Chapter 2 Organic chemistry

IMPORTANT CONCEPTS IN ORGANIC CHEMISTRY

Essential oil chemistry is a part of organic chemistry, which covers a vast range of compounds. Early ideas suggested that organic compounds were all obtained from either plant or animal sources, i.e. that they were natural products, and arose only through ‘vital forces’ inherent in living cells. This definition is no longer true as a result of modern laboratory synthetic methods. The modern definition of organic chemistry is that it is the chemistry of covalently bonded carbon compounds.

AROMAFACT

There is a very dangerous misconception that if a substance is natural it is not harmful, and can be used therapeutically without the fear of any side-effects. Natural products often contain very powerful and toxic compounds and some form a basis for mainstream drugs. Essential oils are very complex mixtures of organic compounds, many of which should be used with great care. Their roles in the plant body are often protective and defensive, e.g. to repel invading organisms. Essential oils contain compounds with varying physiological effects and toxicity. In a genuine aromatherapy-grade essential oil, the more toxic components are often balanced by others that act as ‘quenchers’. There is a phenomenon called synergy whereby the components making up the oil can cooperate to produce their healing effect. A knowledge of the oil components is needed for their safe use and this is why it is vital to use high-grade essential oils in a controlled way

The important points in organic chemistry are outlined below.

1 Carbon and only very few other elements are involved

The number of organic compounds far exceeds that of inorganic compounds, but very few other elements are involved along with the ubiquitous carbon. Most of them are other nonmetals, most commonly carbon (C) and hydrogen (H), which are always present; oxygen (O), nitrogen (N), sulfur (S) and phosphorus (P), which are commonly found; and chlorine (Cl), bromine (Br), iodine (I) and fluorine (F), which are present in other compounds (see Fig. 2.1).

Figure 2.1 Atoms most commonly encountered in organic chemistry: carbon (black), hydrogen (white) and oxygen (grey). In colour the models are carbon (black), hydrogen (white) and oxygen (red). Their valencies are represented by the number of ‘sticks’, which are shown attached to the atoms in the spatial configurations that chemical bonds to these atoms occupy (e.g. pointing to the vertices of a tetrahedron in the case of carbon); the directionality of the bonds produces the shapes of different molecules. These are the basic kinds of atom that build up by covalent bonding to form the organic molecules found in essential oils.

Courtesy Spiring Enterprises Ltd.

2 The bonding is mainly covalent

Most organic compounds are low melting point solids, liquids or gases that are insoluble in water but soluble in organic (sometimes referred to as nonpolar) solvents such as ether, benzene and hydrocarbons. They do not conduct electricity. This is in contrast to ionic or electrovalent compounds with their bonding by electrostatic forces, which usually result in solids that are soluble in inorganic (sometimes referred to as polar) solvents such as water and that will conduct electricity when molten or in solution.

AROMAFACT

Essential oils are most commonly applied to the skin diluted and dissolved in another oil called a carrier for use in a massage. Water is not an appropriate or efficient carrier for massages, although some components may be water soluble. Adding essential oils to a bath will bring the oil into contact with the skin and the hot water will help the oil to evaporate so that the volatile molecules enter the nasal passages by inhalation.

3 Large molecules are common

Compounds are encountered with molecular formulae such as C₂₀H₄₀, with 20 carbon (C) atoms and 40 hydrogen (H) atoms. Many important biological compounds are what are called macromolecules and are very large. They are polymers made up of many repeating units. For example, cellulose (plant cell wall material) is made up of many linked glucose units and has a molecular weight of 150 000–1 000 000. Insulin (a protein) is made up of 51 amino acid residues and has a molecular weight of about 5700.

AROMAFACT

Most essential oils are liquids with constituents that exhibit a range of molecular sizes. Monoterpenes have molecular formulae C₁₀H₁₆, while sesquiterpenes have the formula C₁₅H₂₄. Although quite large molecules are present in essential oils, the oils do not contain any macromolecules.

4 Structures are based on carbon’s valency of 4

The carbon atom has 4 outer electrons, which are usually shared in covalent bonds that point to the vertices of a tetrahedron (a triangular pyramid).

In representing molecules it is common to use a two-dimensional representation with the chemical bonds drawn in the plane of the page, as shown above for methane. However, it is sometimes important to show the three-dimensional tetrahedral arrangement of single bonds around the carbon atom. To do this a ‘wedge’ convention is used. In the 3D representation of methane above, bonds drawn as plain lines are lying in the plane of the page; the black wedge indicates that that bond points out of the plane of the page and the broken wedge indicates that the bond points below the page. The angle between each pair of bonds is about 109 °.

Carbon can form multiple bonds (double or triple bonds; see Ch. 1) between its atoms when forming compounds. This can be shown in some simple hydrocarbons (see Fig. 2.2). In covalent bonds, the sharing of a pair of electrons comprising one electron from each atom involved in the bond is called a single bond and is represented by a single line. The bond is formed by two electrons; for example, in ethane, C₂H₆: