Drawing Good Arrows

Now that we know how to identify good arrows and bad arrows, we need to get some practice drawing arrows. We know that the tail of an arrow must come either from a bond or a lone pair, and that the head of an arrow must go to form a bond or a lone 

we need to know where to put the head of the arrow. We look for any lone pairs or double bonds that are appearing. We see that there is a new lone pair appearing on the oxygen. So now we know where to put the head of the arrow  

It is pretty straightforward to see how to push only one arrow that gets us from one resonance structure to another. But what about when we need to push more than one arrow to get from one resonance structure to another Let s do an example like that. 

EXERCISE 2.13 For the two structures below, try to draw the curved arrows that get you from the drawing on the left to the drawing on the right  

Answer Let s analyze the difference between these two drawings. We begin by looking for any double bonds or lone pairs that are disappearing. We see that the oxygen is losing a lone pair, and the C=C on the bottom is also disappearing. This should automatically tell us that we need two arrows. To lose a lone pair and a double bond, we will need tw o tails. 

Now that we know how to identify good arrows and bad arrows, we need to get some practice drawing arrows. We know that the tail of an arrow must come either from a bond or a lone pair, and that the head of an arrow must go to form a bond or a lone pair. If we are given two resonance structures and are asked to show the arrow(s) that get us from one resonance structure to the other, it makes sense that we need to look for any bonds or lone pairs that are appearing or disappearing when going from one structure to another. For example, consider the following resonance structures  

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Because the intermediates Me and MeiCOH are pretty rmlikely species and they would have to be intermediates in the real reaction too We have already found the first way to recognise a good disconnection it has a reasonable mechanism. Choose a discormection for this molecule, target molecule 3 (TM 3) breaking bond a or b. Draw the arrow and the intermediates. 

Hints. First draw good diagrams of the reagents. NaHC03 is a salt and a weak base—strong enough only to remove which proton Then work out which bonds are formed and which broken, decide whether to push or pull, and draw the arrows. What are the other products ... 

Draw proton transfer with good arrows to a valid Lewis structure and check and charge balance. 

Sometimes when writing the Lewis structure of a species, we may draw more than one possible correct Lewis structure for a molecule. The nitrate ion, N03 , is a good example. The structures that we write for this polyatomic anion differ in which oxygen has a double bond to the nitrogen. None of these three truly represents the actual structure of the nitrate ion—it is an average of all three of these Lewis structures. We use resonance theory to describe this situation. Resonance occurs when more than one Lewis structure (without moving atoms) is possible for a molecule. The individual structures are called resonance structures (or forms) and are written with a two-headed arrow (<- ) between them. The three resonance forms of the nitrate ion are ... 

It makes good sense to draw free-radical mechanisms in the manner shown by these examples. However, shorter versions may be encountered in which not all of the arrows are actually drawn. These versions bear considerable similarity to two-electron curly arrow mechanisms, in that a fishhook arrow is shown attacking an atom, and a second fishhook arrow is then shown leaving this atom. The other electron movement is not shown, but is implicit. This type of representation is quite clear if the complement of electrons around a particular atom is counted each time but, if in any doubt, use all the necessary fishhook arrows. 

We can t draw a single electron-dot structure that indicates the equivalence of the two 0-0 bonds in 03 because the conventions we use for indicating electron placement aren t good enough. Instead, the idea of resonance is indicated by drawing the two (or more) individual electron-dot structures and using a double-headed resonance arrow to show that both contribute to the resonance... 

We last discussed mechanisms in Chapter 5 where we introduced basic arrow-drawing. A lot has happened since then and this is a good opportunity to pull some strands together. You may like to be reminded ... 

It is best to use a HPLC column always in the same direction or to know why the direction of flow has been changed. Many columns have an arrow which indicates the direction in this case it is not clear if it is possible to reverse the column. If the frits at both ends are identical in porosity it is no technical problem to run the column in the other direction, however, frits may differ in pore width and the wide-pore frit would be at the entrance. Flow reversal could not be recommended under these circumstances. If a column has no arrow it is good advice to draw one oneself. The column is then always used in the same direction, therefore fines and non-eluted compounds are concentrated at the inlet. The column is only turned for the purpose of regeneration. 

In the first reaction, the nucleophile is the amine and the electrophile is the methyl iodide, so the arrow is right in the sense that it starts on the nucleophile, where the electrons come from. However, we have stressed that a curly arrow should start on a representation of an electron pair, in other words on a lone pair, a bond or a negative charge. Here the arrow just starts on an atom this is no good, and we must draw in the lone pair. The other problem is at the end of the arrow. It shows a new bond being formed to C, so unless a bond breaks then the C atom will have an impossible five bonds. We need another arrow to show the C-I bond breaking. 

An example of globular tertiary structure of a protein is presented in Figure C5.14. This particular protein is called aspartic protease endothia-pepsin, but its name is not really important for us at the moment. Let s look first at the image C5.14 a. There, the spirals show the strands of a-spirals (there happens to be only a few of them in this particular protein), and the flat arrows represent the pieces which have /S-structure. This way of depicting the protein structure was suggested by the biophysicist Jane Richardson of Duke University. It is good for its clarity if you tried to draw tertiary structure in more detail, the picture would appear too... 

With the intermediates placed in the correct order, the final step is to draw the reagents and curved arrows that show how each intermediate is transformed into the next intermediate. Begin with the acetal, and work forward until reaching the ketone. This requires that your arrow-pushing skills are in good shape. 

PROBLEM 7.61 The following molecules can form conjugate bases by loss of a proton from more than one position (arrows). Start by drawing a good Lewis structure (electron dots are deliberately left out) and then choose which proton wiU be lost more easily. Explain your choice. 

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