What Kinds of Compound Give Odor

Some compounds, though gaseous or very volatile, do not cause smell that is, they are odorless. For example, oxygen (O ), nitrogen (N ) carbon dioxide (CO ), and carbon monoxide (CO) gas are odorless. These compounds are small and rather non-polar and hence would not effectively interact with the olfactory receptor site. 

Most inorganic crystalline compounds including salt (NaCl) and rocks (e.g., metal silicates) are odorless because they are solid at ordinary temperatures and do not give a significant amount of molecules in the gas phase i.e., they are not volatile at all, and hence cannot come to our nose. 

Another series of hydrocarbons exemplified by ethylene (ethene=C3H, CH2=CHj) and propene (CjH, CH3-CH=CHj) has a more reactive site, i.e., C=C double bond, so that they may significantly interact with a receptor site. They do indeed smeU. 

Benzene C H has a shape of hexagon and has alternating single bonds and double bonds (see the previous chapter). The compounds derived from benzene (in the formal sense) are called aromatic compounds. The name itself implies that they have special odor. 

Now let us replace the hydrogen atoms in hydrocarbons with other elements such as chlorine (Cl), oxygen (O), nitrogen (N), and sulfur (S). These elements are more electronegative (attracting electrons more strongly) than carbon atom. As a result, the electrons are not equally distributed between carbon and an atom X of one of 

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