I was first taught curly arrow pushing in 1968, and have myself taught it to many a generation of student since. But the other day, I learnt something new. Nick Greeves was kind enough to send me this link to the origin of curly arrow pushing in organic chemistry, where the following diagram is shown and Alan Dronsfield sent me two articles he co-wrote on the topic (T. M. Brown, A. T. Dronsfield and P. J. T Morris, Education in Chemistry, 2001, 38, 102-104, 107 and 2003, 40, 129-134); thanks to both of them.
This diagram dates from 1924, and is to be found in an article published by Robert Robinson (J. Soc. Chem. Ind., 1924, 43, 1297, a journal difficult to get hold of nowadays). Here, Robinson was trying to explain why the nitroso group is o/p-directing in aromatic electrophilic substitution. Whilst the notation is remarkably modern, some aspects do need explaining.
- Robinson shows the nitrogen lone pair (arrow 1) as a line, and not as we now do, a double dot.
- Similarly, he shows arrow 3 ending at a line. We now do not show this in the starting structure, but reveal it in the final result, as above on the right, and again shown as a double dot.
- Similarly, he shows a + charge on the nitrogen at the start, whereas we now show it as the outcome of the process.
- If Robinson intends to create a +ve charge, then he really should balance that by showing the creation of a negative charge in the p-position of the ring. He does not balance his charges!
- As was the custom at the time, the benzene ring itself is not represented in the Kekule mode (which of course should have been well known in 1924) but as what looks to us now as cyclohexane. It must have been the case in 1924 (and for several decades after) that cyclohexane itself was not regarded as an interesting system, and hence there must have been little confusion about drawing benzene as (modern) cyclohexane. The implied semantic of showing such a ring was that it represented benzene.
- But this way of drawing it leads to really difficult issues. Thus Robinson’s arrow 2 departs from what looks to us like a single bond, in which case no bond would be left. Robinson of course means implicitly that arrow 2 reduces the bond order by one, and if we start with a double bond from a Kekule structure, that the bond is reduced to 1, not zero, as is shown in the modern notation above.
- Likewise, the destination of arrow 2 in Robinson’s notation clearly creates a double bond. Which again is an issue, since he is not showing the double bonds. The trouble really arises because Robinson does not illustrate the outcome of his process.
- Finally, whereas arrow 1 starts at a line representing a lone pair, that line is disconnected from the N. However, the destination of arrow 3 appears to create a bond, not a lone pair.
Now that we have clarified Robinson’s meaning, what else can we say about Robinson’s structure.
- It is important to realise that in 1924, the 3D characteristics of electrons (their wavefunction) were not known. Looking at the modern version of the diagram, chemists realise that when a double line is drawn, the two are not the same. One line represents a σ-bond, the other a π-bond. We recognise that the two have different spatial characteristics. Hückel it was who showed that in planar aromatics, the two sets are in fact orthogonal, and do not mix. At which point we need to sort out what the three arrows in Robinson’s diagram represent. Arrows 2 and 3 we recognise as π-arrows. But what of arrow 1? I decided to do a search of the Cambridge data base for nitrosobenzenes, finding 22 sets of coordinates. In all except one, the two atoms of the nitroso group were co-planar with the six of the benzene ring. We now know of course that this places the nitrogen lone pair firmly in the plane of the eight atoms, and hence of a σ-type. Strictly therefore, it is orthogonal to the π-arrows and cannot be mixed with them. The solution of course is to first rotate the nitroso group by 90° to bring the nitrogen lone pair into conjugation with the π-system, whereupon Robinson’s arrows now “work”.
- On a more minor point, we recognise that the nitrogen lone pair occupies a trigonal position, and so we draw the C-NO group as bent, rather than linear as Robinson did.
- If the co-planarity of the nitroso and benzene rings is retained, then the only way to draw the arrows is in the opposite direction to Robinson, resulting in the creation of a -ve charge on the oxygen and a +ve charge on the p-carbon. This of course is the resonance we now show for the nitro group, and implies m-direction, not o/p.
- Which raises the fascinating question. Why, if the structure of nitrosobenzenes appears to be planar and not rotated, is the nitroso group nevertheless observed to be an o/p director? The answer of course must be in looking at the properties of the transition state, and not the starting material itself. But in 1924, the concept of a transition state itself was not yet recognised.
So this little blast-from-the-past example still gives us lots to think about!