Structural Representation of Organic Compounds

The compounds that contain carbon and hydrogen atoms covalently bonded to each other are known as organic compounds. Although an organic compound has only one chemical formula, the structural representation of compounds can be in various ways. The organic compound’s three structural representation is explained below: entire structure, condensed structure, and bond line structural formulas.

As the hydrocarbons are from many compounds, there is a need to better visualise and understand their molecular structures. Two or hydrocarbon compounds can have the same molecular formula, but how the constituting atoms are arranged may differ widely. Let’s learn some of how these compounds can be represented.

Representation of Organic Compounds: Overview

A molecular formula is the simplest way to represent a compound. In the molecular formula, a compound is represented by counting up all of the different types of atoms and listing them in order.  For example,  in sugar, glucose contains six carbons, \(12\) hydrogens, and six oxygens. The molecular formula would then be written as \({{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}.\)

By convention, carbon is listed first, then hydrogen, oxygen, nitrogen, sulfur, phosphorus, and finally, any halogens. However, the molecular formulae do not provide adequate information for chemical analysis. They simply provide the numbers of each type of atom present in the molecule, but they carry no information about how the atoms are joined together in space.
For example, the molecular formula \({{\rm{C}}_{\rm{4}}}{{\rm{H}}_{{\rm{10}}}}\) tells us there are four carbon atoms and ten hydrogen atoms in a molecule. Still, it doesn’t distinguish between butane and isobutane. A structural formula shows all the carbon and hydrogen atoms and their bonds. Thus, structural formulas identify the specific isomers by showing the order of attachment of the various atoms.

Although an organic compound has only one molecular formula, it can be represented in a number of ways which are as follows–

• 1. Complete Structural Formula
• 2. Condensed Structural Formula
• 3. Bond Line Structural Formulas
• 4. Three dimensional (3D) representation

Complete Structural Formula

To represent the organic compounds through complete structural formula, we need to have knowledge about the Lewis dot structure or the electron dot structure of representation. In Lewis structure:

1. Dots \(\left( {\rm{ \bullet }} \right)\) are used to represent electrons, and

2. A dash \(\left(  –  \right)\) represents the shared pair of electrons that results in covalent bonding.

3. Every single bond, a double bond, and a triple bond is represented by one dash \(\left(  –  \right),\) double dash \(\left(  =  \right),\) and triple dash \(\left(  \equiv  \right),\) respectively.

In a complete structural formula, all the atoms in a molecule, the types of bonds connecting them, and how they are connected are depicted. This formula helps us identify the location of chemical bonds between the atoms of a molecule. A structural formula consists of symbols for the atoms connected by short lines representing chemical bonds—one, two, or three lines standing for single, double, or triple bonds.

For example:

Condensed Structural Formula

For molecules other than the most simple ones, drawing a complete structural formula can be cumbersome and time–consuming, especially all the carbon-hydrogen bonds. These bonds are hence simplified– or condensed. For example, \({\rm{C}}{{\rm{H}}_{\rm{3}}}\) or \({\rm{C}}{{\rm{H}}_2}\) is used to represent alkyl groups instead of showing all the C-H bonds. For example, ethanoic \(\left( {{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{4}}}{{\rm{O}}_{\rm{2}}}} \right)\) acid can be shown in a complete structural formula, a partially condensed form and a fully condensed form.

A condensed structural formula is a more compact way of representing the structural formula of a molecule. The dashes/bonds are removed, and the identical atoms or groups are numerically represented in subscript to that atom. The subscript number equals the count of identical atoms or groups present in the complete structural formula. In the condensed structural formula, every carbon atom is represented individually. For example –

In a completely condensed structure, analysing the bonding patterns can be challenging. The fully condensed form does not contain adequate information about neighbouring atoms’ bonding order and arrangement in space. However, it is more useful than the molecular formula. While expanding a condensed formula of a compound, the octet and duplet configuration of the carbon atom and the hydrogen atom should be satisfied.

While condensed structures are easier to write than complete structural formulas or partially condensed structures, they can prove to be a little more challenging to determine the three-dimensional bonding pattern of the atoms.

Bond Line Structural Formula

A more minimalistic drawing convention, known as the bond-line notation, is also used to represent the structures of organic compounds. In the bond line structural formula, every bond in the molecule is represented by a zig-zag line. In Bond Line Structural Formula:

1. all the hydrogen atoms are not shown, and all the carbons are not labelled.

2. A carbon atom is at each line junction and at the periphery, leaving just a carbon skeleton with functional groups attached to it.

3. If not specified, every terminal is assumed to be a methyl \(\left( {{\rm{ – C}}{{\rm{H}}_{\rm{3}}}} \right)\) group.

4. Heteroatoms (any other atom than carbon or hydrogen) and hydrogens attached to heteroatoms are shown in condensed form.

5. All lone pairs are not shown.

6. This “zig-zag” pattern can be initiated using either an upward or a downward angle.

7. Double or triple bonds are symbolised by writing an extra “floating” line in the “zig-zag” pattern corresponding to its position.

Three Dimensional or \({\rm{3 – D}}\) Representation of Organic Compounds

The \({\rm{3 – D}}\) structure of an organic compound is also known as the Wedge-Dash method of representation. In the wedge-dash method of representation of organic compounds:

1. A solid wedge represents a bond that protrudes out of the plane of paper towards the viewer, denoted by a solid wedge.

2. A dashed wedge is used to represent a bond that projects away from the viewer or into the plane of the paper, and

3. A line is used to represent a bond that lies in the plane of the paper.

Solved Example

Q.1. Draw the bond line notation of the following compound-

Sol:
Step 1: All hydrogen atoms bonded directly to the carbon atom are removed.

Step 2: All carbon atoms are removed.

Step 3: On removing carbon atoms, the skeletal structure looks like a straight line; hence, the straight line is drawn in a zig-zag manner so that the corners are apparent.

Summary

To represent organic molecules, organic chemists use several different notations. The compositional characteristics of an organic compound can be determined by analysing its structure through the expanded structural notation. The connectivity of each atom, bond, and lone pair can be explained through the complete structural formula. Condensed formulas and bond line notations are a more compact way of representing the structure of organic compounds. Through the three-dimensional representation, the spatial arrangement of atoms can be studied and analysed.

FAQs

Q.1. What are the different ways to represent the structure of organic compounds?
Ans: An organic compound has only one chemical formula but different structural formulas. An organic compound can be represented by the following structural formulas – complete structural formula, condensed structural formula, bond line structural formula.

Q.2. What is an expanded structural formula?
Ans: The expanded structural formula shows all bonds present in the constituting atoms of a compound. The expanded structural formula is based upon the Lewis Structures of compounds.

Q.3. What is alkanes general formula?
Ans: The general formula for the alkanes \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 2}}}}\) is where \({\rm{n}}\) is the number of carbon atoms in the molecule.

Q.4. What is the meaning of the structural formula?
Ans: The structural formula represents the location of chemical bonds between the atoms of a molecule. It consists of symbols for the atoms connected by single, double, or triple lines representing single, double, or triple bonds.

Q.5. What is the condensed structural formula of ethanol?
Ans: Ethanol is alcohol consisting of two carbon atoms, five hydrogen atoms and a single oxygen atom. Hence, the condensed structural formula of ethanol is \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}{\rm{.}}\)

Study Carbon and Its Compounds Concept Here

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