Read and be familiar with the Semi-Microscale lab procedure. Omit Step 1 \"Recry

Question

Read and be familiar with the Semi-Microscale lab procedure. Omit Step 1 "Recrystallizing benzil" and begin with Step 2 using 1 g of benzil in a 125-mL Erlenmeyer flask. You will assess the purity of your product by obtaining i) an IR spectrum, ii) TIC, and iii) comparing the melting point of your product to the known compounds. Melting points can be used to assess purity (and identity) In a perfect crystal with no impurities the melting point will occur at one temperature However when impurities arc introduced there is no longer a continuous crystal structure. Instead the solid is made up of different regions, some with more crystal imperfections than others This will cause melting to begin earlier than expected and to occur over a longer range of temperatures. Even water or solvent can be an impurity, that is why it's important to make sure the crystals arc dry. Often in an organic chemistry research lab, one will obtain a "co-spot" of the starting material and product on a TLC plate to ascertain if the reaction has gone to completion Prepare your TLC plate as follows: i) In the left-hand lane spot the starting material; ii) In the right hand lane, spot your product: and iii) In the middle spot both starting material and product (on top of each other). What hazards arc associated with the use of the reagents and solvents used in this lab? How will melting point and IR be useful for identifying the product of your reaction? How many moles of ketone or aldehyde can you reduce with one mole of NaBH_4? What changes would you expect lo observe in the 'HNMR and 'CNMR upon reducing a ketone lo an alcohol? What arc the physical property (mp, bp, R_f) differences between two enantiomers? Diastereomers? Meso compounds? What is the relationship between R_f and polarity?

Explanation / Answer

1. Hazards associated with,

Benzil : It is an irritant and hazardous when comes in contact with skin, eyes or lungs.

Ethanol : Irritant to eye and skin.

2. Melting point of a compound indicates its purity. When the product is pure, the intermolecular forces are strong and thus it melts at higher temperature. Adding impurities lowers the number of intermolecular forces (bonds) and thus lowers the melting point of any compound. Similarly, IR would show peak at 3200 cm-1 for the O-H stretch of starting material If the material is pure in the IR, while no such peak would be visible If the material is pure.

3. Theoretically one mole of NaBH4 can reduce four moles of aldehyde or ketone.

4. The proton attached to the carbon next to an -OH group in the product would appear at about 3-4 ppm region in 1H NMR, which is absent in the ketone. In case of ketone. In the ketone we would see a peak at about 195-205 ppm region corresponding to the carbonyl carbon in 13C, which is absent in the alcohol.

5. The reduction of benzil gives meso diol melting at 137-139 oC. The reduction also gives enantiomeric pair of diol which meltis at much lower 120 oC.

6. As the polarity of a compound goes up, the Rf value also increases. So a less polar compound would have a lower Rf, whereas, a more polar compound would have a higher Rf value.

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