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How to figure out the number of HNMR signals?

July 9, 2019

 

HNMR stands for hydrogen nuclear magnetic resonance. This technique allows us to figure out the structure of many molecules. It is especially useful in pharmaceutical industries for the research in drug discovery.

The number of signals that HNMR gives is equal to the number of different hydrogens. 

Two hydrogens are different when they are on carbons attached to different groups. 

Here is a set of tips that will allow you to solve the HNMR number of signal problems successfully:

1. First, you must show all the hydrogen to see what you are working with. 

2. Look for lines of symmetry. Symmetry can be horizontal, diagonal or vertical. There can be multiple lines of symmetry for the same molecule. When two hydrogens are on opposite sides of the line of symmetry, they are going to give the same signal.

3. Hydrogens in terbutyl and propyl  methyl groups are the same, since their carbons are attached to the same carbon.

4. Hydrogen on the same carbon are the same, with the exception of hydrogens on a double bond end. 

 

Lets take a look at some examples:

 

CH3 - CH2 - CH3       Here the line of symmetry is vertical though the middle. Both methyl groups are the same signal. Therefore this molecule has 2 signals.

 

CH3-CH2-O-CH3         This molecule has no lines of symmetry. Thus, every hydrogen will be different. This molecule has 3 signals in HNMR.

 

(CH3)2CHCH2CH(CH3)2 This molecule has a line of symmetry though the middle. It also has two methyl groups attached to the same carbon, making their hydrogens the same. There are 3 signals in this molecule.

 

For more help with organic chemistry, please contact Transformation Tutoring at 646-407-9078. Our amazing organic chemistry tutors are available in Brooklyn, NYC and online. Mayya, the founder of Transformation Tutoring, graduated from NYU with honors and received A's in both of her organic chemistry classes.

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