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What is chemical equivalence and magnetical equivalence in NMR?

by egpat        Posted on 08-06-2017


In NMR, chemical and magentic equivalence plays important role in determining the number of peaks.

To learn more about number of peaks in NMR,see how to determine the number of NMR peaks

All nuclei which are magnetically equivalent will give single NMR signal. Similarly nuclei which are magnetically non-equivalent will produce different NMR signals.

Chemical equivalence

Let's start with chemical equivalence.

Chemical equivalnce means similar chemical environment. When nuclei are attached by similar linkage they are called as chemically equivalent.

Chemical equivalence=Identical linkage
Chemical equivalence

Let’s take an example of methane.

 

NMR signals in methane

In methane, protons has spin quantum number I=1/2 and herefore can give NMR signal. Here all the protons are attached to carbon in similar way and hence chemically equivalent.

Remember

All the protons attached to carbon are always chemically equivalent.

In methyl group all protons are equivalent as they attached similary to the carbon.

NMR signals in 1-bromopropane

Similarly in methylene group the two protons are equivalent.

methylene radical

 

Now let’s take another example. Consider 1-bromopropane.

NMR signals in 1-bromopropane

Here CH3 protons are not equivalent with other protons hence indicated by ‘a’. Now we can see two types of –CH2 protons. Protons indicated by green color are not attached in similar way as the protons indicated by blue color. The protons indicated by green color are attached as – CH2Br whereas those with blue color as –CH2-.  Hence all these protons are equivalent.

Now consider 2-chlorobutane.

 

NMR signals in 2-chlorobutane

Here CH3 indicated by blue and green color are not equivalent. Protons indicated by green are attached to –CHCl- where as protons indicated by blue are attached to –CH2-.  Similarly protons –CHCl- and –CH2- are also non-equivalent. Hence all the protons above are non-equivalent.

 

Magentic equivalence
magnetical equivalence=Identical CONFIGURATION

When nuclei are arranged in similar way in the space they are called as magnetically equivalent.

Magnetic equivalence in NMR Here protons present in cis and trans position have siilar linkage but different configuration. cis-proton on the same side whereas trans-proton on opposite side of the double bond. Hence these protons are chemcially equivanet, but magntically not equivalent.

 

Compounds will have different configuration when they exhibit restricted rotation. Alkenes and cyclic compounds show restricted rotation hence may show magnetic non-equivalence.

Let's consider vinyl bromide.

NMR signals in vinyl bromide

In vinyl bromide, the protons of –CH2– are not magnetically equivalent. The proton indicated by green is cis and blue is trans to other proton on -CHBr.




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