|
|
 |
Reply
| |
I'm trying to write my lab report for an experiment I did involving the preparation of synthetic pear oil. We mixed hexyl alcohol and acetic acid and produced hexyl acetate along with some other garbage. We got rid of most of the garbage except for the hexyl alcohol. So we distilled the liquid in order to get rid of the alcohol and keep the hexyl acetate.
But, when we heated the liquid, we weren't able to get it hotter than 100 degrees C. (We were trying to get to 170). The thermometer rose to 100, then dropped to 80, rose to 100, dropped to 80... and meanwhile nothing distilled out.
So the TA's told us just to quit and write our report based on what happened. I'm trying to figure out why we weren't able to get the temperature any higher, other than a malfunction in the heating plate (we had it set to max). Are there any other possible reasons?
Thanks! :o) |
|
 First
Previous
2-12 of 12
Next
Last
|
|
Reply
| | From:  ·Steve· | Sent: 11/4/2006 10:23 PM |
It is possible, but hopefully not the case, that you accidentally discarded the hexyl acetate layer and tried to distill the other product of the reaction, water!  If you were using a fractional distillation setup and the upper part of the fractionating column was not able to heat up sufficiently during the distillation, the liquid would boil but just reflux for a long time without actually distilling over. If it was water, you could get the kind of behavior you observed. The temperature would never go over 100° and it could fluctuate if drafts of air in the room intermittently cooled the fractionating column which would affect how much vapor was contacting the thermometer.
After the reaction between acetic acid and hexyl alcohol is complete, you would have two liquid layers remaining, a lower water layer and an upper hexyl acetate layer, since hexyl acetate is insoluble in water and less dense. But it depends on how you do the reaction. Since in this reaction water is the lowest boiling component, it could be removed by distillation as it forms during the reaction, leaving the higher boiling substances behind. At the end of the reaction you would have mainly just the ester product remaining in the original reaction vessel. But on the other hand if you did the reaction in a reflux setup, you would have the two product layers after the reaction was complete and ideally little or no unreacted acetic acid and hexyl alcohol remaining.
So however you did the reaction, if you're sure you saved the correct layer for distillation, it was a bad hot plate. Otherwise....ooops!  Believe me, it happens! 
Steve
|
|
Reply
| |
Thanks for the reply Steve. We just did a simple distillation, not fractional. If, theoretically, we did save the wrong layer, would that kind of behaviour appear in a simple distillation as well?
I'm fairly certain that we saved the right layer... it seemed pretty obvious at the time - one layer was quite watery-looking and the other one looked pretty oily. After heating the alcohol and acetic acid under reflux, we washed the stuff twice, once with water and once with aqueous sodium bicarbonate. Both times, we removed the layer that looked clearer - I'm pretty sure it was the bottom layer.
I wouldn't be surprised if it was a bad hot plate... I'll just add it to the rest of the crappy lap equipment... >:(... but I just wanted to be thorough. Thanks again! |
|
Reply
| | | From: ·Steve· | Sent: 11/4/2006 11:06 PM |
Sounds like that ol' hot plate was the culprit. In a simple distillation, water would have distilled easily for sure. And it certainly looks like you did the washing steps without getting the layers mixed up. Whew! Steve |
|
Reply
| | | From: Norm | Sent: 11/5/2006 2:46 PM |
I have a few suggestions on this since it's in the area of my work. Steve is correct about the two layers remaining after the hexyl alcohol and acetic acid reaction is complete. However, in my experience, that type of esterification can be exceedingly slow and even though you think it's complete, you can come back 24 hours later and find it clouding up again, meaning more esterification is taking place and you're producing more water as a by-product. If that's the case, then your distillation is really working properly and it's distilling off more water, produced even faster because you're heating it again, and even traces of it that you can't necessarily see a lot of condensation from may be coming off. The fact that your temp then drops to 80°C happens frequently in our lab. It generally means your apparatus is losing heat more rapidly than your poor hot plate can put out. We can sometimes (not always!) remedy that by insulating the apparatus as much as possible with towels - make sure they're heat resistant and fireproof! Sometimes labwork is more art than science! -Norman |
|
Reply
| | | From: ·Steve· | Sent: 11/5/2006 8:01 PM |
Good points Norm. I forgot to mention also that the Fischer esterification reaction reaches an equilibrium rather than going 100% products. In one of our experiments we determine the value of the equilibrium constant. Using acetic acid and various primary alcohols, the students usually get values in the 2 to 5 range. Since the reaction is very slow at room temperature, the mixture is refluxed for about two hours, checking for completeness of reaction by titrating aliquots from the reaction mixture to determine how much acetic acid remains.
Steve
|
|
Reply
| |
sounds like your heating mantle is messed up. we have distilled using fractional distillation and soxhlet and it did well. how were they having you heat it and what method of distillation were you using ( eg soxhlet, fractional, soxhlet)? |
|
Reply
| |
ahhh day late and a dollar short on my answer. I wish i was half as smart as steve. tell them to try a fractional set up next time using adapters so you can check pot temp and head temp's. hope things work out for you |
|
Reply
| | | From: Norm | Sent: 11/6/2006 5:07 PM |
Steve, that's what I like about these questions and answers - it breaks up the cobwebs in my mind about some of the principles I learned decades ago. Le Chatelier's Principle - I had to look that one up, I had forgotten all about it. My well-worn, 1969 edition of Skoog & West's Analytical Chemistry text enlightened me about it. One additional suggestion for the distillation problem - if it can be run under vacuum you'd eliminate the high temperature problem. But it takes a whole different setup and apparatus and you need the source of vacuum, sometimes not always available. -Norman |
|
Reply
| |
Hi Steve! Sorry it takes a while for me to reply. I am not often on. Yes Dean Stark is great as well. We use a drying agent in the soxhlet and yes it does help. During the Unit Processes semester we teach fractional, Soxhlet, Dean Stark Trap and test results of them using GC, LC, UV/Vis Hey you deserve the compliment. You helped me when I was having issues learning. Thanks for all of your help!! Mike Richardson (Skippy) |
|
Reply
| | | From: ·Steve· | Sent: 11/13/2006 6:16 AM |
Hey Mike, thanks! Sorry, I can't recall when you were having those "issues learning" (I've had a few of those myself!) but now you're teaching this ol' subject - congratulations! At Houston Community College where I teach we have Chemical Laboratory Technology and Process Technology courses, but they are in a different department from mine so I don't teach any of these. Plus I don't have any on the job experience in process work that would qualify me to teach in that field anyway; all my experience is in the lab. But anyway, good luck and I hope all is going well with your work.  Steve |
|
|
First
Previous
2-12 of 12
Next
Last
|
|
|
Free Forum Hosting
Important Announcement
I assume that your Soxhlet contains a drying agent or molecular sieves to absorb the water. That's the best way to get the maximum yield of ester, removing the water during the reaction and letting the Le Chatelier principle do its thing. 
Return to Organic