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Principles of Organic Chemistry 1. LEWIS STRUCTURES, LEWIS ACIDS AND BASES, AND LA/LB REACTIONS LEWIS STURUCTURE Lewis structure a diagram that shows the bonding between atoms and the lone pairs of electrons of a molecule * Writing Lewis structures 1.Determine # of valence electrons (neutral molecule, cation, anion) 2.Connect the atoms with only single bonds 3.Place a sufficient number of non-bonded electron pairs around each atom (octet) - use double or triple bonds if needed. 4. Determine the formal charge on each atom Ex) C2H4 (ethylene) Ex) C2H2 (acetylene) FORMAL CHARGE * Formal charge The charge which is assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity. Formal charge = X – Y - Z X = # of valence electrons Y = # of non-bonded electrons Z = half # of bonded electrons PRACTICE formal charge PRACTICE nitrosyl chloride formal charge and resonance PRACTICE PRACTICE Common Functional Groups LEWIS ACIDS AND BASES Lewis acid: electron pair acceptor Lewis base: electron pair donor Definition of Acid and Base Arrhenius : H+/ OHBrønsted-Lowry: H+ donor/acceptor Lewis: e- pair acceptor/ donor - Amines are stronger LB than alcohols and ethers. Why? hint: check the electronegativity of N and O - Aliphatic amines are stronger LB than some aromatic amines. Why? In aromatic amines the non-bonding e- pair on N is delocalized over the aromatic ring. LEWIS ACID/BASE REACTIONS 1. Writing the Lewis structures of LA and LB 2. Using a curved arrow to show the electron pair movement 3. Determining the formal charges on the two atoms Ex) LEWIS ACID/BASE REACTIONS- EXAMPLES Strong acids are fully ionized in solution. Represent them as ions. LEWIS ACID/BASE REACTIONS- EXAMPLES Carbonyl carbon (δ+) as a Lewis acid Nucleophile in SN2 reaction as a Lewis base