Questions 1.During a suction filtration, what are the consequences of using a fi

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1.During a suction filtration, what are the consequences of using a filter paper that is larger than the funnel diameter and using one that is too small? What are the consequences for the use of each?

2.What problem occurs if a student turns off the aspirator before detaching the tubing at a connecting point?

3.What criteria should you use to determine whether recrystallization or sublimation will be a better method of purification?

4The reduction of benzil to hydrobenzoin can be followed by TLC or IR or by watching the disappearance of the yellow color. Explain.

5.What is the purpose of adding water to the reaction mixture after the reaction between sodium borohydride and the ketone is complete? .

Explanation / Answer

1. A filter paper should sit flat on the bottom of the funnel with no creases or folds if one uses larger size filter paper there will be creases or folds.

If larger filter paper than the funnel diameter is used the filter paper will not sit on the funnel surface properly creating a space through which solid can pass to the filtering flask.

Using the filter paper to small may not cover all the holes in the base of the funnel which can also allow the solid to pass to the filtering flask as well as it will allow the the solid to get on the funnel surface apart from the filter paper.

2. If the aspirator is detached the tubing at a connecting point?

This will cause the pressure in the filter flask to drop than that in the water aspirator. Allowing water to be drawn from the aspirator stream into the filter flask resulting in contamination of the filtrate.

3.

If the product is soluble in the low boiling solvent partially at the room temperature & has higher solubility at the boliling tempeature then crystallization can be used. Sublimation can be used if the solid has low vapor pressure so that it can be converted to gaseous state from the solid state.

4. Reduction of benzil to hydrobenzoin can be detected by the dissaperance of strong carbonyl peak in the IR (around 1700). The loss of chromophore (carbonyl) in hydrobenzoin can also be seen on the TLC appearance of the reaction by the disappearance of the yellow colour. If the reaction mixture does not give yellow colour on TLC it also indicates the consumption of the starting material.

5.

The alkoxide ion formed by the transfer of hydride ion on the carbonyl carbon of ketone is protonated by the water to give the alcohol product. It also quenches any excess sodium borohydride present in the reaction mixture.

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