I am trying to understand why the experiment asks for raising the pH of a fairly acidic solution. The analytes should not be affected by it, so I thought the silica plate might be. Are silica-plates vulnerable in acidic conditions and, if yes, what happens?

  • $\begingroup$ I should add that the goal pH was between 6 and 7. $\endgroup$
    – waterlemon
    Apr 6 '17 at 9:30
  • $\begingroup$ The pH value of an aqueous solution of $\ce{NH4Cl}$ is mildly acidic, and with an aqueous solution of $\ce{NaHCO3}$ you catch much of acidity, too. If you prepare a long chain alcohol, for example, the quench with aqueous $\ce{NH4Cl}$ is meant to have no longer the alcoholate generated by the Grignard reaction, for example; simultaenously, to remove remains of salt ($\ce{MgBr2}). In this perspective, the then extracted organic phase is pure enough to attempt the TLC, regardless if the pH of an aqueous solution in contact with the organic phase were 7.00 or about 6. $\endgroup$
    – Buttonwood
    Apr 6 '17 at 19:24
  • $\begingroup$ At the stage of monitoring the progress of the reaction, or to monitor the quench and (extractive) workup of the reaction, chromatography does not yet deal with purification; which may be done subsequently by column chromatography, or other means like distillation, recrystallisation. $\endgroup$
    – Buttonwood
    Apr 6 '17 at 19:27

Assuming "normal" TLC with silica as stationary phase, and a mix of ethyl acetate / hexane as eluent, charged compounds (i.e., salts) do not elute well; their adsorption is too tight to move forward.

Hence, if your reaction is performed under acidic conditions (sulfonation reaction, for example), drop a small sample of your reaction into a test tube or small vial, gently quench it against an aqeuous solution of $\ce{Na2CO3}$, extract it with some ether / MTBE / methylene chloride (well...) and spot the extract on the TLC plate. With some training, this works easily around o.5 mL of total volume.

Conversely, if your reaction proceeds under basic conditions (like a Grignard reaction, for example), bring the sample back to neutral, for example with dilute HCl (or, with more fragile products like tertiary alcohols, with an aqueous solution of $\ce{NH4Cl}$), and then spot the organic extract on the plate.

I referred to "normal" TLC as there are other stationary phases than silica, too. Alumina (basic, neutral, acidic) works with the same principle as silica. These two contrast to applications of RP-TLC, were the stationary phase is coated with alkyl chains, and much more polar solvents are deployed as eluents, hence with much lesser issues of free salts / free acids. But the price tag for these plates is higher than for normal phase TLC.


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