Representation of Organic Molecules
Organic compounds, with their complex structures, require various types of formulas for accurate representation.
Chemical Formula
A chemical formula is a symbolic representation of a chemical substance using chemical symbols to denote atoms and subscripts to indicate the number of atoms of each element in a molecule. Chemical formulas are used to depict chemical reactions on a whiteboard or notebook to aid in understanding the reaction process. Representing a chemical reaction with the help of symbols and chemical formulae of reactants and products is called a chemical equation.
Molecular Formula
A molecular formula indicates the actual number of atoms of each element present in a molecule of an organic compound. It provides the precise composition of a molecule.
The following table shows some molecular formulae of organic compounds.
| Compound | Molecular formula | Compound | Molecular formula |
|---|---|---|---|
| Ethanol | C₂H₆O | Ethene | C₂H₄ |
| Ethanoic Acid | C₂H₄O₂ | Benzene | C₆H₆ |
| Glucose | C₆H₁₂O₆ | Ethyne | C₂H₂ |
Empirical Formula
The empirical formula of organic compounds shows the simplest whole-number ratio of atoms of each element present in the molecule.
Organic molecules can have the same or different ratios of atoms within their molecular formulae. For example, ethyne (C₂H₂) and ethane (C₂H₆) illustrate this difference.
The following table shows empirical formulae with corresponding molecular formulae.
| Compound | Molecular formula | Empirical formula |
|---|---|---|
| Ethanol | C₂H₆O | C₂H₆O |
| Ethanoic Acid | C₂H₄O₂ | CH₂O |
| Glucose | C₆H₁₂O₆ | CH₂O |
| Benzene | C₆H₆ | CH |
| Ethyne | C₂H₂ | CH |
Empirical formulae can sometimes be misleading when identifying organic compounds. For example, both glucose and ethanoic acid have the same empirical formula “CH₂O,” which can lead to confusion in distinguishing between them. Similarly, some compounds have the same empirical and molecular formulae, like sucrose (C₁₂H₂₂O₁₁).
Determination of Empirical and Molecular Formulae from Combustion Analysis
Combustion analysis is an experimental technique used to determine the empirical formula of a compound by burning a known mass of the substance and measuring the masses of the products (usually carbon dioxide and water).
Calculation of Empirical Formula
Example 1:
An organic compound has the following percentage composition by mass: C = 54.5%, H = 9.1%, and O = 36.4%. Calculate the empirical formula of this compound.
Solution:
i. Calculate moles of each element.
-
- Moles of carbon = 54.5 / 12 = 4.54
- Moles of hydrogen = 9.1 / 1.0 = 9.1
- Moles of oxygen = 36.4 / 16 = 2.28
The percentage by mass of an element is assumed to be equal to its mass in grams.
ii. Divide all moles by the smallest value of moles to obtain the ratio of atoms (empirical formula).
- Carbon = 4.54 / 2.28 ≈ 1.99 ≈ 2
- Hydrogen = 9.1 / 2.28 ≈ 3.99 ≈ 4
- Oxygen = 2.28 / 2.28 = 1
If whole numbers are not obtained and values cannot be reasonably rounded, multiply to get whole numbers.
iii. The empirical formula of this compound is C₂H₄O.
Example 2:
An organic compound has the empirical formula “CH” and a molecular mass of 78 amu (atomic mass units). Calculate its molecular formula.
Solution:
We know that the molecular formula can be a whole-number multiple of its empirical formula. Therefore, we need to determine that multiple, represented by “n”.
n = Molecular mass / Empirical formula mass
= 78 / 13 = 6
We know that Molecular formula = n (Empirical formula)
Therefore, we can write:
6(CH) = C₆H₆
Both molecular and empirical formulae do not provide a complete identity of organic molecules.
Since a molecular formula cannot guide us in identifying functional groups and the bonding pattern between atoms in organic molecules, we need other formulae that can show a more detailed picture of organic molecules.
Structural Formulae
A structural formula of a molecule shows the arrangement of atoms and the type of functional group in it. There are several types of structural formulae, each providing a different level of detail.
1. Condensed Structural Formula
A condensed structural formula shows the relative positions of all atoms in a molecule, without explicitly showing all single covalent bonds. In this formula, each carbon atom is written individually, along with the hydrogen atoms attached to it. Branches, functional groups, and repeating methylene groups (-CH₂-) in the middle of molecules are shown in brackets.
Key Information:
In the condensed structural formulae of compounds, side chains or functional groups are shown in brackets, alongside the carbon atom to which they are attached.
The condensed structural formula of acetic acid (CH₃COOH) shows that three hydrogen atoms are bonded to the first carbon, and the remaining hydrogen atom is attached to one of the two oxygen atoms. It also indicates that the carboxyl group (-COOH) is the functional group in the acetic acid molecule.
| Alkane | Condensed Formula | Alkane | Condensed Structural Formula |
|---|---|---|---|
| Methane | CH₄ | Hexane | CH₃(CH₂)₄CH₃ |
| Ethane | CH₃CH₃ | Heptane | CH₃(CH₂)₅CH₃ |
| Propane | CH₃CH₂CH₃ | Octane | CH₃(CH₂)₆CH₃ |
| Butane | CH₃(CH₂)₂CH₃ | Nonane | CH₃(CH₂)₇CH₃ |
| Pentane | CH₃(CH₂)₃CH₃ | Decane | CH₃(CH₂)₈CH₃ |
2. Displayed and 2D-Structural Formulae
A full structural formula shows all atoms and the bonds between them in a molecule. It is a two-dimensional formula, also called a 2D displayed formula. This type of formula provides the most complete representation of the connectivity of atoms in a molecule.
Key Information:
A 2D formula illustrates planar molecules, showing the arrangement of atoms in the x and y planes only.
 |
 |
 |
 |
| Butane | Cyclopentene | Propene | Chlorohexane |
3. Skeletal Formulae
The skeletal formula represents the carbon skeleton and functional groups, omitting hydrogen atoms bonded to carbon atoms. These formulas are a simplified way to represent organic molecules, particularly cyclic and complex structures.
Key Information:
- In a skeletal formula, carbon and hydrogen atoms are not explicitly shown.
- Double and triple bonds between carbon atoms are shown in skeletal formulae.
- Functional groups are shown.
In a skeletal formula, lines are drawn where each end and vertex of the line is assumed to represent a carbon atom, unless another atom, like oxygen or a halogen, is present. Each carbon atom is assumed to be bonded to enough hydrogen atoms to complete its valency (the number of bonds it typically forms).
4. Stereochemical Formulae
Stereochemical formulae provide the best representation of the shapes of molecules, showing the three-dimensional arrangement of atoms and bonds. The 3D displayed formula of a molecule gives a complete picture about the arrangement of atoms and bonds of a molecule in three-dimensional space. For example, methane has the following stereochemical formula.
| Name | Molecular formula | Condensed structural formula | Displayed formula | Skeletal formula |
|---|---|---|---|---|
| Propane | C₃H₈ | CH₃CH₂CH₃ |  |
 |
| Propene | C₃H₆ | CH₃-CH=CH₂ |  |
 |
| Propan-2-ol | C₃H₈O | CH₃CH(OH)CH₃ |  |
 |
| Propanal | C₃H₆O | CH₃CH₂CHO |  |
 |
| Propanone | C₃H₆O | CH₃COCH₃ |  |
 |
