Arrow, electron movement and

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

Vague arrows Some mechanisms have arrows going in all sorts of directions. Arrows must flow from start to finish they should not veer off in different directions. Many of the arrows do not represent electron movements, and it would appear that, as a last resort, students have tried to memorize the mechanism rather than rationalizing it. This is both dangerous and really rather unnecessary. The logical approach gets the right answer, requires relatively little effort, and cuts out the need to learn the mechanism. Mechanisms should not be learnt they should be deduced. 

Most undergraduate texts have a short section on curved-arrow notation or electron movement, and these discussions are tied in with the development of reaction mechanisms. 

Now we shall discuss a generalized example of a neutral nucleophile, Nu, with a lone pair donating its electrons to a cationic electrophile, E, with an empty orbital. Notice the difference between the curly arrow for electron movement and the straight reaction arrow. Notice also that the nucleophile has given away electrons so it has become positively charged and that the electrophile has accepted electrons so it has become neutral. 

PROBLEM 4.6 Write an equation for the reaction of ammonia ( NH3) with hydrogen chloride (HCI). Use curved arrows to track electron movement, and identify the acid, base, conjugate acid, and conjugate base. 

Be sure to use the right types of arrows — double arrows for equilibrium, single arrows for reactions, curved arrows or curved half arrows for electron movement, and so on. Also, don t try to combine too many mechanistic steps, especially on an exam. Take it one step at a time and your results will be clearer and easier to grade (and this is a very good thing). Keep in mind, though, that you may not be asked for the mechanism on an exam, just the reaction. In that case, only write the reaction. You can get yourself into trouble by volunteering extra information. 

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... 

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 ... 

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

Further symbols are used to indieate reaction mechanisms, in particular the use of curly arrows (to represent the movement of pairs of electrons) and fish-hooks (to represent the movement of single eleetrons). Students need to understand the precise meaning of these arrows (whieh electrons move, and where from and where to) to appreeiate how they represent stages in reaetion mechanisms. Students who have been taught the formalism are not neeessarily able to identify the outcome of... 

Figure 7-6. Mechanism for catalysis by an aspartic protease such as HIV protease. Curved arrows Indicate directions of electron movement. Aspartate X acts as a base to activate a water molecule by abstracting a proton. The activated water molecule attacks the peptide bond, forming a transient tetrahedral Intermediate. Aspartate Y acts as an acid to facilitate breakdown of the tetrahedral intermediate and release of the split products by donating a proton to the newly formed amino group. Subsequent shuttling of the proton on Asp X to Asp Y restores the protease to its initial state.

Figure 13.12 The protonmotive Q cycle. Electron transfer reactions are numbered and circled. Dashed arrows designate movement of ubiquinol or ubiquinone between centres N and P and of the ISP between cytochrome b and cytochrome c,. Solid black bars indicate sites of inhibition by antimycin, UHDTB and stigmatellin. (From Hunte et al., 2003. Copyright 2003, with permission from Elsevier.)...

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. 

It makes good sense to draw free-radical mechanisms in the manner shown by these examples. However, shorter versions may be encountered in which not all of the arrows are actually drawn. These versions bear considerable similarity to two-electron curly arrow mechanisms, in that a fishhook arrow is shown attacking an atom, and a second fishhook arrow is then shown leaving this atom. The other electron movement is not shown, but is implicit. This type of representation is quite clear if the complement of electrons around a particular atom is counted each time but, if in any doubt, use all the necessary fishhook arrows. 

Arrows from protons Ask yourself how many electrons are there in a proton We trust the answer is none, and you will thus realize that arrows representing movement of electrons can never ever start from a proton. It seems that this mistake is usually made because, if one thinks of protonation as addition of a proton, it is tempting to show the proton being put on via an arrow. With curly arrows, we must always think in terms of electrons. 

Thus, abstraction of a hydrogen atom from HBr generates a bromine radical. Note that, for convenience, we tend not to put in all of the electron movement arrows. This simplifies the representation, but is more prone to errors if we do not count electrons. Our attacking radical has an unpaired electron, and it abstracts the proton plus one of the electrons comprising the H-Br a bond, i.e. a hydrogen atom, and the... 

Schemes are used to depict a series of steps that progress in time. (Note that schemes differ from charts, which list groups of compounds or structures that do not change in time.) Most commonly, schemes are used to illustrate chemical reactions. In such cases, schemes often include arrows (e.g., to denote a forward reaction, resonance, equilibrium, and/or electron movement), intermediates, transition states, reactants, and products. Schemes are numbered in order of appearance in the text (Scheme 1, Scheme 2, etc.). As with tables and figures, the scheme is mentioned in the text before the scheme is encountered. Schemes are perhaps most common in Discussion sections of journal articles (e.g., to illustrate proposed mechanisms) but can appear most anywhere in journal articles, posters, and proposals.

---