Single electrons, movement

In reaction (1.10), the electron pairs are shown flowing in the clockwise direction. They could equally well have been shown as flowing in the anticlockwise direction or as single electron movements. See Problem 1 4... 

Butadiene dimerizes to 4-vinylcyclohexene (4-ethenylcyclohexene, 1) (reaction 7.1). The absence of intermediates suggests a cyclic movement of three electron pairs, (which could equally well have been written in the opposite direction, or as single electron movements). The transition state would involve partial bond-making and breaking in the six-membered transition state as shown. Reactions involving such cyclic transition states are known as pericyclie reactions. However, ethene does not dimerize to cyclobutane (reaction 7.2) under thermal conditions, even though a cyclic movement of two pairs of electrons could have been invoked. 

For single-electron movement, the arrowtail is drawn from (a) a bond made up of two electrons, or (b) from a center having a single valence electron occupying a molecular orbital (e.g radical center, triplet carbene center). 

For single-electron movement, the arrowhead is drawn toward another center with the arrowhead being a half-arrowhead. 

We use single-barbed arrows to depict mechanisms involving single electron movements (see Section 3.1A). 

A regular arrow (double-sided arrowhead) is used to indicate the movement of two electrons, while a line with a single-sided arrowhead (sometimes called a"fish hook arrow") is used for single electron movement involved with radical reactions that are first described in Chapter 8. 

Historically, ethylene potymerization was carried out at high pressure (1000-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as benzoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved half-arrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

The observation of these linear relations is of interest since the energy changes of the initial states are obtained from a process involving displacements of atoms which are effectively equal to two-electron changes. On the other hand, electron transfer involves the movement of a single electron without atom transfer. There is no a priori reason therefore for the correlation. Observation of the correlations also... 

Notice the use of fish-hook arrows to show the movement of a single electron. Such arrows are used a great deal when writing mechanisms for reactions involving radical species. 

A single-headed curly arrow or fishhook arrow indicates the movement of a single electron. 

Here, we have used single-headed curly arrows or fishhook arrows to indicate. the movement of single electrons. The tail of the curly arrow shows the source of the electron and the head shows its destination. Using single-headed curly arrows, the mechanism for the methane/chlorine chain reaction is completed below. 

A covalent bond consists of a shared pair of electrons. Nonbonded electrons important to the reaction mechanism are designated by dots (— OH). Curved arrows (<- ) represent the movement of electron pairs. For movement of a single electron (as in a free radical reaction), a single-headed (fishhook-type) arrow is used ( ). Most reaction steps involve an unshared electron pair (as in the chymotrypsin mechanism). 

Radical reactions involve the correlated movement of single electrons. In a unimolecular reaction like the photolytic cleavage of chlorine, one electron moves to one atom and the other electron moves to the other. In bimolecular... 

Sometimes reactions take place that involve the movement of single electrons rather than pairs of electrons. Such reactions are called radical reactions. For example, a chlorine molecule can be split into two chlorine radicals on treatment with light. One of the original bonding electrons ends up on one chlorine radical and the second bonding electrons ends up on the other chlorine radical. The movement of these single electrons can be illustrated by using half curry arrows rather than full curly arrows ... 

Most chemists still tend to think about the structure and reactivity of atomic and molecular species in qualitative terms that are related to electron pairs and to unpaired electrons. Concepts utilizing these terms such as, for example, the Lewis theory of valence, have had and still have a considerable impact on many areas of chemistry. They are particularly useful when it is necessary to highlight the qualitative similarities between the structure and reactivity of molecules containing identical functional groups, or within a homologous series. Many organic chemistry textbooks continue to use full and half-arrows to indicate the supposed movement of electron pairs or single electrons in the description of reaction mechanisms. Such concepts are closely related to classical valence-bond (VB) theory which, however, is unable to compete with advanced molecular orbital (MO) approaches in the accurate calculation of the quantitative features of the potential surface associated with a chemical reaction. 

Note that an arrow with only half of an arrowhead is used to show the movement of a single electron that occurs in radical reactions, whereas the normal arrow shows the movement of a pair of electrons. 

This is an example of the homolytic cleavage of a bromine molecule to form two bromide radicals. Note the use of single-barbed arrows to describe radical-based mechanisms resulting in the movement of single electrons. For clarity, the bond is elongated. Arrow pushing is illustrated below ... 

Notice the fishhook-shaped half-arrows used to show the movement of single unpaired electrons. Just as we use curved arrows to represent the movement of electron pairs, we use these curved half-arrows to represent the movement of single electrons. These halfarrows show that the two electrons in the Cl—Cl bond separate, and one leaves with each chlorine atom. 

Occupied Molecular Orbital (SOMO), for obvious reasons. The shape of this orbital tells us that the single electron is located on the end carbon atoms. This can also be shown using delocalization arrows (again single-headed arrows to show movement of one electron). 

They indicate the movement of single electrons among orbitals, by analogy with our normal curly arrows, which indicate the movement of electron pairs. 

In a free radical step, each atom contributes one electron to the bond (3 the single-headed arrows represent the movement of a single electron). At least one of the reactants or products must contain an unpaired electron. 

Fishhook arrows are used to show movement of single electrons. 

A concertE d reaction is oub which occurs in a single step, without any intermediates. The electron movements take place simultaneously ... 

Let us illustrate this important claim with a simple model showing how the reactivity pattern of the substrate, toluene 6 (Scheme 2.3), can be controlled by variations in reaction partners and external conditions. There are two reactions of 6 that proceed with the same stoichiometry (equations 1 and 2) but yield isomeric products benzyl bromide 7 and p-bromotoluene 8. Under the appropriate conditions it is possible to carry out each reaction selectively with almost complete exclusion of the alternate process. In order to understand how this can be accomplished, it is necessary to analyse the mechanisms of these conversions. The concise description of reaction mechanisms requires the use of special symbols such as the curved pronged or half-pronged arrows shown below. These arrows indicate the movement of an electron pair or a single electron, respectively, within the dynamics of a reaction process. 

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,... 

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