A new sort of reaction

Most organic reactions are ionic. Electrons move from an electron-rich atom towards an electron-poor atom anions or cations are intermediates. Formation of a cyclic ester (a lactone) is an example. 

The reaction involves five steps and four intermediates. The reaction is acid-catalysed and each intermediate is a cation. Electrons flow in one direction in each step—towards the positive charge. This is an ionic reaction. 

This chapter is about a totally different reaction type. Electrons move round a circle and there are no positive or negative charges on any intermediates—indeed, there are no intermediates at all. This type of reaction is called pericyclic. The most famous example is the Diels-Alder reaction. 

In Chapter 39 you will meet a third category—radical reactions—in which one electron instead of two is on the move. 

Otto Diels (1876-1954) and his research student Kurt Aider (1902-58) worked at the University of Kiel and discovered this reaction in 1928. They won the Nobel PHze in 1950. Diels also discovered the existence of carbon -suboxide. C203 (see p. 372). 

OttoDi stl876-19S4t and .hiS . research studarft Kuit  

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To address this sort of reaction we needed to develop a new approach to catalytic antibodies or enzymes in general that differed significantly from the noncovalent approach of Pauling and Jencks. We sought a method that would allow for the programming of detailed aspects of chemical reaction mechanisms, down to the level of the chemical identity... 

The first arrow from the nucleophile makes a new G bond and the last breaks the carbonyl k bond The middle arrow just n moves the C-C Jt bond along the molecule. If you inspect the product you will see that its structure follows precisely from the arrows. The middle arrow starts in tire middle of a n bond and ends in the middle of a G bond. All it does is to move the Jt electrons along the molecule. It turns the old 71 bond into a g bond and the old C bond into a Jt bond. We shall discuss this sort of reaction in Chapter 10. 

One of the most exciting recent advances in organic and organometallic chemistry has been the development of new catalysts that produce polypropylene with high stereochemical purity. Both isotactic and syndiotactic polypropylene are now made commercially with a new class of metallocene catalysts, prototypes of which are shown below. The mechanism of the polymerization reaction is discussed in Chapter 17. Here we will focus on the stereochemistry, because symmetry principles of the sort we discussed above were crucial in the design of this chemistry. 

The local dynamics of tire systems considered tluis far has been eitlier steady or oscillatory. However, we may consider reaction-diffusion media where tire local reaction rates give rise to chaotic temporal behaviour of tire sort discussed earlier. Diffusional coupling of such local chaotic elements can lead to new types of spatio-temporal periodic and chaotic states. It is possible to find phase-synchronized states in such systems where tire amplitude varies chaotically from site to site in tire medium whilst a suitably defined phase is synclironized tliroughout tire medium 51. Such phase synclironization may play a role in layered neural networks and perceptive processes in mammals. Somewhat suriDrisingly, even when tire local dynamics is chaotic, tire system may support spiral waves... 

The modes of thinking about structures and reactions and intermediates facilitated or even demanded by these conceptual innovations meshed productively with new tools of a different sort to drive the progress of recent decades. Advances in electronics and computers, all sorts of spectroscopy, laser optics and physics, chromatography, mass spectrometry, quantum theory and computational strategies, molecular beam experiments, and so on, radically expanded the limits of experimental and theoretical investigations. 

In an exploratory sort of way the F.E.M. has been used to study catalytic reactions on metal surfaces. While such studies are far from complete, they clearly indicate that this new tool can reveal details of the mechanism of such reactions to a much greater extent than is possible by any other known technique. We shall here discuss only two systems H2, O2 and H2O on W and C and O2 on W. 

Contrary to potentiometric methods that operate under null current conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions and organic compounds that can either be reduced or oxidised electrochemically. Polarography, the best known of voltammetric methods, is still a competitive technique for certain determinations, even though it is outclassed in its present form. It is sometimes an alternative to atomic absorption methods. A second group of methods, such as coulometry, is based on constant current. Electrochemical sensors and their use as chromatographic detectors open new areas of application for this arsenal of techniques. 

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