Expt' 8'1A

1

• Expt. 8.1A B
• Nucleophilic Substitution Reactions

2
Lab Overview

• Virtual lab (done on the computer in SCI 242)
• Observe results of computer simulated SN1 and SN2
  reactions.
• The factors you will study, and their effects on
  SN2 and SN1 reactions are
• Structure of the alkyl halide
• Nature of the leaving group
• Size of the nucleophile
• Polarity of the solvent

3
Pre-lab Preparation

• Read Background Expt. 8.1
• Parts A B will be done on the computer.
• Nothing needs to written in lab notebook.
• Print out a copy of the datasheet and bring it
  with you to lab. You will use this to take notes
  on.

4
In Lab

• Work in pairs.
• Work through pre-lab, experiment analysis.
• Record observations directly on datasheet.
• Complete the datasheet.
• Then submit a clean, typed datasheet in one week.

5
Alkyl Halide Examples
6
SN2 Reaction

• Substitution Nucleophilic Bimolecular
• Concerted, one-step reaction
• Backside attack
• Inversion of configuration at chiral carbon
• P. 262 in lab text

7
Reactions SN2 Factors

• R-X NaI R-I
  NaX(ppt)
• (colorless) (colorless)
  (precipitate)
• Alkyl halide (1º, 2º, 3º steric hindrance)
• Leaving group (Br- or Cl-)
• Rate law vary RX NaI
• acetone solvent

8
SN1 Reaction

• Substitution Nucleophilic Unimolecular
• Formation of carbocation
• Attack of the nucleophile to form intermediate
• Proton transfer from intermediate to solvent to
  form final product.
• Racemization of chiral carbon

9
Reactions SN1 Factors
AgNO3

• R-X CH3CH2OH R-OCH2CH3
  AgX(ppt)
• Alkyl halide (1º, 2º, 3º)
• Leaving group (Br- or Cl-)
• Polarity of solvent
• Rate law vary RX CH3CH2OH
• AgNO3 coordinates with the leaving group and
  helps form a carbocation

10
Solvents

• Protic solvents
• Solvents that have OH groups that allow them to
  form hydrogen bonds H2O, EtOH, MeOH, Acetic acid
• Aprotic solvents
• Solvents that lack OH groups (next slide)

11
Common Aprotic Solvents
12
Chemical Kinetics

• Kinetics is concerned with speed or rate of the
  reaction.
• Chemical reaction rate
• the change in the concentration of reactants or
  products per unit time.
• Units are typically molarity per second(M/s)

13
Chemical Kinetics

• Consider the general reaction
• aA bB ? cC dD
• The rate law has the general form
• Rate kAmBn
• where k is the rate constant. The exponents m
  and n are the reaction orders of reactants A
  and B respectively.

14
Chemical Kinetics

• Exponents m and n have no necessary
  relationship to reaction coefficients a and
  b.
• m and n must be experimentally determined.
• To determine the reaction orders we hold one
  reactant concentration constant while we change
  the other.
• Then we see what effect the change had on the
  rate.

15

• Rate kAmBn (general form)
• Data double A ? the rate doubles
• double B ? the rate doubles
• triple A ? the rate triples
• triple B ? the rate triples
• The rate is directly proportional to the
  concentrations of A and B. The reaction is first
  order in A and first order in B. m and n both
  equal 1. The rate law is
• Rate kAB

16

• A different reaction
• Data double A ? the rate quadruples
• double B ? the rate doubles
• The reaction is second order in A (m2) and first
  order in B (n1). The rate law is
• Rate kA2B

17

• Another different reaction
• Data double A ? no change in rate
• double B ? the rate doubles
• The reaction is zero order in A and first order
  in B. m 0 and n 1.The rate law is
• Rate kB

18
Chemical Kinetics

• This is the kind of data used to support
  mechanisms!!
• If a reaction is first order in A and first order
  in B, then A and B must both be involved in the
  rate determining step.
• What mechanism does that sound like?
• If a reaction is first order in A and zero order
  in B, then only A is involved in the rate
  determining step.
• What mechanism does that sound like?