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Unformatted text preview: Copyright, Arizona State University Brnsted Acid/Base s First Reactions (more...) 1 Brnsted Acidity A Brnsted acid DONATES a proton A Brnsted 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 Brnsted Acid Strength Measured in terms of pKa (usually in water) HA + 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 Brnsted 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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