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Number of signals in HNMR

October 12, 2018

 

HNMR (hydrogen nuclear magnetic resonance) is a technique used by organic chemists in the lab to figure out the structure of the molecule. As the name implies, HNMR is all about Hydrogens. Students studying organic chemistry will encounter HNMR numerous times in their classes as well as MCAT.

 

One of the most important features of HNMR is the number of signals. 

Number of signals represents the number of different hydrogens. What does it mean? Hydrogens will be the same if they are connected exactly to the same molecules, so as we go to the left and to the right, there is no difference. Thus, molecules with a lot of symmetry will have a lot of the same hydrogens and less signals. For example a cyclohexane molecule has only one signal since it is so symmetric. Examples of the same hydrogens also include tertbutyl group, where are all nine hydrogens are on the carbons that are connected to the same carbon and to the rest of the molecule. 

Let's do a simple example: CH3CH2CH3 (propane). How many different signals would it give us? Well the molecule is symmetrical through the middle, and therefore, both CH3 groups are the same giving us one signal and the hydrogens on CH2 are different, giving us signal number two.

How about 1-methyl butane? (CH3)2CH2CH2CH3. Again, both methyl groups are connected to the same carbon and are the same, unlike the rest of the hydrogens. Therefore, we will get four signals for this molecule. 

 

CNMR will also be important in telling us how many different carbons a molecule has. Again, a molecule that is very symmetric will have less signals since more carbons will be the same.

 

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