IR Spectra

[JDHK]

So I see the peak right of 3000, indicating alkane bonds. So the answer is apparently octane, the only alkane bond, because there is no other functional groups shown anywhere else. But what about the peak at 1500? Doesnt that indicate a benzene?

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[Eri3]

So I see the peak right of 3000, indicating alkane bonds. So the answer is apparently octane, the only alkane bond, because there is no other functional groups shown anywhere else. But what about the peak at 1500? Doesnt that indicate a benzene?

Anything below 1500 is considered the fingerprint region. This region is very specific for each molecule so it is helpful when comparing two or more IR spectra and you're trying to figure out which one is which based on comparing it to other spectra of that compound. For example 1-propanol will have almost the same IR spectrum as 2-propanol but their fingerprint regions are specific.

benzene has C==C so these will show slightly above 3000. C---H bonds show below 3000. That's how you can tell if you have an aromatic system or not.

So this one does not have a ring b/c it doesn't have any peaks above 3000. Aromatic hydrocarbons would also show peaks at 1600-1585 which this spectrum does not have either.
When you solve these type of problems you have to be kind of like a detective and write down what you have evidence for and the more evidence something has the more likely it will be the right compound.

 

[Glimmer1991]

Though it's been a long time since my ochem days, I seem to remember benzene rings being at 1600. Sometimes the peaks were pronounced; sometimes they were quite subtle.

Also, Eri3 is right that you'd have to have some C=C bonds showing up right above 3000 (slightly lower than the normal C=C bond because they're conjugated, right?).

 

[JDHK]

Got it! Thank you!

 

[Eri3]

Though it's been a long time since my ochem days, I seem to remember benzene rings being at 1600. Sometimes the peaks were pronounced; sometimes they were quite subtle.

Also, Eri3 is right that you'd have to have some C=C bonds showing up right above 3000 (slightly lower than the normal C=C bond because they're conjugated, right?).

Exactly 😀

Also, take this with a grain of salt- I don't think questions on the DAT will ask you to identify molecules based on a spectrum. That would take way too much time for a standardized test. I think it is more important to remember at what range typical peaks show up.