Oh o oh phenol acetic acid hybrid orbital effect

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Unformatted text preview: it is more acidic than its uncharged counterpart. more acidic than H3O+ H2O NH4+ ‘Nuff said. more acidic than NH3 Resonance: Resonance is typically a source for stability. If a molecule loses resonance when protonated, the original molecule would be a weak base. If a molecule gains resonance when deprotonated, it can be a strong(er) acid. Acetaldehyde has two types of protons, only one is somewhat acidic. O O O H H H H enolate 20 O O H No resonance 45 The enolate resonance puts the negative charge on the oxygen. Oxygen can better stabilize negative charge. Hence, the sp3 carbon next to a carbonyl is unusually acidic. If you add a second carbonyl, the proton becomes even more acidic due to more delocalization. O O O O O O pKa ~ 10 O O The number of resonance structure is important, but the quality of resonance is more so. The conjugate base of phenol has more resonance structures than the conjugate base of acetic acid. Phenol’s oxygen shares the negative charge with three carbons. Acetic acid shares the charge between two oxygens evenly, while the majority of the charge on the phenoxide is still centered on the one oxygen. The pKa of acetic acid is approximately 5, while the pKa of phenol is approximately 10. OH O OH phenol acetic acid Hybrid Orbital Effect This is a specialized effect. Hybrid orbitals have different electronegativities. As the amount of s character increases, the electronegativity increases: sp3 sp2 sp electronegativity increases Lone pairs in sp orbitals are less reactive than normal. As a result, terminal alkynes and hydrogen cyanide are more acidic than typical carbons. HCN is especially acidic as the inductive effect also plays a role. Inductive Effect This is based on electronegativity. It is a weak effect and its effect diminishes the further away from the acidic proton the electronegative group is. Essentially, electronegative groups pull on electrons through sigma (single) bonds. This eventually will pull at the electrons from a sigma bond to a hydrogen, making the hydrogen more positive and more likely to be pulled off by a base. (The values listed below are from DeWick Essentials of Organic Chemistry.) Cl...
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This note was uploaded on 02/14/2014 for the course CHEMISTRY 343 taught by Professor Clauss during the Spring '10 term at University of Wisconsin.

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