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How To Predict Structures Of Molecules Using HNMR Graph

HNMR stands for Hydrogen Nuclear Magnetic Spectroscopy and is a laboratory technique that allows us to predict the structure of a molecule. If you are an organic chemistry student, chances are you will have a problem on your test where you would need to figure out the structure of the molecule based on the HNMR graph given. It is certainly a puzzle that you can get better at solving through practice and some rules. Let's look at my steps for doing this kind of problem!



Explanation:

To predict the structure of a molecule based on the HNMR graph, we need to follow the following steps:


1. Calculate degrees of unsaturation


Degrees of unsaturation = ((#C's*2+2) - #H's)/2

O= doesn't count

Halogen (Group 17 elements) = 1H

N = .5C


* 1 Degree of Unsaturation means that the molecule has either one double bond or a ring.


* 2 Degrees of Unsaturation means that the molecule has either 2 double bonds, or 2 rings, or a triple bond or one ring and one double bond.


* If there are 4 or more Degrees of Unsaturation, most likely the molecule has a benzene ring (one benzene ring = 4 degrees of unsaturation).

2. Write out the HNMR graph including the number of hydrogens, splitting, and ppm shift for every signal.

3. Splitting follows n+1 rule, where n = to the number of neighboring hydrogens.

Number of neighboring hydrogens n+1

0 1 SINGLET

1 2 DOUBLET

2 3 TRIPLET

3 4 QUARTET

4 5 PENTET

5 6 SIXTET

6 7 HEPTET

Sometimes, 5 or more peaks are called multiplet.

4. Use the ppm chart to see what functional group each set of hydrogens is next to.

The ppm number is important in indicating what is the next to the hydrogens we are looking at. For example, hydrogens with ppm around 2 are next to a c=o group, and hydrogens with 6.5-7.5 ppm are on the benzene ring.




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