Organic Chemistry Help: Deciphering an NMR [with study guide]

 

Happy New Year!  It has been a while since we have put a new organic chemistry post up, so I thought I would put a little guide up now that finals are over. 

 

The question is:  How did you decipher an H¹ NMR spectrum?  Well, here is a good, uniform way to tackle the problem. 

 

Step 1: Calculate the degree of unsaturation in the molecule.  This is sometimes called the Sum of Double Bonds and Rings or SODAR.  You will most times be given a molecule formula, and can calculate your total number of double bonds and rings in the molecule using the formula (2#C + 2 – #H – #X + #N)/2 where

 

            #C = the number of Carbons

            #H = the number of Hydrogens

            #X = the number of Halogens

            #N = the number of Nitrogens

 

In this, you do not count the oxygen or sulfur atoms.  For example, the molecular formula C6H6NOCl would be (2*6 + 2 – 6 –1 +1)/2 = 4, meaning that there are 4 double bonds and/or rings.  It is helpful to remember that benzene rings equal to 4 on the SODAR scale, so if you have a SODAR that is 4 or larger, think benzene ring.

 

Step 2: Look for arene protons.  The number of protons between 6ppm-7.5ppm, known as the AR region, can give many clues to your molecule.  A mono-substituted benzene ring will have 5 protons in the AR region.  A di-substituted benzene will have 4 protons in the AR region.  However there are even clues to what type of di-substituted benzene it is.  If the peaks in the AR region are 2 perfect doublets, it is most likely para substituted.  If you have a singlet in the AR region, you most likely have a meta-substituted benzene.  If you just have a mess, it is most likely ortho substituted. 

 

Step 3: Look for the 2 A’s, aldehydes and alcohols.  This is actually simpler than it sounds, and can give you some nice clues.  Aldehydes are sharp singlet peaks that show up past 9ppm.  Alcohols are broad singlets that can show up anywhere in the spectrum, but will “exchange” with D2O, meaning that they will disappear if D2O is added.  Most organic chemistry profs will signify this by writing “exchange” over your spectrum.

 

Step 4:  Add up the integrations in your spectrum and make sure it equals the number of protons that you have.  For example, if you have 10 H’s in your formula, but can only have an integration equal to 5 on your spectrum, you need to realize that each integration is equal to 2 protons.

 

Step 5: Start to make fragments and then add up the fragments.  Using the integration and splitting of each peak, you can start to write down fragments of the molecule.  For example, if you have a singlet with an integration of 3, you know that you have a methyl group (3 H’s) next to something with no protons.  If you have a doublet with an integration of 2, you have a CH2 that is next to a CH.  Once you have all of your fragments, start to piece them together and you will be figure out what your molecule is. 

 For some good practice tests, please see organic chemistry.