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Lec-37-Organic - Organic Reactions I and II CHE 131...

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Organic Reactions I and II CHE 131- Lectures 37 & 38 Web Supplements Brønsted-Lowry definition of acids and bases An acid is a proton donor. A base is a proton acceptor. Lewis definition of acids and bases An acid is an electron pair acceptor. A base is an electron pair donor. Cl H O H H Cl H O H H Lewis acids metal cations (Cu 2+ ) Lewis bases all Brønsted bases (HO , NH 3 , H 2 O etc.) molecules with high-energy filled orbitals, such as lone pairs Even very weak Lewis acids will react with very strong Lewis bases. Very weak Lewis bases will react with very strong Lewis acids. all Brønsted acids (H 3 O + , CH 3 CO 2 H, etc.) molecules that have vacant valence orbitals (BH 3 ) molecules with polar π bonds (CO 2 , SO 3 , CH 3 COCl) Lewis acids: Electron pair acceptors Any molecule with a polar bond can act as a weak Lewis acid. If the Lewis-acidic atom has a complete octet, it cannot form a new bond without breaking an old bond. H N H H H N H H C H H H Cl Cl C H H H In this reaction, a new bond forms between nitrogen and carbon. At the same time, the bond breaks between carbon and chlorine.
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What happens when an acid (HA 1 ) and base ( A 2 ) are mixed together? If pK a (HA 1 ) << pK a (HA 2 ), proton transfer will occur. If pK a (HA 1 ) >> pK a (HA 2 ), there will be no proton transfer reaction. If pK a (HA 1 ) pK a (HA 2 ), there will be a mixture of both acids and both bases present. CH 3 OH + OH CH 3 O + H 2 O pK a: 18 15.7 Substitution vs. Elimination Substitution: Lewis base attacks the C-X bond directly. Cl + O H OH + Cl Cl + O H H + O H H + Cl One set of starting materials can produce both products!!! Substitution and elimination are competing reactions. Elimination: Lewis base attacks the proton on a carbon adjacent to the C-X bond. Summary Substitution & Elimination : Alkyl halide starting material acts as a Lewis acid Lewis base leads to either: Substitution – new bond to C is formed with loss of halide Elimination – deprotonates Lewis acid to form a carbocation – loss of halide leads to double bond Addition : • C–C π bond acts as a Lewis base Lewis acid is HBr, Br 2 , other Lewis acids. Addition-Elimination : Acid chloride acts as a Lewis acid Lewis base attacks sp 2 C, breaks π bond, forms sp 3 C π bond reforms with elimination of the chloride Many synthetic reactions are simple Lewis acid-base reactions To understand these reactions we need to understand the manner in which bonds are broken and formed. X Y + XY Addition Elimination H X :B + + HB + X X B :B + + :X Substitution Addition-Elimination O Cl B + O B + Cl
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Curved-Arrow Notation We can show the changes in bonding during a reaction by using curved-arrow notation . • Each curved arrow represents a pair of electrons • Each arrow starts at a source of electron density, either a bond or a lone pair.
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