Theory of Knowledge

Organic Chemistry obviously lends itself to many different connections to TOK. The core part of the syllabus only has two specific links. Sub-topic 10.1 Fundamentals of organic chemistry talks about the use (or misuse) of labels. The origin of the label 'Organic chemistry' was based on the misconception that some vital force was needed to explain the chemistry of carbon compounds. The syllabus questions whether there are other similar examples elsewhere and whether the label should be changed once the truth of the matter is recognised. The second example referred to in the same sub-topic is based on Kekulé's claim that his explanation for the cyclic structure of benzene emanated from a dream. It questions the role played by less analytical ways of knowing in the acquisition of scientific knowledge.

One interesting aspect of TOK and the Nature of Science is the value of models. This is mentioned specifically in the AHL sub-topic 20.3 Stereoisomerism particularly ways of representing three-dimensions using two-dimensional diagrams. I have written about modelling in the page on Computer modelling under ICT in practical work but this is more for computer simulations. It is certainly worth looking at modelling in chemistry in more depth and this is covered below in Example 1 and, separately in the Key terms & concepts of NOS. Sub-topic 20.3 also refers to optical isomerism and links it to the indirect evidence that enantiomers provide to confirm the existence of tetrahedrally-bonded carbon atoms. The point being that the power of reasoning allows chemists to gain access to the molecular scale. This is illustrated well by Walden inversion during S_N2 reactions and I have included it below as Example 2.The role of language is also important, particularly the use of the IUPAC system of naming organic compounds. This is a tool which enables chemists to clarify and distinguish between different organic compounds and their structures. I have written a separate page on the Language of Chemistry in the TOK section on this site and also linked this to a page specifically on IUPAC.

There is a further reference to TOK in the Additional Higher Level material under sub-topic 20.2 which questions the roles of imagination, intuition and reasoning in finding solutions to practical problems.

1. Modelling organic molecules

When you ask students to reflect on the types of models they have come across they may have encountered some more abstract forms of modelling in subjects like Economic but most will think of dolls, model aeroplanes, model boats etc. etc. From their childhood they will think of models as being smaller than the objects they represent. In chemistry the opposite is true. The models we have of molecules are many times bigger than the molecules they represent. Think of the word ‘methane’. It will sum up many images and much information and in a sense the word methane is itself a model. Certainly just writing methane does not mean we have methane itself magically on the page. The surrealist painter René Magritte realised this in his famous painting ‘Ceci n'est pas une pipe’.

There are many different models of methane that are commonly used. Some of them are:

Five of those given above will be familiar to Standard and Higher Level students and the sixth (image 5) to Higher Level students only. You could ask students what information about methane is carried in each of the six pictures because they do all represent a different aspect of methane’s attributes. Students will also realise that they make assumptions themselves about the models and what they represent. For example, the 90^o angles in the second model are known to represent 109^o 28’ but are drawn as 90^o for ease of drawing in two dimensions etc. Which is the true model? Model number 5 is perhaps the most sophisticated as it tries to convey the concept of sp³ hybridization and shape but it is only one small step further towards representing methane. Of course in reality none of them is the real methane. Each gives some specific information about methane but we are limited by the constraints of dimension and size (to name just two of the many constraints) to give anything like a true representation of a methane molecule.

2. Walden inversion

As you can see from the TOK aspects of Topics 6 & 16 it is possible to disprove a mechanism from the experimentally determined rate equation but it is not possible to prove it. You can only say that the proposed mechanism is consistent with the rate equation. A Latvian chemist, Paul Walden (1863 - 1957), applied fantastic logic to 'prove' the S_N2 mechanism for certain nucleophilic substitution reactions and in the process also confirmed the tetrahedral structure of halogenoalkanes. He reasoned that if the initial halogenoalkane was optically active then if the mechanism went through an S_N1 type mechanism then the product would be a racemic mixture of the two enantiomeric alcohols as the hydroxide ion could attach to either side of the carbocation intermediate in a 50:50 ratio. However if it went by an S_N2 type mechanism, via a five-membered transition state, the resulting alcohol would rotate the plane of plane-polarized light in the opposite direction to the starting material. The product did indeed rotate the plane of plane-polarized light in the opposite direction to the reactant.