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# Lesson17 - Recognizing Acids and Bases Acids and bases will...

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Recognizing Acids and Bases Acids and bases will be one of two types as illustrated by the two equations below. Acids: •Must have a hydrogen atom to donate •The most acidic hydrogen is often bound to a heteroatom (N, O, S, X) •In rare (but important!) cases, the acidic hydrogen is bound to carbon •Will be neutral or positively charged Bases: •Must have a lone pair to accept a hydrogen •Will be neutral or negatively charged K a K a H––X H X + acid proton base H––X H X + acid proton base Type I Type II

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Henderson-Hasselbalch eqn. Acid-Base Equilibria: Concepts & Definitions Many protonation and deprotonation steps are reversible. The equilibrium can be expressed in terms of pK a . Let’s review the meaning of pK a : HA + H 2 O H 3 O + + A - K eq K eq = [H 3 O + ][A - ] [HA][H 2 O] Assuming the [H 2 O] to be constant (55.5 M) i.e., [H 2 O]>>[H 3 O + ], we can define a new constant _________________ which allows us to drop water from the above equilibrium expression. This is the basis of the widely used pK a convention: K a = K eq [H 2 0] HA H + + A - K a K a = [H + ][A - ] [HA] log K a = log [H+] + log [A - ] [HA] Or -log K a = -log [H+] + log [A - ] [HA] pK a pH = + log [A - ] [HA] definition pH pK a = 5.2 HA A - Relative conc. vs. pH
Determining the Position of Equilibria K eq K eq can be calculated by analyzing this reaction as a sum of two acid / base equilibria K a (1) K a (2) pK a ____ Relevant equilibria _______________ -2.5 -1.7 K eq = ––––– 1 K a (1) · K a (2) pK a = - log K a 10 -(-1.7) 10 -(-2.5) = 0.16 K eq = ––––– Based on the magnitude of K eq , can we conclude that protonated alcohol is a reasonable species?

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Estimating pK a Values of Organic Structures MarvinSketch has a very useful tool for estimating pK a values of organic molecules. Load MicroMarvinSketch: http://www.widgetbox.com/widget/micromarvin To determine the pK a of an organic structure (e.g., of methylacetamide) 1) draw the structure 2) Call pK a from Tools…protonation… 3) Accept default values and click OK •Red values are pKa
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