Rules to construct the electronic structure of organic molecules. (1) Count the total number of valence electrons. Add one for every negative charge and subtract one for every positive charge. (2) Attach the atoms in the correct order using single bonds, being careful to give no atom more than four bonds (duet for H). Each bond uses two valence electrons. (3) Place the extra valence electrons on the heteroatoms (atoms that are different from C and H) or between pairs of neighboring electron-deficient atoms, being careful to give no more than a total of 8 electrons to each atom. (Each bond to an atom counts as two electrons.) Usually C makes a total of four bonds, N three, O two, and H and the halogens one. (4) Are any atoms electron-deficient by two or more electrons? If so, take a lone pair of electrons on a next-door neighbor and turn it into a bond between the electron-poor atom and its neighbor. If you have a choice of neighbors from which to borrow a pair of electrons, choose it from the neighbor that lacks its "normal" number of bonds. (5) Assign formal charges. The formal charge for any atom is calculated by (valence electrons – number of unshared electrons – number of bonds). Rules (in the order shown) to determine the best resonance structure. 1. Resonance structures in which any atom has more than an octet (duet for H) are not acceptable. 2. It is acceptable to draw a resonance structure in which electron-deficient atoms are present, but the fewer the electron-deficient atoms, the better the resonance structure. (Cf. structures 1 and 2 of methyl acetate above.) If there must be electron-deficient atoms, they should be relatively electropositive ones (C, B) rather than electronegative ones (N, O).3. Structures with fewer formal charges are better than those with more formal charges, unless this violates rule #2. 4. If there must be formal charges, the negative charges should be located on atoms that are as electronegative as possible, and the positive charges should be located on atoms that are as electropositive as possible. Rules for generating resonance structures 1. Any double bond A=B may be broken and the two electrons in that bond given to atom A or atom B. Both electrons may be given to one atom, or one electron may be given to each atom. 2. The lone pair on any atom may be shared with its next-door neighbor to make a new double bond if and only if that neighbor is electron-deficient. 3. The lone pair on an atom may be shared with an electron-saturated neighbor if and only if a double bond to that neighbor breaks at the same time, with the electrons in that bond moving to the neighbor's neighbor. The purpose of this rule is to prevent any atom from acquiring more than an octet. 4. A C atom bonded to four different atoms does not participate in resonance. N bound to three atoms or O bound to two atoms can donate a pair of electrons to a neighbor, but it cannot accept a pair of electrons. 5. In a ring of alternating double bonds, the double bonds can be moved around to generate a new structure that is different from the original
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