Carboxylic acids are everywhere around you - from the vinegar... Show more
Chemistry157 views·Updated May 16, 2026·4 pagesUnderstanding Carboxylic Acids - Chapter 26.3
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Carboxylic Acids - Structure and Basics
The functional group that defines carboxylic acids is -COOH, which gives them their distinctive properties. When naming these compounds, you simply add the suffix -oic acid to the end (like ethanoic acid for vinegar).
If a molecule has two carboxylic acid groups, you use -dioic acid instead. Think of it as doubling up on the acidity! The general structure shows a carbon atom bonded to both a hydroxyl group and a carbonyl group .
The geometry around the carbon is particularly important - you'll find three regions of electron density creating bond angles of 120°, whilst the oxygen in the -OH group has two bonded pairs and two lone pairs, giving it a 104.5° angle.
Key Insight: The -COOH group combines two familiar functional groups (carbonyl and hydroxyl), which explains why carboxylic acids are so reactive and versatile.
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Solubility and Physical Properties
Hydrogen bonding is the secret behind carboxylic acids' unique physical properties. The polar O-H bond allows these molecules to form strong hydrogen bonds with each other, creating dimers (pairs of molecules stuck together).
This hydrogen bonding also means carboxylic acids can bond effectively with water molecules. Carboxylic acids with up to 4 carbon atoms dissolve completely in water, but solubility drops dramatically as the carbon chain gets longer.
Here's the pattern you need to remember: longer carbon chains mean stronger London forces due to more electrons. This increases boiling points but decreases water solubility because the non-polar carbon chain dominates. However, if you've got more than one -COOH group, solubility increases again.
Exam Tip: Remember that melting and boiling points are unusually high for carboxylic acids due to that hydrogen bonding - this often comes up in exam questions!
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Acid Behaviour and Reactions
Carboxylic acids are weak acids, which means they only partially dissociate in water. For example, methanoic acid (HCOOH) splits into H⁺ and HCOO⁻ ions, but not completely - that's why we use the equilibrium arrow.
Since they're acids, they undergo typical acid reactions. With metals, you get a salt plus hydrogen gas (a redox reaction). With bases, you get neutralisation reactions producing a salt plus water.
The naming pattern for the salts is straightforward - just change the '-oic acid' ending to '-oate'. So ethanoic acid becomes ethanoate, propanoic acid becomes propanoate, and so on.
Memory Aid: Think of carboxylic acids as "polite acids" - they donate protons, but not too aggressively, unlike strong acids like hydrochloric acid.
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Specific Reactions You Need to Know
When carboxylic acids react with metals like magnesium, you get a classic redox reaction. For instance, propanoic acid + magnesium produces magnesium propanoate and hydrogen gas. The metal gets oxidised (loses electrons) whilst the hydrogen ions get reduced.
Neutralisation reactions follow predictable patterns. With metal oxides like calcium oxide, you get the metal carboxylate plus water. With hydroxides like sodium hydroxide, same result - salt plus water.
The test tube test for carboxylic acids uses sodium carbonate. If you add Na₂CO₃ to a carboxylic acid, you'll see fizzing as CO₂ gas is produced. This fizzing is your definitive test for the presence of a -COOH group.
Practical Note: That fizzing test with sodium carbonate is dead useful for identifying unknown compounds in your practicals - if it fizzes, you've likely got a carboxylic acid!
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Chemistry157 views·Updated May 16, 2026·4 pagesUnderstanding Carboxylic Acids - Chapter 26.3
Carboxylic acids are everywhere around you - from the vinegar in your chips to the citric acid in your fizzy drinks. Understanding their structure and reactions is crucial for your A-level chemistry success, and once you grasp the basics, you'll... Show more
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Carboxylic Acids - Structure and Basics
The functional group that defines carboxylic acids is -COOH, which gives them their distinctive properties. When naming these compounds, you simply add the suffix -oic acid to the end (like ethanoic acid for vinegar).
If a molecule has two carboxylic acid groups, you use -dioic acid instead. Think of it as doubling up on the acidity! The general structure shows a carbon atom bonded to both a hydroxyl group and a carbonyl group .
The geometry around the carbon is particularly important - you'll find three regions of electron density creating bond angles of 120°, whilst the oxygen in the -OH group has two bonded pairs and two lone pairs, giving it a 104.5° angle.
Key Insight: The -COOH group combines two familiar functional groups (carbonyl and hydroxyl), which explains why carboxylic acids are so reactive and versatile.
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Solubility and Physical Properties
Hydrogen bonding is the secret behind carboxylic acids' unique physical properties. The polar O-H bond allows these molecules to form strong hydrogen bonds with each other, creating dimers (pairs of molecules stuck together).
This hydrogen bonding also means carboxylic acids can bond effectively with water molecules. Carboxylic acids with up to 4 carbon atoms dissolve completely in water, but solubility drops dramatically as the carbon chain gets longer.
Here's the pattern you need to remember: longer carbon chains mean stronger London forces due to more electrons. This increases boiling points but decreases water solubility because the non-polar carbon chain dominates. However, if you've got more than one -COOH group, solubility increases again.
Exam Tip: Remember that melting and boiling points are unusually high for carboxylic acids due to that hydrogen bonding - this often comes up in exam questions!
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Acid Behaviour and Reactions
Carboxylic acids are weak acids, which means they only partially dissociate in water. For example, methanoic acid (HCOOH) splits into H⁺ and HCOO⁻ ions, but not completely - that's why we use the equilibrium arrow.
Since they're acids, they undergo typical acid reactions. With metals, you get a salt plus hydrogen gas (a redox reaction). With bases, you get neutralisation reactions producing a salt plus water.
The naming pattern for the salts is straightforward - just change the '-oic acid' ending to '-oate'. So ethanoic acid becomes ethanoate, propanoic acid becomes propanoate, and so on.
Memory Aid: Think of carboxylic acids as "polite acids" - they donate protons, but not too aggressively, unlike strong acids like hydrochloric acid.
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Specific Reactions You Need to Know
When carboxylic acids react with metals like magnesium, you get a classic redox reaction. For instance, propanoic acid + magnesium produces magnesium propanoate and hydrogen gas. The metal gets oxidised (loses electrons) whilst the hydrogen ions get reduced.
Neutralisation reactions follow predictable patterns. With metal oxides like calcium oxide, you get the metal carboxylate plus water. With hydroxides like sodium hydroxide, same result - salt plus water.
The test tube test for carboxylic acids uses sodium carbonate. If you add Na₂CO₃ to a carboxylic acid, you'll see fizzing as CO₂ gas is produced. This fizzing is your definitive test for the presence of a -COOH group.
Practical Note: That fizzing test with sodium carbonate is dead useful for identifying unknown compounds in your practicals - if it fizzes, you've likely got a carboxylic acid!
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Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
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