Electron pairs movement

The terf-butyl (2-methylprop-2-yl) cation, which is the electrophile in reaction (5.2), has a vacant orbital which can accept the electron pair from the nucleophile without the need for any further movement of electrons. However, in many reactions a further electron pair movement is needed. The electrophilic proton in H30 +, HBr or H20 (reactions 5.3-5.5) can only support one covalent bond, so as the nucleophile attacks with its electron pair, the bond from the proton to the oxygen or bromine atom has to break, with the electrons from that bond forming a lone pair on the oxygen or bromine atom. 

Note a more detailed description of this mechanism using curly arrows to show electron pair movement is given in Chapter 20.)... 

The individual resonance forms are connected by donble-headed arrows and are placed within one set of square brackets. They have the characteristic property of being interconvertible by electron-pair movement only, indicated by red arrows, the nuclear positions in the molecule remaining unchanged. Note that, to turn A into B and then into C, we have to shift two electron pairs in each case. Snch movement of electrons can be depicted by curved arrows, a procedure informally called electron pnshing. ... 

The use of curved arrows to depict electron-pair movement is a nseful technique that will prevent us from making the common mistake of changing the total nnmber of electrons when we draw resonance forms. It is also advantageons in keeping track of electrons when formulating mechanisms (Sections 2-2 and 6-3). 

New bond derived from electron-pair movement... 

Identify each of the following species as either a Lewis acid or a Lewis base, and write an equation illustrating a Lewis acid-base reaction for each one. Use curved arrows to depict electron-pair movement Be sure that the product of each reaction is depicted by a complete, correct Lewis structure. 

Reexamine your answers to Problem 34. Rewrite each one in the form of a complete equation describing a Lewis acid-base process, showing the product and using curved arrows to depict electron-pair movement. [Hint For (b) and (d), start with a Lewis structure that represents a second resonance form of the starting organic molecule.]... 

The nucleophilic substitution of a haloalkane is described by either of two general equations. Recall (Section 2-2) that the curved arrows denote electron-pair movement. 

Draw representations of the hypothetical frontside and backside displacement mechanisms for the Sn2 reaction of sodium iodide with 2-bromobutane (Table 6-3). Use arrows like those shown in Figures 6-2 and 6-3 to represent electron-pair movement. 

Write out two step-by-step mechanisms for the addition of HI to cyclohexene shown above (p. 489, margin). In the first, use a free proton as the electrophile. In the second, use undissociated HI in the electrophilic addition step. Make sure to include all necessary curved arrows to depict electron-pair movement. 

Wnte an equation for the Brpnsted acid-base reaction that occurs when each of the fol lowing acids reacts with water Show all unshared electron pairs and formal charges and use curved arrows to track electron movement... 

A Lewis base donates an electron pair to a Lewis acid. We therefore need to locate the electron lone pairs on acetaldehyde and use a curved arrow to show the movement of a pair toward the H atom of the acid. 

As we saw in Section 2.11, chemists indicate the movement of an electron pair during a polar reaction by using a curved, full-headed arrow. A curved arrow shows where electrons move when reactant bonds are broken and product bonds are formed. It means that an election pair moves from the atom... 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the more common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to... 

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. 

Combinations of the Above. Note that arrows are used to show movement of electrons. An arrow always follows the motion of electrons and never of a nucleus or anything else (it is understood that the rest of the molecule follows the electrons). Ordinary arrows (double-headed) follow electron pairs, while single-headed arrows follow unpaired electrons. Double-headed arrows are also used in pericyclic reactions for convenience, though in these reactions we do not really know how or in which direction the electrons are moving. 

The two major differences are (i) the absence of any complex or ester in the second mechanism, and (i7) the direction of the movement of the electron pair of the C-H bond, the hydrogen becoming proton-like in the transition state of the Westheimer scheme but hydride-like in that of the RoCek mechanism. 

We shall, however, subsequently write canonical structures, e.g. (19a) and (19b), linked by a double-headed arrow, but without curved arrows. These will be reserved for indicating a real movement of electron pairs, i.e. as happens during the forming, and breaking, of bonds in the course of a real reaction. 

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. 

In addition to simple electron transfers in which no chemical bond is either broken or formed, numerous organic reactions, previously formulated by movements of electron pairs, are now understood as processes in which an initial electron transfer from a nucleophile (reductant) to an electrophile (oxidant) produces a radical ion pair, which leads to the final products via the follow-up steps involving cleavage and formation of chemical bonds [11-23], The follow-up steps are usually sufficiendy rapid to render the initial electron transfer the rate-determining step in an overall irreversible transformation [24], In such a case, the overall reactivity is determined by the initial electron-transfer step, which can also be well designed based on the redox potentials and the reorganization energies of a nucleophile (reductant) and an electrophile (oxidant). 

To identify Lewis acids and bases, track the movement of electron pairs. Draw a Lewis dot structure to locate the atom with a lone pair available to donate. This is the Lewis base. 

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