Drawing reaction mechanisms

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... 

The benzene ring is usually depicted as one of two mesomeric structures (2.1). The double arrow indicates that the true structure of the molecule lies somewhere in between the two drawn structures. It is therefore more accurate to use structure 2.2, since the six C-C bonds of the ring are identical, with the 7t-electrons delocalized over the entire ring. The configuration shown in 2.2 is, however, less convenient for drawing reaction mechanisms. 

The resonance-delocalized picture explains most of the structural properties of benzene and its derivatives—the benzenoid aromatic compounds. Because the pi bonds are delocalized over the ring, we often inscribe a circle in the hexagon rather than draw three localized double bonds. This representation helps us remember there are no localized single or double bonds, and it prevents us from trying to draw supposedly different isomers that differ only in the placement of double bonds in the ring. We often use Kekule structures in drawing reaction mechanisms, however, to show the movement of individual pairs of electrons. 

MasteringChemistry allows students to draw reaction mechanisms in a step-wise manner. Ranging in difficulty levels, the new mechanism problem types provide students with detailed, immediate feedback after each step of their mechanism or, if assigned, feedback after completion of an entire multipart mechanism as to where they made their first mistake. Professors maintain control over the grade value of each mechanistic step and can limit student attempts as well as assign a more challenging mechanistic problem for credit alone. Every individual student attempt is recorded within the grade-book and can be accessed by professors as they work with students to identify their misconceptions. 

When you first start drawing reaction mechanisms, rewrite any intermediate structure before you try to manipulate it further. This avoids confusing the arrows associated with electron flow for one step with the arrows associated with eiectron flow for a subsequent step. As you gain experience, you will not need to do this. It will also be helpful to write the Lewis structure for at least the reacting atom and to write lone pairs on atoms such as nitrogen, oxygen, halogen, phosphorus, and sulfur. 

Molecules are three-dimensional objects, and as such they have shapes. You must always keep the three-dimensional shapes of organic compounds in mind when you draw reaction mechanisms. Often something that seems reasonable in a flat... 

Common error alert When drawing reaction mechanisms involving carbocations, it is very important to obey Grossman s rule. If you don t obey Grossman s rule, you are guaranteed to lose track of which atoms have three groups attached and which have four. 

Extensive problem sets are found at the end of all chapters. The only way you will learn to draw reaction mechanisms is to work the problems If you do not work problems, you will not learn the material. The problems vary in difficulty from relatively easy to very difficult. Many of the reactions covered in the problem sets are classical organic reactions, including many name reactions. All examples are taken from the literature. Additional problems may be found in other textbooks. Ask your librarian, or consult some of the books discussed below. 

Benzene can be written with a circle within the ring to show the delocalisation of electrons (see Section 7.1). However, this does not show the six Jl-electrons, which makes drawing reaction mechanisms impossible. A single resonance form, showing the three C=C bonds, is therefore most often used. 

Benzene has six re-electrons, which are delocalised around the ring, and a circle in the centre of a 6-membered ring can represent this. However, it is generally shown as a ring with three C=C bonds, because it is easier to draw reaction mechanisms using this representation. 

Some unusual organizational decisions have been made, too. SrnI reactions and carbene reactions are treated in the chapter on polar reactions under basic conditions. Most books on mechanism discuss SrnI reactions at the same time as other free-radical reactions, and carbenes are usually discussed at the same time as carbocations, to which they bear some similarities. I decided to place these reactions in the chapter on polar reactions under basic conditions because of the book s emphasis on teaching practical methods for drawing reaction mechanisms. Students cannot be expected to look at a reaction and know immediately that its mechanism involves an electron-deficient intermediate. Rather, the mech-... 

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