Three-Dimensional Bond-Line Structures and Resonance in Organic Chemistry - Prof. Alexande, Study notes of Organic Chemistry

Download Three-Dimensional Bond-Line Structures and Resonance in Organic Chemistry - Prof. Alexande and more Study notes Organic Chemistry in PDF only on Docsity! 2.6 Three-Dimensional Bond-Line Structures 1. 3-D bond-line structures are used to represent acyclic (no ring), cyclic (one ring), and bicyclic (two rings) compounds a. wedges represent a group coming out of the page b. dashes represents a group going behind the page 2. Other types of 3-D drawings a. Fischer projections are used only for acyclic compounds (no ring) b. Haworth projections are used only for cyclic compounds (one ring) c. Bicyclic projections are used only for bicyclic compounds 2.7 Introduction to Resonance 1. The inadequacy of bond-line structures a. Bond-line structures misrepresent a pair of bonding electrons as always represented as a line that is drawn between two atoms, which implies that the bonding e- are confined to a region of space directly between the two atoms i. P orbitals must be represented as a “conduit,” allowing the two pi e- to be associated between all carbon atoms in a model ii. Using Molecular Orbital Theory, the entire molecule is treated as one entity and all of the electrons in the entire molecule occupy regions of space called molecular orbitals 1. Two e- are placed in each orbital, starting with the lowest energy orbital, until all e- occupy orbitals 2. The lowest energy level in MO has NO NODES 3. The next highest energy level has ONE node- non-bonding molecular orbital 4. The highest energy is called the anti-bonding molecular orbital, which has TWO NODES a. The pi e- of the allyl system will fill these MO’s starting with the lowest energy level iii. MO theory suggests that the positive charge is associated with the two ends of the system, rather than just one end iv. Any single bond-line structure we draw will therefore be inadequate 2. Resonance a. Resonance is a method that chemists use to deal with the inadequacy of bond-line drawings i. in this approach, we draw more than one bond-line structure and mentally meld them together ii. These drawings are called resonance structures are a series of structures that are melded together (conceptually) to circumvent the inadequacies of bond-line drawings iii. We separate resonance structures with a straight, two-headed arrow and we place brackets around the structures. This indicates the drawing are OF ONE ENTITY, known as resonance hybrid. 1. NOTE: This is not flipping back and forth between the different resonance structures. 2. Since no single drawing adequately describes the nature of the e- density spread out over the molecule, we draw several drawing and then meld them together to create a mental hybrid of the situation 3. Resonance Stabilization a. The spreading of charge or lone pair is called delocalization. i. The delocalization of either a positive charge or a negative charge stabilizes a molecule b. Resonance Stabilization is the stabilization associated with the delocalization of e- via resonance 2.8 Curved Arrows 1. Curved arrows are tools used when drawing resonance structures. They show the flow of e- density during each step of a reaction mechanism. a. Every curved arrow has a head and a tail. i. Tail shows where the e- are coming from ii. Heads shows where the e- are going 2. Curved arrows used for drawing resonance structures do not represent the motion of e- a. They help us draw resonance structures more easily b. These tools treat the e- as if they were moving , even though the e- are not moving at all 3. Rules to follow a. Avoid breaking a single bond i. by definition, resonance structures must have all the same atoms connected in the same order ii. Breaking a single bond would change this iii. The tail of an arrow should never be placed on a single bond b. Never exceed an octet for second-row elements i. Second-row elements (C, N, O, and F) must have only four orbitals in their valence shell ii. Each orbital can either form a bond or hold a lone pair iii. Therefore, for second-row elements the total number of bonds plus the number of lone pairs can never be more than 4 iv. Make sure to look for hidden hydrogens! 1. This is only considered a violation if a second-row element has MORE than an octet of e-. 2. It is not a violation if a second-row element has less than an octet of e- v. Whenever more than one curved arrow is used, all curved arrows must be taken into account to determine if the above two rules have been violated 1. It may be possible to add or remove a curved arrow and still be ok