Organic Chem Melting Point Lab

[FrankP]

Hello again everyone!

 

So this post is less dramatic and doomsday-ish then all my others have been as it pertains to chemistry at least.

 

Lab 1 was Monday for me, our aim was to use the Thomas-Hoover Apparatus to discover the melting point of our unknowns. So I wanted to first state that I understand how to perform the lab, as a small group we did well on the lab experiment (except for one minor error which happened on our first trial but that was quickly adjusted). I just wanted to post to get more insightful opinions on how I can formulate my ideas on why things went the way they did.

 

The chemicals we used for my group were:

• biphenyl [Melting Point] 69-72 °C
• 2,5-dimethyl phenol [Melting Point] 68-71°C

 

So my group suspected that our compound was 2,5-dimethyl phenol because trial one of the strict non-mixed MP test showed a range of 69-71. Then trial 2 was a mixed where we took biphenyl and the unknown as well as 2,5-dimethyl phenol and the unknown and ran 2 separate trials to see if the melting point was altered at all within the trials. We discovered that the trial with biphenyl had a slightly higher melting point as was 2,5-dimethyl phenol + unknown. So we were able to confirm our original hypothesis that our unknown was 2,5-dimethyl phenol.

 

So my question is (drumroll please lol jk) given the situation that happened above in our experiment would it be scientifically accurate for me to make my conclusion as such?

 

The unknown compound was confirmed as 2,5-dimethyl phenol after 2 trials of mixed melting point testing. This is confirmed on the grounds that the melting point across the two trials did not differ from the original non-mixed melting point test of our unknown; thus confirming that there were no impurities and the mixed melting point test was in fact a mixture of the unknown substance with itself.

 

While further testing confirmed that biphenyl mixed with the unknown compound altered the melting point by 1 degree. This while apparently insignificant at first upon further testing in trials 2 and 3 of the same mixture confirmed a melting point shift of 1 degree. Therefore this test showed an impurity was mixed with the original unknown, which was made clear by the shift in melting point.

 

Since this is the first lab experiment and assignment I have had for any organic based chemistry class I want to make sure that my rational is at least correct.

 

Thanks again!

[hypervalent_iodine]

In the context of the particular lab you were doing, I would say that your analysis is fine. My only comment is that 2 trials per test would not usually be considered enough, though I assume that this is how many you were expected to do? I would normally advise my undergrads to do between 3 and 5 replicates in these sorts of experiments, but that number is heavily influenced by time pressures.

 

In a more general setting, you probably would use melting point to discern two substances that were so close together. Something like GCMS or NMR might be more appropriate. I say this for insterest's sake only, of course.

[FrankP]

In the context of the particular lab you were doing, I would say that your analysis is fine. My only comment is that 2 trials per test would not usually be considered enough, though I assume that this is how many you were expected to do? I would normally advise my undergrads to do between 3 and 5 replicates in these sorts of experiments, but that number is heavily influenced by time pressures.

 

I tend to agree we ended up having to do only 2 trials of each I think because the lab was such a short period. We did about an hour of intro, expectations etc.. so that significantly ate into the time plus we had to confirm all the contents for our lab lockers which took about 25 minutes. I would have liked to get more conclusive data to be honest because I feel a little funny making a conclusion based off 2 trials.

[hypervalent_iodine]

I tend to agree we ended up having to do only 2 trials of each I think because the lab was such a short period. We did about an hour of intro, expectations etc.. so that significantly ate into the time plus we had to confirm all the contents for our lab lockers which took about 25 minutes. I would have liked to get more conclusive data to be honest because I feel a little funny making a conclusion based off 2 trials.

 

So long as you understand that, then it doesn't matter very much for this lab. Depending on the sorts of questions you need to answer for assessment on it, you may like to mention the possibility of performing more replicates to account for any random errors, and develop a more statistically valid conclusion.