Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: Woopy on February 25, 2013, 01:28:05 AM

Title: Naming a compound given NMR information
Post by: Woopy on February 25, 2013, 01:28:05 AM

Ok, so I have no idea where to begin with this problem. I don't want anyone to give me the answer, and I don't even have a noble attempt at the solution, but I just would like if someone could point me in the right direction for these types of problems to get my brain thinking straight, because I am basically unable to do anything with these, and I am 100% certain it will be on my exam. If I see this it's basically the deer in headlights look.

So here is an old exam question released by the professor.

A hydrocarbon is determined to have a 106 m/z molecular ion peak, exactly two H NMR signals, and exactly three C NMR signals. Draw and name the compound and briefly explain.

Title: Re: Naming a compound given NMR information
Post by: discodermolide on February 25, 2013, 01:43:50 AM

Well this must be quite a simple compound. With such a simple NMR it must be symmetrical.
It's not a chain of 3 carbon atoms, so what is left?

Title: Re: Naming a compound given NMR information
Post by: Woopy on February 25, 2013, 02:06:01 AM

A 3 carbon ring?

Title: Re: Naming a compound given NMR information
Post by: discodermolide on February 25, 2013, 02:08:13 AM

How do you reconcile that with a molecular weight of 106?
Hint: it is a ring structure.

Title: Re: Naming a compound given NMR information
Post by: tomek on February 25, 2013, 08:41:14 AM

Take a look at the actual spectrum and pay attention to chemical shifts - that's a big hint:
http://www.sigmaaldrich.com/spectra/fnmr/FNMR005940.PDF

Title: Re: Naming a compound given NMR information
Post by: discodermolide on February 25, 2013, 09:29:21 AM

With all due respect I do not think the OP has the original spectra!
He/she posted all the info they were given.

Title: Re: Naming a compound given NMR information
Post by: orgopete on February 25, 2013, 09:58:53 AM

The first thing I would do is determine a MF. You should be able to determine possible formulae that only contain carbon and hydrogen with a MW 106. From that you should be able to cdetermine the number of rings/double bonds.

With that information, you can begin to consider how that number of atoms must have the correct degree of symmetry in order to have the number of atoms, yet so few peaks in the spectra.

Title: Re: Naming a compound given NMR information
Post by: Alexis7 on February 26, 2013, 12:35:22 AM

There is a great site to use for NMR determination:

http://sdbs.riodb.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

I use it all the time.

Title: Re: Naming a compound given NMR information
Post by: orgopete on February 26, 2013, 08:22:35 AM

Quote from: Alexis7 on February 26, 2013, 12:35:22 AM

There is a great site to use for NMR determination:

http://sdbs.riodb.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

I use it all the time.

Read the original poster's question. I rather doubt this database will be an exam option. However, spectral questions that involve symmetry are common, especially 13C-NMR. For this reason, I used to go over determining the number of signals in a spectrum for practice. You don't need a spectrum for this exercise.