Pi Bonds and Conjugated Systems

In Level 2 chemistry, you are not expected to understand "pi bonds" or "conjugated systems". However, a basic knowledge of these will help you explain and discuss the structure, properties and development of polyacetylene.

PI BONDS

Alkenes and alkynes have a type of bond between the carbon atoms called a "pi bond".

The first bond between the carbon atoms is called a "sigma bond" and it holds the carbon chain together.

The next bond formed between the same two carbon atoms is called a "pi bond". It is not as strong as a sigma bond, but it is what prevents rotation of the double (or triple) bond.

Alkenes have one pi bond per carbon-carbon double bond (as well as one sigma bond).

Alkynes have two pi bonds per carbon-carbon triple bond (as well as one sigma bond).

We need to understand pi bonds to understand the next concept (conjugated systems).

CONJUGATED SYSTEMS

When a molecule has alternating single and double bonds, we call this a conjugated system. This means it also has a "layer" of pi bonds, usually drawn above and/or below the sigma bonds.

In cyclic compounds (such as benzene, shown in the video), this is represented as a circle inside the cyclic structure. We do not do the same thing with non-cyclic substances (such as polymers), but we need to keep in mind that the entire structure "shares" these bonding electrons.

A polymer with conjugation will allow for movement of these pi bond electrons  You need to link this key idea to the properties and (proposed) uses of such a polymer.