![]() Here’s an important new concept: When atoms with a lone pair are adjacent to an atom with an empty orbital, formation of a π bond will be favored. Due to their high electronegativity they take electrons away from whatever they’re attached to. ![]() Why is this weird? Because up to this point, you’re probably used to thinking of atoms like F, O, Cl, and N as the “electron Scrooges” of the periodic table. Positive Formal Charges On Oxygen Or Nitrogen Are OK So Long As It Has A Full Octet In the process we put a positive charge on that atom.Ģ. The best resonance form has a new π bond that has been formed through the donation of a pair of electrons on the adjacent atom (O, N, Cl, F). Notice how in each one, the second-best resonance form has a carbocation – that is, a carbon with six valence electrons. Let’s look at the “best” and “second-best” resonance forms for these positively charged species. It’s probably this first principle which causes the most confusion of all. To be more specific, there are three main ways this plays out when evaluating resonance structures. Electron poor atoms are destabilized by adjacent electron-poor atoms.Electron-poor atoms are stabilized by adjacent electron-rich atoms.Recall the one sentence summary of chemistry: opposite charges attract, like charges repel. There are really one main principle to think about when deciding which site will stabilize positive charge the best. Electron-Deficient Atoms Are Stabilized By Electron-Rich Neighbors Carbocations Are Destabilized By Adjacent Electron Withdrawing Groups That Lack Lone Pairs (Such As CF3, NR3+, And C=O)ġ.When Placing Positive Charges On Carbon, The Resonance Form Where The Positive Charge Is On The Most Substituted Carbon Will Be The Most Important.Positive Formal Charges On Oxygen Or Nitrogen Are OK So Long As It Has A Full Octet.Electron-Deficient Atoms Are Stabilized By Electron-Rich Neighbors.Today I’ll talk about the opposite situation: where to put a positive charge. ![]() Last time I talked about the principles involved in deciding where best to put a negative charge: on the least basic atom. However, sometimes you can’t get around it: you have to put a charge somewhere. Resonance Structures With Positive Charge: Four Key PrinciplesĪs I mentioned before, the resonance form(s) of lowest energy are those where the charges are minimized.
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