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Synthesis of Isopentyl Acetate question

  1. Oct 27, 2006 #1
    Hi,

    In the isopentyl acetate synthesis from acetic acid and isopentyl alcohol, what do you extract it with? All the things I've read online say to extract with water, then 2x sodium bicarbonate, then saturated NaCl. However, the procedure we are supposed to modify this from is different. In the synthesis of methyl benzoate, we extracted first with saturated NaCl, then with everything else the same as above.

    Is it necessary to extract teh isopentyl acetate first with sat. NaCl? What most optimizes yield (1st priority) and purity (2nd priority)

    Please help!!!
     
  2. jcsd
  3. Oct 28, 2006 #2

    GCT

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    Extracting with water is going to promote further formation of the ester, according to La Chatelier's principle, since water is a product.......resulting in higher yields. The bicarbonate would extract the alcohol and the acid, you may have used one or the other as the limiting reagent. The NaCl is going to promote the extraction of all ionic substances or with compounds which would favorably be transferred to the saturated NaCl in consideration of its partition coefficient. It's been a while since I've encountered this experiment, so the above may not be accurate.
     
  4. Oct 20, 2007 #3
    Yes, water is a product, so the reaction is going to shift *left*, toward the reactants, which *lowers* the yield.

    Can someone please post what the extraction order should be?
     
  5. Oct 22, 2007 #4

    chemisttree

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    The order in the OP is correct. The first extraction will extract out the acid catalyst, the alcohol and the acetic acid. These will be in water now and so the esterification reactions effectively stops at that step. What remains is the ester product and some traces of the acid, acetic acid and the alcohol. The bicarb extraction neutralizes the last traces of the acid catalyst and deprotonates the acetic acid. These salts are very effectively transferred into the bicarb phase. Avoid too vigorous an extraction since it is easy to form a fairly stable emulsion and it will take quite long to break cleanly (all the while the product is hydrolyzing!).
    The last step is a drying step with NaCl. This removes most of the remaining water from the organic phase and aids in obtaining a clean 'break' during this last extraction.

    The ultimate step is a drying step with a solid dessicant such as sodium sulfate or magnesium sulfate.
     
    Last edited: Oct 22, 2007
  6. Oct 27, 2007 #5
    Thank you chemisttree; your response was most lucid.

    Do you have any tips on how to increase yield in this experiment, or in orgo lab in general? I am often unable to remove all of the product from my flask during vacuum filtration, for instance.
     
    Last edited: Oct 27, 2007
  7. Oct 27, 2007 #6

    chemisttree

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    There are tips too numerous and situation specific to share in detail. Some general tips are to:
    1. Avoid splashing. You will have to learn techniques specific to your situation here.
    2. Rinse with minimum solvent volume. A little solvent used over several steps is better than one iteration using lots of solvent.
    3. If you are permitted, use excess reagents when possible if they are cheap. Same goes for catalyst.
    4. Slow and careful is almost always better than quick and dirty.
    5. For your vacuum filtration use a chaser solvent... dropwise. If you are collecting a volatile or semi-volatile fraction (like isoamyl acetate), do it cold with the flask in an ice bath.
    6. If you are learning to use TLC to follow your reaction, stop the reaction when significant rate of change in concentration of starting materials levels out. When the reaction slows way down, byproducts often begin to appear. If byproducts don't start to appear, let the reaction go longer.
    7. Use the right size glassware. Don't use a 500 mL flask when doing a 10 g scale reaction, for example. Small funnels are best for small amounts. The more glass is wetted by your product, the more loss you will have.
    8. Be completely prepared before class. Visualize doing the reaction and all of the steps in your mind at least twice before actually doing it. Be very familiar with the amounts of reagents and solvents to use at each step of the reaction and understand why you are using them at the time. Mistakes like adding too much solvent usually slow things down and can kill a yield if recrystallization is used.
    9. Focus on what you are doing now and prepare for the next step. Neat and organized both in mind and on your bench.

    Good luck.
     
  8. Oct 29, 2007 #7
    Isopentyl Acetate Synthesis

    Sorry I wasn't 100% clear on the order of the extractions for the synthesis of isopentyl acetate. My main priority is yeild, but purity is important as well. I wasn't sure whether or not I should use water in the extraction but here is the order of the extractions I have so far:

    1) saturated NaCl
    2) sodium bicarbonate
    3) sodium bicarbonate (again until acid catalyst is neutralized)
    4) saturated NaCl to remove additional impurities
    5) small amount of magnesium sulfate to remove any excess water

    Okay so I talked to one of my TAs who said that water should be used before the sulfuric acid catalyst is is added b/c if it isn't then there could possibly cause an explosion. However, he was referring to another esterification lab (benzoic acid) and one of my main priorities in this lab is yield. So, my concern with adding water was that due to Le Chatelier's principle, the water would cause the reaction to shift left and reduce the yield, which is what i don't want. My main question is whether or not water should be used at all in this experiment, and if so, after which step. Also, how can I estimate the volume of each extraction? if anyone could help me out with this, i would REALLY appreciate it :-)
     
  9. Oct 31, 2007 #8

    chemisttree

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    What solvent are you using for the saturated NaCl solution? The bicarb? Its water, yes?

    Your TA didn't want you to add bicarb directly to a reaction containing a lot of acid, stopper it and shake the extraction flask. That could be very dangerous for obvious reasons!!!! The water wash (or saturated NaCl) will dilute the acid and remove it to another phase for removal before you treat with bicarb. You could use saturated NaCl or water in the first step but if you use saturated NaCl you won't remove as much of the unreacted alcohol and acetic acid. You also won't remove as much of the ester product which might help you with the yield a little. Using saturated NaCl will also produce a cleaner and more rapid break which is also good. I have used 50% saturated NaCl solutions as a compromise in these cases.

    You should be using a protocol for the volumes of each extraction solution. I would recommend following those instructions.
     
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