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Curly Arrows: Writing Organic Reaction Mechanisms

General information

Technical Details

Review

This set of WWW pages and Java applets provides a wonderful introduction to writing mechanisms. I began by going through the 'Getting Started' pages, which outlined the aims of the resource. It is intended to be used alongside a lecture course, and to help to provide the practice which is required to be able to make use of this key skill in understanding organic chemistry. Further pages then provided a clear introduction to the use of curly arrows, and lead on to notes on example reactions.

So far, the resource provides a well presented description of an important topic, but it does not do anything which many books on organic chemistry can also do. However, now I had read the introductory pages, I could go on to investigate the Java applets, which provide facilities that no book on organic chemistry could possibly do. So far, I had used Netscape 4.73 and Internet Explorer 5 on a Macintosh to access the pages. At this point, I had to switch over to a PC, as the Java applets were not designed to run on a Macintosh. Internet Explorer 5.5 on a PC running Windows 98 worked very well.

fig 1: Instructions to users for the Curly Arrows application

Without reading the instructions I went straight to the first exercise. A picture of hydrogen bromide appeared, and a series of radio buttons marked with a lone pair, a partial positive charge and a partial negative charge. I quickly was able to highlight the parts of the molecule which had partial positive and partial negative charges, and pressed the "Done" button. I was told I should have marked the lone pairs too, so, suitably chastened, I returned to the instructions, which explained all of this. I now went through a series of ten molecules, marking all the lone pairs, partial positive and partial negative charges. I had a few minor differences of opinion with the program, about whether the hydrogens of formaldehyde had partial positive charges or not, and whether the methyl group of a methyl ester had a partial positive charge. In the first case, I was corrected, and in the second the program refused to let me put a partial charge where I thought one should go. The corrections were very clear and helpfully presented. Eventually I scored 94%! Exercise two was harder, but now I had worked out what sort of answers the program wanted. I went up to 96%, and I would like to have a discussion with the authors about the missing 4%! Exercise three was designed to be harder still, presenting more complex systems with more complicated mixtures of functionality.

This exercise would be very valuable for students. Beginners have an interactive way of exploring the electronic effects displayed by a series of increasingly complicated molecules. More advanced students may take the opportunity to argue with their teachers and with each other over minutiae of the program's assignments.

I then moved on to the curly arrows section, after reading the instructions. The diagrams showed a reaction, and I used the mouse to draw in the curly arrows. When I got this correct, a series of helpful notes on the reaction appeared. If I dragged the arrow to an incorrect position, then some helpful text appeared explaining my mistake, or else it told me that it had not been dragged to a defined area.

I found the curly arrows more interesting than assigning partial charges, perhaps because the conventions used were closer to those I am used to using. The text explaining the reaction and providing background information, once the curly arrows had been drawn correctly, was well thought out and very helpful. Students learning to use curly arrows may stare at structures wondering where to start. This program enables them to experiment, to find the answer, and then to build on from their first curly arrow. A student learning on their own may not get as far as the first arrow. A student being watched by a teacher may feel under some pressure to perform. This program removes the pressure, but maintains the positive feedback of confirming success and advising after errors, and so provides a new learning environment which is accessible at the student's convenience.

Java and the internet are shown to be a very powerful set of tools for this approach to teaching. However, the technology by itself would be useless without a well chosen and carefully explained set of examples. The team which produced this program is to be congratulated both on providing a good software basis for developing a course on curly arrows, and also for thinking through the information and examples which will be helpful for this. It would be useful to allow for some customisation so that slightly different conventions can be accommodated, but it could also be argued that exposure to competing ways of drawing reactions is a useful experience.

This resource is a very valuable teaching tool which deserves to be widely used. Students can proceed at their own pace, getting helpful feedback, and repeating the exercises as often as possible. I will certainly be encouraging my first and second year undergraduates to use it.

A resource as good as this immediately stimulates new ideas about how it could be developed. Could the students be marked automatically? Could they be timed? Could new reactions be entered to illustrate a particular course? It is a mark of the excellence of these web pages that they excite so many ideas about the possibilities that the technology opens up.

There is no doubt that this package does more than any textbook on curly arrows could hope to do. Computer-aided teaching is likely to be influential in the development of chemistry courses, and this is an example of computer-aided teaching at its best. Everyone who teaches organic reaction mechanisms should look at this web site, and evaluate how much it could help their students. Everyone who is learning about organic reaction mechanisms could benefit from experimenting with these examples.