Structural representations of organic compounds
In this module, you will learn about various structural representations of organic compounds.
The structures of organic compounds can be represented in several different ways. They are:
1. Lewis Structure or Dot Structure,
2. Dash Structure,
3. Complete and Condensed structure and
4. Bond Line Structure.
Let’s take a closer look at each type of structure in detail.
Lewis structure is the simplest method of representing any molecules by showing the involvement of electron between the bonds. However, writing these structures is quite time consuming.
Here, Lewis structure of Methane and Methanol is represented by the structural formulae. The electron sharing is represented by cross mark or by dot.
However, these structures can be simplified by representing the two-electron covalent bond by a dash (-). In this representation, a single dash (-) is used to represent single bond and two dashes (=) is used to represent double bond. Similarly, three dashes are used to represent triple bond.
These type of structural representation are called complete structural formulae or graphic or displayed formulae.
Further, these structural representations can also be simplified by representing the arrangements of atoms and not the bonds between them.
For example, Methane and Methanol can be represented as CH4and CH3OH. These structural representations are called condensed structural formulae.
Sometimes these structural formulae can be further condensed by enclosing the repetitive structure unit within a bracket and placing an integer as a subscript indicating the number of times the structural unit is repeated.
For example, CH3CH2CH2CH2CH2CH2CH2CH3 can be further condensed to CH3 (CH2)6CH3 Similarly, CH3CH2CH2CH2CH2CH2CHO can be shown as CH3 (CH2)5CHO.
Among all these types of structural representation of organic compounds, the simple, short and convenient method is Bond-line structural formulae.
In this method, lines have drawn in a zigzag fashion and carbon atoms by line ends and intersections show carbon-carbon bonds.
Here, carbon atoms are not shown but all atoms other than carbon and hydrogen atoms such as Oxygen, Nitrogen, and Halogen are shown on the zigzag line.
Further, it is assumed that each carbon on the line end or intersection is attached to required number of hydrogen atoms. That is termini denote CH3 groups and unsubstituted intersection denotes a CH2 group.
For example, 3-methylocatane can be represented as
Similarly, Hexane and 2, 3- Methyhexane can be represented by bond line structural formula.
However, there are many organic compounds, in which the carbon atoms are not joined in a chain but are joined in a ring. These are called cyclic compounds and are usually represented by polygon without showing carbon and hydrogen atom.
Here, the corner of a polygon represents a carbon atom and the sides of a polygon denote a carbon-carbon bond.
If an atom or group of atoms other than hydrogen is attached to carbon, then that atom or gropu of atoms is shown in the structure.
For example, chorocyclohexane can be represented as shown here.
Well now, we have learned much about the various structural representations of organic compounds.
Now, let’s see about the three-dimensional representation of organic molecules in detail.
The three dimensional structure of organic molecules are very difficult to be drawn on a two dimensional paper.
To overcome this problem, certain graphic conventions have been proposed. The simplest convention is solid and dashed wedge formula.
What is solid and dashed wedge formula?
A solid wedge or heavy line is used to indicate a bond lying above the plane of the paper and projecting towards the observer.
The dashed wedge is used to represent a bond lying below the plane of the paper and projecting away from the observer.
The bonds lying in the plane of the paper are represented by normal lines.
For example, the 3-D structure of methane molecule on a paper can be represented as shown here.
Such a representation, which completely describes the actual positions of various atoms of a molecule in space, is called a spatial formula or three-dimensional structure.
In addition to these structural representations, nowadays computers are widely used for creating molecular models.
Molecular models are used for a better visualization and perception of 3D structures. These are mainly of three types:
1. Framework models
2. Ball and stick modes and
3. Space filling models.
Let’s look at the framework models.
Here in this model, the atoms themselves are not shown but only the bonds connecting the atoms of a molecule are shown. This type of model is used only to study the pattern of bonds in a molecule while ignoring the size of atoms.
Let’s look at the ball and stick modes.
In the ball-and-stick model, both the atoms and bonds are shown. Whereas balls represent the atoms but sticks denote the bonds. Unsaturated compounds containing carbon double bond carbon and carbon triple bond carbon are better represented using springs instead of sticks.
Let’s look at the space filling modes.
In space filling models, bonds are not shown but importance is provided on the relative size and volume occupied by each atom depending upon its vander waal’s radius.