WK 14 Organic Chemistry Experiment Sn1 and Sn2 Reactions Lab Report

The following is an example of an investigative lab report. Most of theexperiments in Organic Chemistry I will follow this lab report format.
Everything shown in bold below is what should actually be written in your
report.
Both your name and date should appear on separate lines on the right hand side of
the page.
Your Name
Date
Title of the Experiment Centered in the Page
Experiment 2 Extraction and Evaporation. Separating the Components of
“Panacetin”
I. Introduction
The introduction should be in your own words and explain the purpose behind the
experiment. This will only require at most 10 sentences. The purpose can be found
in the reading section from the departmental handout.
II. Safety
This section should have the NFPA rating for each chemical presented in the
demonstration video. This will include the Health, Flammability, Reactivity, and
other hazard ratings. You can find these ratings by doing an internet search of the
chemical.
III. Experimental Procedure
While watching the demonstration video for the experiment, write the procedure
stepwise, numbering your steps, in your own words. There should be enough detail
such that anyone who reads your report could do the experiment without the use of
a lab textbook.
IV. Observations and Results
This section will include things such as boiling range, melting point, color, time for
reaction, spectra and etc. This section will include results related to your
observations such as melting point and purity relationship, percent yield, percent
recovery etc.
V. Theory and Conclusions
This section should indicate how your results related to the purpose of the
experiment and what can be interpreted from your results. This section should
indicate how your results prove the theory behind the reaction or procedure. Most
of the information on the theory section can be found in your lecture book or in the
prereading section of the lab experiment as well as the reading sections from the
textbook, and research using the ACC library particularly AccessScience, KirkOthmer Encyclopedia of Chemical Technology, Van Nostrand’s Scientific
Encylopedia, and Gale ebooks. This section will be graded the hardest and
rambling results in grade deduction. You should be able to make your point in
3 to 5 pages. YOU MUST CITE YOUR SOURCES USING SUPERSCRIPTS TO
INDICATE ANOTHER AUTHOR’S WORK.
VI. Citations
Any format is acceptable, I tend to use MLA.
Tests of Substitution Mechanisms
PURPOSE
To test some aspects of nucleophilic substitution reaction mechanisms.
LEARNING OBJECTIVES
Students will learn to:
• Identify the effects of substitution patterns on nucleophilic substitution reactions
• Identify the effects of solvent on nucleophilic substitution reactions
READING ASSIGNMENT
There is no material to read in “Laboratory Techniques in Organic Chemistry” (4th ed.) by Mohrig,
et al. You may wish to review your lecture materials related to nucleophilic substitution
reactions of alkyl halides.
BACKGROUND AND DISCUSSION
Some of the substitution reactions discussed in the lecture this semester include
nucleophile substitution of alkyl halides.
Nucleophilic Substitution
Nucleophilic substitution involves replacing a leaving group (typically a halogen) on an
carbon atom with a nucleophile.
sp3-hybridized
R-X + Nuc-
____>
R-Nuc + X-
It has been well established that nucleophilic substitution occurs by two different
mechanisms: SN1 and SN2.
(1) SN2 Mechanism
The SN2 mechanism is a concerted process where the nucleophile replaces the leaving
group in a single step, with no reaction intermediate.
Reactivity of the substrate goes as follows: methyl > 1o > 2o > 3o, where 3o substrates
rarely react by SN2. This order is driven by transition state stability. In the transition state, there
are 5 groups surrounding carbon, resulting in significant steric hindrance. With smaller groups,
less crowding occurs, resulting in a faster reaction.
(2) SN1 Mechanism
The SN1 mechanism is a two-step process and involves a carbocation intermediate.
Looking at the mechanism, you will see that the first step is rate-limiting. In this step a
carbocation is formed. Its stability affects reaction rate. Carbocations are fundamentally
electron deficient and alkyl groups inductively donate electron density, increasing their stability.
The more stable the carbocation, the faster it forms, and the faster the rate of the reaction. As
a result, reactivity of the substrates follows the pattern: 3o > 2o > 1o > methyl.
(3) Effect of solvent
Solvent plays and important role in nucleophilic reactions. To begin with, solvents can be
polar or nonpolar. In nucleophilic substitution reactions, polar solvents are necessary because
the nucleophile, leaving group, transition states and intermediates are all polar. Remember the
rule “like dissolves like”.
The next consideration is that polar solvents can be protic or aprotic. Protic solvents are
those that undergo hydrogen bonding (e.g., H2O, ROH, NH3). Aprotic solvents do not undergo
hydrogen bonding (e.g., CH2Cl2, CH3CN, acetone).
Experiments suggest that polar protic solvents hinder the SN2 process, because they
strongly solvate the nucleophile, inhibiting its ability to react. Polar aprotic solvents do not
solvate the nucleophile as strongly, allowing it more freedom to react with the substrate.
Reactions occurring by the SN1 mechanism are favorably affected by polar protic
solvents. The transition state and intermediates are more polar than in SN2. Plus, it is likely that
the solvent assists in carbocation formation, by pulling the leaving group away from carbon.
In this experiment, you will study the effect of solvents on two different nucleophilic
substitution reactions.
In the first reaction, you will observe the reaction of alkyl bromides with silver nitrate
dissolved in ethanol (EtOH). A balanced equation for the process is as follows:
R-Br + EtOH + AgNO3
____>
R-OEt + AgBr ↓
The visual indicator is the formation of AgBr precipitate. The rate of precipitation is the
indicator of how fast the reaction occurs. Note, ethanol serves as both the solvent and the
nucleophile for this reaction. AgNO3 does not participate directly in the substitution part of the
reaction. It is added because it is soluble in ethanol, but the product, AgBr, is not.
In the second experiment, you will observe the reaction of alkyl bromides with sodium
iodide dissolved in acetone.
R-Br + NaI
____> R-I
+ NaBr ↓
The visual indicator here is the formation of NaBr precipitate. NaBr is insoluble in
acetone, whereas NaI is soluble.
Before you begin, fill out the prelab section of the report form, according to your
instructor’s directions.
SAFETY INFORMATION
There are a variety of hazards associated with all of the chemicals used in these
experiments. Dichloromethane and the alkyl bromides are toxic and are harmful if inhaled.
Many are suspected carcinogens. Bromine is toxic and corrosive and produces dangerous
vapors. Sodium iodide in acetone produces a lachrymator so avoid the fumes.
EXPERIMENTAL PROCEDURE
Perform all experiments in a well-ventilated area.
A.
Nucleophilic Substitution
(1) Polar protic solvents
Obtain 5 clean, dry test tubes and label with the following names: 1-bromobutane, 1chlorobutane, 2-bromobutane, 2-chlorobutane, and 2-chloro-2-methylpropane. Add 2 mL of a
0.1 M silver nitrate in ethanol solution to each test tube. Then add 1 drop of each alkyl halide to
the appropriately labeled test tube, one after the other, as quickly as possible. Once all have
been added, carefully shake the test tubes. You are looking for a precipitation reaction. Record
the time required for appearance of the first sign of cloudiness. If, after 10 minutes, a relative
order of reactivity cannot be established, put the test tubes in a warm water bath for up to 10
minutes. If after the time has elapsed, no reaction occurs, then record this result.
(2) Polar aprotic solvents
Obtain 5 clean, dry test tubes and label with the following names: 1-bromobutane, 2chlorobutane, 2-bromobutane, 2-chlorobutane and 2-chloro-2-methylpropane. Add 1 mL of a
15% sodium iodide in acetone solution to each test tube. Then, add 2 drops of each alkyl halide
to the appropriately labeled test tube, one after the other, as quickly as possible. Once all have
been added, carefully shake the test tube. Again, you are looking for a precipitation reaction.
Record the time required for appearance of the first sign of cloudiness (discoloration without
precipitate is not a sign of reaction). If an order of reactivity is not established after 10 minutes,
place the test tubes in a warm water bath for up to an additional 10 minutes. After this time has
passed, record the final results.
OBSERVATIONS AND RESULTS
Observations are in the table below and include if a precipitate formed and if elevated
temperature was required. Complete the expected mechanism column as Sn1 or Sn2.
Alkyl Halide
Expected
Mechanism
NaI
ppt
formed
Time to
React
3 min
No heat
AgNO3
EtOH
No rxn
Time to
React
Expected Products if
Reaction Occurs
I
Br
O
ppt
formed
Cl
ppt
formed
Br
No rxn
Cl
5 min
total
3 min at
RT
2 min
heat
7 min
total
3 min at
RT
4 min
heat
No rxn
I
O
ppt
formed
3 min
I
O
ppt
formed
4 min
I
O
No rxn
Cl
ppt
formed
10 sec
I
O
THEORY AND CONCLUSIONS
For the theory and conclusions section, give a short synopsis of Sn1 and Sn2. Also for any
of the alkyl halides that reacted, write out a mechanism using arrow notation and any
necessary transition states to show the reactions that took place. The nucleophiles as
chosen in each case were weak nucleophiles and as such will only follow a substitution
reaction if a reaction occurred.