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:

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Write about the following: Did you observe the trends in Sn2 & Sn1 reactions you learned in class?

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List what you observed/learned If not, what did you observe instead?

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Any ideas for the why?