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Different types of Hydrogen

The issues to understand from this page are:

This may seem a trivial concept, but the idea of establishing the number of kinds of hydrogens (or other atoms such as C) is a very important useful concept, especially in NMR spectroscopy where for example, different types of H typically give rise to a different signal.


What do we mean?
A hydrogen atom is "different" to another hydrogen atom if it is not in an identical environment to the other hydrogen. This could mean it's attached to a different type of atom (e.g. CH vs OH, or sp3 CH vs sp2 CH) or due to the number of adjacent H (e.g. CH3- vs -CH2- ) or just at a different point in a chain (e.g. compare the H in the methylene (CH2) groups in CH3CH2CH2OH

Counting different types of Hydrogen
There are three methods that can be used to count the number of kinds of H (each achieve the same result).  You will probably find it best to master the first of these methods and over time you will find yourself moving towards the 3rd method)

1. Substitution method (simplest but slowest)
The idea is that you replace each H in the molecule in turn with a "dummy" atom (for example a -Cl) to see if you get a different product (i.e. one that will require a different name). Each new product, indicates a different type of  H. This idea is related to the radical chlorination of alkanes where some of each possible product is usually obtained.

For example:

Qu : If you remove one H atom from chloroethane and substitute it for another Cl atom, how many different molecules can you make ?
Ans : Two products, 1,1-dichloroethane and 1,2-dichloroethane. Hence there are two types of H in chloroethane

2. Verbal description
The verbal method requires that you describe the position of each H within the molecule.  If you need to use different words to describe two H atoms, then they represent different types of H.

For example:


3.
Symmetry (most difficult but fastest)
The symmetry method is the most sophisticated but the quickest method and requires that you look for mirror planes, rotation axes or inversion centers that interchange H atoms.  H atoms that can be interchanged are equivalent to each other.

 
CAUTION

  • Remember that rotation about σ-bonds produces different conformations only and not different molecules.
  • Remember to consider the three dimensional nature of the molecule

QUESTIONS

  • How many different types of H are there in each of the following hydrocarbons: propane, 2-methylbutane and propene ?

ANSWER
ANSWER
ANSWER
  • How many types of H are there in bromomethane ?



ANSWER

Terminology
 
Homotopic
 
 
Replacement of the groups gives the same product
 
For example, it doesn't matter which of the H atoms in bromomethane is replaced with chlorine, we always get bromochloromethane. Hence these three H are said to be homotopic.
 
Enantiotopic
 
 
 
 

 

Replacement of the groups gives enantiomers
 
 
 

 

Consider the H atoms in the methylene group in bromoethane. If we replace one of those H with a Cl, we create a chirality center. Therefore depending on which of the two H is replaced, we get one enantiomer or the other. Hence these two H are said to be enantiotopic

enantiotopic H in bromoethane

Diastereotopic

Replacement of the groups gives diastereomers
Consider the H atoms in the methylene group in 1-bromo-1-chloropropane. There is already a chirality center at C1.  If we replace one of those H with a Cl, we create a new chirality center.  Therefore depending on which of the two H is replaced, we get one diastereomer or the other. Hence these two H are said to be diastereotopic
diastereotopic H in 1-bromo-1-chloropropane

QUESTIONS


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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary