Mechanisms and Arrow Pushing

11 Each of the following compounds exhibits two electrophilic centers. Identify both centers in each compound. (Hint You will need to draw resonance structures in each case.) 

Recall that we also used a handful of patterns for arrow pushing to master resonance stmctures in Chapter 2. 

Recall from Chapter 3 that a mechanism shows how a reaction takes place using curved arrows to illustrate the flow of electrons. The tail of every curved arrow shows where the electrons are coming from, and the head of every curved arrow shows where the electrons are going  

In order to master ionic mechanisms, it will be helpful to become familiar with characteristic patterns for arrow pushing. We will now learn patterns of electron flow, and these patterns will empower us to understand reaction mechanisms and even propose new mechanisms. There are only four characteristic patterns, and all ionic mechanisms are simply combinations of these four steps. Let s go through them one by one. 

The first pattern is characterized by a nucleophile attacking an electrophile for example  

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Mechanisms and Arrow Pushing (section 6.8) Nucleophilic Attack (section 6.8)... 

Reversing the direction of one or more arrows during a chemical step is the most common mistake made by students when writing organic reaction mechanisms. Backward arrow pushing usually derives from a student thinking about the movement of atoms, not the movement of electrons. Hence, to avoid this mistake it is important to remember that arrows depict how electrons move, not where atoms move, within or between chemical structures. Further, one can avoid this mistake by remembering that every arrow must start at an electron source (a bond or lone pair) and terminate at an electron sink (an atom that can accept a new bond or lone pair). 

Referring to Sections 7-6 and 9-3, write a mechanism for the preceding reaction. (Caution As emphasized repeatedly, when writing mechanisms, use arrow pushing to depict electron flow write out every step separately formulate complete structures, including charges and relevant electron pairs and draw explicit reaction arrows to connect starting materials or intermediates with their respective products. Don t use shortcuts, and don t be sloppy )... 

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... 

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. 

We begin by bringing you up to speed on mechanisms and reminding you how to push electrons around with those curved arrows. We jog your memory with a discussion of substitution and elimination reactions and their mechanisms, in addition to free radical reactions. Next you review the structure, nomenclature, synthesis, and reactions of alcohols and ethers, and then you get to tackle conjugated unsaturated systems. Finally, we remind you of spectroscopic techniques, from the IR fingerprints to NMR shifts. The review in this part moves at a pretty fast pace, but we re sure you can keep up. 

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. 

Figure 2.7 Diffusion of atoms or ions in crystalline solids can occur by at least three possible mechanisms illustrated here. In the vacancy mechanism (bottom arrow), an atom in a lattice site jumps to an adjacent vacant lattice site. In the interstitial mechanism (middle arrow), an interstitial atom jumps to an adjacent vacant interstitial site. In the intersitialcy mechanism (top two arrows), an interstitial atom pushes an atom residing in a lattice site into an adjacent vacant interstitial site and occupies the displaced atom s former site. (After Lalena and Cleary, 2005. Copyright John Wiley Sons, Inc. Reproduced with permission.)...

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... 

This is an example of an aldol condensation between an acetone anion and acetaldehyde. Note the mechanism proceeds through addition of an anion to an aldehyde carbonyl. Arrow pushing is illustrated below ... 

Additionally, as mentioned in Chapter 1, concerted reaction mechanisms can be described using arrow pushing. As illustrated below, both the starting bromide and one of the trienes can undergo Cope rearrangements to form new products. While these reactions are not within the scope of this book, it is important to recognize these reactions. For more detailed information, readers are referred to their introductory organic chemistry textbooks. 

This is an addition -elimination reaction involving addition of methylamine to the acid chloride and elimination of hydrochloric acid. The mechanism is illustrated below using arrow pushing. 

Next, the carboxylate anion participates in an addition-elimination reaction with isobutyl chloroformate. Elimination of a chloride anion results in formation of intermediate A. These reactions are generally facilitated by the introduction of an amine base such as triethylamine (not shown in this problem). The mechanism is illustrated below using arrow pushing, and the illustrated product belongs to a class of compounds known as mixed carbonic anhydrides. 

This is an example of a Diels-Alder reaction. This is an electrocyclic reaction where no charges are involved. While no charges are involved, electron pairs do move and their movement can be illustrated using arrow pushing. The mechanism, illustrated below, involves aligning cyclopentadiene (a diene) with methyl vinyl ketone (a dienophile) such that all three double bonds define a six-membered... 

The ene reaction is an electrocyclic reaction similar to the Diels-Alder reaction and the Claisen rearrangement. In this reaction, a hydrogen atom is participating in the electrocyclic process. The mechanism, illustrated below using arrow pushing,... 

In this book, there have been many references to oxidation and reduction reactions. While these reactions are not within the scope of the discussions of this book, their mechanisms do involve the processes presented herein. In the case of the Swem oxidation, the first step is an addition-elimination reaction between dimethyl sulfoxide and oxallyl chloride. This process, illustrated below using arrow pushing, involves addition of the sulfoxide oxygen to a carbonyl with subsequent elimination of a chloride anion. 

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