Arrow formalism curved arrows

Arrow formalism, curved arrow formalism, or electron pushing (Section 1.4) A mapping device for chemical reactions. The electron pairs (lone pairs or bond pairs) are pushed using curved arrows that show the bonds that are forming and breaking in the reaction. 

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. 

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... 

Write an equation for the Lewis acid-Lewis base reaction between boron trifluoride and dimethyl sulfide [(0)3)25]. Use curved arrows to track the flow of electrons and show formal charges if present. 

The curved arrows show how one resonance structure relates to another. Notice that the formal negative charge is located on the ortho and para positions, exactly where reaction takes place most quickly. Other ortho- and para-directing groups include —NH2, —Cl, and —Br. All have an atom with a lone pair of electrons next to the ring, and all accelerate reaction. 

Now we have all the tools we need. We know why we need resonance structures and what they represent. We know what curved arrows represent. We know how to recognize bad arrows that violate the two commandments. We know how to draw arrows that get you from one structure to another, and we know how to draw formal charges. We are now ready for the final challenge using curved arrows to draw resonance structures. 

Some atoms, even in covalent compounds, carry a formal charge, defined as the number of valence electrons in the neutral atom minus the sum of the number of unshared electrons and half the number of shared electrons. Resonance occurs when we can write two or more structures for a molecule or ion with the same arrangement of atoms but different arrangements of the electrons. The correct structure of the molecule or ion is a resonance hybrid of the contributing structures, which are drawn with a double-headed arrow () between them. Organic chemists use a curved arrow (O) to show the movement of an electron pair. 

The curved-arrow formalism is universally used for keeping track of the flow of electrons in reactions. We have also used this device (in Section 1-9, for example) to keep track of electrons in resonance structures as we imagined their flow in going from one resonance structure to another. Remember that electrons do not flow in resonance structures they are simply delocalized. Still, the curved-arrow formalism helps our minds flow from one resonance structure to another. We will find ourselves constantly using these (red) curved arrows to keep track of electrons, both as reactants change to products and as we imagine additional resonance structures of a hybrid. 

The strictly mathematical basis of the quasi-equilibrium rate theory is absent from other, qualitative theories of mass spectrometric fragmentation which are based mainly on empirical rules and are discussed more fully in Section VI on the classification of mass spectra. One empirical classification, the charge-localization treatment, has achieved an aura of theoretical respectability through the use of curved arrows and analogies with the formalism of resonance theory in solution chemistry (Budzikiewicz et al., 1967a). The bonds in a molecular ion vibrate with excess energy but since an electron has been removed on ionization, the vibration frequencies are not the same as in the original intact molecule. 

The curved-arrow convention is used to show how electrons in one resonance structure can be moved around to generate a new resonance structure. The curved arrows are entirely a formalism electrons do not actually move from one location to another, because the real compound is a weighted average of the different resonance structures, not an equilibrium mixture of different resonance structures. The curved arrows help you not to lose or gain electrons as you draw different resonance structures. 

When lone pairs of heteroatoms are omitted in structural drawings, a formal negative charge on a heteroatom can double as a lone pair. Thus, a curved arrow will often begin at a formal negative charge rather than at a lone pair. 

The curved arrow shows how the pair of electrons moves from the nucleophile to the electron-deficient center. The nucleophilic atom increases its formal charge by 1, and the leaving group decreases its formal charge by 1. The minus sign on Nu indicates both a formal charge and a pair of electrons. When the nucleophile is uncharged, the lone pair is usually drawn. 

Electron flow paths are written in the language of Lewis dot structures and curved arrows. Lewis dot structures are used to keep track of all electrons, and curved arrows are used to symbolize electron movement. You must be able to draw a proper Lewis structure complete with formal charges accurately and quickly. Your command of curved arrows must also be automatic. These two points cannot be overemphasized, since all explanations of reactions will be expressed in the language of Lewis structures and curved arrows. A Lewis structure contains the proper number of electrons, the correct distribution of those electrons over the atoms, and the correct formal charge. We will show all valence electrons lone pairs are shown as darkened dots and bonds by lines. 

PROBLEM 18.4 Represent the reaction of chlorine with each of the enol forms of 2-butanone (see Problem 18.3) according to the curved arrow formalism just described. 

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