acidsbasesg

acidsbasesg - Copyright Arizona State University Brønsted...

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Unformatted text preview: Copyright, Arizona State University Brønsted Acid/Base s First Reactions (more...) 1 Brønsted Acidity • A Brønsted acid DONATES a proton • A Brønsted base ACCEPTS a proton Example from general chemistry H + – Cl + Na + – OH + Na + – Cl acid base conjugate acid conjugate base H H O new bond Example from organic example C O O H 3 C + N CH 3 CH 3 H 3 C C O O H 3 C + N CH 3 CH 3 H H CH 3 organic acid organic base conjugate acid conjugate base new bond • hydrogen moves from the acid to the base, but leaves its electron "behind" (is given to the oxygen). A hydrogen with no electron is a proton, therefore a proton is transferred from the acid to the base. 1.1 Brønsted Acid Strength Measured in terms of pKa (usually in water) H–A + H 2 O A + H O H H K a K a = [ A – ] [H 3 O + ] [ HA] [H 2 O] pK a = - log (K a ) breaks a bond makes a bond smaller pK a = stronger acid • Bronsted acidity involves HETEROLYTIC cleavage of a bond to hydrogen, with liberation of a proton • stronger Bronsted acids have smaller pKa values 1.2 Range of Acidities in Organic Chemistry • We will encounter acids with a VERY WIDE RANGE of acidities • We will consider the acidity of some protons that are so weakly acidic that they would not normally be considered as acids • We will also encounter some organic species that are a lot stronger acids than sulfuric acid (although they will not normally be very long lived, especially in water!) • REMEMBER, the pKa scale is logarithmic! So, an acid with a pKa of 10 is not ten times stronger than an acid with pKa of 20, it is 10 ORDERS OF MAGNITUDE STRONGER!! Some Example Acids We will Meet H 3 C H H 3 C H 2 N H H 2 N C H HC C HC H 3 CO H H 3 CO N C HO S O O O H C H 3 C H O H C H 3 C H O N O O O H C H 3 C O O H ~ 50 ~ 35 ~ 25 ~ 15 ~ 4 ~ –1 ~ –5 ~ –8 ~ –10 C H 3 C O O N O O O HO S O O O H 3 C N H C H 3 C methanol ammonia methane acetylene protonated nitrile protonated carbonyl sulfuric acid nitric acid acetic acid Acid Structure Conjugate Base pKa (in water) 1.3 Factors Controlling Brønsted Acidity (more) • Absolute acidity (pKa) is difficult to predict, relative acidity is much easier!! • Focus attention on the electrons that are involved, BEFORE and AFTER bond heterolysis • Think of acidity as a reaction. The stronger acid "wants" to go from left to right more than a weaker acid H + electrons "before" in a bond A H electrons "after" on an atom generic acid conjugate base A • note, HETEROLYTIC BOND CLEAVAGE (this is NOT given the bond dissociation energy) • in general, an acid will be stronger if the electrons in the bond are high in energy, i.e. if the bond is weak • in general, an acid will be stronger if the energy of the now non-bonding electrons on the conjugate base (usually on an ANION) are lower in energy, e.g. if A is an electronegative atom • PROBLEM, these two effects sometimes OPPOSE, how to know which one "wins" • A STRONGER ACID has a more stable conjugate base anion, thus WEAKER BASE...
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This note was uploaded on 01/07/2012 for the course CHM 233 taught by Professor Skibo during the Fall '08 term at ASU.

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acidsbasesg - Copyright Arizona State University Brønsted...

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