13 Intro to Structure and Reactivity-Acids and Bases

Organic Chemistry: Structure and Function

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1 Introduction to Structure and Reactivity: Acids and Bases - Part 1 Lecture Supplement: Take one handout from the stage
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2 Midterm 2 High score = 100 Average = 76.0 Grade scale posted on course web site Exam Return : Last Name Returned By A - Cl Farnosh Co - He Robyn Ho - Le Steve Li - Ng JC Ni - Sl Richard Sk - Z Dr H
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3 Chemistry 14C Part 3 Structure Controls Everything Physical properties Boiling point Solubility etc. Chemical properties Reactions Reactivity Biological properties Drug effects etc. + mple: Enzyme function ydrogen atom (proton) transfer common step in enzyme reactions OH SH O OH Serine Cysteine Aspartic acid + substrate OH SH O O + H-substrate Why most acidic?
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4 Structure versus Acidity and Basicity Supplementary reading : OCATSA chapter (email for access) Begin with a simple example: autoionization of water Brønsted acid (BA) : hydrogen atom donor Hydrogen atom = proton Brønsted base (BB) : proton acceptor Avoid this common misconception H 2 O HO - + H + 2 H 2 O HO - + H 3 O + BA, BB BB BA Balanced equation does not reveal how or why autoionization occurs. X “naked proton”
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5 Autoionization of Water: A Deeper Look δ + δ + δ - δ + δ + δ - Hydrogen bond Electrostatic attraction Collision Moment of collision Must have sufficient energy Must have correct orientation Bonds made/broken via e - changes
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6 action mechanism : Step-by-step description of bond changes escription can be lengthy implify with curved arrows electron source electron destination O H H O H H O H O H H H Autoionization of Water: Reaction Mechanism Form H-O bond Break O-H bond Single step or many steps Useful to explain and predict reaction products Important organic chemistry tool
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7 Autoionization of Water: Reaction Mechanism Nucleophiles and Electrophiles Most reactions initiated by... electrostatics (opposite charges attract) electron donor interacting with electron acceptor Electron donor Electron acceptor Electron poor: +, δ + , open octet “electron loving” = electrophile Lewis acid •Examples: H 3 O + , H 3 C + , H 3 C-Cl Electron rich: -, δ - , lone pairs, pi bonds “nucleus loving” = nucleophile Lewis base •Examples: H 2 O, HO - , H 2 C=CH 2 O H H O H H O H O H H H δ + δ -
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8 Autoionization of Water: Reaction Mechanism Functional Groups and Mechanisms Mechanisms are powerful predictive tools Similar functional groups similar charges similar reactions Example: autoionization of HOH suggests autoionization of CH 3 OH O H 3 C H O H CH 3 O H 3 C O H CH 3 H O H H O H H O H O H H H
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9 CH 3 O H OH 2 O CH 3 O O H OH 2 Proton Transfer Equilibrium Position ow can we determine equilibrium position for proton transfer reaction? Quantified by K eq = equilibrium constant Example: Acetic acid ionization in water K eq = [products] [reactants] = [CH 3 CO 2 - ] [H 3 O + ] [CH 3 COOH] [H 2 O] [H 2 O] ~ constant K a = [CH 3 CO 2 - ] [H 3 O + ] [CH 3 COOH] = acid ionization constant Quantifies extent of acid ionization [unionized] = [ionization products]
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10 K a and p K a H 2 SO 4 K a = 10 9 H 3 O + K a = 1.7 x 10
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