MWF_12 - Organic Organic Lecture Series CH 310/318 M...

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Unformatted text preview: Organic Organic Lecture Series CH 310/318 M LECTURE 12 Textbook Assignment: Chapter 4-Cont’d Today’s Topics: pKa ThermochemistrypKa Thermochemistry-Mechanisms Notice & Announcements: Exam 1: Thursday, 9/23, 7-9PM WEL 2.246 for last name beginning A-L and FAC 21 for last name beginning with M-Z, from 7-9 PM. 36 Organic Lecture Series Thermochemistry Thermochemistry & Mechanisms • For reactions at constant pressure, the change in Gibbs free energy, ∆G° is used. , – This kind of energy controls the rates and equilibrium of reactions. – The change in Gibbs free energy between reactants and and products is related to the equilibrium constant through this equation: ∆G°= -RT ln Keq where R = 8.31J/K mol and T is the temperature in kelvins. – Free energy of activation: The difference in energy between reactants and the transition state. 37 Organic Organic Lecture Series Molecular Structure and Acidity • The overriding principle in determining the relative acidities of uncharged organic acids is the stability of the anion, A-, resulting from the loss of a proton. the more stable the anion, the greater the acidity of HA 38 38 Organic Lecture Series Molecular Structure and Acidity • Ways to stabilize anions include having the negative charge: 1. 2. 3. 4. 5. on a more electronegative atom. on a larger atom. delocalized through resonance. delocalized by the inductive effect. in an orbital with more s character. 39 Organic Organic Lecture Series Molecular Structure and Acidity 1. Electronegativity of the atom bearing the negative charge Within a period, the greater the electronegativity of the atom bearing the negative charge, the more strongly its electrons are held, the more stable the anion is, and the stronger stronger the acid. 40 Organic Lecture Series Molecular Structure and Acidity 2. Size of the atom bearing the negative charge – Within a column of the Periodic Table, acidity is related to the size of the the atom bearing the negative charge. – Atomic size increases from top to bottom of a column. – The larger the atom bearing the charge, the greater its stability. CH 3 S H Methanethiol p Ka 7.0 (stronger acid) + CH 3 O – CH 3 S – + CH 3 O H Methanol p K a 16 (weaker acid) Methoxide ion (stronger base) Methanethiolate ion (weaker base) 41 Organic Organic Lecture Series Molecular Structure and Acidity 3. Resonance delocalization of charge in A– The more stable the anion, the farther the position of equilibrium is shifted to the right. – Compare the acidity alcohols and carboxylic acids. acids. – Ionization of the O-H bond of an alcohol gives an anion for which there is no resonance stabilization: CH3 CH2 O-H + H2 O A n al co ho l CH3 CH2 O - + H3 O + pK a = 15.9 42 An al k oxi de i o n Organic Lecture Series Molecular Structure and Acidity – Ionization of a carboxylic acid gives a resonance-stabilized anion. – The pKa of acetic acid is 4.76 O CH3 C OH + H2 O O CH3 C O CH3 C O + H3 O + O equ ivalen t contrib uting structu res ; the carb oxylate anion is stab ilized by delocalization of the n egative ch arge. – Carboxylic acids are stronger acids than alcohols as a result of the resonance stabilization of the carboxylate anion. 43 Organic Organic Lecture Series Molecular Structure and Acidity 4. Electron-withdrawing inductive effect – The polarization of electron density of a covalent bond due to the electronegativity of an adjacent covalent bond. bond. H H C-CH2 O-H H F F C-CH2 O-H F Ethanol p K a 1 5.9 2,2,2-Tri fl uoroethano l p K a 12.4 44 Organic Lecture Series Molecular Structure and Acidity 4. Electron-withdrawing inductive effect – Stabilization by the inductive effect falls off rapidly with increasing distance of the electronegative atom from the site of negative negative charge in the conjugate base. CF3 - CH2 - OH 2,2,2-Trifluoroethan ol (pK a 12.4) CF3 -CH2 - CH2 -OH 3,3,3-Trifluoro-1prop anol (p Ka 14.6) CF3 - CH2 -CH2 - CH2 - OH 4,4,4-Trifluoro-1butan ol (pK a 15.4) 45 Organic Organic Lecture Series Molecular Structure and Acidity – the operation of the inductive effect in the acidity of halogen substituted carboxylic acids is also observed: O OH Bu tanoic acid pK a 4.82 Cl O OH Cl O OH O OH Cl 4-Ch lorob utan oic 3-Ch lorobutan oic 2-Chlorob utanoic acid acid acid pK a 3.98 pK a 2.83 p Ka 4.52 46 Organic Lecture Series Molecular Structure and Acidity 5. Hybridization – For anions differing only in the hybridization of the charged atom, the greater the percentage s character to the hybrid orbital of the charged atom, the more stable the anion. – Consider the acidity of alkanes, alkenes, and alkynes (given for comparison are the acidities of water and ammonia). Weak Acid Water Alkyne HO- H HC C H Conjugate pK a Base Increasing acidity HO – HC C– H2 N – 15.7 25 38 Ammonia H2 N - H Alkene Alkane CH2 = CH- H CH2 = CH– 44 CH3 CH2 - H CH3 CH2 – 51 47 Organic Organic Lecture Series Lewis Acids and Bases • Lewis acid: Any molecule of ion that can form a new covalent bond by accepting a pair of electrons. • Lewis base: Any molecule of ion that can form a new covalent bond by donating a pair of electrons. -+ A + :B AB Lewis acid Lewis base new covalent bond formed in this Lewis acid-base reaction 48 48 Organic Lecture Series Lewis Acids and Bases – Examples H CH3 -C + C-CH3 H : Br : : + : Br : : - : CH3 -C C-CH3 HH sec- Butyl cation (a carbocation) Bromide ion HH 2-Bromobutane CH3 CH 2 :O: CH3 CH 2 Diethyl ether (a Lewis base) + F BF F CH3 CH 2 + F - :O B-F CH3 CH 2 F A BF3-ether complex Boron trifluoride (a Lewis acid) 49 ...
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This note was uploaded on 10/10/2010 for the course CH 54600 taught by Professor Colorpart during the Fall '10 term at University of Texas.

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