Esters are widespread in nature. In particular, these compounds are responsible for pleasant aromas — Organic Chemistry Chemistry Question
Synthesis of diethyl ester of succinic acid
Esters are widespread in nature. In particular, these compounds are responsible for pleasant aromas of flowers, fruits and berries. Low molecular weight esters, flammable liquids with low boiling points, are applied as solvents for varnishes and paints, flavoring additives in food industry, etc. Esters of higher monocarboxylic acids and higher alcohols are referred to as waxes. Fats and vegetable oils, vitally important storage compounds, are esters of polyhydric alcohol glycerol and higher carboxylic acids.
Several synthetic schemes are available for ester preparation. The acid catalyzed esterification of a carboxylic acid and an alcohol is among the most used in laboratory practice. Since the reaction is reversible, special attention should be given to shifting the equilibrium towards the ester formation. This can be achieved by either introduction of dehydrating agents or removal of the products (ester and/or water) from the reaction mixture. In the case of starting compounds with relatively low boiling points azeotropic removal of water is possible. In this work you will follow the latter approach to ester synthesis.
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Chemicals and reagents
* Succinic acid, 18 g,
* Ethanol (absolute), 55 cm3,
* Toluene (dry), 24 cm3,
* Sulfuric acid (concentrated), 0.5 cm3,
* Potassium carbonate (calcined), 25 g
Equipment and glassware
* Graduated cylinders, 5 and 50 cm3,
* Round-bottom flask, 250 cm3,
* Long Vigreux distilling column with a downward condenser,
* Vacuum adapter
* Hoses for the condenser,
* Glass capillaries or boiling granules,
* Receiver flask, 100 cm3,
* Thermometer with a tapered joint,
* Magnetic stirrer with heating or hot plate,
* Claisen flask (150 cm3) with a downward condenser,
* Capillary for Claisen flask,
* Filter paper or fritted glass filter,
* Beaker, 100 cm3,
* Three-way vacuum adapter,
* Receiver, 50 cm3 (3 ea.),
* Water-jet air pump,
* Manometer,
* Analytical balances (± 0.001 g),
* Refractometer,
* Teflon sleeves for tapered joints or vacuum grease,
* Spatula,
* Pressure nomogram.
Procedure
A. Assembly of the apparatus
Assemble the apparatus as shown in the hereunder picture. Equip every joint with the Teflon sleeve or apply vacuum grease.
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Note: the tip of the thermometer should be positioned slightly below the outlet to the condenser.
B. Preparation of the reagents and synthesis of the product
Weigh out and admeasure the required amounts of succinic acid (18 g), absolute ethanol (55 cm3), toluene (24 cm3), and concentrated sulfuric acid (0.5 cm3). Place all the materials into a 250 cm3 round-bottom flask.
Heat the reaction mixture constantly monitoring the temperature. The fraction with the azeotropic mixture of the alcohol, water and toluene should start distilling off at 74 – 80°C. Collect the distillate in the 100 cm3 receiver flask and terminate distillation (turn off the heater plate and disconnect the receiver flask with the collected distillate). Add 25 g of calcined K2CO3 to the receiver flask to dry the distillate. Shake the flask and leave it for 1 hour. Then separate the liquid from the desiccant by filtration through any filter enlisted in the “Equipment and glassware” section. Place the dried distillate back to the reaction mixture in the round-bottom flask, switch on heating and collect once again the fraction with the azeotropic mixture of the alcohol, water and toluene distilled off at 80 °C.
C. Vacuum distillation of the residue
Label and take weights of all empty 50 cm3 receiver flasks. Transfer the residue from the round-bottom flask to the Claisen flask and add glass capillaries or boiling granules. Assemble the apparatus for vacuum distillation as shown in the hereunder picture. Equip every joint with the Teflon sleeve or apply vacuum grease. Connect the vacuum line to the vacuum adapter.
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Distill the residue under vacuum using the water-jet pump. Collect the fractions into the receiver flasks. Maintain the rate of distillation of about 1 - 2 drops of the distillate per second. Once the temperature reaches 103 °C (as read on the thermometer) at 15 mm Hg, collect about 10 drops in the first receiver flask and change it to the second one by turning the adaptor. Collect the fraction up to 107 °C at 15 mm Hg. Then change to the third receiver flask and switch off the heating. Let the apparatus cool down before disconnecting the vacuum line and letting air in.
Note. Use the pressure nomogram to recalculate the temperatures if you observe pressure other than 15 mm Hg.
Record the pressure and the temperature of distillation.
Attention!
Never use flat-bottomed flasks as receivers!
Always wear protective glasses or mask when doing vacuum distillation!
D. Analysis of the product
Weigh the receiver flask with the required fraction and find the mass of the product. Calculate the yield.
Determine the refractive index nD 20 of the product using the refractometer. If you perform the measurement at a different temperature, reduce the obtained value to 20 oC.
What is the role of toluene in the above process?
Model Answer
Toluene forms a ternary azeotrope with water and ethanol with the boiling temperature of 75 °C. The boiling point of diethyl succinate is 218 °C. The difference in the temperatures allows removing water from the reaction mixture, thus shifting the equilibrium towards the reaction product formation.
Write down the reaction mechanism.
Model Answer
The reaction mechanism is as follows:
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O
OH
R H
+ O
OH
R
H +
H +-
O
O
CH3
C2H5OH OH
OH
R +
+ ..
CH3
OH O +
OH
OH H
CH3 -H2O
R
OH
O
CH3
R +
R
Suppose 18O-labeled ethyl alcohol and 18O-labeled tert-butyl alcohols are involved into the reactions instead of unlabeled ethyl alcohol. Which compound(s) will be the 18O atom found in when the reactions with the labeled alcohols are complete? Prove your choice.
Model Answer
The tert-butyl alcohol molecule is protonated first, which is followed by a water molecule release. As a result, a relatively stable tert-butyl cation reacts with the acid molecule. Consequently, the isotopic label will be found in the H2O molecule:
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In the other case it is the acid molecule which is protonated first at the oxygen atom. Then one of the C-O bonds is cleaved giving the carbocation, which further attacks the alcohol molecule followed by the proton elimination. Thus, the isotopic label remains in the ester molecule:
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