General Chemistry II – CHEM 2124
Lab #9 – Major Lab 2
Dehydration of t-Butyl Alcohol
In this laboratory exercise, you will be performing a simple organic chemistry reaction. This reaction is
an acid-catalyzed dehydration of an alcohol. The products of this reaction are an alkene and water.
Specifically, you will convert t-butyl alcohol into two alkene products: 2,4,4-trimethyl-1-pentene and
2,4,4-trimethyl-2-pentene. You will be performing two laboratory techniques in this part of the
experiment: a reflux and simple distillation.
Figure 1 – Dehydration of t-butyl alcohol
+ H+
+
Objectives:
1. Prepare a mixture of two alkene products (2,4,4-trimethyl-1-pentent and 2,4,4-trimethyl-2pentene) by dehydrating t-butyl alcohol.
2. Perform some basic analytical techniques commonly utilized in an organic chemistry laboratory:
refluxing and distillation.
3. Gain a basic understanding of organic chemistry reaction mechanisms and explain the steps in
the mechanism of the reaction described in objective #1.
4. Use the analytical technique of gas chromatography-mass spectrometry to analyze the product
obtained after performing the reaction described in objective #1.
Procedure:
1. In a 50-mL round bottomed flask, place 7 mL of distilled water and add cautiously with swirling
7 mL (12 g, 0.12 mole) of concentrated sulfuric acid. (CAUTION – H2SO4 is extremely
corrosive!)
2. Cool the diluted acid to about 50°C.
3. SLOWLY add 5 grams (6.3 mL) of t-butyl alcohol.
4. Attach at once a reflux condenser (figure 2) and boil the material gently for 30 minutes.
5. Remove the reaction vessel from the heat and cool the mixture to room temperature.
6. Transfer the reaction mixture to a separatory funnel (figure 3) and carefully remove the lower
aqueous layer into a waste beaker.
7. Wash the remaining hydrocarbon layer with water to remove traces of acid and dry it with about
0.5 gram (a pinch) of anhydrous calcium chloride. Remove the water layer into a waste beaker.
8. Decant the dried liquid into a small flask arranged for distillation.
9. Set up the simple distillation apparatus (figure 4) and distill your sample.
10. Collect the fraction that boils over into the collection flask (a 10-mL graduated cylinder)
11. The yield will be about 1.8-2.0 grams (2.5-3.0 mL)
Figure 2 – Reflux apparatus
Figure 3 – Separatory funnel
Figure 4 – Simple
distillation apparatus
Introduction to Organic Reaction Mechanisms
One of the goals of organic chemistry is to figure out the “mechanism” of reactions. A mechanism is
the set of exact steps by which a molecule is converted to another. There are several ways to describe
these steps and one way is shown below. It focuses on atom transfers between reactants and the
resulting charges and structures that occur. Another way is to follow the flow of electrons, a process
sometimes referred to by chemists as “electron pushing.” The atom transfer method and electron
pushing method are complementary and result in the same steps and results.
A very common pathway (but by no means the only pathway) is for an initial attack to go through a
“transition state” to form an intermediate which is then further transformed into the final product. In a
general form this looks like:
Step 1 A + B
[transition state]
Step 2 I + C
P (final product)
I (the intermediate)
The details of how these transformations happen are indicated by a series of arrows and dotted lines
indicating bond breaking/making activities. The charge formed and importantly their placement reveals
a lot about trends in various compound reactivities. A specific example, detailing the steps of the
hydrobromination of an alkene compound is shown below.
In this particular laboratory exercise, the student performed an acid-catalyzed dimerization via
dehydration of isobutyl alcohol. The mechanism of this specific reaction is detailed below:
Play by play notes for
each step:
Initial Alkene Formation
Step 1: Protonation of t-butyl alcohol: a reversible equilibrium
Concentrated sulfuric
acid and refluxing favors
the product side
Higher concentration of
protonated alcohol
favors primary
carbocation formation
and subsequent
elimination
Step 2: Rate limiting step: Decomposition of the protonated
alcohol to a primary carbocation
Alkene is more stable
than the carbocation but
not more stable than the
alcohol..so heating is
needed..but, the initial
alkene is volatile (B.P. of
-7°C so gentle heating is
needed.
Step 3: Proton transfer from primary carbocation to water to
form the initial alkene
Dimerization of Primary Carbocation with Initial
Alkene to form the Pentene Mixture
The dimerization here is
favored by high
concentrations of the
primary carbocation and
alkene, and the
secondary carbocation
may further polymerize.
Step 4: Secondary attack of carbocation on initial alkene to
Form a secondary carbocation
Step 5: Final elminiation of hydrogen cation to form two
possible alkene products
LESS
SUBSTITUTED
MORE
SUBSTITUTED
The major product here
is determined
theoretically by Zaitsev’s
Rule, which predicts that
the major product will be
the one more substituted
around the C=C bond.
Does this really happen
in this reaction?
Introduction to Refluxing and Distillation
Most organic reactions do not occur quickly at room temperature but require a period of heating. If the
reaction were heated in an open container, the solvent and other liquids in the system would soon
evaporate; if the system were closed, pressure could build up and it could explode when heated.
Chemists have developed a simple method of heating a reaction mixture for extended periods of time
without loss of reagents. This process is called refluxing which simply means boiling a solution while
continually condensing the vapor by cooling and returning the liquid to the reaction flask. A condenser
mounted vertically above the reaction flask provides the means of cooling the vapor so it condenses
and flows back into the reaction flask.
In a simple distillation, only one or two vaporizations and condensations occur. The condensed liquid
is called the distillate or condensate. One purpose of performing a simple distillation is to separate
two products with different boiling points. However, simple distillation does not effectively separate a
mixture of liquids whose boiling points differ by less than 60-70°C. Thus, organic chemists use simple
distillations in two commonly encountered situations: (1) when the last step in the purification of a liquid
compound involves simple distillation to obtain a pure product and determine its boiling point; (2) when
simple distillation is used to remove a low-boiling solvent from a dissolved organic compound with a
high boiling point.
Introduction to Gas Chromatography-Mass Spectrometry
Gas chromatography-mass spectroscopy (GC-MS) is one of the so-called hyphenated analytical
techniques. As the name implies, it is actually two techniques that are combined to form a single
method of analyzing mixtures of chemicals. Gas chromatography separates the components of a
mixture and mass spectroscopy characterizes each of the components individually. By combining the
two techniques, an analytical chemist can both qualitatively and quantitatively evaluate a solution
containing a number of chemicals. The uses for GC-MS are numerous. They are used extensively in
the medical, pharmacological, environmental, and law enforcement fields.
Gas Chromatography
In general, chromatography is used to separate mixtures of chemicals into individual components.
Once isolated, the components can be evaluated individually. In all chromatography, separation occurs
when the sample mixture is introduced (injected) into a mobile phase. In gas chromatography (GC),
the mobile phase is an inert gas such as helium. The mobile phase carries the sample mixture through
what is referred to as a stationary phase. The stationary phase is a usually chemical that can
selectively attract components in a sample mixture. The stationary phase is usually contained in a tube
of some sort. This tube is referred to as a column. Columns can be glass or stainless steel of various
dimensions. The mixture of compounds in the mobile phase interacts with the stationary phase. Each
compound in the mixture interacts at a different rate. Those that interact the fastest will exit (elute from)
the column first. Those that interact slowest will exit the column last. By changing characteristics of the
mobile phase and the stationary phase, different mixtures of chemicals can be separated. Further
refinements to this separation process can be made by changing the temperature of the stationary
phase or the pressure of the mobile phase.
Our GC has a long, thin column containing a thin interior coating of a solid stationary phase (5%
phenyl-, 95% dimethylsiloxane polymer). This 0.25 mm diameter column is referred to as a capillary
column. The capillary column is held in an oven that can be programmed to increase the temperature
gradually (or in GC terms, ramped). This helps our separation. As the temperature increases, those
compounds that have low boiling points elute from the column sooner than those that have higher
boiling points. Therefore, there are actually two distinct separating forces, temperature and stationary
phase interactions mentioned previously.
As the compounds are separated, they elute from the column and enter a detector. The detector is
capable of creating an electronic signal whenever the presence of a compound is detected. The greater
the concentration in the sample, the bigger the signal will be. The signal is then processed by a
computer. The time from when the injection is made (time zero) to when elution occurs is referred to as
the retention time (RT).
While the instrument runs, the computer generates a graph from the signal. (See figure 1). This graph
is called a chromatogram. Each of the peaks in the chromatogram represents the signal created when
a compound elutes from the GC column into the detector. The x-axis shows the RT, and the y-axis
shows the intensity (abundance) of the signal. In Figure 5, there are several peaks labeled with their
RTs. Each peak represents an individual compound that was separated from a sample mixture. The
peak at 4.97 minutes is from dodecane, the peak at 6.36 minutes is from biphenyl, the peak at 7.64
minutes is from chlorobiphenyl, and the peak at 9.41 minutes is from hexadecanoic acid methyl ester.
Figure 5: Chromatogram generated by a GC.
If the GC conditions (oven temperature ramp, column type, etc.) are the same, a given compound will
always exit (elute) from the column at nearly the same RT. By knowing the RT for a given compound,
we can make some assumptions about the identity of the compound. However, compounds that have
similar properties often have the same retention times. Therefore, more information is usually required
before an analytic al chemist can make an identification of a compound in a sample containing
unknown components.
Mass Spectroscopy
As the individual compounds elute from the GC column, they enter the electron ionization (mass spec)
detector. There, they are bombarded with a stream of electrons causing them to break apart into
fragments. These fragments can be large or small pieces of the original molecules. The fragments are
actually charged ions with a certain mass. The mass of the fragment divided by the charge is called the
mass to charge ratio (M/Z). Since most fragments have a charge of +1, the M/Z usually represents the
molecular weight of the fragment.
A group of four electromagnets (called a quadrupole) focuses each of the fragments through a slit and
into the detector. The quadrupoles are programmed by the computer to direct only certain M/Z
fragments through the slit. The rest bounce away. The computer has the quadrupoles cycle through
different M/Z's one at a time until a range of M/Z's are covered. This occurs many times per second.
Each cycle of ranges is referred to as a scan. The computer records a graph for each scan. The x-axis
represents the M/Z ratios. The y-axis represents the signal intensity (abundance) for each of the
fragments detected during the scan. This graph is referred to as a mass spectrum (see Figure 6).
Figure 6: Mass-spectrum of a hydrocarbon compound.
The mass spectrum produced by a given chemical compound is essentially the same every time.
Therefore, the mass spectrum is essentially a fingerprint for the molecule. This fingerprint can be used
to identify the compound. The mass spectrum in Figure 6 was produced by dodecane. The computer
on our GC-MS has a library of spectra that can be used to identify an unknown chemical in the sample
mixture. The library compares the mass spectrum from a sample component and compares it to mass
spectra in the library. It reports a list of likely identifications along with the statistical probability of the
“match.”
Notes on Writing up Experiment #9 (Synthesis of 2,4,4-trimethyl-1-& 2-pentenes) as a Major Lab
DUE FRIDAY, APRIL 20 by 4PM to the brown box on the floor outside of AG 237
(NO LATE PAPERS ACCEPTED!!!) – 45 points total
THEORY
In an organic synthesis write-up, the theory amounts to an explanation of the reaction and the
mechanism of how the overall reaction is supposed to occur, using your own words. A good
theory is a clear and easy-to-read combination of reaction pictures and descriptive language
that demonstrates your understanding of how the reaction occurs, and in particular, provides
insights into the steps you performed in the actual experiment. A large part of your grade
arises from how you do this.
Some things to ponder as you write up your THEORY section:
1) What type of reaction did you carry out? (we discussed this in lecture-how do you
transform an alcohol into an alkene – what is that type of reaction called)?
2) What are the exact step-by-steps to the elementary reaction mechanism in order
as presented in class? I do not expect you to explain all of the details of each step,
but I should see a diagram indicating what the five steps are in this reaction
mechanism along with a few notes about each step. (see previous pages)
3) What role does the sulfuric acid play?
4) What is a carbocation and where is one formed in this mechanism? (Explain this in
the mechanism section)
5) What is refluxing and what does it do in this reaction? You should include a brief
discussion on the theory of refluxing (how it works) and a diagram of a reflux
condenser here.
6) What is distillation and what role does it play in this type of reaction? You should
include a brief discussion on the theory of distillation (how does it work?) and a
diagram of a simple distillation apparatus here.
7) Which of the two products should be the major one and which should be the minor
one? Why?
8) A brief description of how the tests performed allow you to identify the products
formed. Please do not describe gas chromatography and mass spectrometry in
detail but display to me that you have a basic knowledge of how you can separate
components of a mixture and then identify them through the instrumental analysis
performed. This should be no more than 4-6 sentences in length.
DO NOT ANSWER THESE QUESTIONS IN LIST FORM. YOU MUST WRITE SENTENCES AND
PARAGRAPHS LABELLED WITH DIAGRAMS!!!
You will also need to include a table of pertinent materials data for all of the reactants and products of
this reaction like the one shown below. You will need to use an outside source to find the information
needed to fill in the table. This should be included within your theory section.
Table of Pertinent Materials Data for Synthesis of 2,4,4-trimethyl-1 & 2-pentenes
(Copy this table into your paper but note that you must fill in the blanks and note which product is
predicted to be major and which is predicted to be minor – Include a title for the table!)
Compound
MW (g/mol)
Density
(g/mL)
bp (°C)
Amount
used (g or
mL)
Moles
Used
t-butanol
Notes
Limiting
Reagent
Catalyst
Sulfuric acid
2,4,4-trimethyl-1pentene
N/A
N/A
product
2,4,4-trimethyl 2pentene
N/A
N/A
product
RESULTS
Unlike the previous major lab report (acids and bases) you will include a section entitled
RESULTS. This section will contain details of how you analyzed your final products to ensure
that what you actually did obtain is what you wanted to obtain. Specifically, report the following:
1) Chemical Analysis of Product
Br2 test for alkenes – describe the results and conclusions of this test.
2) Instrumental Analysis of Product
Attach the collected instrumental analysis printouts of your product to the end of your
report (GC/MS). Make sure to annotate (HAND WRITE) these with your name and
the date. You should also include the instrumental conditions somewhere in this
section. See later in this handout for the instrumental conditions you MUST include in
your report.
Some overall suggestions:
- You should write this paper up as if you are actually writing a scientific paper – that is
each section should be clearly defined with a heading and should be easy to find.
- You should include a title at the top of your report which is the title of this lab (Synthesis of
2,4,4-trimethyl-1 & 2 pentenes from t-Butyl Alcohol)
-
You will be graded on neatness, general workmanship and evidence of care – that is, did
you put an effort into this paper or did you throw it together at the last minute?
Check your grammar, spelling and use of punctuation – this will be graded as well.
DO NOT COPY THE MATERIAL PRESENTED IN HANDOUTS – YOU WILL RECEIVE A
MUCH HIGHER GRADE IF THE WORDS ARE YOUR OWN AND NOT COPIED. This
applies specifically to the theory of refluxing, theory of distillation, steps in the reaction
mechanism.
PLEASE DO NOT WAIT UNTIL THE LAST MINUTE TO WRITE THIS
PAPER—YOU WILL NOT BE HAPPY!!!!
Grading rubric for Experiment #9 – 45 Total Points
(Synthesis of 2,4,4-trimethyl-1 & 2 pentenes)
Theory: (35 points)
 Descriptive sentence of the type of reaction carried out here (2)
 Presentation of the exact step-by-step elementary reactions in the reaction mechanism as
presented in class – diagrams should be neat, clear and easy to follow in ALL 5 steps
and some notes about each step should be present – Make sure you note which step is
the rate-limiting step (12) – Max of 5 points if diagrams not included.
 Explanation of the role of sulfuric acid (1)
 Definition of a carbocation and what role it plays in this reaction (2)
 Explanation of what refluxing is – theory and diagram of reflux mechanism we used must be
present, clear, neatly drawn and properly labeled (4)
 Explanation of what distillation is – theory and diagram of distillation apparatus we used must be
present, clear, neatly drawn and properly labeled (4)
 Sentence(s) describing which product is the major one, which is the minor one, and a clear
explanation of the rule which determines this in this type of reaction (4)
 Paragraph describing the role of instrumental analysis in the determination of the identity of
various components in a mixture. (6)
Results: (5 points)
 Chemical Analysis of product – you should explain what test(s) you performed and what the
results are for your product(s) – Example, why did you add Br2? What color change are you
looking for? Did you see this color change? If not, why not? (3)
 Instrumental Analysis of product – Append the collected GC/MS printouts here. They can be
added as pages at the end or you may cut and paste them to you report. Either way, they
should be clearly marked. They should be annotated with your name and the date. Also
include the instrumental conditions. (2)
Overall Presentation: (5 points)
 You will be graded on proper use of the English language including spelling, grammar, and
punctuation
 You will also be graded on the overall neatness of the report including diagrams and tables
 General laboratory workmanship and evidence of care also will be included in the final grade
GC/MS Conditions for Analysis of Product
Instrument Used:
Agilent GC/MS 5975MSD
Oven Program:
75°C for 1 minute, then 10°C per minute to 130°C for 0.5 minutes, total
run time of 8 minutes
Injector Temperature:
250°C
Flow Rate of Carrier Gas:
189.38 ml/minute
Split Ratio:
200:1
Column Type:
DB5, 30 meter x 250 µm, 0.25 µm
MS Scan Parameters:
30.0 amu (low mass) – 350.0 amu (high mass)
MS Source Temperature:
230°C
AS A FINAL REMINDER, THIS LAB REPORT IS DUE BY
4PM ON FRIDAY APRIL 20th TO THE BROWN BOX ON THE
FLOOR OUTSIDE OF PROFESSOR BENSLEY’S OFFICE
(AG 237) – NO LATE PAPERS WILL BE ACCEPTED AND
YOU WILL BE ASSIGNED A GRADE OF ZERO WITHOUT
EXCEPTION FOR ANY LATE SUBMISSION.