Esterification

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Transcript Esterification

How well
are you
doing?
Where did the class fall down on the last
past paper?
Esters, Equilibria and Testing for ions
Esterification and
Dehydration
Success Criteria: Be able to
• Describe the esterification of alcohols with carboxylic acids.
• Produce a sample of a pure Phenyl Benzoate ester using a
range of advanced chemical techniques.
• Explain the process of re-crystallisation.
• Extension:
• Describe elimination of H2O from alcohols to form alkenes.
Numeracy Starter
• What is the
resolution of
measuring
cylinder A?
Numeracy Starter
• What is the
resolution of
measuring
cylinder B?
• S&C why is
there a
temperature
on the
cylinder?
Which measuring cyclinder would
you use to measure 2cm3 of
liquid? Why?
TASK : Calculate the % error in each case.
Numeracy Starter
Answer 3
• The 500cm cylinder had a resolution
of +/-2.5cm3. Therefore to measure
2cm3 the error would be
(2.5/2.0)x100 = 125%
• The 5cm3 measuring cylinder had a
resolution of +/- 0.1cm3
Therefore to measure 2cm3 the error
would be (0.1/2.0) x 100 = 5%
AFL
• On your post-it note write down
• A/ How you would make the
ester phenyl benzoate.
• B/ How you would separate and
purify the product.
• If you don’t know say so. That is
fine!
Name the compound…
Name the compound
The esterification reaction between
propanoic acid and methanol
Diagram to show the bonds broken
during esterification
Naming esters
Simple laboratory preparation of an ester
Making Pentyl Ethanoate
2cm3 of Pentan-1-ol,
2cm3 of glacial
ethanoic acid. Add a
few drops of
concentrated
sulfuric acid.
HAZARDOUS
Separating the ester
Boiling points
118oC
65oC
Miscible with water?
Yes
Yes
57oC
No
100oC
Yes
Suggest two ways that you could separate the
ester from the reaction mixture.
Pour your reaction
mixture into water
Ester floats on the
water as it is less
dense and immiscible
Sulfuric acid,
ethanoic acid and
pentan-1-ol are
soluble in water
Mini Plenary
• How do you make an ester?
• What is the catalyst used?
• How do you separate the ester
from the reaction mixture?
• Name the ester made from
ethanol and propanoic acid.
Homework
• Describe and explain the
formation of phenyl benzoate
from benzyl chloride and phenol
including a detailed explanation
of recrystallization.
Spaced Learning Activity
• You have four minutes to use
the modelling putty to make a
model of a bunch of grapes.
Esters in grapes
• Pembrokeshire has a vineyard which produces
wines with fine flavours produced by esters.
• Research has identified 11 different esters in grape
juice, but wine can have up to 83; this affects the
wine’s flavour and complexity in proportion
to their presence. Esters are formed when an acid
reacts with alcohol, eliminating a water molecule in
the process.
Ethyl esters have a strong influence on a wine’s
aroma here are three examples;
• ethyl hexanoate, fruity, strawberry, green apple and
anise
• ethyl octanoate, with sweet, fruity, ripe fruit,
burned and beer characteristics
• ethyl decanoate, which imparts an oily, fruity and
floral character.
Sweet-smelling esters
Explain how you would make Octyl Ethanoate
The esterification reaction is both slow and
reversible. The equation for the reaction
between ethanoic acid and ethanol is:
1180C
780C
540C
1000C
Q1 What does reversible mean?
DH of the forward reaction is -20kJ.mol-1
Q2 How can we favour the forward reaction and make
more Ester?
Q3 Why do we warm the reaction mixture?
Starter Synthesis of
PhenylBenzoate
• Draw the two compounds that you would
need to react together to form
phenylbenzoate.
Answer
• Benzoic Acid
Phenol
How to get around reversible reactions..
By using a derivative of carboxylic acids called an
acid chloride in the presence of sodium ions we
minimise the back reaction from taking place to gain
a better yield of the ester. We also do not need the
acid catalyst if we do this.
Risk Assessment
• Hazard- Phenol is corrosive and
causes burns to the skin.
• Risk – Phenol getting on skin during
weighing/ handling.
• Wear gloves, lab coats and safety
glasses. Inform teacher immediately
if phenol gets on your skin.
Method
1/ Weigh 1.0g of Phenol into a small conical flask. HAZARD
2/Add 18cm3 of 1M NaOH(aq) and bung the flask.
3/ HAZARD In a fume cupboard add 2cm3 of Benzoyl chloride in
small quantities at a time.
4/ Fit a bung and shake vigorously with occasional cooling under
the tap or in ice water. Releasing the gas formed into the fume
cupboard every few minutes.
5/ After 15 minutes the solid benzoate separates out: the solution
should be alkaline at the end of the reaction; if not alkaline, or if
oily, add a solid pellet of sodium hydroxide and shake again.
6/ Buchner filtrate the phenyl benzoate and wash thoroughly with
ice cold water. and re-crystallise from ethanol (NO FLAMES!).
Buchner filtration
Impurity 1
Remains in
filtrate.
Product
Product
low
solubility at
low temp.
Product
very
soluble at
high temp
Impurity 2
Impurity can be
filtered out while
solution is hot
Numeracy starter
If a saturated
solution of
phenylbenzoate in
ethanol was
cooled from 55oC
to 20oC what
mass of the ester
would be
precipitated?
Recrystalisation to purify your
product.
• What is your product contaminated with?
• How can we remove this?
Recrystallisation
• → Solvent added (clear) to compound and soluble
impurities (orange) → Solvent heated to give
saturated compound solution (orange) with
impurities in solution → solution (orange) allowed
to cool over time to give crystals of product
(orange) and a solution of impurities (pale-orange).
Method
1/ Warm 2/3 of a boiling tube of ethanol to 70oC.
2/ Place the impure Phenyl Benzoate crystals into a
second boiling tube and place the tube in the water
bath.
3/ Add the minimum quantity of hot ethanol to the
crystals to get them to dissolve. Work with a glass
rod.
4/ When all the crystals have dissolved take the tube
out of the water bath and place in a test tube rack.
5/ Observe with care DO NOT DISTURB.
6/ Note your observations.
7/ Buchner filtrate your crystals when cool.
Phenyl Benzoate crystals
Dehydration of alcohols
to form Alkenes.
Dehydration of cyclohexanol to form cyclohexene
Dehydration of ethanol to form ethene
© Pearson Education Ltd 2008
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Extension questions