Chapter 34 – The carboxylic acids
Section A – Structure of the carboxylic acids
The carboxylic acids are a homologous series with the functional group: __________, this means that
they all contain a carbonyl group (________), attached to a hydroxyl group (_______________)
Use:
Methanoic acid
Use:
Ethanoic acid
Use:
Propanoic acid
Benzoic acid
Butanoic acid
Use: ___________________
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Chapter 34 – The carboxylic acids
Section B – Physical Properties
Melting and boiling points
Carboxylic acids have very _____________ melting and boiling points – even higher than their
corresponding alcohols This is due to the presence of BOTH the carbonyl group (___________) and
the hydroxyl group which allows the formation of dimers – two hydrogen bonds intermolecular
between two carboxylic acid molecules
Solubility in water
The lower members of the carboxylic acids are _____________ in water. The presence of the
hydroxyl group allows them to _______ bond to water and allows for very efficient dissolving
(Hydrogen bonding can occur)
The higher members of the carboxylic acids are ____________ soluble in water. This is because in
these the non polar alkyl chain gets longer and its effect becomes more significant.
Section C – Formation from alcohols
In the alcohols chapter we studied the formation of carboxylic acids by the ____________ of
__________ alcohols, using acidified ___________ dichromate. In this next experiment an
alternative method of producing carboxylic acids y the oxidation of alcohos using acidified potassium
permanganate solution is carried out.
Section D: Mandatory experiment: To oxidise phenyl methanol (benzyl
alcohol) to benzoic acid from potassium permanganate solution under
alkaline conditions
Introduction: The following reaction will be carried out:
3C6H5CH2OH+ 4KMnO4
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3C6H5COOH+ 4MnO2 +H2O+ 4KOH
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Chapter 34 – The carboxylic acids
Procedure:
1. Some phenylmethanol, a ________________ solution of potassium permanganate and a
solution of _________________ __________________ are mixed together in a conical flask and
heated in a hot water bath for about 20 minutes .
*A saturated solution of potassium permanganate is used to ensure all of the phenyl methanol is
_____________ to benzoic acid.
**The sodium carbonate is used to ensure alkaline pH conditions, which maximise the _________ of
the reaction.
**During the reaction a colour change of ___________ to ____________ is observed. This is due to
the reduction of the purple Mn(7) ions to brown Mn (4) ions occurs as the reaction proceeds. A
brown precipitate is formed in the conical flask as the ___________ is not soluble in water.
2. The reaction mixture, which contains sodium benzoate, is cooled by running ______________
over the sides of the conical flask.
3. Some concentrated ___________________ acid is added to the conical flask (do this in a
________ cupboard to avoid inhaling dangerous fumes)
* The acid is required as it converts sodium benzoate to ____________ acid.
***The hydrochloric acid also _______________any excess sodium carbonate in the reaction mixture
or potassium hydroxide that is made as a product of the reaction.
****It is helpful for the reaction mixture to be cold as the benzoic acid is not soluble in this cold
solution and _________________ out.
4. Add some _____________ sulfite solution to the conical flask with a ________________. The
brown ________________ will disappear and white crystals of benzoic acid will appear.
* Sodium sulphite is a reducing agent and causes the reduction of Mn(4)- a brown precipitate to
Mn(2) ions – colourless and soluble in water.
**The Hydrochloric acid added in the last step provides the acidic conditions needed for this reaction
to occur
***It is important that the brown precipitate of __________ is removed from the reaction mixture as
it would cause contamination of the ___________ acid product.
5. The conical flask is placed in an _________ bath.
*The benzoic acid is ______________ in cold water and so the crystals precipitate out of the solution,
thus increasing the ______________ of the reaction.
6. The crystals are filtered using a Buchner flask, funnel and filter paper.
*This is a quick method of filtration and also dries the crystals in the process.
7. The filtrate can be used to rinse out the conical flask, and this is re- filtered.
8. The crystals on the Buckner flask’s filter paper are rinsed with ice-cold water.
*This will dissolve any impurities on the filter paper that are soluble in this cold water.
Calculations – Find the % yield of benzoic acid from the reaction.
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Chapter 34 – The carboxylic acids
Example 1: A student carried out the preparation of benzoic acid via the oxidation of 2cm3 of phenyl
methanol ( density 1.04g/cm3) with 6.32g of acidified potassium permanganate under basic
conditions.1.05 g of benzoic acid was produced at the end of the reaction. The balanced equation is
provided below: C6H5CH2OH+ 4KMnO4
3C6H5COOH+ 4MnO2 +H2O+ 4KOH
Answer
(i)
Which of the reactants was the limiting reagent?
 To find this out we must work out the moles of each reactant present in the reaction mixture:
Moles of phenylmethanol present
Moles of potassium permanganate present
According to the balanced equation for the reaction :
1 mole of phenylmethanol requires ___________ moles of potassium permanganate to react with.
0.019 moles of phenylmethanol would require______________ moles of potassium permanganate.
Since 0.04 moles of potassium permanganate is actually present we can see that it is present in
_______________, therefore phenylmethanol is the _______________ reagent.
(ii)
What was the % yield of benzoic acid?
% YIELD = _________________________________ X 100
Find the theoretical yield:
According to the balanced equation for the reaction :
1 mole of phenylmethanol will produce ______________ moles of benzoic acid.
0.019 moles of phenylmethanol will theoretically produce ______________ moles of benzoic acid.
Find the actual yield:
The actual mass of benzoic acid produced was ____________________g,
Therefore the actual yield in moles is ________________
% YIELD = _________________________________ X 100
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Chapter 34 – The carboxylic acids
Section E – Mandatory experiment: To recrytallise a sample of benzoic acid
and measure its boiling point
Part a – Rerystallisation of benzoic acid
1. A sample of impure benzoic acid crystals is dissolved in the ____________ amount of
____________water ( the water must be near its boiling point).
* It is necessary to use hot solvent in order to produce a solution that will be
supersaturated at room temperature. On cooling, the substance recrystallizes
from solution.
**The minimum amount of solvent is used because this reduces the quantity of
the substance that can remain dissolved when the hot filtered solution is
allowed to cool. The yield of recovered crystals would be very low if too much
solvent were used.
2. Pour the boiling solution quickly through the filter paper in small portions. This is called a
______ filtration.
This step is carried out to remove any __________ impurities ( any substances
present in the sample that do not dissolve in hot water) – they remain on the
filter paper. The rest of the sample ( the _________ acid and other
____________ impurities passes through the ___________ paper into the flask
as the filtrate.
*It must be carried out quickly to keep the mixture hot so that any benzoic acid
present stays dissolved in the solution and does not start to crystallise at this
stage.
**If any crystals form in the filter paper, add a ___________ boiling water to dissolve them.
3. Allow the filtrate to cool to room temperature and then placed in an ice bath. Crystals of
benzoic acid will appear in the beaker.
*Any benzoic acid in the filtrate will precipitate out of the solution because it is
not soluble in cold water. The ______________ impurities present will stay
dissolved in the solution, eve n at this colder temperature.
**To encourage crystallisation scratching the side of the dish can be useful, or
evaporating off more of the _______________.
4. To recover the crystals, carefully filter the recrystallised mixture, by vacuum filtration. This step
is called a ______________ filtration. Wash the crystals with a small amount of ice cold water.
*The soluble impurities are washed down into the filtrate, and the benzoic acid
srytals remain on the filter paper.
5. Using the spatula, remove the crystals from the funnel and transfer them to
a previously weighed dry ________________ glass. Dry the crystals by placing
the clock glass in an oven set at _______________for 24 hours.
Calculations – Find the percentage yield of purified benzoic acid
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Chapter 34 – The carboxylic acids
Part b – Measuring the melting point of benzoic acid
Measuring the melting point of a substance can give an indication of its ____________ and can also
bee used as a method of identification.
One of the physical properties of a pure substance is its melting point range( the narrow band of
temperatures between the temperature at which melting _____________ and the temperature at
which all the solid has _______________.
The melting point range of an impure substance is ___________ and ___________________then
that recorded for a pure sample of the same substance.
Procedure
1. Seal the end of a capillary tube by passing it over the blue
flame of a Bunsen burner.
2. A small amount of the sample to be tested is placed into the
capillary tube and it is inserted into an ______________ block
alongside a _____________ thermometer.
3. Heat the block slowly using a Bunsen burner or on a hot
plate.
4. Observe the melting point range and compare to the correct value of 121 – 122 OC
Example 1 –
2.5g of impure benzoic acid was recrystallised following the normal procedure. After drying 2.25g of the
purified crystals remained. What is the percentage yield of the purified crystals?
Answer:
% yield = ______________________________ x
% yield =
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Chapter 34 – The carboxylic acids
Section F – Chemical reactions of the carboxylic acids
1. Carboxylic acids reacting as acids
Carboxylic acids are stronger acids than alcohols, and this can be explained by
 The Inductive effect
The carbonyl group pulls the electrons away from the O-H bond in the hydroxyl group, and makes the
loss of the H+ ion easier
 the stability of the anion that is formed after the H+ is lost from the molecule. The anion has extra
stability due to the way the negative charge can be spread out over three atoms (it is
_____________________)
Reaction with magnesium metal
Ethanoic acid
+ Magnesium
Magnesium ethanoate +
Hydrogen gas
Reaction with sodium hydroxide
Ethanoic acid
+ Sodium hydroxide
Sodium ethanoate
+
Water
Reaction with sodium carbonate
Ethanoic acid + Sodium carbonate
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Sodium carbonate + Water + Carbon dioxide
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Chapter 34 – The carboxylic acids
2. Reduction reactions
Carboxylic acids can be reduced to their corresponding aldehydes and even further to their
corresponding alcohols by reacting with hydrogen gas passed over a hot Nickel catalyst. This
reaction is an example of ___________________ catalysis.
Ethanoic acid
Ethanal
Ethanol
3. Esterification reactions
When an alcohol reacts with a carboxylic acid under acidic conditions, an ester and water are made.
These are types of ___________________ reactions ( condensation reactions)
Ethanol +
Ethanoic acid
Ethyl ethanoate
+
water
** Note that the geometry of the carbonyl carbon does NOT change in this reaction.
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Chapter 34 – The carboxylic acids
Section G: Mandatory experiment – to study the following reactions of
Ethanoic acid..
Part 1: Reaction of ethanoic acid with sodium carbonate
1. Some sodium carbonate powder is placed in a test-tube
2. Some ethanoic acid is added to the test tube
Result: _____________ is observed. When the gas is held over a lighted splint it ____________.
When the gas is bubbled through limewater it goes _____________.
Explanation: The following reactions occur:
Part 2: Reaction of ethanoic acid with magnesium metal
1.
Some magnesium metal is placed in a test-tube
2.
Some ethanoic acid is added to the test tube
Result: _____________ is observed. When the gas is held over a lighted splint it ____________.
Explanation: The following reactions occur:
Part 3: Reaction of ethanoic acid with ethanol
1. Some ethanol is placed in a test-tube
2. Some ethanoic acid is added to the test tube
3. A few drops of _____________ sulfuric acid are added
4. The test tube is warmed in a hot beaker of water
Result: The product of the reaction has a __________ smell.
Explanation: An ______________________ is made ( ethyl ethanoate)
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Chapter 34 – The carboxylic acids
Check your learning of
The Carboxylic acids
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Do you know about…
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Structure of carboxylic acids
Carboxylic acids: structure and names up to C-4 Geometry of carbon
atoms.
Presence of a carbonyl group which is polar
Physical properties of the carboxylic acids
[physical state and explanation of boiling points of the members of the
family compared to corresponding alkanes or alcohols and each other}
Solubility (qualitative only) in water and explanation and in non-polar
solvents
Uses of acids
Use of methanoic acid, ethanoic acid, propanoic acid and benzoic acid
Reactions of acids
Reduction of carbonyl compounds using H2/Ni catalyst. (equations
required).
Reactions with alcohols and acid to make ester (equations required)
Reactions with Magnesium metal (equations required)
Reactions with sodium hydroxide
Reactions with sodium carbonate
Mandatory experiment: to investigate the reactions of Ethanoic acid
with sodium carbonate, magnesium and ethanol
Mandatory experiment : To oxidise phenyl methanol (benzyl alcohol) to
benzoic acid from potassium permanganate solution under alkaline
conditions
Mandatory experiment - Rerystallisation of benzoic acid (+ calculations
of % yields)
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Chapter 34 – The carboxylic acids
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