Carboxylic acids
The general formula for the first members of this homologous series is C_nH_{2n+1}COOH. The naming convention replaces the '-e' ending of the corresponding alkane with '-oic acid'. For example, a 2-carbon chain (ethane) becomes ethanoic acid.
Building on the concept of functional groups, the carboxyl group is responsible for the acidic properties of these molecules. The hydrogen atom in the -OH part can dissociate in water to form H^+ ions, making the solution acidic.
When asked to draw a displayed formula, you must show all atoms and all bonds. A common error is omitting the bond between the oxygen and hydrogen in the -OH group or drawing the structure as an ether (C-O-C) instead of a carboxylic acid.
The general equation is:
\text{Carboxylic Acid} + \text{Alcohol} \rightleftharpoons \text{Ester} + \text{Water}
Naming Esters:
The name of an ester has two parts:
- The first part comes from the alcohol (ending in '-yl').
- The second part comes from the carboxylic acid (ending in '-oate').
For example, ethanoic acid + ethanol \rightarrow ethyl ethanoate.
| Salt Formula |
|---|
| CH_3COONa |
| CH_3COONa |
| CH_3COONa |
| CH_3COONa |
The ethanoate ion is CH_3COO^-. When writing salt formulae, ensure the charges balance. For example, calcium forms Ca^{2+} ions, so it requires two ethanoate ions: calcium ethanoate (CH_3COO)_2Ca.
Ethanoic acid can be formed by the oxidation of ethanol (C_2H_5OH). There are two main methods:
Laboratory Method: Ethanol is warmed with acidified aqueous potassium manganate(VII) (KMnO_4) or acidified potassium dichromate(VI). The solution changes colour (e.g., purple to colourless for KMnO_4, or orange to green for dichromate).
Industrial/Biological Method: During vinegar production, ethanol is oxidized by bacteria in the presence of oxygen from the air. This is a slower process occurring at room temperature.
In the laboratory oxidation of primary alcohols to carboxylic acids, the mixture is heated under reflux. Reflux is necessary because ethanol and the intermediate aldehyde (ethanal) are volatile. Heating under reflux ensures that these vapours condense and return to the flask, preventing them from escaping before they can be fully oxidized to ethanoic acid.
Identify the components:
- Carboxylic acid: Propanoic acid (3 carbons)
- Alcohol: Butan-1-ol (4 carbons)
Name the ester:
- From alcohol 'butan' \rightarrow butyl
- From acid 'propanoic' \rightarrow propanoate
- Ester Name: Butyl propanoate
Determine the Formula:
- Propanoic acid part: CH_3CH_2COO-
- Butan-1-ol part: -C_4H_9 (specifically -CH_2CH_2CH_2CH_3)
- Combined Ester Formula: CH_3CH_2COOC_4H_9
Condensed Formula Convention:
We use CH_3CH_2COO- to represent the propanoate group derived from the acid, and -C_4H_9 for the butyl group derived from the alcohol. The oxygen atom in the ester linkage (-COO-) comes from the carboxylic acid.
Students often write the salt name correctly but fail to provide the correct formula. For example, for calcium ethanoate, writing CH_3COOCa is incorrect because it does not balance the charges (Ca^{2+} vs CH_3COO^-). The correct formula is (CH_3COO)_2Ca.
Mistake 2: Swapping the Ester Oxygen
When drawing esters, students sometimes draw the linkage as R-O-C(=O)-R'. This is incorrect. The carbonyl carbon (C=O) must be directly attached to the oxygen from the acid, not the alcohol. The correct linkage is R-C(=O)-O-R'.
Mistake 3: Omitting Bonds in Displayed Formulae
When drawing ethanoic acid, students often omit the covalent bond between the oxygen and hydrogen in the -OH group. Always show all bonds explicitly.
When asked to identify the reactants that form a specific ester, split the name at the space. The first word (ending in '-yl') is the alcohol. The second word (ending in '-oate') is the carboxylic acid.
Example: If the ester is propyl ethanoate:
- 'Propyl' \rightarrow Reactant is propan-1-ol.
- 'Ethanoate' \rightarrow Reactant is ethanoic acid.
This phrasing is accepted because it directly addresses the nomenclature rules where the alkyl group from the alcohol precedes the alkanoate group from the acid.
When describing the formation of ethanoic acid from ethanol, always mention acidified potassium manganate(VII) or dichromate(VI). Simply saying 'potassium manganate' is insufficient because the acid provides the H^+ ions necessary for the redox reaction. Examiners look for the term acidified to confirm understanding of the reaction conditions.