{ note the same warning about subscripts, superscripts, and Greek letters as indicated for Chapter 6 }
Acid/Base Catalysis
in a typical acid-catalyzed reaction, a reactive protonated intermediate is formed
A + H+ <---(k1, k-1)---> AH+
AH+ + B ---(k2)---> product
specific acid catalysis: step 2 is the RDS, step 1 is an acid-base equilibrium
Rate = k2 [AH+] [B]
[AH+] = (k1/k-1) [A] [H+]
Rate = (k1 k2 / k-1) [H+] [A] [B] (third-order)
the rate depends only on the concentration of the specific acid, H+ (pH)
general acid catalysis: step 1 is the RDS and any general acid can provide H+
A + HX ---(k3)---> AH+ + X-
Rate = k1 [A] [H+] + k3 [A] [HX]
the rate depends not only on pH but on total acid concentration
distinguishing general vs. specific acid catalysis:
buffer solutions of constant pH but variable total acid
e.g., pH 5 buffer solutions made from 0.1M or 1 M acetate/acetic acidgeneral acid catalysis - observe higher rate (more general acid)
specific acid catalysis - observe same rate
(same pH, same equilibrium conc. of protonated forms)
solvent isotope effects
D2O (and D3O+) are stronger acids than H2O (and H3O+)
specific acid catalysis - observe higher rate in D2O
(stronger acid, more protonated forms at equilibrium)general acid catalysis - observe higher rate in H2O
(primary KIE favors transfer of H+ in the RDS)
Bronsted Catalysis Law:
for general acid catalysis - measures the catalytic effect of general acids as a function of their acid strength (Ka)
log kcat = a log Ka + b ( 0 < a < 1 )
a represents the sensitivity of the catalysis to acid strength
e.g., hydrolysis of enol ether, where a = 0.8
the acid leveling effect - in any solvent S, the strongest acid is SH+ (usually represented as H+ )
thus, H+ is a stronger acid than any general acid HX and k1 is faster than k3
mechanistic interpretations:
larger values of a suggest greater proton transfer in the transition state
Base Catalysis:
completely analogous considerations for base-catalyzed reactions lead to:
specific base catalysis: step 2 is the RDS, step 1 is an acid-base equilibrium
Rate = (k1 k2 / k-1) [OH-] [A] [B]
general base catalysis: step 1 is the RDS and any general base can remove H+
Rate = k1 [A] [OH-] + k3 [A] [Base]
Bronsted Catalysis Law:
log kcat = b log Kb + c ( 0 < b < 1 )
Hammett acidity function:
many organic compounds require strongly acidic or basic solvent systems
(outside the range of normal aqueous pH measurements)
the concept of pH is extended to nonaqueous systems using weak bases as indicators of the protonating ability of a strong acid (activity of H+)
B + H+ <==> BH+
K = ( aBH+) / ( aB) ( aH+) = [BH+] gBH+ / [B] gB aH+
Ho = aH+ = [BH+] gBH+ / [B] gB K
measure the ratio of [BH+] / [B] for a base of known K in aqueous solution
assume that the ratio of activity coefficients ( gBH+ / gB ) is constant
use a graded series of bases to extend to more and more acidic solvents
examples - 20% H2SO4 has Ho = -1, 98% H2SO4 has Ho = -10.4
Lewis acid catalysis:
formation of donor-acceptor complexes can enhance reactivity
e.g. BF3 , AlCl3 , TiCl4 complexes with carbonyl cmpds
Diels-Alder dienophiles are more reactive as D-A complex
Gas-Phase Acidity and Basicity
proton affinity, defined as - delta H for the reaction B + H+ ---> BH+
Carbonyl Addition Reactions
hydration
oximes (pH effects)
enolization
halogenation
(note - base-promoted implies stoichiometric amount of base needed - not catalytic)
Carboxyl Substitution Reactions
ester hydrolysis - mechanistic classifications
acid-catalyzed
base-catalyzed
tetrahedral intermediate
nucleophilic catalysis