CHEM 2211 1st Edition Lecture 24 Outline of Last Lecture I II III IV V Know your mechanisms Intro to Retrosynthesis Example 1 Example 2 Practice Problem Outline of Current Lecture I II III IV V VI Delocalized Electrons Bonding in benzene Drawing Resonance structures Factors that decrease stability Delocalization adds stability Criteria for Aromaticity Current Lecture I II III Delocalized Electrons A Electrons are shared by 3 or more atoms B All hydrogens are equivalent to each other benzene Bonding in benzene A Has bond angle of 120 aka PLANAR molecule B P orbitals overlap neighboring p orbitals Drawing Resonance structures A Resonance hybrids actual structure with delocalized electrons drawn as a dashed line Resonance Hybrid B RULES i Only electrons move ii Only pi electrons and lone pairs can move iii Sigma bonds are not broken and substituents are not removed Example of Resonance IV Factors that decrease stability A Atoms with incomplete octets B Negative charge on an atom that is not the most electronegative atom in the molecule or a positive charge on the most electronegative molecule C Charge separation REMEMBER that resonance increases stability and makes the molecule less reactive and more acidic V Criteria for Aromaticity A Must have an uninterrupted cyclic cloud of pi electrons above and below molecule s plane B Must have an odd number of electron pairs or a Huckel number i Huckel s rule 4n 2 where n is any integer C If the molecule does not fit both requirements it is NOT aromatic Aromatic NOT Aromatic
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