Purification of caffeine
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Purification of caffeine
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[post_date] => 2025-01-14 05:50:06
[post_date_gmt] => 2025-01-14 10:50:06
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Caffeine is most valued for its ability to temporarily ward off drowsiness and restore alertness. It belongs to a large class of compounds known as alkaloids, compounds that contain mostly basic nitrogen atoms. These are of plant origin and can be extracted from tea bags, for example, through a series of purification steps.
Figure 1. Caffeine Molecule
Extraction Purification
After brewing tea bags in water, the liquid extract is filtered to remove any insoluble material. The remaining supernatant can then be separated through liquid/liquid extraction. The two solvents used in this extraction step are water and dichloromethane.
The two solutions then separate into two layers; a brown aqueous solution and an organic layer with dichloromethane. Evaporation of the organic solvent yields a caffeine compound with impurities such as green chlorophyll.
Sublimation Purification
Further purification of caffeine is possible through sublimation if impurities are nonvolatile. Crude material is placed on the bottom of a sublimation chamber below a cool surface test tube. Impure caffeine extract is then heated to sublimation. A vacuum is applied to the airtight environment by sucking all of the air out of the chamber through the vacuum/gas line as shown in Figure 2.
(1) Cooling water goes in the test tube.
(2) Cooling water goes out of the test tube, generating a cold surface.
(3) Vacuum/gas line creates a near zero pressure environment. It constantly removes gas it is exposed to from the system.
(4) Crude caffeine material is placed inside the base of the sublimation apparatus.
(5) When heated, sublimed caffeine gas rises from the base of the apparatus, forming solid crystals when it makes contact with the cold surface test tube.
(6) Solid non-volatile impurities are left at the base of the apparatus after sublimation.
(7) External heating drives the sublimation process. The gaseous caffeine will be separated from the less volatile impurities and then forms crystal caffeine deposits along the cool surface. The caffeine deposits are then collected for the next step in processing.
Figure 2. Sublimation apparatus
This vacuum is key to the process of sublimation. For a substance to change phase into a gas, its molecules must reach a vapor pressure equal to that of its external air pressure. Because a vacuum lowers the air pressure, the caffeine can reach this vapor pressure at a lower temperature and bypass the liquid phase.
Figure 3. Caffeine Phase diagram with the arrow indicating the path of the caffeine sublimation.
[post_title] => Purification of caffeine
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[question] => How would the caffeine sublimation phase diagram be affected if the vacuum in the apparatus allowed more air pressure in the chamber?
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[answer] => 3
[description] => Reason for the Correct Answer:
A phase diagram shows how the phases of a substance are determined by its temperature and external pressure.
The vacuum assists in the sublimation by keeping the external pressure on the caffeine as low (close to zero) as possible.
Nothing indicates that the increase in temperature from the heating process was affected.
When a higher external pressure is present inside the apparatus while a solid is undergoing a phase change, the solid will more likely change into a liquid before changing into a gas, moving the arrow up to a higher pressure.
Thus the new caffeine sublimation phase diagram will be:
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[answers] => Array
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[0] => Array
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[each_answer] => A.
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[1] => Array
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[each_answer] => B.
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[2] => Array
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[each_answer] => C.
)
[3] => Array
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[each_answer] => D.
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which of the following types of hybridizations describe carbon atoms on the caffeine molecule?
I. sp
II. sp2
III. sp3
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[answer] => 4
[description] => Reason for the Correct Answer:
The sp2 hybrid atomic orbitals have a trigonal planar geometry.
The sp3 hybrid atomic orbitals have a tetrahedral geometry.
The methyl (CH3) substituent is tetrahedral.
The sp hybrid atomic orbital has a linear geometry.
Three carbons on the caffeine molecule have sp3 hybrid orbitals at the methyl groups, while the rest of the carbons have sp2 hybrid orbitals where they are connected to other atoms at approximately 120° angle.
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[answers] => Array
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[0] => Array
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[each_answer] => A.I, II and III
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[1] => Array
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[each_answer] => B.I and II only
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[2] => Array
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[each_answer] => C.I and III only
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[each_answer] => D.II and III
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Why was dichloromethane (CH2Cl2) used in the above extraction?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
The dichloromethane was used as the organic solvent in the liquid/liquid extraction procedure.
In extractions, solvents are liquids used to separate compounds of interest using their chemical and physical characteristics.
Compared to dichloromethane, water has a strong dipole through hydrogen bonding and would tend to segregate with more polar and charged molecules
Miscibility describes the mixing of two substances while each retaining their chemical structure.
Dichloromethane is immiscible with water and is miscible, but not chemically reactive, with caffeine, separating it from the aqueous layer.
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[answers] => Array
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[0] => Array
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[each_answer] => A.It is immiscible with water
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[1] => Array
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[each_answer] => B.It has a binding affinity with water
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[each_answer] => C.It is immiscible with caffeine
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[each_answer] => D.It has a binding affinity with caffeine
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[quiz_unique_key] => 2377279144
[question] => If the sublimation purification were to be performed with the cool surface tube situated higher than the original setup as shown in the left image of Figure 2, how would the results be affected?
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[answer] => 2
[description] => Reason for the Correct Answer:
The cool surface tube does not affect the phase diagram because its role happens after sublimation.
Sublimation is the transition from solid to gas.
The cool surface tube in the sublimation apparatus collects the sublimated compound.
Gases move freely at high speeds and assume the shape of its container.
The left image on Figure 2 shows how, in contrast with the right image, the sublimation chamber does not encompass the cool surface test tube’s shape as effectively.
This decreases the amount of gas that makes contact with the tube and increases the amount of caffeine lost to the outside environment. The vacuum line, now exposed, would rid the system of the caffeine.
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[each_answer] => A.There would be more actual yield as gaseous caffeine will take more time to reach ambient temperature before reaching the cool surface where it will be crystallized
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[1] => Array
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[each_answer] => B.There would be less actual yield as much of the gaseous caffeine will exit the sublimation chamber without making contact with the cool surface
)
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[each_answer] => C.The sublimation arrow in the phase diagram would move left due to the cooler temperatures experienced by the gaseous caffeine
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[each_answer] => D.The sublimation arrow in the phase diagram would move right due to the warmer temperature experienced by the gaseous caffeine
)
)
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[quiz_unique_key] => 2261298308
[question] => Despite heating the apparatus, the scientist detects no temperature change for a short period during the sublimation step. Which of the following explains this observation
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
Chemical sublimation is the second purification step.
Sublimation is a phase transition.
Compounds like caffeine undergoing phase transition do not undergo temperature change as energy is used to break intermolecular bonds instead of increasing the kinetic energy of molecules.
)
[answers] => Array
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[0] => Array
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[each_answer] => A.The energy added was used to increase the pressure inside the apparatus
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[1] => Array
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[each_answer] => B.The energy added had a negligible effect on temperature, because caffeine has a high specific heat capacity
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[2] => Array
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[each_answer] => C.The energy added was used to break caffeine’s intermolecular bonds
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[each_answer] => D.The energy added was stored as chemical potential energy
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[560232|1] => C
[560232|2] => D
[560232|3] => A
[560232|4] => B
[560232|5] => C
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