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Counting atom types

The connectivity and molecular motion due to bond rotations within a molecule can result in atoms that are considered to be equivalent or non-equivalent types.  For example, the six hydrogen atoms in ethane are considered to be chemically equivalent (i.e. of the same type). Each individual hydrogen atom is in an identical environment (attached to a carbon atom that is linked to 2 other hydrogens and one methyl group). The ability to recognise the number of types of H (or indeed other atoms such as C) is a very important and a useful concept.  For example, counting types of H is very important in spectroscopy (especially nuclear magnetic resonance) and in reactions (e.g. radical halogenation of alkanes).  It will be revisited several times in later questions in this exercise and applied in other components of the course. 

There are three methods to establish the number of kinds of H (similar methods can be used for other atoms such as C):

  1. Substitution method. This method is based on the idea is that you replace each H in turn with a "dummy" atom to see if you get a different product (i.e. one that will require a name that differs by more than just E/Z. cis/trans or R/S, e.g. 1-chlorobutane and 2-chlorobutane). If you have a new product, then the H was different to those already considered.

  2. By "verbal" description.  The verbal method requires that you describe the position of the 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 an -OH is different to a -CH (based on what they are attached to), and a -CH­3 is different to a -CH2- (because the number of H at that C are different).  Other differences could be position on a chain, across a ring or double bond, hybridisation etc.

  3. Symmetry. 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.

Try replacing different H atoms in ethane, propane, butane, pentane, 1-propanol and 2-propanol determine how many different types of H there are in each.

CAUTION: Remember that rotation about bonds produces different conformations (conformational isomers or conformers) only, not different molecules.


  Dr. Ian Hunt, Department of Chemistry, University of Calgary