Systematic Naming: The names for carboxylic acids follow the pattern 'alkan- + -oic acid'. For example, a two-carbon chain uses the stem 'ethan-' resulting in the name ethanoic acid. It is crucial to count the carbon within the -COOH group as part of the total chain length when identifying the correct prefix (meth-, eth-, prop-, but-).
The First Four Acids: The series begins with methanoic acid (), followed by ethanoic acid (), propanoic acid (), and butanoic acid (). Each subsequent member adds a unit to the alkyl chain. This predictable structural increase leads to gradual changes in physical properties like boiling point across the series.
Reaction with Metals: Carboxylic acids react with reactive metals, such as magnesium, to produce a metal salt and hydrogen gas (). The reaction is typically slower than with strong mineral acids because carboxylic acids only partially dissociate in water. The resulting salts are named with the suffix '-anoate', so ethanoic acid forms ethanoate salts.
Reaction with Carbonates: When a carboxylic acid reacts with a metal carbonate, the products are a salt, water, and carbon dioxide gas (). Effervescence is observed during the reaction as is released, which can be verified by the limewater test. This reaction serves as a standard identifier for the presence of the acidic carboxyl group in an organic substance.
| Acid Name | Salt Name Suffix | Example Salt |
|---|---|---|
| Methanoic Acid | Methanoate | Sodium methanoate |
| Ethanoic Acid | Ethanoate | Magnesium ethanoate |
| Propanoic Acid | Propanoate | Potassium propanoate |
Salts of Divalent Metals: When reacting with Group 2 metals like magnesium, the formula of the salt must account for the charge of the metal ion. For instance, magnesium ethanoate is written as because two ethanoate ions (each ) are required to balance one magnesium ion. Balancing these charges correctly is essential for accurate chemical equations.
Concentration vs. Strength: Carboxylic acids like ethanoic acid are weak acids, meaning they do not fully ionize in solution, whereas vinegar is simply a specific dilute aqueous solution of ethanoic acid (about 5%). It is important to distinguish the inherent chemical strength of the acid from the concentration of a given solution.
The 'n' Value Pitfall: In the general formula , students often incorrectly set 'n' to the total number of carbons. Always remember that , because the carbon atom in the carboxyl group is counted separately from the alkyl chain. For methanoic acid, , resulting in the formula .
Predicting Products: When writing equations for reactions with metals or carbonates, identify the acidic hydrogen attached to the oxygen as the only one that is replaced. The hydrogen atoms bonded to the carbon atoms in the alkyl chain (e.g., the in ethanoic acid) remain unchanged and do not participate in the acidic reaction.
Identifying Salt Names: Exams frequently ask for the name of the salt produced. Always convert the '-oic acid' name to '-anoate' and pair it with the metal (e.g., butanoic acid plus potassium carbonate yields potassium butanoate).
