Curved arrows pushing

TUTORIAL An Exercise in Drawing Curved Arrows Pushing Electrons 225... 

Now we know what curved arrows are, but how do we know when to push them and where to push them First, we need to learn where we cannot push arrows. There are two important rules that you should never violate when pushing arrows. They are the two commandments of drawing resonance structures ... 

PROBLEMS For each problem below, draw the intermediate that would result from pushing the curved arrows as shown. 

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. 

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. 

Chemical reactions entail the breaking and forming of covalent bonds. The flow of electrons in the course of a reaction can be depicted by curved arrows, a method of representation called "arrow pushing." Each arrow represents an electron... 

Full-Headed Curved Arrow Moves Two Electrons Half-Headed Curved Arrow Moves One Electron Electron Source Electron Sink Charge Is Conserved Direction of Electron Flow Good Arrow Pushing Habits Common Errors... 

Berthelot and colleagues have estimated the pATes values of the phenohc OH group in the intramolecular hydrogen-bonded systems 1, 2 and 3. The higher basicity pATes of 1 and 2 compared to phenol (—0.07) can be explained by cooperative effects involved in hydrogen-bond formation the oxygen electron pairs are more basic in OH- B than in the free OH group . The push-puU effect shown by the curved arrows in 3 opposes the cooperativity effect and pATes falls. 

Chemical reaction mechanisms, which trace the formation and breakage of covalent bonds, are communicated with dots and curved arrows, a convention known informally as electron pushing. ... 

The use of curved arrows in this way is commonly referred to as electron pushing. 

HOWTO Draw Curved Arrows and Push Electrons in Creating Contributing Structures... 

Curved arrows show the marmer in which valence electrons are redistributed from one contributing structure to the next. Use of curved arrows in this way is commonly referred to as electron pushing. Curved arrows always show movement of electron pairs, never atoms. 

Earlier we used curved arrows to show the flow of electrons that takes place in the process of bond breaking and bond forming in the Diels-Alder reaction. As discussed, these reactions involve a four-carbon diene and a two-carbon dienophile and are termed [4 + 2] cycloadditions. We can write similar electron-pushing mechanisms for the dimerization of ethylene by a [2 + 2] cycloaddition to form cyclobutane and for the dimerization of butadiene by a [4 + 4] cycloaddition to form 1,5-cyclooctadiene. 

In this example, there are two curved arrows. The first arrow pushes one of oxygen s lone pairs to form a bond, and the second arrow pushes the it bond to form a lone pair on a carbon atom. When both arrows are pushed at the same time, neither of the rules is violated. So, let s focus on how to draw the resonance structure by following the instructions provided by the curved arrows. We delete one lone pair from the oxygen and place a Tt bond between the carbon and oxygen. Then we must delete the C—C 7t bond and place a lone pair on the carbon ... 

This perspective is perhaps more accurate, but we must keep in mind that both curved arrows are showing just one arrow-pushing pattern nucleophilic attack. 

Regardless of how we view the process, it is important to recognize that all of these curved arrows together show only one arrow-pushing pattern loss of a leaving group. 

The third pattern for arrow pushing has already been discussed in detail in Chapter 3. Recall that a proton transfer is characterized by two curved arrows ... 

Make sure that all the curved arrows you draw accompUsh one of the four characteristic arrow-pushing patterns. 

We begin by identifying which characteristic arrow-pushing pattern to use in this case. Look carefully, and notice that H3O+ is losing a proton. That proton has been transferred to a carbon atom. Therefore, this is a proton transfer step, where the it bond is functioning as the base to deprotonate H3O+. A proton transfer step requires two curved arrows. Make sure to properly place the head and the tail of each curved arrow. The first cunred arrow must originate on the it bond and end on a proton ... 

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