Carboxylic acids
You will have come across a range of carboxylic acids before, both in everyday life and in your chemistry studies.
Consider carboxylic acids in wine. The video below shows how the presence of carboxylic acids in wine can be tested. You will see that although the carboxylic acids are generally weak acids, they can be detected by using chemical indicators.
Text alternative to the carboxylic acid video
A scientist describes how chromatography is used to test the presence of carboxylic acids in samples of wine. The main aim is to ensure that there is no malic acid in the wine. A chromatography experiment is set up with a standard of malic acid. This is compared to a number of wine samples.
Once the chromatograph is developed, any wines that show a spot that matches the position of the malic acid standard are shown to contain the unwanted malic acid.
Chromatography is used to measure the amount of malic acid (C4H6O5) there is in the wine. In the red wine they need to go through monolactic fermentation, so we have to get rid of all the malic acid. The colours on the chromatograph are produced by the indicator which we have in the solution here and that is bromocresol green. The solvent used in the experiment is n-butyl alcohol (CH3CH2CH2 CH2OH).
This choromatograph has been set up with seven different wines and a standard of malic acid so that we could see where the spot for the malic acid should be in the choromatograph. These five wines do not have malic acid in them. These two wines still have the malic acid in them, because they have the spot, which is aligned with the malic acid standard.
These are the tartaric acid (C4H6O6), this is where the tartaric acid is and these are the lactic acid (C3H6O3) plus various other minor acids in the wine.
Click here to open the video in a new window
Ethanoic acid (or acetic acid) is the acid present in vinegar. This is a monoprotic acid, with only one -COOH functional group.
Lactic acid is also a monoprotic acid, but this also contains the -OH functional group, which will affect its properties.
Diprotic carboxylic acids contain two -COOH functional groups, such as tartaric acid, which is present in wine.
Malic acid can be produced during the production of wine but must be removed before the wine is bottled.
In this section we only need to consider monoprotic carboxylic acids. The first thing to consider is how they can be produced from the oxidation of primary alcohols.
You should recall that primary alcohols can be oxidised to form aldehydes. The following reactions show an example of this process.
(oxidation of ethanol to ethanal) CH3CH2OH → CH3CHO + 2 H+ + 2 e-
(reduction of dichromate) Cr2O72- + 14 H+ + 6 e- → 2 Cr3+ + 7 H2O
(overall reaction) Cr2O72- + 3 CH3CH2OH + 8 H+ → 2 Cr3+ + 3 CH3CHO + 7 H2O
Draw the structure of the ethanal produced in this reaction and then check the answer.
Check your version against the answer here.
The ethanal can undergo further oxidation, as shown by this half-equation: CH3CHO + H2O → CH3COOH + 2 H+ + 2 e-
Therefore, as long as there is enough of the oxidising agent, oxidation will continue because the aldehyde is oxidised to form the carboxylic acid. Write an equation for this reaction in your notes.
Check your version against the overall equation here.
Cr2O72- + 8 H+ + 3 CH3CHO → 2 Cr3+ + 3 CH3COOH + 4 H2O
Because aldehydes can undergo further oxidation to carboxylic acids, it is sometimes easier to combine the oxidation half-reactions of the primary alcohol and the aldehyde into a single step.
This can be written for the example of ethanol as shown here.
CH3CH2OH + H2O → CH3COOH + 4 H+ + 4 e-
Similar half-equations can be written for other primary alcohols. Try to write the half-equation for the complete oxidation of butan-1-ol.
Click here for the oxidation of butan-1-ol half-equation
CH3CH2CH2CH2OH + H2O → CH3CH2CH2COOH + 4 H+ + 4 e-
Now try to write an overall equation for the complete oxidation of propan-1-ol by hydrogen peroxide (H2O2). (Use the half-equation for H2O2 from the data sheet.)
Click here to reveal answer
(oxidation of propan-1-ol )
CH3CH2CH2OH + H2O → CH3CH2COOH + 4 H+ + 4 e-
(reduction of peroxide)
H2O2 + 2 H+ + 2 e- → 2 H2O
(overall reaction – note the cancelling out of H2O and H+)
CH3CH2CH2OH + 2 H2O2 → CH3CH2COOH + 3 H2O
Primary, secondary and tertiary alcohols
The oxidation reactions of alcohols can be summarised as shown below:
- primary alcohols → aldehydes → carboxylic acids
- secondary alcohols → ketones
- tertiary alcohols → not oxidised.
You will be using this information later in the topic when completing identification of alcohol questions.
Complete the worksheet covering Oxidation reactions of alcohols 
.
The answers can be accessed here .
