Understanding Organic Formulas
Organic chemistry mainly studies compounds based on carbon chains. Carbon forms 4 bonds, which allows it to create complex structures with other elements, mainly hydrogen, oxygen, nitrogen, sulfur, and halogens.
Types of Chemical Formulas
Molecular Formula
Shows only the number of atoms of each element in the compound.
Examples: CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane)
Condensed Formula
Groups the atoms according to their arrangement in the molecule, without showing the bonds.
Examples: CH₄, CH₃CH₃, CH₃CH₂CH₃
Semi-developed Formula
Shows the arrangement of atoms and some bonds, especially between carbons.
Examples: CH₄, CH₃-CH₃, CH₃-CH₂-CH₃
Developed Formula
Represents all the bonds between atoms, showing the complete structure.
Example for propane (C₃H₈):
To simplify chemical formulas, the most common atoms, carbon and hydrogen, are represented in a simplified way. All bonds that are not shown are C-H bonds, and each carbon forms 4 bonds. Bonds where atoms are not indicated are C-C bonds. In this way, a chain is drawn with angled bonds, indicating that at each vertex there is a carbon and at each bond not shown there is a hydrogen.
Example for methane (CH₄):
The C-H bonds are omitted, and it is understood that there is a C at each vertex.
Elements in Organic Formulas
Carbon (C)
Carbons are usually not shown, and it is understood that there is a carbon at each bond where no element is specified. Sometimes they are shown in small molecules or at terminal carbons.
Hydrogen (H)
Hydrogens are usually not shown except when they are bonded to other elements, such as in alcohols or aldehydes. Since carbons form 4 bonds, the number of hydrogens can be determined by observing the number of bonds not shown for each carbon.
Other elements (O, N, S, Cl, ...)
They are always shown and indicated in the formula.
Degree of Unsaturation
The number of unsaturations is used to help draw chemical structures. It indicates the number of double bonds, triple bonds, or rings in a molecule. A molecule is considered saturated when it does not have these types of bonds.
To calculate, we can think of a simple molecule as a carbon chain. The carbons in the chain will have 2 bonds to hydrogen each since they are bonded to 2 other carbons. The terminal carbons in the chain will have 3 bonds to hydrogen, since they are bonded to only one carbon.
Unsaturations can be calculated with the formula: $I = \frac{2C + 2 - H - X + N}{2}$
Where each letter is the number of atoms of each element.
For example, C₇H₆O has: $I = \frac{2 \times 7 + 2 - 6}{2} = 5$ unsaturations.
This is benzaldehyde, which has 3 unsaturations in 3 C=C double bonds, 1 C=O double bond, and 1 ring.