Compounds 1-3 were analyzed by normal phase TLC (shown) using ethyl acetate as the eluent. Assign spots on the TLC plate to compounds 1-3.
1: 2-acetoxybenzoic acid 2: 2-hydroxybenzoic acid 3: benzoic acid

tlc plate and structural formulae of compounds 1 - 3

I asked a question about TLC and I received some mixed answers, so I'd like to see if my understanding of TLC is fine.

We're using normal phase TLC so lower Rf compounds will be more polar. $0.47$ is the most polar compound, $0.70$ is the least polar compound.

Ethyl acetate is polar. Does the eluent polarity just determine where the compounds will run on the TLC?

  • $\begingroup$ The stationary phase is polar so it will be something like R-O-H or R-(OH)2 where the H's will hydrogen bond. So in terms of affinity for the stationary phase I think it depends on the number of oxygen molecules here. So 1 (blue) >2 (green) >3 (red). $\endgroup$
    – MaxW
    Feb 26 '17 at 21:19
  • $\begingroup$ @MaxW Why do you think compound 1 is the most polar? $\endgroup$
    – Jason
    Feb 26 '17 at 22:16
  • $\begingroup$ In part it seemed that 4 oxygen atoms offered more chances to hydrogen bond than 3. $\endgroup$
    – MaxW
    Feb 26 '17 at 22:52
  • $\begingroup$ @MaxW " I think it depends on the number of oxygen molecules here." is definitely not correct. An ester is usually less polar than an alcohol or phenol but has more oxygen atoms. The big question here is whether 1 or 3 is less polar, and here it get's quite tricky, since compound 1 can do an intramolecular hydrogen bond. $\endgroup$
    – DSVA
    Feb 26 '17 at 23:04
  • $\begingroup$ @DSVA - Shouldn't both 1 and 2 form an intramolecular hydrogen bond? I was flipping a coin based on oxygen count. $\endgroup$
    – MaxW
    Feb 26 '17 at 23:13

TLC is only about polarity. Since all three compounds share the benzoic acid functionality, we only have to consider the rest. Then we have three compounds: $\ce{R-OCOCH3}$, $\ce{R-OH}$ and $\ce{R-H}$.

In general the polarity is:

alkane < ester < alcohol

That is what we have here also: 3 < 1 < 2.

The one with the lowest polarity will have the highest R$_f$. To show that it works, I have run them on a silica TLC plate with AcOEt:

enter image description here

AcSA: acetylsalicylic acid; SA: salicylic acid; BA: benzoic acid.

  • $\begingroup$ It's more accurate to say that TLC is comparing the eluate's differential interactions with the stationary phase and the mobile phase, where the primary interaction is polarity. $\endgroup$
    – Zhe
    Feb 21 '18 at 22:34

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