Substitution Reactions 2: The Relative Substitution Reactions 2: The Relative Rates of Nucleophilic Substitution Rates of Nucleophilic Substitution

ReactionsReactions Experiment 8.1 A & BExperiment 8.1 A & B

Organic Chemistry Lab II, Spring 2010Dr. Milkevitch

Feb 1 & 3, 2010

Substitution ReactionsSubstitution Reactions

TodayToday: Explore the rates of Sn: Explore the rates of Sn22 and Sn and Sn1 1 reactionsreactions– Two fundamental substitution reactions of alkyl halidesTwo fundamental substitution reactions of alkyl halides

What you will learn/observe:What you will learn/observe:The preferred structure of alkyl halides undergoing SnThe preferred structure of alkyl halides undergoing Sn22

The preferred structure of alkyl halides undergoing SnThe preferred structure of alkyl halides undergoing Sn11

Effect of the leaving group on reaction rates for both types Effect of the leaving group on reaction rates for both types of reactionsof reactions

Effect of the size of the nucleophile in SnEffect of the size of the nucleophile in Sn22 reactions reactions

The effect of solvent polarity on the rate of SnThe effect of solvent polarity on the rate of Sn11 reactions reactions

Rate laws for both types of reactionsRate laws for both types of reactions

The SnThe Sn22 Reaction Reaction

General mechanism:General mechanism:

Nu _

= nucleophile (“nucleus loving”), species that seeks a + charge

L = leaving group, the species that leaves

substrate

Factors That Affect SnFactors That Affect Sn2 2

ReactionsReactions

Strength of nucleophileStrength of nucleophile– Stronger is betterStronger is better

Structure of substrateStructure of substrate– Least hindered is betterLeast hindered is better

Nature of solventNature of solvent– Polar aproticPolar aprotic

Nature of the leaving groupNature of the leaving group– Better LG, faster rxnBetter LG, faster rxn

The SnThe Sn11 Reaction Reaction

Some similarities with the SnSome similarities with the Sn22

– But with a critical differenceBut with a critical difference

Substitution, nucleophilic, Substitution, nucleophilic, unimolecularunimolecularTwo step reaction with carbocation Two step reaction with carbocation intermediate.intermediate.Rate is first order in the alkyl halide, Rate is first order in the alkyl halide, zero order in the nucleophilezero order in the nucleophileRacemization can occurRacemization can occur

SnSn11 Mechanism I Mechanism I

(CH3)3C Br (CH3)3C+

+ Br-

Formation of carbocation (slow)Formation of carbocation (slow)

SnSn11 Mechanism II Mechanism II

Nucleophilic attackNucleophilic attack

(CH3)3C+

+ H O H (CH3)3C O H

H

(CH3)3C O H

H

H O H+ (CH3)3C O H + H3O+

Loss of H+ (if needed)

Rates of SRates of SNN1 Reactions1 Reactions

Substrate: 3° > 2° > 1° >> CHSubstrate: 3° > 2° > 1° >> CH33XX– Order follows stability of carbocations Order follows stability of carbocations

(opposite to S(opposite to SNN2)2)

– More stable ion requires less energy to formMore stable ion requires less energy to form

Better leaving group, faster reactionBetter leaving group, faster reaction – (like S(like SNN2)2)

Polar protic solvent best: Polar protic solvent best: – It solvates ions strongly with hydrogen It solvates ions strongly with hydrogen

bonding. bonding.

ProcedureProcedure

All procedural information is on the All procedural information is on the worksheetworksheet

Follow the instructions on the worksheetFollow the instructions on the worksheet

Remember: no formal lab report requiredRemember: no formal lab report required

Your ReportYour Report

Complete worksheetComplete worksheet Conclusions SectionConclusions Section::– Write about the following:Write about the following:

Did you observe the trends in Sn2 & Sn1 reactions you Did you observe the trends in Sn2 & Sn1 reactions you learned in class?learned in class?

– List what you observed/learnedList what you observed/learned

If not, what did you observe instead?If not, what did you observe instead?– Any ideas for the why?Any ideas for the why?