Doping is not a chemical process that needs to be understood in depth in Level 2 Chemistry. However, a basic understanding of this process will help explain and discuss the properties, uses and developments of polyacetylene.
Doping is a chemical process that adds or removes electrons, changing the conductive properties of a substance (making it a better electrical conductor). It is much like rubbing a plastic ruler (or rod) with a cloth, except that the change is more long-lasting.
Doping usually involves one of two chemical processes:
OXIDATION: loss of electrons/removal of electrons
REDUCTION: gain of electrons/addition of electrons
The removal of electrons works by "making space" for electrons from a power source.
The addition of electrons works by overloading each atom with electrons. These electrons go into higher energy levels, so are easily "moved on" to the next atom by electrons from a power source.
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.
All of the Organic Chemistry that we have worked through so far has been done to give us a deeper understanding of a common chemical in our everyday lives - plastic.
This video gives a good overview of the history of plastics:
Plastics are an amazing technology, but their use has led to soooooo many problems. What is the future of plastics? How can we continue to use plastics without creating such a big waste issue for our planet?
There is a variety of plastic used in our everyday life:
The main advantage of plastics are their ability to be recycled...but this isn't the case for every plastic (for example, polystyrene). Also, recycling is expensive, and we do not do it as often as we should. That has led to a new movement - REFUSE single-use plastics.
Over the next two weeks, we will be looking at the past, present and future of one plastic - polythene (also called PE, polyethene and polyethylene).
Polymer = macro-molecule (huge molecule) made up of many repeating units Monomer = the starting molecule for a polymer (mono = "one"; mer = "unit") Repeating Unit = the basic structural component of the polymer; the part that is repeated continually in the structure of the polymer Addition Polymerisation = polymerisation where the monomers are simply added together when the double (or triple) bond of neighbouring monomers contribute one electron each to the formation of a new bond.
When given the name of an alkene, we need to be able to draw them. For this assessment, it is useful to draw them around the C=C double bond, as this is where polymerisation occurs. This video goes through how you draw an alkene from its name.
It has "ene" in the name, so draw a C=C double bond, and four bonds (2 off each carbon atom)
Use the number in the alkene part of name to number the far-left carbon atom
Use the prefix (but-, pent-, hex- etc.) to build the rest of the carbon chain. Number every carbon.
If it says cis, make sure the carbon chain is all on the same side of the C=C bond (above it or below it on the page. If it says trans, make sure the carbon chain goes across the C=C bond, like in the picture and video.
Add any side chains to the appropriate carbon(s) - you had already numbered the carbon atoms so this should be easy :)
Fill in all of the hydrogen atoms to the vacant bonds.
Because the C=C double bond cannot freely rotate in space, alkenes have a special type of isomerism, called geometric isomerism (or cis/trans isomerism).
