The Carboxyl Group
- Contains the carbonyl functional group and a hydroxyl group
- Organic acid have acidic behaviours due to the close proximity of the carbonyl group with the hydroxyl group
- Weak acids in equilibrium (OH group capable of donating a proton, H+, only a small fraction of molecules ionise) partially dissociate in water
- Found in medicines, fruit juices, vinegar and rhubarb leaves
- Carboxylic acids and their derivatives are used in organic synthesis as starting materials or intermediates in the formation of more useful compounds e.g. the painkiller aspirin can be synthesised from salicylic acid
Naming Carboxylic Acids
- Prefix is the same as alkanes name of the longest carbon chain, remove the ‘e’ at the end and add Suffix ‘-oic acid’
The COOH functional group takes priority and the Carbon atoms is ALWAYS Carbon 1
Solubility of Carboxylic Acids
- Do you remember the content of AS/A-level Chemistry regarding hydrogen bonds?
- Carboxylic acids up to 4 Carbons long are very soluble in water
- C=O and O-H bonds in carboxylic acids are polar so they form hydrogen bonds with water molecules
- Oxygen is more electronegative than Hydrogen and Carbon
- However, as the number of Carbon atoms increases, solubility decreases
- This is because the non-polar Carbon chain has a greater effect on the overall polarity of the molecule
- Dicarboxylic acids have 2 polar carboxyl groups to form hydrogen bonds. They are solids at room temperature and dissolve readily in water
Acid Reactions of Carboxylic Acids
- Redox: with metals
- Neutralisation: with bases (alkalis, metal oxides and hydroxides, carbonates)
- Form carboxylate salts, named by removing ‘-ic acid’ and adding ‘-ate’
- e.g. carboxylic acid ➔ carboxylate, carboxylate ions are ionic and so will react with a metal ion
- Carboxylate ions have a 1- charge so when they form salts with group 2 metals, there are 2 carboxylate ions to every 1 metal ion
- E.g. magnesium methanoate
The reactions below are often seen inAS/A-level Chemistry exam, make sure you know the equations!👨🏫
Redox Reactions of Carboxylic Acids with Metals
- Aqueous solutions of carboxylic acids react with metals in a redox reaction to form hydrogen gas and carboxylate salt
Carboxylic acid(aq) + metal(s) ➔ carboxylate salt(aq) + H2(g)
Observations:
- metal disappearing
- effervescence: hydrogen gas evolved
- e.g. propanoic acid + magnesium ➔ magnesium propanoate + H2
2CH2CH2COOH + Mg ➔ (CH3CH2COO-)2Mg2+ + H2
Neutralisation Reactions of Carboxylic Acids and Metal Oxides and Hydroxides
- Carboxylic acid(aq) + metal oxide(s) ➔ salt(aq) + water(l)
e.g. ethanoic acid + calcium oxide ➔ calcium ethanoate + water
2CH3COOH(aq) + CaO(s) ➔ (CH3COO-)2Ca2+(aq) + H2O(l)
- Carboxylic acid(aq) + metal hydroxide(aq) ➔ salt(aq) + water(l)
e.g. ethanoic acid + sodium hydroxide ➔ sodium ethanoate + water
CH3COOH(aq) + NaOH(aq) ➔ CH3COO-Na+(aq) + H2O(l)
Neutralisation Reactions of Carboxylic Acids and Metal Carbonates
- Carboxylic acid(aq) + carbonate(aq) ➔ salt(aq) + water(l) + CO2(g)
e.g. ethanoic acid + sodium carbonate ➔ sodium ethanoate + water + carbon dioxide
2CH3COOH(aq) + Na2CO3(aq) ➔ 2CH3COO-Na+(aq) + H2O(l) + CO2(g)
- Carbon dioxide gas is evolved
- If the carboxylic acid is in excess, a solid carbonate would disappear
Test for Carboxyl Group
- Only common organic compounds are sufficiently acidic to react with carbonates
- Useful for distinguishing carboxylic acids from phenols as phenols are not acidic enough to react with carbonates
- The neutralisation reaction of carboxylic acids with carbonates e.g. sodium carbonate, provides chemists with a method of distinguishing carboxylic acids from any other organic compounds
- Produces CO2 gas bubbles
This is the end of topic!
Drafted by Cherry (Chemistry)