How do I tell the difference with an IR spectrum of 2-hexanol and 1-hexanol?

Question

  It seems like the OH peak is less intense on 2-hexanol...why?

Answer 1

The fingerprint region is a descriptive name. To actually tell the difference without guessing, you need an IR spectrum taken from known 1-butanol and known 2-butanol. 

 

If you match them up (like fingerprints), then you can tell the difference. 

 

It takes a PhD to be able to tell the difference and even then, that is from repeated exposure, not from some set of empirical rules. IR is a useful but not a very precise analytical technique. 

 

Nevertheless, if you consider that a secondary alcohol has more inductive donation to it from carbon relative to 1-butanol, then you should realize that the secondary alcohol is slightly less acidic than the primary alcohol. If that is the case, the bond between O and H is stronger, shorter and therefore has less stretch. Make sense?

Answer 2

IR spectroscopy is innately associated with how symmetrical a molecule is. The more symmetrical the molecule the less complex a spectrum looks and vice-versa.

 

I would not go based on the intensity because that can differ from sample to sample, based on how it was prepared. However, if you look carefully between 1600-500 cm-1 you will find out that the spectrum of 2-hexanol is a lot more complex than that of 1-hexanol. This is how you can tell to which molecule the spectrum belongs.

 

Beyond this suggestion, it would be nearly impossible to distinguished between the two spectra with the naked eye without prior knowledge of what the molecules are.