Wednesday, September 28, 2011

Functional groups (general ideas)

All substances have properties that impart specific physical and chemical characteristics. I'm talking about simple things like color, taste, smell, physical state, solubility, etc..
But how do these different features are explained at the molecular/atomic level? What makes a substance liquid at room temperature and other solid, for example? The answer involves the composition of molecules. But I'm not just talking about size, which, of course, is a very important parameter. I am also speaking about the presence of certain atoms, and the relative proportions of the atoms that constitute the molecule... Features such as smell, color or taste, are a direct consequence of the atomic composition of molecules. But other characteristics, for example, physical state, boiling point and melting point also relates to the composition, although in this case the relationship is more indirect.
In general, the characteristics of a substance depend on the presence of certain functional groups in it. But what are functional groups? Basically, they refer to atoms arranged in submolecular structures that alter the properties of substances. They determine the interactions established within the molecules and with neighboring molecules. Moreover, they are responsible for the reactivity of molecules. That is, a functional group always reacts the same way with another (for example, a carboxylic group always originates an ester when it reacts with an alcohol...), regardless of the remaining composition of the molecule where it is inserted. At the same time, it is the presence of functional groups that will make a molecule more or less reactive. Therefore, one can say that the functional groups determine the reactivity and the types of chemical reactions that molecules can suffer.
When it comes to functional groups, there is always some ambiguity about what can be considered  to belong or not to that class. Some authors argue that the functional groups must have at least one atom other than C or H (except for the phenyl group), while others include also groups such as methyl, ethyl, etc.
Soon I will start posting details about the main functional groups ...