LINEAR VS. BRANCHED HYDROCARBONS

March 17, 2017

BACKGROUND:

One of the reasons why there are millions of organic compounds is that carbon can form long, stable linear chains of carbon. This makes it possible to produce polymeric chains containing thousands to millions of carbon atoms in it. Even with these lengths of the polymer chains, the compounds still have high stability.

Due to the tetravalency of the carbon atom, it can form bonds with up to four other carbon atoms. This property of carbon enables formation also of branched chains. Formation of linear and branched chains contributesto the variety of organic compounds possible.

Using the Lewis structures, linear chains are drawn with all carbon atoms forming a straight line. On the other hand, replacing H atoms attached to middle carbons with carbon atoms represents a branched chain. The Lewis representation of n-butane (normal butane) and isobutane only shows 2D perspective of the difference in the linear and branched carbon chains. These two compounds are both highly flammable and are used in the petroleum industry specifically in gasoline blending to increase the octane number of gasoline. They are isomers of each other that only differ in their structures, as n-butane is linear, while isobutane is branched.

Molecular models can provide 3D representations of these two types of chains. In terms of perspective, there is a slight difference in describing linear and branched chains using molecular models. In this post, the Duluth Labs Molecular Set will be used to show the difference between linear and branched chains of carbon.

MODELLING: Let’s Walk Through Building A Linear Chain

Note: We are building a Linear Chain using n-Butane as an example. These examples will all use carbon atoms with 4 bonding sites (which are called tetrahedral carbons).

1

1. Get a carbon atom (black) and connect three Hydrogen atoms (white) using short connectors.
2

2. Attach another carbon atom using a medium connector
3

3. To the Second Carbon, attach two hydrogen atoms using short connectors then attach one carbon atom using medium connectors.
4

4. To the Third Carbon, attach two hydrogen atoms using short connectors and then attach one carbon atom using a medium connector.
5

5. Complete the structure by attaching three hydrogen atoms around the fourth carbon using short connectors.

Linear Structure

Note: This structure is considered linear as if you place the model atop of a table, and look at the structure as shown below; the carbon atoms seem to follow a straight line.

n-butane (normal butane)

MODELLING: Let’s Walk Through Building A Branched Chain

Note: We are building a Branched Chain using isobutane as an example

1

1. To the structure you prepared in Part I, detach thewhole Carbon 4 group.
2

2. Attach 2 hydrogen atoms using small connectors and then attach one carbon atom using a medium connector.
3

3. To the 2nd Carbon added, use 2 medium connectors to attach 2 carbon atoms to either side. After doing this, use 4 short connectors to attach 2 hydrogen atoms to either carbon.
4

4. To complete the structure, use short connectors to attach hydrogen atoms.