Nucleophilic substitution reactions (SN2) :
Objective: To determine the factors that affect the rate of SN2 reactions
Procedure
All test tubes must be CLEAN and dry.
Section I: Effect of substrate structure
Objective : To determine the influence of the structure of the substrate (primary/secondary/tertiary halide) on the rate of substitution
Substrates to be used (A) 1-bromobutane, (B) 2-bromobutane and (C) 2-bromo-2-methylpropane
Reaction conditions: RT, Solvent – acetone, Nucleophile-Iodide(in Excess)
Place three clean dry test tubes in a test tube rack and label them for each substrate
Measure 1 mL of 0.80M sodium Iodide in acetone into each of the test tubes.
As you start a stop watch, add 50ul of substrate A into the first test tube. [Make sure that you place the tip of the pipet into the liquid in the test tube before you deliver the substrate]. Mix the contents of the test tube by shaking the test tube gently and frequently. Watch closely for appearance of cloudiness or formation of a white precipitate. Note down the time you observed the precipitate or cloudiness
Note down the time in the stop watch as you add substrate B into the second test tube, as described in step 3.
Note down the time in the stop watch as you add substrate C into the third test tube, as described in step 3.
Effect of substrate structure
Reaction conditions
Substrate Used Time in the stop watch at addition Time in the stop watch when you saw the cloudiness or precipitate Observations Reaction time
1-bromobutane 0;0;0 0;0;25 Thick yellow cloudy milky laser stopped 25 sec
2-bromobutane
0;0;0 0;06;45 Thick lase fattened cloudiness faintly light 6 min 45 sec
2-bromo-2,ethylpropane
0/01/05 0;10;21 Laser fattened 9min 16 sec
Analysis – Arrange the substrate reactions in order of increasing rates. Generalize your conclusion
Section 2 : Steric effects
Objective : To determine the influence of the crowding at or near the reaction center on the rate of substitution
Substrates to be used (1) 1-bromobutane and (2) 1-bromo-2-methylpropane
Reaction conditions: RT, Solvent-acetone, Nucleophile -Iodide in excess
Place two clean dry test tubes in a test tube rack and label them for each substrate
Measure 1 mL of 0.80M sodium Iodide in acetone into each test tube.
As you start a stop watch, add 50ul of the first substrate into the first test tube. [Make sure that you place the tip of the pipet into the liquid in the test tube before you deliver the substrate]. Mix the contents of the test tube by shaking the test tube gently and frequently. Watch closely for appearance of cloudiness or formation of a white precipitate. Note down the time when you first observed the cloudiness or precipitate
Note down the time in the stop watch as you add the second substrate into the second test tube, as described in step 3.
Steric effects
Substrate Used Time in the stop watch at addition Time in the stop watch when you saw the cloudiness or precipitate Observations Reaction time
1-bromobutane
0;0;0 0;0;39 Cloudy yellow solution, laser did not shine thru. White precipitate formed. 39 sec
1-bromo-2-methyl propane
0;0;0 0;05;00 No reaction 5 minute
Analysis – Arrange the substrate reaction in order of increasing rates. Generalize your conclusion
Section 3 : Effect of leaving group
Objective : To determine the influence of the nature of the leaving group on the rate of substitution
Substrate to be used (1) 1-bromobutane and (2) 1-chlorobutane
Reaction conditions RT, Solvent – acetone, Nucleophile-iodide (in excess)
Place two clean dry test tubes in a test tube rack and label them for each solvent
Measure 1 mL of 0.80M sodium Iodide in acetone into each test tube.
As you start a stop watch, add 50ul of substrate A into the first test tube.[Make sure that you place the tip of the pipet into the liquid in the test tube before you deliver the substrate]. Mix the contents of the test tube by shaking the test tube gently and frequently. Watch closely for appearance of cloudiness or formation of a white precipitate. Note down the time in the stop watch when you first observed the cloudiness or precipitate
Note down the time in the stop watch as you add substrate B into the second test tube, as described in step 3.
Effect of leaving group
Reaction conditions
Substrate used Time in the stop watch at addition Time in the stop watch when you saw the cloudiness or precipitate Observations Reaction time
1-chlorobutane
0:00:00 0:00:35 Laser did not go thru solution. Solution became cloudy yellow 35 seconds
1-bromobutane
0:00:00 0:00:45 Laser did not pass thru tube. Solution got cloudy and very yellow and laser band got very thick in solution. No precipitate formed 45 sec
Analysis – Arrange the reactions in the two solvents in order of increasing rate. Generalize your conclusion, referencing the leaving group
Section 4 : Determining the rate law
Overall reaction :
The rate law expression for this reaction can be written ask is the rate constant for the reaction,
m- order of the reaction with respect to substrate
n- order of the reaction with respect to nucleophile
Substrate -alkyl halide Nucleophile- Sodium iodide
The experimental task is to determine how the rate of the reaction is affected by the change in concentration of the substrate (determine the value of m) and change in concentration of sodium iodide (determine the value of n)
Objective – to determine how the rate changes when the concentration of each reactant was varied and derive a rate law for the reaction based on the results.
Effect of the concentration of the substrate -alkyl halide, on the rate (determine the value of m)
Substrates to be used (A) 2.0M 1-bromobutane and (B) 1.0M 1-bromobutane
Reaction conditions: RT, solvent -acetone, nucleophile -iodide
Place two clean dry test tubes in a test tube rack and label them for each concentration
Measure 1 mL of 0.80M sodium Iodide in acetone into each test tube.
As you start a stop watch, add 100ul of substrate A into the first test tube. [Make sure that you place the tip of the pipet into the liquid in the test tube before you deliver the substrate]. Mix the contents of the test tube by shaking the test tube gently and frequently. Watch closely for appearance of cloudiness or formation of a white precipitate. Note down the time in the stop watch when you first observed cloudiness or precipitate
Note down the time in the stop watch as you add 100uL of substrate B into the second test tube, as described in step 3.
Effect of substrate concentration
Reaction conditions:
Reaction Substrate concentration Time in the stop watch at addition Time in the stop watch when you saw the cloudiness or precipitate Observations Reaction time
1
2.0M
0.182M
0:00:00 0:03:10 Time 1
2
1.0M
0.0910M 0:00:00 0:06:40 Time 2
Analysis: Determine how doubling the concentration of the substrate changes the rate?
Calculate the order of the reaction with reference to substrate concentration, m
OR
Effect of change in sodium iodide concentration
Substrate to be used 1.0M 1-bromobutane
Reagents to be used :(A) 0.80M sodium iodide and (B) 0.40M sodium iodide
Reaction conditions R, Solvent -acetone, Nucleophile – Iodide
Place two clean dry test tubes in a test tube rack and label them for each concentration
Measure 1mL of 1.0M 1-bromobutane into each of the test tubes.
As you start a stop watch, add 100uL of 0.80M sodium iodide into the first test tube[Make sure that you place the tip of the pipet into the liquid in the test tube before you deliver the substrate]. Mix the contents of the test tube by shaking the test tube gently and frequently. Watch closely for appearance of cloudiness or formation of a white precipitate. Note down the time in the stop watch when you first observed cloudiness or precipitate
Note down the time in the stop watch and add 100uL of 0.40M sodium iodide into the test tube, as described in step 3.
Effect of sodium iodide concentration
Reaction conditions: Reactants used, solvent and temperature
Reaction Sodium Iodide concentration Time in the stop watch at addition Time in the stop watch when you saw the cloudiness or precipitate Observations Reaction time
1 0.0730M 0:00:00 0:01:30 Time 1
2 0.0365M
0:00:00 0:02:20 Time 2
Analysis: Determine how doubling the concentration of the sodium iodide changes the rate ?
Calculate the order of the reaction with reference to silver nitrate concentration, n
OR
Write your rate law expression using your values of m and n
