1. Resonance (section 1.4). Arguably the most important technique to rationalize outcomes in the science majors organic chemistry series

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1 Chemistry 261 Fall 2017 Lecture 2 Emphasis 1. Resonance (section 1.4). Arguably the most important technique to rationalize outcomes in the science majors organic chemistry series a. Requires an introduction to the curved arrow approach, where a double headed arrow shows the motion of a pair of electrons from the tail of the arrow to the head of the arrow (a single barbed arrow is used to denote the motion of a single electron). For example, resonance nicely explains why acetic acid is acidic, when we consider resonance in the conjugate base, which allows the negative charge to be shared between the 2 oxygens equally 2. Notice the curved arrow approach is also used to show the motion of electrons between molecules, in order to describe the mechanism of reaction definitely not resonance! A nice example is the reaction between acetic acid and sodium hydroxide which leads to the acetate shown above Spectator ion Na + not shown 3. It is extremely important to remember resonance is a means of best describing an actual chemical entity as a hybrid of 2 or more forms the actual molecule is somewhere in between! 1

2 The Rules of Resonance Resonance is a means of depicting the stabilization of a molecule by the delocalization of electrons within the molecule In organic chemistry we often speak in terms of the carbon skeleton composed of C-C single bonds. In order to have resonance we must have - double bonds, such as those found in hydrogen sulfate HSO 4 There are 3 principal resonance forms associated with the hydrogen sulfate anion 1. All structures must be bona fide Lewis structures; i.e. maintain noble gas electronic configurations for all atoms where possible, though the possibility of resonance stabilizing an electron deficient species (e.g. carbocation) certainly exists 2. Since we are trying to find a way to depict an overall structure, we leave the nuclei in a fixed position and try to move electrons around. Example, the resonance structures of chlorobenzene 2

3 Notice that in this case we went from a case of no charge to charge separated species which is energetically unfavorable. In this case resonance helps predict reactivity at the ortho and para position, but the bulk of the electron density still resides on chlorine (see rule number 2 on page 4) 3. All atoms participating in resonance must lie in a plane or very nearly so (this will become apparent when we discuss hybridization and bonding molecular orbitals) 4. All forms must have the same number of unpaired electrons. Example, the resonance forms of butadiene The charge separated resonance structure contributes very little to the overall hybrid. Compare this to the resonance stabilization used to predict product outcome when acids of the form HX add across the double bond in this conjugated system. X - X - In this case resonance nicely rationalizes the 2 principal addition products as X - attacks the electron deficient carbocation Question: The benzylic cation is especially stable as far as carbocations go (you are still not going to be able to isolate it). It has the structure shown below. Using the approach shown for chlorobenzene, can you rationalize why it is more stable than other carbocations? 3

4 Not all resonance forms contribute equally the most stable forms contribute disproportionately, with the actual molecule bearing the closest resemblance to the most stable form. The question then becomes how do we ascertain the most stable form? A few hints 1. Structures with more covalent bonds are typically more stable than those with fewer 2. Stability is decreased by an increase in charge separation 3. Structures with formal charges are less stable than uncharged structures (structures with more than 2 formal charges do not contribute, nor do structures with like charges on adjacent atoms) 4. Structures carrying a negative charge on an electronegative element are favored; similarly, structures carrying a positive charge on a less electronegative atom are also favored 5. Structures with distorted bond angles or lengths are unfavored Resonance is best learned by practicing drawing structures. Let s consider the resonance of a series of conjugate bases of usual acids to get a feel for the stabilizing effect of resonance: Sulfuric acid hydrogen sulfate sulfate Nitric acid nitrate 4

5 Acetic acid acetate Phosphoric acid dihydrogen phosphate A couple of interesting examples of the consequences of resonance: Phenolphthalein: Notice that the color change associated with phenolphthalein occurs at ph = 9 and [an isolated] phenol has a pk a = 10 (pk a s may be viewed as the ph at which an acid is 50 % ionized via the Henderson-Hasselbalch equation) 5

6 L-ascorbic acid: pka = 4.1 pka = 11.6 Vitamin C has a pk a value very similar to that of a carboxylic acid but definitely does not have the RCO2H structural motif (e.g. acetic acid on page 1 with a pk a = 4.76) why the hydroxyl on the left is so much more acidic than the hydroxyl on the right is readily explained by resonance: 6

Chapter 9 Bonding. Dr. Sapna Gupta

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