CHEM 109C – Zhang
CLAS, F10
Catalysis I: Definition & Types - KEY
1. What factors contribute to the rate of a chemical reaction?
2. (Ch 23, #32) Indicate the type of catalysis that is occurring in the slow step of
each of the following reactions.
a.
Intramolecular Nucleophilic Cat.
b.
Intramolecular General Acid Cat.
3. What kind of catalyst is shown in each of the following? And, predict the
products.
a.
Metal ion cat.
complexes with N and carbonyl O, making the carbonyl carbon
more electrophilic/susceptible to attack by methoxide ion.
Co2+
b.
Intramolecular Nucleophilic cat. or General Base
cat. (See also Ch 23 # 33)
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CHEM 109C – Zhang
CLAS, F10
Catalysis I: Definition & Types - KEY
c.
Intramolecular Nucleophilic cat.
4. Rank the following in order of increasing reaction rate and explain your
reasoning.
a.
b.
c.
d.
See Table 23.2
Uncat. rxn
6-mem ring
6-mem ring
+ steric
hinderence
keeping
reactive
groups close
5-mem ring
5-mem ring
+ steric
hinderence
5-mem ring
+ steric
hinderence
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CHEM 109C – Zhang
CLAS, F10
Catalysis I: Definition & Types - KEY
5. (Ch 23 #41) At pH = 12, the rate of hydrolysis of ester A is greater than the rate
for hydrolysis of ester B. At pH = 8, the relative rates reverse (ester B hydrolyses
faster than ester A). Explain these observations and show the mechanisms for the
fastest hydrolysis at each pH.
At pH = 12 the nucleophile is HO- and the negative charge that develops on
the carbonyl O is stabilized by the N+ in ester A, so ester A reacts faster.
At pH = 8 the nucleophile is water and the neutral N with its lone pair can
act as a general base cat. to increase the nucleophilicity of water, so ester B
reacts faster.
Catalyst: A substance that increases the rate of a chemical reaction without itself being
consumed or changed in the reaction. Catalysts can increase the rate of reaction by
decreasing the activation energy barrier in one of three ways…
a. By converting the reactant into a less stable species…
__________
is the uncatalyzed pathway
- - - - - - is the catalyzed pathway
Ex. Specific Acid or Base Catalysis
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CHEM 109C – Zhang
CLAS, F10
Catalysis I: Definition & Types - KEY
b. By providing a similar mechanism with a more stable transition state…
Ex. General Acid or Base Catalysis
c. By completely changing the mechanism and providing a pathway with a smaller
activation energy barrier than the uncatalyzed reaction…
Ex. Nucleophilic or Covalent Catalysis
Types of Catalysts
1. Nucleophilic/Covalent – forms a new/covalent bond
2. Acid – donates a proton
3. Base – removes/accepts a proton
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CHEM 109C – Zhang
CLAS, F10
Catalysis I: Definition & Types - KEY
Specific vs. General Acid or Base Catalysis
Proton is completely transferred BEFORE the r.d.s.
Occurs under strongly acidic or basic cond.
Proton is transferred DURING the r.d.s.
Occurs under weakly acidic or basic cond.
4. Metal Ion/Lewis Acid
can increase the rate of reaction by making a better electrophile (electrophilic cat./A),
making a better LG (B) or decreasing the pKa of water/making water a better Nuc (C).
Similar to Table 23.1 pKas of metal-bound water.
M-H2O
Water/H2O
Ca2+
Mg2+
Cd2+
Mn2+
Ni2+
Co2+
Zn2+
Fe2+
Cu2+
Be2+
pKa
14
12.7
11.8
11.6
10.6
9.4
8.9
8.7
7.2
6.8
5.7
5. “Intramolecular” Catalysis
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CHEM 109C – Zhang
CLAS, F10
Catalysis I: Definition & Types - KEY
6. Enzyme Catalyzed Reactions (see “Catalysis II handout)
Examples:
Decarboxylation of dimethyloxaloacetate cat. by Cu2+ or Al3+ (A)
Hydrolysis of trifluoroacetate cat. by Zn2+ (C and B)
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