1. Substitution Reactions 2: The Relative Rates of Nucleophilic Substitution Reactions Experiment 8.1 A & B Organic Chemistry Lab II, Spring 2010 Dr. Milkevitch Feb 1 & 3, 2010

2. Substitution Reactions Today: Explore the rates of Sn 2 and Sn 1 reactions –Two fundamental substitution reactions of alkyl halides What you will learn/observe: –The preferred structure of alkyl halides undergoing Sn 2 –The preferred structure of alkyl halides undergoing Sn 1 –Effect of the leaving group on reaction rates for both types of reactions –Effect of the size of the nucleophile in Sn 2 reactions –The effect of solvent polarity on the rate of Sn 1 reactions –Rate laws for both types of reactions

3. The Sn 2 Reaction General mechanism: Nu _ = nucleophile (“nucleus loving”), species that seeks a + charge L= leaving group, the species that leaves substrate

4. Factors That Affect Sn 2 Reactions Strength of nucleophile –Stronger is better Structure of substrate –Least hindered is better Nature of solvent –Polar aprotic Nature of the leaving group –Better LG, faster rxn

5. The Sn 1 Reaction Some similarities with the Sn 2 –But with a critical difference Substitution, nucleophilic, unimolecular Two step reaction with carbocation intermediate. Rate is first order in the alkyl halide, zero order in the nucleophile Racemization can occur

6. Sn 1 Mechanism I Formation of carbocation (slow)

7. Sn 1 Mechanism II Nucleophilic attack Loss of H + (if needed)

8. Rates of S N 1 Reactions Substrate: 3° > 2° > 1° >> CH 3 X –Order follows stability of carbocations (opposite to S N 2) –More stable ion requires less energy to form Better leaving group, faster reaction –(like S N 2) Polar protic solvent best: –It solvates ions strongly with hydrogen bonding. –It solvates ions strongly with hydrogen bonding.

9. Procedure All procedural information is on the worksheet Follow the instructions on the worksheet Remember: no formal lab report required

10. Your Report Complete worksheet Conclusions Section: –Write about the following: Did you observe the trends in Sn2 & Sn1 reactions you learned in class? –List what you observed/learned If not, what did you observe instead? –Any ideas for the why?