Chapter 13 - Types of H

Chapter 13: Spectroscopy

Different types of Hydrogen

The issues to understand from this page are:

What do we mean by different types of hydrogen ?
Terminology

This may seem a trivial concept, but the idea of establishing the number of kinds of hydrogens (or other atoms such as C) is an important useful concept, especially in NMR spectroscopy.

There are three methods to establish 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 mving 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.

As examples:

an -OH is different to a -CH (based on what they are attached to)
a -CH₃ is different to a -CH₂- (because the number of H at that C are different)
an sp³C-H is different to an spC-H.
Other differences could be position on a chain, across a ring or double bond etc.

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 s-bonds produces different conformations only and not different molecules.
Remember to consider the three dimensional nature of the molecules.

Questions

How many different types of H are there in each of the following hydrocarbons: propane, 2-methylbutane and propene ?
	show H types propane	show H types pentane	show H types propene
Note that in pentane there are two different types of CH₂ units becausen they are attached to different groups

How many types of H are there in bromomethane shown to the below ?

If you got it wrong, it might be because you thought that the H opposite to the Br is different to the other 2, but if you consider it in 3D, you should realise that all the H are the same. Use the CHIME image below to check.

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

How are the H in bromochloromethane related ?
How are the H in the methyl group in bromoethane related ?

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*.
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*.