The proposed work
aims to design experiments using state-of-the-art computer-controlled
analytical instruments to expand and strengthen the undergraduate
organic chemistry laboratory. These methodologies include
but are not limited to the following: Uv-vis near infrared,
fluorescence, and Fourier-transform infrared spectrophotometry;
proton nmr spectrometry, and high performance liquid chromatography.
Using automated instrumentation will allow students to
spend less time conducting routine laboratory procedures
while concentrating on developing their analytical skills
correlating and interpreting experimental data.
The project started with a close examination of the curriculum
in the organic chemistry laboratory to determine how instrumentation
can be incorporated to complement existing experiments. Unlike
general chemistry, the organic lab is highly sequential in that
certain basic techniques must be covered regardless of the textbook
that is being used in lecture. Consequently, care must be exercised
in deciding at what point instrumental analysis can be most effectively
used to teach the subject.
After careful consideration of the course objectives, goals,
and overall methodology involved in conducting each experiment,
it was determined that at least three instruments could be used
to facilitate learning in the organic lab. These instruments
are a proton nmr spectrometer, a Fourier-transform infrared spectrometer,
and an ultraviolet-visible spectrophotometer.
Nevertheless, because of the empirical nature of methods development,
it was possible to develop a protocol for only one instrument,
namely, the UV-vis spectrophotometer. In particular, it was determined
that the kinetics of solvolysis of tert-Butyl chloride, which
is conducted by the students during the first semester of organic
lab (CHM 2210L), could be significantly improved using spectrophotometric
Project Goals. The kinetics lab is currently performed by the
students under conditions which are primitive. For example, the
experiment depends on accurately controlling the concentration,
temperature, and endpoint of each run. Temperature control and
endpoint detection are particularly problematic and errors proliferate
throughout resulting in lower student performance.
The use of our Perkin-Elmer Lambda 20 spectrophotometer drastically
improves the effectiveness of the kinetics experiment as follows:
a) The instrument is many times more sensitive than the unaided
human eye for detecting the color change of bromophenol blue
which is used as an indicator dye. Therefore, using a spectrophotometer
dramatically improves endpoint detection, that is, the time at
which the kinetic run must be stopped; b) The instrument’s
thermostated cell holder can provide temperature control within
0.1 degrees as compared with the typical student setup (an open
water bath) in which temperature fluctuations exceeding one degree
or more are considered “normal”; c) Because the spectrophotometer
is computer-controlled it can automate data acquisition to allow
teachers to train students to use standard programs such as Microsoft
Excel for subsequent analysis and correlation of their results.
Outcomes. To facilitate integration into the teaching lab, the
protocol was developed in a modular fashion making it possible
for instructors to use the method completely, or perhaps only
certain parts of it, according to their preference. The Lambda
20 spectrophotometer will allow our students to accomplish the
following objectives in order of increasing difficulty:
• To directly observe a first order kinetic decay and obtain a plot of
absorbance versus time. Students can then graphically evaluate the reaction half-life
and thereby calculate the corresponding rate constant.
• To linearize the unimolecular decay equation and graphically evaluate
the rate constant from the decay data.
• To linearize the unimolecular decay equation
and use LINEST in Microsoft Excel to evaluate the rate
constant from the decay data.
• To use the SOLVER in Microsoft Excel for non-linear regression of the
decay to evaluate the rate constant under the following conditions: a) in various
acetone-water mixtures, b) in the presence of different leaving groups, such
as bromide and iodide, and c) at different temperatures.
• To use LINEST in Microsoft Excel to measure the Arrhenius activation
Assessment and Evaluation. In order to integrate a new experiment
and/or methodology into the teaching laboratory, it is of the
utmost importance to carefully test and validate the procedure.
Accordingly, most of the Summer term 2001-3,4 was dedicated to
optimize the spectrophotometric analysis.
Approximately one hundred experiments were conducted in which
the rate of solvolysis was measured under various conditions.
For example, the effect of the solvent was assessed by measuring
the rates in mixtures of water/acetone; the effect of the leaving
group was also assessed using tert-butyl bromide and iodide,
respectively; finally, the effect of temperature on the reaction
rate was assessed. The data generated from these experiments
were found to be in excellent agreement with published literature
values, thereby validating the method.
The final question remained as to whether beginning students
with limited skills would be able to implement the method and
obtain meaningful results. Although the protocol that was developed
has not been “field” tested, it is noteworthy that
two assistants (former organic chemistry students) helped to
develop the method and actually ran many experiments producing
high quality data without encountering major difficulties.
||The results of this
project will be disseminated at three different levels. At
the departmental level, the results will be shared with colleagues
in the form of a typewritten report which gives a detailed
description of the method. As pointed out above, the procedure
was developed in modular form so that instructors have the
option to use the method completely, or perhaps only certain
parts of it, according to their preference.
The project will be disseminated College-wide in the form of a
50-minute presentation during Professional Development Day in March
2003. The presentation will enable the author to share the results
with chemistry faculty elsewhere at the College.
Finally, and perhaps most importantly, a project can only be considered
a true innovation when it is accepted outside the boundaries of
the institution in which it originated. Although the subject matter
is not original – since the solvolyses experiments described
here are well documented in the published literature – the
methodology for measuring the rate constant is original. To the
best of the author’s knowledge, the reaction of alkyl halides
with water in acetone has not been previously studied spectrophotometrically,
for pedagogical purposes.
Accordingly, a manuscript describing this work will be submitted
to the Journal of Chemical Education. The J. Chem. Ed. is the world’s
premier journal for dissemination of innovative teaching methodologies
in chemistry. It will probably represent the first time that an
article from mdc has been submitted (and hopefully accepted) for
publication in the journal.