Transcript Evaluation preparation

Evaluation preparation
Starter
1. Draw methyl benzoate
2. What are the 2 different conditions for the
hydrolysis of an ester?
Starter
1. Draw methyl benzoate
2. What are the 2 different conditions for the
hydrolysis of an ester? Acid or alkaline.
Learning objectives
• Recall how to carry out hydrolysis on an ester
• Describe the different stages of the process
• Evaluate the practical
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
HCOOH
+
METHANOIC
ACID
C2H5OH
ETHANOL
ETHYL METHANOATE
CH3COOH
ETHANOIC
ACID
METHYL ETHANOATE
+
CH3OH
METHANOL
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic
CH3COOCH3 + H2O
CH3COOH + CH3OH
alkaline
CH3COOCH3 + NaOH  CH3COO¯ Na+ + CH3OH
If the hydrolysis takes place under alkaline conditions,
the organic product is a water soluble ionic salt
The carboxylic acid can be made by treating the salt with HCl
CH3COO¯ Na+ + HCl  CH3COOH + NaCl
• T:\Science\KS5\Chemistry\Hydrolysis of an
Ester cut and stick.doc
Thinking about the experiment.
1. The solution is acidified after the reflux, why?
2. Explain why ethanoic acid is water soluble but benzoic acid is not soluble
in cold water.
3. Suggest what might be the most significant procedural error and suggest a
modification for it.
4. Give two reasons why your percentage yield is less than 100%.
5. A suggested mechanism for the hydrolysis of the ester involves the
hydroxide ion as a nucleophile. Complete the mechanism below, putting
dipoles and curly arrows where necessary.
1. Alkaline hydrolysis of an ester produces the carboxylate. The solution was
acidified to convert the carboxylate into the carboxylic acid.
2. The polarity of the bonds in the COOH group in ethanoic acid produces
interactions with polar water. As well as dipole-dipole interactions there
will be hydrogen bonding between the acid and water. In benzoic acid the
large benzene ring is non- polar. The delocalised π-electrons are attracted
to the carbon in the COOH group, which reduces the polarity of the bonds.
This means that the intermolecular interactions and the solubility of
benzoic acid are also reduced.
3. The volume of methyl benzoate is measured in a 10 cm3 measuring
cylinder and as the yield is based on this measurement it would more
appropriate to measure it in a graduated pipette.
4. The yield will be less than 100% because: The methyl benzoate may not be
completely hydrolysed when it is refluxed. Some of the product is lost during
each step of the purification process.
Learning objectives
• Recall how to carry out hydrolysis on an ester
• Describe the different stages of the process
• Evaluate the practical
Starter
• Draw the mechanism to show the nitration of
methyl benzoate
Starter
• Draw the mechanism to show the nitration of
methyl benzoate
Practical
• nitration of methyl benzoate ss.doc
Clamp a round bottomed
flask into an ice bath
15
Into a boiling tube, pour 3cm3
of conc nitric acid and 3 cm3
of conc sulfuric acid, place the
boiling tube into a beaker of
ice and allow to cool until the
solution reaches 5oC
16
Pour 5cm3 of conc sulfuric acid
into the round bottomed flask
and cool it until it reaches 5oC.
Slowly add 3cm3 of methyl
benzoate to the sulfuric acid,
keeping the temperature at
5oC.
17
Pour the nitrating mixture
into the dropping funnel
and place the funnel into
the central ground glass
joint, allowing the
temperature to be
monitored through the
other glass joint
May have to remove the bung to
reduce the pressure and allow all the
nitrating mixture to drip out
18
When all the nitrating
mixture has been added,
leave the mixture for
about 10 mins at room
temperature.
19
Cool a test tube of
methanol
20
Pour the solution into a small
beaker half filled with ice. Allow
the product to precipitate out
21
Filter the solid under
reduced pressure and
wash with a MINIMUM of
cooled methanol.
22
Heat a test tube of
methanol for
recrystallisation.
23
Recrystallise the nitrated
ester with the MINIMUM
methanol. Filter under
reduced pressure and allow
to dry
24
Find the mass of your product then
determine the melting point.
The melting point should be
between 76-80oC
25
Answers to the questions
• Mass of methyl benzoate used: 3.27 g
• Expected yield of methyl 3-nitrobenzoate: 4.35 g
• percentage yield = actual yield / expected yield ×
100%
• Melting point methyl 3-nitrobenzoate: 76–77 °C
1. HNO3 + H2SO4  NO2+ + HSO4 – + H2O
or
HNO3 + H2SO4  H2NO3+ + HSO4 –
2. It is necessary to control the temperature at which nitration
occurs to ensure that only mono-nitration takes place. Trinitro compounds are unstable.
3. Electrophilic substitution
The proton substituted by the nitro group reacts with
the hydrogen sulfate ion and produces sulfuric acid. As the
sulfuric acid has been regenerated at the end of the
reaction, it has acted as a catalyst.
H+ + HSO4-  H2SO4
5.
The ester group must deactivate the ring for
substitution to occur at carbon-3. The carbonyl group is
polarised and the carbon carries a δ+ charge. The
delocalised π-electrons in the benzene ring are attracted to
the carbon atom which decreases the electron charge
density of the ring.
4.