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A question requires me to distinguish between propan-2-ol and ethanol from IR spectra. To me, they both look nearly identical, even though I know both of them contain the same functional group and that IR spectra are only capable of identifying functional groups.

My question is, how do you distinguish the two, without using a mass spectrometer?

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  • $\begingroup$ The isopropyl group shows characteristic coupled peaks around ~1350 cm-1 $\endgroup$ – Eashaan Godbole Mar 4 '18 at 19:49
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Here is the spectrum for ethanol:

IR spectrum of ethanol

Here is the spectrum for propan-2-ol:

IR spectrum of propan-2-ol

Here are the tabulated peak positions ($\pu{cm^{-1}}$) and percent transmittance (%T) for the two. I've tried to match the appropriate peaks.

$$ \begin{array}{cccc}% \hline% \text{ethanol position} & \text{ethanol %T} & \text{propan-2-ol position} & \text{propan-2-ol %T} \\ \hline% 3358 & 12 & 3346 & 9 \\ & & 3334 & 9 \\ 2974 & 13 & 2972 & 5 \\ 2927 & 38 & 2933 & 19 \\ & & 2907 & 23 \\ 2887 & 35 & 2884 & 20 \\ & & 2722 & 60 \\ & & 2669 & 60 \\ & & 2521 & 74 \\ & & 2408 & 79 \\ & & 2387 & 81 \\ & & 2198 & 84 \\ & & 1903 & 85 \\ & & 1766 & 84 \\ 1455 & 59 & 1467 & 24 \\ & & 1409 & 29 \\ 1381 & 54 & 1379 & 16 \\ & & 1368 & 17 \\ 1330 & 68 & 1341 & 28 \\ 1274 & 74 & 1309 & 23 \\ & & 1162 & 10 \\ 1090 & 22 & 1130 & 9 \\ 1050 & 7 & 1110 & 15 \\ & & \color{red}{954} & 4 \\ 881 & 37 & 818 & 13 \\ 669 & 67 & 660 & 37 \\ & & 654 & 37 \\ & & 490 & 66 \\ & & 436 & 68 \\ \hline% \end{array}% $$

One way of differentiating between the two is that I don't believe the peak at 954 wavenumbers in propan-2-ol appears in ethanol. Another better way that doesn't require looking at the fingerprint region is to see that the hydrogen-bonding peaks (the broadest ones to the left) are red-shifted in propan-2-ol compared to ethanol.

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