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So there was this exercise in my book (picture below) enter image description here a compound

I got to the conclusion that the compound might be either 2,6-di-tert-butyl-4-methylphenol or 3,5-di-tert-butyl-4-methylphenol. I was searching for some clue that would help me to distinguish between both of these compounds and their spectra's but I just couldn't. I think that the only difference between them is just the chemical shift values and it wouldn't be even that significant.

Am I just really missing something? Or maybe the latter one is much more rarer or hard to synthesize and not as common as the first one (I wanted to look up both of these compound's NMR spectra's but i could find only of the first one and the second compound seems like 100 times less mentioned in the internet and very unpopular). I would really appreciate some help because I have been thinking about this for quite some time and I can't figure out what is wrong.

Oh and the correct answer is 2,6-di-tert-butyl-4-methylphenol; the second one isn't even mentioned.

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    $\begingroup$ The -OH peak might be more diffuse in the 3,5- isomer because the less hindered proton might hydrogen bond. Kudos for looking more deeply into the problem. $\endgroup$
    – jimchmst
    Apr 21 at 19:01

1 Answer 1

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Indeed the shifts and integrals match up with those reported for butylated hydroxytoluene (BHT), a very common antioxidant used as a preservative, see for instance the list of NMR chemical shifts of common laboratory solvents as trace impurities in Gottlieb et al., J. Org. Chem., 62(21), 1997:

proton mult CDCl3 (CD3)2CO (CD3)2SO C6D6 CD3CN CD3OD
ArH s 6.98 6.96 6.87 7.05 6.97 6.92
OHc s 5.01 6.65 4.79 5.20
ArCH3 s 2.27 2.22 2.18 2.24 2.22 2.21
ArC(CH 3)3 s 1.43 1.41 1.36 1.38 1.39 1.40

The difference in shifts between BHT and 3,5-di-tert-butyl-4-methylphenol lies mainly in the effect of the hydroxyl group on the neighboring aromatic protons (Ar-H). The OH has a stronger electron-withdrawing (inductive) effect at the meta than the ortho position seen for instance in phenol in a 0.4 ppm downfield shift of meta relative to ortho H.

Aromatic substituent effects on shifts are approximately additive and have been tabulated (you can see a table here). Estimating shifts of Ar-H for the two compounds this way agrees with the previous conclusion. You can try to memorize a table of substituent effects (I looked it up) or focus on some key ones.

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