\text{COOH} \). This group consists of a carbonyl group (\( \text{C}=\text{O} \)) directly bonded to a hydroxyl group (\( -\text{OH} \)). They represent an important class of weak organic acids with wide industrial utility, serving as precursor molecules for esters, amides, and acyl chlorides.
🔑 Key Principle
The carboxyl group exhibits unique reactivity because the carbonyl oxygen withdraws electron density from the \( \text{O-H} \) bond. This polarises the \( \text{O-H} \) bond and makes the hydrogen atom easier to lose as a proton (\( \text{H}^+ \)) compared to the hydrogen in alcohols.
Hydrogen Bonding and Solubility
Carboxylic acids can form strong hydrogen bonds with water molecules. This is due to the presence of two polar sites: the electronegative carbonyl oxygen (\( \text{C}=\text{O}^{\delta-} \)) and the polar hydroxyl group (\( \text{O}^{\delta-}-\text{H}^{\delta+} \)). Water molecules can interact with both regions, facilitating dissolution.
An organic compound containing the carboxyl functional group, \( -\text{COOH} \), located at the end of a carbon chain.
Solubility behaves in a predictable manner based on molecular structure:
- Short-chain carboxylic acids: Methanoic, ethanoic, propanoic, and butanoic acids are completely miscible in water because hydrogen bonding dominates.
- Long-chain carboxylic acids: As the length of the non-polar hydrocarbon chain (alkyl group) increases, solubility decreases. The hydrophobic carbon chain does not form hydrogen bonds and disrupts the hydrogen bonding network of the water molecules around it.
Weak Acid Properties
Carboxylic acids behave as weak acids in aqueous solution. They undergo partial dissociation, establishing a state of dynamic equilibrium:
An acid that only partially dissociates into its constituent ions in aqueous solution, resulting in a low concentration of free \( \text{H}^+ \) ions.
The acidic properties are caused by the stability of the carboxylate anion, \( \text{RCOO}^- \). The negative charge is not localized on a single oxygen atom. Instead, \( \pi \)-electrons delocalise across the entire \( \text{O-C-O} \) system, spreading the negative charge equally over both oxygen atoms. This makes the anion stable enough to exist in equilibrium, although the backwards reaction is still heavily favoured.
Acid Reactions of Carboxylic Acids
Carboxylic acids undergo typical acid reactions, forming carboxylate salts. You need to be familiar with the following reactions:
1. Reaction with Bases
Carboxylic acids react with metal hydroxides (soluble bases) in a standard neutralisation reaction to produce a soluble ionic carboxylate salt and water:
In this reaction, ethanoic acid reacts with sodium hydroxide to form sodium ethanoate and water. The ionic equation is:
2. Reaction with Carbonates
Carboxylic acids are strong enough to liberate carbon dioxide gas from metal carbonates, producing a salt, water, and carbon dioxide:
Effervescence is observed as carbon dioxide gas bubbles out of the solution.
3. Reaction with Hydrogen Carbonates
Similarly, carboxylic acids react with metal hydrogen carbonates (bicarbonates):
An ionic compound containing a carboxylate anion (\( \text{RCOO}^- \)) and a metal or ammonium cation, formed by the neutralisation of a carboxylic acid.
The reaction with sodium hydrogen carbonate (\( \text{NaHCO}_3 \)) is the standard chemical test to confirm the presence of a carboxylic acid. Alcohols and phenols are not acidic enough to react with carbonates or hydrogen carbonates. Therefore, observing effervescence that turns limewater cloudy when adding \( \text{NaHCO}_3 \) rules out other hydroxyl-containing species.
Solution:
1. Identify the chemical formulas: propanoic acid is \( \text{CH}_3\text{CH}_2\text{COOH} \) and sodium carbonate is \( \text{Na}_2\text{CO}_3 \).
2. Balance the molecular equation:
\[ 2\text{CH}_3\text{CH}_2\text{COOH(aq)} + \text{Na}_2\text{CO}_3\text{(s)} \rightarrow 2\text{CH}_3\text{CH}_2\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \]3. Write the ionic equation: sodium ions are spectator ions, and propanoic acid is a weak acid (written in molecular form):
\[ 2\text{CH}_3\text{CH}_2\text{COOH(aq)} + \text{CO}_3^{2-}\text{(s)} \rightarrow 2\text{CH}_3\text{CH}_2\text{COO}^-\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \]4. Observations: The solid sodium carbonate dissolves, and vigorous effervescence (bubbling) occurs as carbon dioxide gas is evolved.
Esterification
Carboxylic acids react with alcohols when heated under reflux in the presence of a concentrated strong acid catalyst (typically concentrated \( \text{H}_2\text{SO}_4 \)) to produce an ester and water:
A reversible condensation reaction between a carboxylic acid and an alcohol, catalysed by concentrated acid, to form an ester and water.
The naming of the ester is derived from both reactants:
- The first part of the name comes from the alcohol alkyl group (e.g. methanol gives methyl).
- The second part comes from the carboxylic acid carboxylate stem (e.g. ethanoic acid gives ethanoate).
In esterification, the water molecule is formed from the \( -\text{OH} \) group of the carboxylic acid and the \( -\text{H} \) of the alcohol's hydroxyl group. Remember this when drawing structures of esters or using isotopes to track reaction pathways.
Solution:
1. Write the equation using structural formulae:
\[ \text{CH}_3\text{CH}_2\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} \]2. Identify the name: The alcohol is ethanol, giving "ethyl". The acid is propanoic acid, giving "propanoate". The product is ethyl propanoate.
3. Structure of the ester link: The ester link consists of a carbonyl carbon directly bonded to an oxygen: \( \text{-C}(=\text{O})\text{-O-} \).
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