Properties of Carboxylic Acids
Carboxylic acids are a family of organic compounds defined by the -COOH functional group. For the ESAT, you must understand their naming, their behaviour as weak acids, and their key reaction to form esters with alcohols.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Carboxylic acids form a homologous series with the general formula C(n)H(2n+1)COOH, where n starts from 0 (for methanoic acid).
- They are named using the standard hydrocarbon prefix for the total number of carbons (including the one in -COOH) and the suffix '-oic acid'. For example, CH3COOH has two carbons, so it is ethanoic acid.
- As weak acids, they partially dissociate in water to release H+ ions: RCOOH ⇌ RCOO- + H+.
- They undergo typical acid reactions: with reactive metals to produce a salt and hydrogen; with bases for a salt and water; and with carbonates for a salt, water, and carbon dioxide.
- In a reaction called esterification, carboxylic acids react with alcohols in the presence of a strong acid catalyst (like concentrated H2SO4) to form an ester and water.
Formulae
C(n)H(2n+1)COOH To determine the molecular formula of a straight-chain carboxylic acid when n (the number of carbons in the alkyl chain) is known. Note n can be 0.
Acid + Metal → Salt + Hydrogen Predicting the products of a reaction between a carboxylic acid and a reactive metal like magnesium or zinc.
Acid + Carbonate → Salt + Water + Carbon Dioxide Predicting the products of a reaction between a carboxylic acid and a metal carbonate, which produces a characteristic effervescence.
R-COOH + R'-OH ≤> R-COO-R' + H2O Representing the general reversible reaction for ester formation (esterification).
Definitions
- Carboxylic Acid
- An organic compound containing the carboxyl functional group, -COOH.
- Weak Acid
- An acid that only partially ionises or dissociates in aqueous solution, establishing an equilibrium.
- Esterification
- A condensation reaction between a carboxylic acid and an alcohol, typically requiring an acid catalyst, which produces an ester and a molecule of water.
- Homologous Series
- A series of compounds with the same functional group and similar chemical properties, in which successive members differ by a -CH2- group.
Worked example
Butanoic acid is warmed with ethanol in the presence of a few drops of concentrated sulfuric acid. Give the name and structural formula of the organic product formed.
- 1
Identify the reactants:
Butanoic acid (CH3CH2CH2COOH) and ethanol (CH3CH2OH).
- 2
Recognise the reaction type:
This is an esterification reaction, which forms an ester and water.
- 3
Determine the structure of the ester.
The -OH from the carboxylic acid and the -H from the alcohol's hydroxyl group are removed to form water.
The remaining parts join.
- 4
The butanoic acid part becomes the butanoate group (CH3CH2CH2COO-).
The ethanol part becomes the ethyl group (CH3CH2-).
- 5
Combine these to form the ester:
The ethyl group attaches to the single-bonded oxygen of the butanoate group, giving the structural formula:
CH3CH2CH2COOCH2CH3.
- 6
Name the ester.
The name comes from the alcohol first ('ethyl') followed by the carboxylic acid with an '-oate' ending ('butanoate').
- 7
The full name of the organic product is ethyl butanoate.
Answer: Name: Ethyl butanoate. Structural Formula: CH3CH2CH2COOCH2CH3
Common mistakes
- ×When naming carboxylic acids, students often forget to include the carbon atom from the -COOH functional group in the main chain count. For example, CH3CH2COOH has 3 carbons, making it propanoic acid, not ethanoic acid.
- ×Incorrectly determining the formula of salts with divalent cations (e.g., Mg2+, Ca2+). The carboxylate ion (RCOO-) has a -1 charge, so two are needed to balance a +2 ion, e.g., (CH3COO)2Mg.
- ×Confusing the naming convention for esters. Remember the first part of the name comes from the alcohol (e.g., ethanol → ethyl) and the second part from the carboxylic acid (e.g., propanoic acid → propanoate).
No-calculator tips
- ✓To quickly derive the formula of a salt, identify the charge of the metal cation (e.g., Na is +1, Mg is +2) and remember the carboxylate ion (RCOO-) is always -1. Use this to balance the charges.
- ✓When balancing reaction equations, count the atoms systematically. Start with carbon, then hydrogen, then oxygen. This ordered approach helps to quickly spot any imbalances.