Arrows half-headed

Double-headed arrow Full-headed curved arrow Half-headed curved arrow (fishhook)... 

Two half-headed arrows are used for the equilihrium reactions. A longer arrow indicates that the equilihrium favours the formation of acetate ion (CH3CO2 ) and ammonium ion ( NH4). Because acetic acid (CH3CO2H) is a stronger acid than ammonium ion ( NH4), the equilihrium lies towards the formation of weak acid and weak base. 

For reactions of radicals (homolytic reactions) arrows are used to indicate motion of single electrons rather than of electron pairs. It is desirable to use arrows with half-heads. For example, reaction of a superoxide radical... 

Half-heads on arrows indicate one-electron movement... 

Fig. 1.11. Radical initiators and their mode of action (in the "arrow formalism" for showing reaction mechanisms used in organic chemistry, arrows with half-heads show where single electrons are shifted, whereas arrows with full heads show where electron pairs are shifted).

The peroxy radical RO" abstracts H from the HBr to give ROH, leaving behind a new radical Br. We have described this process using arrows with half-heads (also known as fish-hook arrows ). 

There is often more than one correct way of drawing a radical mechanism using half-headed arrows. For... 

Many kinds and combinations of arrows can be used. For example, two full arrows in opposite directions ( ) indicate a reaction that is proceeding in both directions. Two arrows with half heads in opposite directions ( ) indicate a reaction in equilibrium. A single arrow with heads on both sides (<- ) indicates resonance structures, not a reaction. 

To illustrate the movement of a single electron, use a half-headed curved arrow, sometimes called a fishhook. 

Two half-headed curved arrows are needed for two single electrons. 

Sample Problem 6.1 Use full-headed or half-headed curved arrows to show the movement of electrons in each equation. 

A radical contains an atom that does not have an octet of electrons, making it reactive and unstable. Radical processes involve single electrons, so half-headed arrows are used to show the movement of electrons. One half-headed arrow is used for each electron. 

A curved half-headed arrow indicates the movement of a single electron in the direction of the arrowhead... 

Note that the arrows used to show the flow of unpaired electrons (radicals) have only a half head. Also, the intermediate in the reaction is a diradical. (Radicals are discussed in more detail in Chapter 5.) Rotation about the highlighted single bond takes place fast enough that the stereochemistry of the starting olefin is lost. 

In chemical structures, the unpaired electron of a radical is represented by a dot. Radical mechanisms are depicted in one of two ways. Most commonly, each individual step of the mechanism is written without the use of arrows to show electron movement. The resulting series of equations shows the order of events, and it is assumed that one-electron transfers are taking place throughout. A second method uses curved, half-headed arrows ( - ) to show electron movement. The half-headed arrow is used to denote movement of a single... 

Half-headed arrows (fishhooks) are used to show the movement of single electrons. 

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... 

A full-headed curved arrow indicates the movement of two electrons from the tail of the arrow to the head. A half-headed curved arrow indicates the shift of one electron likewise. The two ways that a bond can break are heterolytic (two electrons) or homolytic (one electron). Homolytic processes are unusual and will be treated separately in Chapter 11. 

Since almost all stable species are even electron and spin paired (molecular oxygen is the obvious exception), radical processes must start either by single electron transfer or by homolytic cleavage. It is important to recognize these initiation steps in order to know when to use the one-electron paths from this chapter rather than the two-electron paths described previously. Half-headed arrows are used to symbolize the movement of one electron. Homolytic cleavage is the simple extension of a bond-stretching vibration (Fig. 11.4). The process is always endothermic, for the barrier must be at least equal to the strength of the bond cleaved. 

---