Electron movement, curved arrows

We keep track of electron pairs by noting changes in their location in molecules by means of curved arrows. The curved arrow depicts movement of an electron... 

Remember that curved arrows show movement of electrons, always from the nucleophile (electron donor) to the electrophile (electron acceptor). For example, protonation of a double bond must show the arrow going from the electrons of the double bond to the proton—never from the proton to the double bond. Resist the urge to use an arrow to point out where the proton (or other reagent) goes. 

Where appropriate, solved problems (highlighted by a beige background) are provided to show how you might approach a particular type of problem and what kind of answer is expected. For example, a solved problem might work through a mechanism to show how it is broken down into individual steps and how red curved arrows show movement of electrons. 

Figure 1.27. (a) Double-headed arrow, used for resonance forms only. The material A on the left has a different-type font than that on the right. They are different representations of the same letter and both are correct, (b) Two arrows, generally used to indicate equilibriinn. These two arrows are not to be used for resonance. A and B are different from each other, (c) Curved arrows, indicating movement of an electron pair. A pair of electrons originating on A is used to form a bond from A to B. A new compound is formed ( A-B ), and A has become poorer in electrons (they were used to make the bond) and B has become richer. 

Figure 1.28. Some resonance forms for the carbonate anion (COs ) (compare with Figure 1.26) connected by resonance arrows and with curved arrows indicating movement of pairs of electrons showing a path from one resonance form to another.

Figure 10.8 (a) Electrophilic addition reaction, (b) Nucleophilic addition reaction. Curved arrows indicate movement of electrons. (Nu represents a nucleophile.)... 

In every case the curved arrows start from either a lone electron pair on an atom or the center of a bond. Curved arrows never start at electron-deficient atoms, such as H (last equation) The movement of a proton is depicted by an arrow pointing from an electron source (lone pair or bond) toward the proton. Although this may seem counterintuitive at first, it is a very important aspect of the cnrved-arrow formalism. Curved arrows represent movement of electrons, not atoms. 

In Summary Curved arrows depict movement of electron pairs in reaction mechanisms. Electrons move from nucleophilic, or Lewis basic, atoms toward electrophilic, or Lewis acidic, sites. If a pair of electrons approaches an atom already containing a closed shell, a pair of electrons must depart from that atom so as not to exceed the maximum capacity of... 

Resist the temptation to use curved arrows to show the movement of atoms Not only is this con trary to general practice but it is also less reasonable Electrons are much more mobile than atoms so it makes sense to focus on them... 

Use curved arrows to track electron movement and identify the acid base con jugate acid and conjugate base... 

Potassium hydride (KH) is a source of the strongly basic hydride ion ( H ) Using curved arrows to track electron movement write an equation for the reaction of hydride ion with water What is the conjugate acid of hydride lon ... 

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 curved arrow shown as a single barbed fishhook signifies the movement of one electron Normal curved arrows track the movement of a pair of electrons... 

Robinson won the 1947 Nobel Prize in chemistry for his studies of natural products. He may also have been the first to use curved arrows to track electron movement. 

For each reaction, plot energy (vertical axis) vs. the number of the structure in the overall sequence (horizontal axis). Do reactions that share the same mechanistic label also share similar reaction energy diagrams How many barriers separate the reactants and products in an Sn2 reaction In an SnI reaction Based on your observations, draw a step-by-step mechanism for each reaction using curved arrows () to show electron movements. The drawing for each step should show the reactants and products for that step and curved arrows needed for that step only. Do not draw transition states, and do not combine arrows for different steps. 

Write a detailed mechanism for this condensation using only the molecules whose models are provided. Treat all proton transfers, nucleophilic additions, and elimination reactions as separate steps, and use curved arrows to show electron movement. Which of these steps do you think will be favorable Unfavorable Why ... 

Resonance forms differ only in the placement of their tt or nonbonding electrons. Neither the position nor the hybridization of any atom changes from one resonance form to another. In the acetate ion, for example, the carbon atom is sp2-hybridized and the oxygen atoms remain in exactly the same place in both resonance forms. Only the positions of the r electrons in the C=0 bond and the lone-pair electrons on oxygen differ from one form to another. This movement of electrons from one resonance structure to another can be indicated by using curved arrows. A curved arrow always indicates the movement of electrons, not the movement of atoms. An arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow. 

Active Figure 2.5 The reaction of boron trifluoride, a Lewis acid, with dimethyl ether, a Lewis base. The Lewis acid accepts a pair of electrons, and the Lewis base donates a pair of nonbonding electrons. Note how the movement of electrons from the Lewis base to the Lewis acid is indicated by a curved arrow. Note also how, in electrostatic potential maps, the boron becomes more negative (red) after reaction because it has gained electrons and the oxygen atom becomes more positive (blue) because it has donated electrons. Sign in atwww. thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

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... 

A Add curved arrows to the mechanism shown in Problem 5.34 to indicate the electron movement in each step. 

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. 

Dichlorocarbene can be generated by heating sodium trichloroacetate. Propose a mechanism for the reaction, and use curved arrows to indicate the movement of electrons in each step. What relationship does your mechanism bear to the base-induced elimination of HC1 from chloroform ... 

Treatment of 4-penten-l-ol with aqueous Br2 yields a cyclic bromo ether rather than the expected bromohydiin. Suggest a mechanism, using curved arrows to show electron movement. 

Electron movement, curved arrows and, 44-45, 57-58 Electron shell, 5 Electron-dot structure, 9 Electron-transport chain, 1127 Electronegativity, 36... 

The resonance interaction of chlorine with the benzene ring can be represented as shown in 13 or 14, and both of these representations have been used in the literature to save space. However, we shall not use the curved-arrow method of 13 since arrows will be used in this book to express the actual movement of electrons in reactions. We will use representations like 14 or else write out the canonical forms. The convention used in dashed-line formulas like 14 is that bonds that are present in all canonical forms are drawn as solid lines, while bonds that are not present in all forms are drawn as dashed lines. In most resonance, a bonds are not involved, and only the n or unshared electrons are put in, in different ways. This means that if we write one canonical form for a molecule, we can then write the others by merely moving n and unshared electrons. 

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