Writing Mechanisms Using Curly Arrows

To generate a tautomer, remove a proton to give an anion, write a resonance form of the anion, and then replace the proton. 

The proportions of particular tautomers at equilibrium depend on the relative stability of the various structures, but a low proportion of a particular tautomer does not rule out its being important in the reactivity of the molecule. 

We first encountered the curly (or curved) arrow when we looked at resonance forms, as a method to show the movement of electrons around the molecule to give new resonance forms. I have sneaked a few reaction arrows in since, but we now need to look at these more formally. The curly arrow, 8.16, means the movement of two electrons from one end of the arrow to the other, and it is very important to draw your arrows carefully and accurately. This is one of the times that the human hand is better than any computer drawing program—computers have a clear view of what curves are allowed and what is not—but the hand has infinite flexibility. Your arrows will start where there is a pair of electrons, on a species that we describe as a nucleophile—a nucleus seeker. Nucleophiles are species that seek either a positive charge or a region of electron deficiency. In principle, any pair of electrons can be used as a nucleophile, but some work much better than others. The arrow should end where a new bond is made or on an atom to which you are giving a pair of electrons. 

Draw mechanisms for the protonation of HjS, CH3OH, PhjP, and diethyl ether. 

All of these molecules have a lone pair of electrons on the heteroatom, and this is what is used to make the new bond to hydrogen (shown in red). 

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Once you have done that, you are well on the way to writing a reasonable mechanism using curly arrows. We ll take as an example the reaction of triphenylphosphine with methyl iodide. 

Note that if we choose not to put in all the curly arrows, we could write the mechanism in two ways either considering the radical as the attacking species or the double bond as the electron-rich species. The first version is perhaps more commonly used, but it is much more instmctive to compare the second one with an electrophilic addition mechanism (see Section 8.1). The rationalization for the regiochemistry of addition parallels that of carbocation stability (see Section 8.2). 

This representation does present a slight problem to modern organic chemists, however it is not possible to draw mechanisms using the delocalized representation of benzene. The curly arrows we use represent two electrons. This means that in order to write sensible mechanisms we still draw benzene as though the double bonds were localized. Keep in mind though that these double bonds are not really localized and It does not matter which way round we draw them. 

The alternative drawing on the left shows the n system as a ring and does not put in the double bonds you may feel that this is a more accurate representation, but it does present a problem when it comes to writing mechanisms. As you saw in Chapter 5, the curly arrows we use represent two electrons. The circle here represents six electrons, so in order to write reasonable mechanisms we still need to draw benzene as though the double bonds were localized. However, when you do so, you must keep in mind that the electrons are delocalized, and it does not matter which of the two arrangements of double bonds you draw. 

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