acidity - Chapter 8: Organic Acids and Bases Acid/Base...

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Chapter 8: Organic Acids and Bases
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Acid/Base Reactions Some organic chemicals have exchangeable protons (acids) or lone pair electrons which can accept hydrogen (bases) Ionized form of these compounds acts very differently from the neutral form (different HLC, K ow , etc) Proton transfer reactions are usually very fast and reversible, so we can treat them as an equilibrium process
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Acidity Constant Organic acids : HA + H 2 O H 3 O + + A - choosing pure water as a reference state: H + + H 2 O H 3 O + by convention, G ° = 0, K = 1 Thus, HA H + + A - ; lnK a = -( G ° /RT)
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[ ] ( 29 [ ] ( 29 ( 29 [ ] ln ln ' ' ' K H A HA G RT a H A HA o = = - + + - - γ At equilibrium: K a = acid dissociation constant typically measure activity of H + , and conc of HA, A - (mixed acidity constant) at low ionic strength, γ 2245 1 [ ][ ] [ ] ln ln K H A HA a = + - [ ] [ ] log log A HA K pH pH pK a a - = + = - when pH = pK a , [A - ] = [HA]
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For organic bases , treatment is similar: B + H 2 O BH + + OH - [ ] ( 29 [ ] ( 29 [ ] ( 29 K OH BH B b OH BH B = - - + + γ ' ' ' written as acidity constant: BH + B + H + [ ] ( 29 [ ] ( 29 [ ] ( 29 + + + + = BH B H K BH B H a ' ' ' pK a + pK b = pK w = 14 at 25 ° C K a * K b = K w = 10 -14 at 25 ° C
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acids bases carboxylic acids phenols amines heterocycles with N
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O Important functional groups: Acids: CH 3 -OH alcohols (pKa > 14) Carboxylic acids (pKa = 4.75) CH 3 -C-OH Phenols (pKa = 9.82) benzoic acids (pKa = 4.19) Bases: NH 3 ammonia (pKa = 9.25) CH 3 NH 2 primary amine (pKa = 10.66) (CH 3 ) 2 NH secondary amine (pKa = 10.73) (CH 3 ) 3 N tertiary amine (pKa = 9.81) anilines (pKa = 4.63) pyridines (pKa = 5.42) N
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a - = 2 1 0 1 2 1 1 ln T T R H K K r T ia T ia recall that the effect of temperature on any equilibrium constant: for strong acids, r H o is very small r H o
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acidity - Chapter 8: Organic Acids and Bases Acid/Base...

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