Exp 18: Phase Transfer Catalysis

Before you begin, have you mastered:

  • catalysts
  • formal charges
  • ionic salts
  • polarity as it pertains to miscibility
  • alkanes
  • benzenes
  • SN2 reactions
  • reflux technique
  • extraction technique
  • IR spec technique

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PURPOSE:  To discover how a phase transfer catalyst can be used to facilitate the synthesis of an ester

“In [this experiment] you will use an SN2 reaction, catalyzed by a quaternary ammonium salt in a two-phase system, to synthesize butyl benzoate; then you will characterize it by IR spectroscopy (27)”

THEORY:  We have to make an ester. However, most reactions are done in a homogenous reaction medium where everything is the same. If you try to mix a nonpolar organic something (like hexane)  within a polar inorganic something (like LiAlH4), they’re going to mix like Dr. Dre at a Hillary Duff concert (not well).

For example, using two miscible solvents like nonpolar THF and polar water will result in a slurpy mess. So what can we do? The answer you’re looking for is the brand new method developed by experts and professionals alike! What is it?

PHASE TRANSFER CATALYSIS!!!

This handy-dandy detergent called Aliquat 336 (trioctylmethylammonium chloride) will amaze you with its ability to blend freely with BOTH polar and nonpolar compounds of your desire. It has an AMAZING quaternary ammonium salt structure where the nitrogen atom has a formal positive charge and bonds to four nonpolar insanely large alkyl groups that are quite hydrophobic. So you see, it has both polarities covered. It transfers between each phase!

Now that you have a perfect catalyst that will increase the concentration of the benzoate anion in the organic layer, you can have easy and fast SN2 reactions.

THE REACTION

  • benzoate acts as the nucleophile, it displaces a bromide ion from 1-Bromobutane
  • the phase transfer catalyst, Aliquat 336, moves the benzoate ion to the organic layer and speeds up the SN2 by making it a  stronger nuke
  • Blank

The phase transfer starts when Aliquat 336′s positive charge cation forms an ion pair with the benzoate anion. It’s super large nonpolar character helps to sneak the bonzoate anion into the organic layer. Once its job is finished, it reacts with 1-bromobutane and goes back to the water layer.

[Illustrate the phase transfer catalysis here]

Questions & Answers

1.  What is the solid that precipitates when the reaction misture is cooled to room temperature?

-Whatever is not soluble will be the one that precipitates. And so that would be…?

2.  How is the phase transfer catalyst removed from the product?

3.  What purpose does washing with 15% (half saturated) NaCl solution serve?

4.  There is a slight excess of sodium benzoate in the reaction.  How is it removed from the product?

5.  Why is the benzoate anion a stronger nucleophile in 1-bromobutane than in water solution?

6.  What peak in the IR spectrum suggests the presence of water in the product? What could be done to remove the water?

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One Response

  1. Quaternary Ammonium Salt structure where the nitrogen atom has a formal positive charge and quaternary ammonium salt structure where the nitrogen atom has a formal positive charge and bonds to four nonpolar insanely large alkyl groups that are quite hydrophobic.

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