Double-headed curly arrows

Notice here that we use a double-headed curly arrow because it indicates the movement of a pair of electrons. The tail shows the source of the electron pair and the head indicates the destination. 

You will gain more experience of using double-headed curly arrows later. 

A double-headed curly arrow indicates the movement of a pair of electrons. 

We have now encountered a number of different types of arrow routinely used in chemistry to convey particular meanings. We have met curly arrows used in mechanisms, double-headed resonance arrows, equilibrium arrows, and the simple single arrows used for reactions. This is a convenient point to bring together the different types and provide a checklist for future reference. We are also showing how additional information about a reaction may be presented with the arrow. 

A shorthand addition-elimination mechanism sometimes encountered is also shown. This employs a double-headed curly arrow to indicate the flow of electrons to and from the carbonyl oxygen we prefer and shall use the longer two-step mechanism to emphasize the addition intermediate. 

When we made the allyl cation from allyl bromide, the bromine atom left as bromide ion taking both the electrons from the C-Br bond with it—the C-Br bond broke heterolytically. What if the bond broke homolytically—that is, carbon and bromine each had one electron A bromine atom and an allyl radical (remember a radical has an unpaired electron) would be formed, This reaction can be shown using the singleheaded fish hook curly arrows from Chapter 5 normal double-headed arrows show the movement of two electrons single-headed arrows show the movement of one. 

Electron movement is symbolized by a double-headed curly arrow for the movement of an electron pair, and a single-headed arrow or fishhook for the movement of a single electron. In representing electron movement, the arrow must start from the bond or atom that provides the electron(s) and the arrow should end where the electron movement terminates, either to form a bond or on the particular atom or group that receives the charge. Thus if the electron movement creates a bond,... 

If -M groups are introduced at the 2-, 4- and/or 6-positions, the anion can be further stabilised by delocalisation through the Jt-system, as the negative charge can be spread onto the -M group. We can use double-headed curly arrows to show this process (see Section 4.3). 

Curly arrows can be used to represent bond cleavage. A double-headed arrow represents the movement of two electrons (and is used in ionic mechanisms). A single-headed arrow (or fishhook) is used to represent the movement of a single electron (and is used in radical mechanisms). Curly arrows therefore always depict the movement of electrons. Heterolysis (use one double-headed arrow)... 

This double-headed curly arrow short-hand is explained on p. 217 of the textbook. 

Double-headed curly arrows are introdueed in Section 4.1... 

Draw a double-headed curly arrow from the nucleophilic atom to the electrophilic atom to make a new bond. The arrow can start from a negative charge, a lone pair of electrons or a multiple bond. If a new bond is made to an uncharged H, C, N or O atom in the electrophile then one of the existing bonds should be broken and a second curly arrow should be drawn (pointing in the same direction as the first). 

For polar (ionic) reactions make sure that your curly arrows are double-headed single headed arrows are used for radical reactions (Section 4.1)... 

Some molecules and ions have structures that cannot he represented by one Lewis structure (electron dot diagram) only. For example, the gas trioxygen (ozone), O3, consists of molecules that can be represented by two Lewis (electron dot) structures as shown in Figure 4.40. The double-headed arrow represents resonance and the curly arrows show movement of electron pairs. 

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