How To Predict Structures Using HNMR Graph
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.