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

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

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

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

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

Sn 1 Mechanism I Formation of carbocation (slow)

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

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.

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

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?