Download Key - UCSB CLAS

CLAS – Chem 109C - Ch 23 - Key
Website: http://clas.sa.ucsb.edu/staff/terri/
E-mail: [email protected]
1. Explain the differences between an acid and base catalyst? acid catalysts protonate the substrate and can make
electrophiles stronger and leaving groups better
whereas bases catalyst deprotonate the substrate which can make nucleophiles stronger and tetrahedrals collapse
faster
2. Distinguish between specific vs. general acid (or base) catalysis. Specific catalysts will protonate (or deprotonate)
before the slow step whereas general catalyst will protonate (or deprotonate) during the slow step – strong acids
or bases tend to be specific whereas weak acid or bases tend to be general
3. How does an acid catalyst speed up the following reaction? If you protonate the nitrogen it makes the ring more
susceptible to attack by a nucleophile and it makes the nitrogen a weaker base aka a better leaving group
4. Propose a mechanism for the following reactions that involves a general acid catalyst:
a.
b.
5. Propose a mechanism for the following reaction that involves a general base catalyst:
a.
b.
6. Explain how the presence of iodide ion speeds up the following reaction:
CH3CH2Cl + HO-  CH3CH2OH + ClIodide ion is a stronger nucleophile than hydroxide and will displace the chloride faster – in addition the iodide is
a better leaving group than the chloride and will be displaced by hydroxide faster – therefore iodide ion is a
nucleophilic catalyst
7. What role does imidazole play in catalyzing the following?
8. Explain the relative rates for the hydroylysis reaction starting with cis vs. trans – what would the relative rate
chlorocyclohexane vs. the 2-thiosubstituted chlorocyclohexane?
9. Rationalize the relative rates for the following table:
B is faster than A because intramolecular is faster than intermolecular – C is faster than A because the sterics of
the R groups push the functional groups closer – D is faster than B because fewer single bonds between
functional groups – E is faster than D because the double can’t rotate – F is faster than E because the bond
angles for sp3 are smaller than sp2 and therefore the functional groups are closer
10. List three ways a metal ion catalyzes reactions:
11. Show how cobalt (II) ion catalyzes the following:
12. Explain the difference between the lock and key model vs. the induced fit model.
the lock and key model implies that the substrate has a specific shape that fits into the enzyme’s active site –
whereas the induced fit implies that the enzyme and substrate will alter their shapes in order to fit together
13. List 3 ways an enzyme affects the rate of reaction:
1) Reacting groups are brought together at the active site in the proper orientation for reaction
2) Some of the amino acids in the enzyme serve as catalytic groups; many enzymes have metal ions as catalysts
3) Groups on the enzyme can stabilize the transition state of the reaction
14. Explain site specific mutagenesis.
15. What three amino acids make up the “catalytic triade” in serine proteases?
16. Analyze the catalysis of carboxypeptidase A:
In the first step the zinc ion complexes with the water which lowers the water’s pka – in addition the glu-270 acts
as a general base catalyst removing a proton from water – In the second step the zinc ion stabilizes the
tetrahedral intermediate and the glu-270 acts a general acid catalyst protonating the leaving group as the
tetrahedral collapses
15. Analyze the catalysis of chymotrypsin:
In the first step his-57 acts a general base catalyst deprotonating the ser-195 in order to make it a stronger nucleophile –
in step 2 the asp-102 acts as an electrostatic catalyst stabilizing the (+) charge on the his-57 – in addition the his-57 is
acting as a general acid catalyst protonating the leaving group as the tetrahedral collapses – in step 3 his-57 acts a
general base catalyst deprotonating the water – in step 4 the asp-102 acts as an electrostatic catalyst stabilizing the (+)
charge on the his-57 – in addition the his-57 is acting as a general acid catalyst protonating the ser leaving group as the
tetrahedral collapses