1. Organic Chemistry Lab II, Fall 2009
Substitution Reactions 1: The Sn2 Reaction The Synthesis of 1-Bromobutane
Organic Chemistry Lab II, Fall 2009
Dr. Milkevitch
November 9 & 12, 2009

2. Substitution Reactions
Today: Learn about an Sn2 reaction
Type of reaction: Substitution
One thing substitutes for another
Primarily a reaction of alkyl halides

3. 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

4. Mechanism of the Sn2 Reaction
General mechanism:
substrate Nu _
= nucleophile (“nucleus loving”), species that seeks a + charge L
= leaving group, the species that leaves

5. More Detail Transition state + L Nu _ Nucleophile substitutes for
substrate Nu _
Nucleophile
substitutes for
L group

6. Factors That Affect Sn2 Reactions
Strength of nucleophile
Structure of substrate
Nature of solvent
Concentration of reactants
Nature of the leaving group

7. 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

8. 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

9. Synthesis of 1-bromobutane from 1-butanol
Acid catalyzed reaction

10. Mechanism
H2SO4 + NaBr
HBr + NaHSO4
Good leaving
group Br
+ H2O
+ H

11. Reflux Reflux: continual boiling Water cooled condenser is used
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

12. 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 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.

13. 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

14. Your Report This is a 2 week experiment This week: the synthesis
Next week: purification & characterization

15. Organic Chemistry Lab II, Spring 2009
Substitution Reactions 1: The Sn2 Reaction The Synthesis of 1-Bromobutane Part II: Workup
Organic Chemistry Lab II, Spring 2009
Dr. Milkevitch
November 16 & 18, 2009

16. 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)

17. 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

18. 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