Structure Determination

Introduction to Structure Determination

Introduction to Structure Determination

With so many organic compounds in existence it can be hard to figure which compounds, or compound combinations, are in a sample. Chemical tests can distinguish between functional groups and data for melting and boiling points can identify further information including the purity of a compound. However, this data usually only confirms possible structures rather then actually identifies a compound. In addition, chemical tests generally require quite a large sample and are not always effective at distinguishing between compounds which are similar.

Due to this, chemists have developed other analytical techniques which are more precise and can determine exact structures of organic compounds. Three techniques you should be aware of are:

  • mass spectrometry
  • infra-red spectrometry
  • nuclear magnetic resonance (nmr) spectrometry

As you will have already covered infra-red spectrometry at AS level, this chapter will look at the other two techniques in more detail.

For A level these techniques are used to find out the structure of organic molecules which contain carbon, oxygen and hydrogen and tend to be composed of six carbon atoms or less. You will analyse either an alkene, alkane, ether, alcohol, ester or carboxylic acid.

Before you begin your analysis you will usually be provided with the empirical formula first. This is generally provided in the form of composition by mass.

For example: a compound contains 6.7% hydrogen, 40.0% carbon and 53.3% oxygen.

Mole ratio:

C

H

O

40/12 = 3.36.7/1 = 6.753.3/16 = 3.3
Simplest whole number ratio:

C

H

O

3.3/3.3 = 16.7/3.3 = 23.3/3.3 = 1

So, empirical formula = CH2O

To work out the structure and molecular formula of the molecule you can now can use one or more of the analytic techniques.