Below is the synthesis of a chalcone derivative via an aldol condensation reacti
ID: 486547 • Letter: B
Question
Below is the synthesis of a chalcone derivative via an aldol condensation reaction: i. Propose a quench and work-up for this synthesis. ii. Explain why recrystallization is appropriate for the purification of the chalcone product. iii. Imagine that you want to test a few solvents to see which one is the most suitable for the recrystallization of the chalcone product on the previous page. Which solvents would you choose to test and why? iv. During recrystallization of the product, you see two layers in the beaker after heating. What happened? How can this be fixed? b. Why is slow cooling necessary when recrystallizing? c. After cooling on ice, how should one go about isolating the purified product? Should you let the solvent evaporate? What type of apparatus should you use and why? d. What role does temperature play in the solubility of a compound in a given solvent? What is the ideal situation for recrystallization in regards to a chosen solvent and temperature? e. How much solvent should be used during recrystallization? How do you know if you've used too MUCH solvent or too LITTLE solvent?Explanation / Answer
ii) Recrystallisation is an important prurifying technique principal is based on the fact that the amount of solute dissolved in solvent increase with increase in temperature. The product (Crude including impurities) is allowed to dissolve in an similar nature solvent (Likely with same polarity) at high temperature and a solution is formed. The condition is that solute should not be soluble in solvent at less temperature. As the solution cools down, the solute forms a crystall lattice structure leaving impurities in solution.
This process is effectivelly used specially for the molecules with some biological properties.As chalcones belongs to class of natural products, termed as "Flavonoids" found naturally in some fruits and vegetables.Chalcones are known to possess anti-malarial, anti - inflammatory, anti micobial and anti-oxidant activities.
(iii) Recrystallisation works effectively only when appropriate solvent is used. The solvent selection should be like this that solute must be insoluble in solvent at room temperature and more soluble with increase in temperature. However, the impurities should be soluble in solvent at room temperature, as the impurities stay in solution as the solution cool leaving pure solid crystal (as described in recrystallisation procedure above). for example if you want to purify a sample '1' which is contaminated with impurity '2', solvent selection should be such that, impurity 2 should be soluble in solvent at room temperature and sample '1' should have minimum solubility at room temperature and much more solubility with increase in temperature. As we know "like disslove like" , solvent most likely to have similar properties as of solute can be appropraite solvent.
(iv) Can you elaborate this part more ?
Since after heating, only one uniform solution of substance with impurities in solvent is found. And upon cooling pure substance take the form of crystal lattice and impurities left behind in solution( as impurities are soluble at room temperature in solvent). This Crystall lattice is desired pure substance which then filtered out.
(b) slow cooling is an important step in recrystallisation. This impact on the purity of crystal. Slow cooling tends to form pure crystals.A very pure crystall is formed in recrystallisaion process upon cooling. In the process, a small crystall is formed first ehich then grow layer by layer. In a manner a crystall 'select' the pure molecule from solution. When cooling done rapidly, appropraiate selection could not happen and impurities trap in the crystall lattice tend to less pure material.
(c) After cooling, the purified product in the form of crystalls can be isolated using vaccuum filteration.
(d) Solubility of a compounds depends upon temperature. In most of the cases, solubility increases with increase in temperature. The ideal selection of solvent and temperature is such that the solute should be minimum soluble in solvent at room temperature and its solubility should increase with increase in temperature. However, impurities should be soluble in solvent at room temperature so that when pure substance crystallice upon cooling, impurities should remain in solvent.
(e) You should use as musch less solvent as possible for recrysatllisation. Because if large quantity of solvent will used, desired product (which we want to crystallice) will stay dissloved in solution upon cooling. Hence the desired product will lost in too much solvent since the solubility is still not 0 at room temperature.
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