Transcript Exercise C2 Melting Point & E1 Refractive Index
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: substrate Nu _ = nucleophile (“nucleus loving”), species that seeks a + charge L = leaving group, the species that leaves
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) (CH 3 ) 3 C Br (CH 3 ) 3 C + + Br -
Sn
1
Mechanism II
Nucleophilic attack (CH 3 ) 3 C + + H O H (CH 3 ) 3 C O H H Loss of H + (if needed) (CH 3 ) 3 C O H H + H O H (CH 3 ) 3 C O H + H 3 O +
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.
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?