Drawing Curved Arrows

Drawing the resonance stmcture of the following compound requires one curved arrow. The head of this curved arrow is placed on the oxygen atom, and the tail of the curved arrow can only be placed in one location without violating the rules for drawing curved arrows. Draw this curved arrow. 

Using one curved arrow, draw the carbocation rearrangement. 

STEP 2 Using one curved arrow, draw a carbocation rearrangement that forms a more stable carbocation, via either a hydride shift or a methyl shift. 

Every step has two curved arrows. Draw them precisely. 

Following the curved arrows, draw a resonance structure for the HCONH ion. 

Thalidomide is converted to the drug CC-4047 by substitution of an amino group for one of the H atoms on the aromatic portion of the molecule. Using curved arrows, draw the mechanism for this reaction and all the resonance structures for the carbocation intermediate. [144 Sample Problem 10.7]... 

Use curved arrows to show the bonding changes in the reaction of CIS 4 tert butylcyclohexyl bromide with potassium tert butoxide Be sure your drawing correctly represents the spatial relationship between the leaving group and the proton that is lost... 

For each reaction, plot energy (vertical axis) vs. the number of the structure in the overall sequence (horizontal axis). Do reactions that share the same mechanistic label also share similar reaction energy diagrams How many barriers separate the reactants and products in an Sn2 reaction In an SnI reaction Based on your observations, draw a step-by-step mechanism for each reaction using curved arrows () to show electron movements. The drawing for each step should show the reactants and products for that step and curved arrows needed for that step only. Do not draw transition states, and do not combine arrows for different steps. 

Worked Example 5.2 gives another example of drawing curved arrows. 

First, look at the reaction and identify the bonding changes that have occurred. In this case, a C—Br bond has broken and a C-C bond has formed. The formation of the C-C bond involves donation of an electron pair from the nucleophilic carbon atom of the reactant on the left to the electrophilic carbon atom ol CH Br, so we draw a curved arrow originating from the lone pair on the negatively charged C atom and pointing to the C atom of CH3Br. At the same time the C—C bond forms, the C-Br bond must break so that the octet rule is not violated. We therefore draw a second curved arrow from the C-Br bond to Br. The bromine is now a stable Br- ion. 

A. Ammonia reacts with acetyl chloride (CH3COCI) to give acetamide (CH3CONII2). Identify the bonds broken and formed in each step of the reaction, and draw curved arrows to represent the flow of electrons in each step. 

CURVED ARROWS THE TOOLS FOR DRAWING RESONANCE STRUCTURES ... 

In the beginning of the course, you might encounter problems like this here is a drawing now draw the other resonance structures. But later on in the course, it will be assumed and expected that you can draw all of the resonance structures of a compound. If you cannot actually do this, you will be in big trouble later on in the course. So how do you draw all of the resonance structures of a compound To do this, you need to leam the tools that help you curved arrows. 

Here is where it can be confusing as to what is exactly going on. These arrows do not represent an actual process (such as electrons moving). This is an important point, because you will leam later about curved arrows used in drawing reaction mechanisms. Those arrows look exactly the same, but they actually do refer to the flow of electron density. In contrast, curved arrows here are used only as tools to help... 

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 ... 

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 ... 

Now we have all the tools we need. We know why we need resonance structures and what they represent. We know what curved arrows represent. We know how to recognize bad arrows that violate the two commandments. We know how to draw arrows that get you from one structure to another, and we know how to draw formal charges. We are now ready for the final challenge using curved arrows to draw resonance structures. 

Now that we have some of the ground rules down, let s just have a quick review of curved arrows, and the different types of arrows that you can draw. Every curved arrow has a head and a tail. It is essential that the head and tail of every arrow be drawn in precisely the proper place. The tail shows where the electrons are coming from, and the head shows where the electrons are going ... 

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

The overall result is the addition of H and X across the double bond. We have specifically used a starting alkene that avoids issues of regiochemistry or stereochemistry we will soon see other examples in which we must explore both of those issues. For now, focus on the curved arrows used in both steps. It is absolutely critical to master the art of drawing curved arrows properly. Let s quickly practice ... 

One curved arrow is drawn coming from the alkene and pointing to the proton (take special notice of this arrow, as it is a very common mistake to draw this arrow in the wrong direction). The second curved arrow is drawn coming from the H—Cl bond and pointing to Cl. 

Notice that there are three curved arrows here. For some reason, students drawing this mechanism commonly forget to draw the third curved arrow (the one that shows the expulsion of Br ). The product of this hrst step is a bridged, positively charged intermediate, called a bromonium ion ( onium because there is a positive charge). In the second step of our mechanism, the bromonium ion gets attacked by Br (formed in the hrst step) ... 

C21-0030. The reaction between CO2 and H2 O to form carbonic acid (H2 CO3) can be described in two steps formation of a Lewis acid-base adduct followed by Brcjmsted proton transfer. Draw Lewis structures illustrating these two steps, showing electron and proton movement by curved arrows. 

In the preceding mechanism, the carbocation was an intermediate (a species that exists for a short time during the reaction). The form of the intermediate is often essential to understanding the mechanism. The curved arrows help you in drawing the intermediate. Because you can use curved arrows in only three ways (bond to lone pair, bond to bond, and lone pair to bond), you have limited options for drawing intermediates. 

The reaction of PC14+ with Cl- is a Lewis acid-base reaction. Draw electron-dot structures for the reactants and products, and use the curved arrow notation (Section 15.16) to represent the donation of a lone pair of electrons from the Lewis base to the Lewis acid. 

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