Arrow pushing

Now that we know what kinds of arrows are acceptable, we can begin to practice drawing them (or pushing them, as its called). To do this, we need to learn how to analyze a step in a mechanism, and train our eyes to look for all of the lone pairs and all of the bonds. We have said that all arrows are coming from or going to either lone pairs or bonds. So it makes sense that we need to be able to look at a step in a mechanism and determine which bonds have changed and which lone pairs have changed. Let s see this in an example. 

EXERCISE 8.8 Complete the mechanism of the following reaction by drawing the proper arrows in each step  

Answer We need to look for all changes for bonds or lone pairs. In the first step, the double bond is disappearing, one of the carbon atoms of the double bond is forming a new bond to a proton (H ), and we are breaking the H—Cl bond to expel Cl. So we have broken two bonds (C=C, and H—Cl) and we have formed one bond (C—H) and one extra lone pair (on Cl). Therefore, we will need two arrows to make this happen. Where do we start  

In the next step, again we look for all changes to lone pairs or bonds. We see that the Cl is giving up one of its lone pairs to form a bond with a carbon (C ). So, we need only one arrow, from a lone pair to form a bond  

PROBLEMS For each transformation below, complete the mechanism by drawing the proper arrows. 

---

Never exceed an octet for second-row elements. Elements in the second row (C, N, O, F) have only four orbitals in their valence shell. Each of these four orbitals can be used either to form a bond or to hold a lone pair. Each bond requires the use of one orbital, and each lone pair requires the use of one orbital. So the second-row elements can never have five or six bonds the most is four. Similarly, they can never have four bonds and a lone pair, because this would also require five orbitals. For the same reason, they can never have three bonds and two lone pairs. The sum of (bonds) + (lone pairs) for a second-row element can never exceed the number four. Let s see some examples of arrow pushing that violate this second commandment ... 

Arrow pushing is much like riding a bike. If you have never done it before, watching someone else will not make you an expert. You have to leam how to balance yourself. Watching someone else is a good start, but you have to get on the bike if you want to leam. You will probably fall a few times, but that s part of the learning process. The same is trae with arrow pushing. The only way to leam is with practice. 

Now it s time for you to get on the arrow-pushing bike. You would never be stupid enough to try riding a bike for the first time next to a steep cliff. Do not have your first arrow-pushing experience be during your exam. Practice right now ... 

In summary, we have seen the following five arrow-pushing patterns ... 

From an arrow-pushing point of view, all acid-base reactions are the same. It goes like this ... 

The mechanisms that you will learn in the first half of your course are the most critical ones. This is the time when you will either master arrow pushing and mechanisms or you will not master them. If you don t, you will struggle with all mechanisms in the rest of the course, which will turn your organic chemistry experience into a nightmare. It is absolutely critical that you master the mechanisms for the early reactions that you cover. That way, you will have the tools that you need to understand all of the other mechanisms in your course. 

In this chapter, we will not leam every mechanism that you need to know. Rather, we will focus on the tools that you need to properly read a mechanism and abstract the important information. You will leam some of the basic ideas behind arrow pushing in mechanisms, and these ideas will help you conquer the early mechanisms... 

Following the procedure for vicinal interactions illustrated in Example 1.4 and Section 3.3.1, we can associate each geminal ctab— obc donor-acceptor interaction with an arrow-pushing diagram and partial admixture of an alternative resonance diagram, as shown in Fig. 3.78. The formal two-electron transfer from gab to obc results in the NBO configuration... 

Figure 3.78 Generic arrow-pushing diagrams (left) and secondary resonance structures (right) for vicinal (upper) and geminal (lower) NBO donor-acceptor interactions.

The Cope, oxy-Cope, and anionic oxy-Cope rearrangements belong to the category of [3,3J-sigmatropic rearrangements. Since it is a concerted process, the arrow pushing here is only illustrative. Cf. Claisen rearrangement. 

The Diels-Alder reaction, inverse electronic demand Diels-Alder reaction, as well as the hetero-Diels-Alder reaction, belong to the category of [4+2]-cycloaddition reactions, which are concerted processes. The arrow pushing here is merely illustrative. 

Such a species has the charge distribution expected of a state. It allows a simple series of arrow pushes to lead to a charge-separated bicyclohexane[3.1.0] structure analogous to 32. One of the attractions of this scheme is that it is well known in ground-state chemistry that 1,2 shifts of alkyl groups are quite facile in carbonium-ion species but highly improbable in free-radical species. However, it is not apparent that such restrictions need apply to excited biradical species. 

Now we have all the tools we need. We know why we need resonance structures and what they represent. We know about what curved arrows are and where not to draw them. We know how to recognize bad arrows that violate the two commandments. We know how to draw arrow s that get you from one structure to another, and w e know how to draw in formal charges. We are now ready for the final challenge drawing curved arrows w hen we do not know what the next resonance structure looks like. Now that you know when you can and cannot push arrows, you need to practice using arrow pushing to determine by yourself how to draw the other resonance structures. 

Rule 3 (p. 87) states an acceptor A (which can be viewed as an electrophile ) then prefers the nonsubstituted carbon atom. Arrow pushing could have predicted a heterolytic cleavage ... 

All mechanisms have been examined carefully for inclusion of all steps with arrow pushing, proper reagents, and conditions. Each mechanism is clearly labeled and easily identified by a tan background and steps are numbered and annotated. 

Arrow Pushing in Organic Chemistry An Easy Approach to Understanding Reaction Mechanisms. By Daniel E. Levy... 

Scheme 1.7 Illustration of arrow pushing applied to the Cope rearrangement.

Scheme 1.8 Application of arrow pushing to homolytic cleavage using single-barbed arrows.

Scheme 1.9 Application of arrow pushing to heterolytic cleavage using double-barbed arrows.

Having presented the concept of arrow pushing in context of the steps that initiate chemical reactions, some factors impacting the flow of electrons leading from starting materials to products can now be explored. 

In this chapter, the basic principle of arrow pushing was introduced in the context of organic reactions driven by homolytic cleavage, heterolytic cleavage, or concerted mechanisms. Furthermore, the concept of polarity was introduced using heteroatoms and common organic functional groups. This discussion led to the definitions of nucleophiles and... 

---