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Substitution Reactions 1: The Sn 2 Reaction The Synthesis of 1-Bromobutane. Organic Chemistry Lab II, Fall 2009 Dr. Milkevitch November 9 & 12, 2009. Substitution Reactions. Today : Learn about an Sn 2 reaction Type of reaction: Substitution One thing substitutes for another

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substitution reactions 1 the sn 2 reaction the synthesis of 1 bromobutane
Substitution Reactions 1: The Sn2 ReactionThe Synthesis of 1-Bromobutane

Organic Chemistry Lab II, Fall 2009

Dr. Milkevitch

November 9 & 12, 2009

substitution reactions
Substitution Reactions
  • Today: Learn about an Sn2 reaction
  • Type of reaction: Substitution
      • One thing substitutes for another
  • Primarily a reaction of alkyl halides
the sn 2 reaction
The Sn2 Reaction
  • Sn2 = Substitution, nucleophilic, bimolecular
      • Substitution: One things substitutes for another
      • Nucleophilic: Nucleophile does the substituting
        • Something must leave
          • Called the leaving group
      • Bimolecular:The kinetics of the reaction involve the concentrations of 2 reactants
mechanism of the sn 2 reaction

_

Nu

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

Mechanism of the Sn2 Reaction
  • General mechanism:

substrate

L

= leaving group, the species that leaves

more detail

_

+ L

Nu

Transition state

More Detail

substrate

Nucleophile

substitutes for

L group

factors that affect sn 2 reactions
Factors That Affect Sn2 Reactions
  • Strength of nucleophile
  • Structure of substrate
  • Nature of solvent
  • Concentration of reactants
  • Nature of the leaving group
kinetics of the sn 2 reaction
Kinetics of the Sn2 Reaction
  • Reaction kinetics: how fast a reaction goes
      • Appearance of product per unit time
  • Rate Law:
    • Rate = k{A}x{B}y
  • Reaction rate dependent on concentrations of reactants
      • First order reaction: rate dependent on the concentration of one reactant
      • Second order reaction: rate dependent on the concentration of both reactants
kinetics of the sn 2 reaction8
Kinetics of the Sn2 Reaction
  • “2” means the kinetics are second order
  • Rate of reaction dependent on concentration of both reactants
  • Double concentration of either one, rate doubles
mechanism

Br

Good leaving

group

Mechanism

H2SO4 + NaBr

HBr + NaHSO4

+ H2O

+ H

reflux
Reflux
  • Reflux:continual boiling

of a solution in a vial or flask

where solvent is continually

returned to the reaction vessel

from a condenser atop the vial

or flask

  • Water cooled condenser is used
  • Possible to heat a reaction at the boiling point of the solvent for extended periods
procedure
Procedure
  • Weigh out 12.5 g of NaBr and add it to 12 ml of ddH2O in a 100 ml RB flask (with stir bar)
  • Clamp to ring stand, stir until NaBr dissolves
  • Add 10 ml of 1-butanol to this flask
  • Place in an ice bath and continue to stir
  • Measure out 11 ml of concentrated H2SO4 into a clean 50 ml erlenmeyer flask
  • Put this in its own ice bath
  • Carefully add the acid to the solution of NaBr and 1-butanol in small amounts (maybe 0.5 ml) with stirring. Both solutions must be in ice baths.
  • Once completed, remove the RB flask (with acid, 1-butanol in it) and fit a heating mantle with condenser.
  • Reflux for 45 min with stirring.
  • When done refluxing, remove the condenser and fit a still head for distillation.
  • Using a 50 ml RB flask for collection, distill until 20-25 ml of distillate has been collected.
  • You should have 2 layers in the receiving flask.
  • Parafilm the receiving flask and leave it until next week. We will complete the experiment then.
acid addition
Acid Addition
  • Set up your solution of NaBr/H2O/1-butanol for acid addition as follows:

RB flask

50 ml erlenmeyer flask

with acid, in an ice bath

Crystalling

Dish w/ ice

Stir plate

your report
Your Report
  • This is a 2 week experiment
    • This week: the synthesis
    • Next week: purification & characterization
substitution reactions 1 the sn 2 reaction the synthesis of 1 bromobutane part ii workup
Substitution Reactions 1: The Sn2 ReactionThe Synthesis of 1-BromobutanePart II: Workup

Organic Chemistry Lab II, Spring 2009

Dr. Milkevitch

November 16 & 18, 2009

procedure ii
Procedure II
  • Remove parafilm, pour your distillate (solution in RB flask) into a 125 ml separatory funnel
  • Add 50 ml of ddH2O to the separatory funnel, stopper and shake like you did in the extraction lab
  • Allow layers to separate, draw off bottom organic layer into a 125 ml erlenmeyer flask
  • Pour off the upper (aqueous) layer and set aside (do NOT throw away yet)
  • Transfer the organic layer back into the separatory funnel and add 25 ml of H2O
  • Stopper and shake the separatory funnel again, let layers separate
  • Draw off the lower organic layer again, into a 125 ml erlenmeyer flask
  • Pour off the upper (aqueous) layer and set aside (again do NOT throw it away yet)
  • Transfer the organic layer back into the separatory funnel, add 25 ml of saturated sodium bicarbonate solution (the bicarb solution is in the hood)
procedure iii
Procedure III
  • Swirl the separatory funnel, notice CO2 escaping. Carefully shake the separatory funnel with frequent venting
  • Allow the layers to separate
  • Draw off the bottom organic layer into a 125 ml erlenmeyer flask
  • Pour off the upper water layer and set aside (again, do NOT throw away yet)
  • Filter the organic layer through a layer of anhydrous magnesium sulfate.
  • Collect the dried organic layer
  • Place the dried organic layer in a pre-weighed 50 ml RB flask with a spin bar
  • Distill the organic layer until most of the liquid in the distilling flask has distilled over (about 105 deg C)
  • Weigh the receiving flask, determine weight of product
characterization
Characterization
  • Analyze a sample of your product by GC
  • Analyze by TLC :
    • Spot standards: 1-bromobutane,1-butanol, dibutyl ether
    • Spot your distilled product also
    • Mobile phase: ethyl acetate
    • Analyze under UV light in UV light box
  • If you have enough product, acquire an IR spectrum:
    • Consult with Dr. M.
  • Complete reaction worksheet