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#1 2011-07-14 06:00:39


Nitration of acetanilide and impurities in sample

So I was doing a basic Nitration experiment and I have some questions about what could have happened during the experiment since I had some strange observations. I will post the experimental procedure below and tell you what happened after which specific steps.

1.   In a 125-mL Erlenmeyer flask dissolve 6.8g (0.05 mole) of pure acetanilide in 8 mL of glacial acetic acid by warming it gently.

2.     Cool the warm solution until crystals begin to form and then add slowly, while swirling the solution, 10 mL of ice-cold concentrated sulfuric acid.

3.   Prepare in nitrating mixture by adding 3.5mL (5g, 0.06 mole) of concentrated nitric acid to 5 mL of cold concentrated sulfuric acid; cool the solution to room temperature and transfer it to a small separatory funnel.

4.   Cool the acetanilide solution to 5◦ in an ice bath, remove the flask from the bath, and add the nitrating mixture slowly, drop by drop. Swirl the reaction mixture to obtain good mixing in the viscous solution and do not permit the temperature to rise above 20-25◦C.

During this step, the solution was dark orange, almost purple? everyone elses was a lot lighter color. In my lab report, what should I write that could account for this?

5.   Allow the solution to stand at room temperature for about 40 min (but not longer than 1 hr) to complete the reaction. Pour the solution slowly while stirring into a mixture of 100 mL of water and 20-25g of chipped ice.

The liquid that was filtered into the beaker of the vacuum filter was a dark orange and a sort of foamy solute? The lab instructor thought maybe our filter was punctured from us stirring the filtered paste with the stir rod, but when we looked later, there was no hole. Every other group's filtered liquid was light orange and not cloudy at all.   

We asked the lab instructor what could have happened she he said he honestly do not know. The thing is, I prepared the nitrating solution in step 3, while another group prepared step 1 and 2. I asked them if they are sure they didn't make a mistake but they swore they did everything by the book. Any insight and help would be great! I don't think there was any contaminant but it was our first lab and the beakers may not have been washed very well...

I read the rules so I am not asking for answers, I just want some educated theories. Of course I can just write some BS on my lab report about what could have happened but I am genuinely curious what could have happened?

Re: color
A little impurity can give a lot of color. The key I see in the description is the, "I don't think there was any contaminant but it was our first lab and the beakers may not have been washed very well..." Let me add here that I doubt the beakers were washed with an organic solvent to remove any organic impurities or even that this sentence is a euphemism for, "they looked clean so they weren't washed at all". No matter, impurities or not, getting a little or a lot of color from a nitration is not very unusual.

Re: yield (and purity)
The yield is the easiest to answer. Your sample is wet.

The cloudiness of the filtrate (compared to others) shows there are insoluble materials in the filtrate (probably organic in this experiment). The fact that the filtrate are colored or more colored shows they are impurities or simply not the least insoluble (to be in the filtrate). The extent to which they will appear will depend on the ratios of temperature, impurities, solvent, and solvent make up. The funnel and the filtrate will cool from evaporation when vacuum filtering. That is why it is generally difficult to remove impurities by vacuum filtration during crystallization procedures. They will cool and either crystallize in the funnel, the flask, or oil out. Adding more solvent will keep the impurities in solution. High yields increases possible impurities to be present while a low yield could, could (could, not do) correspond with a more pure product. 

From experience, a quick if not the quickest way to dry a sample is to suck air through the sample. Evaporation of solvent is easily noticed by the temperature of the funnel. If the funnel is cool to the touch, water or other solvent is evaporating. When it reaches room temperature, it is dry. When you weigh the sample, the filter paper should also be dry. (I'll bet that wasn't recorded in the notebook, but if you were wondering if you sample could be wet or dry, this would be a clue, no?)

Finally, personal opinion, the poster is asking a basic question, "What happened?" Even the instructor offered no opinion. As a person who worked in a lab for much of their career, I am virtually of the opinion of "Who cares?" While teaching, I emphasized, "What did you get?" Most students assume they got "the product" while in fact, this student got a "dark orange and sort of foamy" solid. If the poster had analyzed the product by tlc, they would have found that it contained a mixture. I dare say that I think evidence of water would have been found in the IR or NMR. I prefer experiments in which some doubt can be raised about the starting material, such as starting with Sample A or B (N-methyl acetanilide or propionyl anilide) as the starting material. A student could not know what the product was until further analysis and comparison of data. This experiment should give some ortho and para nitration, I don't know the ratio. I think a simple tlc would have been helpful in this experiment and with authentic samples, would have allowed a student to identify whether their sample contained any.


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