verify their conclusions.
A Guided Inquiry Experiment for the Organic Chemistry Laboratory: Elimination followed by Dichlorocarbene Addition
The elimination of water from 2-methyl-3-butyn-3-ol followed by the addition of dichlorocarbene to the elimination product affords an excellent reaction sequence for the application of spectroscopic methods for the analysis of organic reaction products. The simplicity and multifuntionality of the reactants and products combine to make the ir and nmr spectra rich with absorptions and straightforward, educational and facile to interpret. The system includes two reactions, elimination and carbene addtion that are commonly included in undergraduate organic laboratory courses. In addition to the spectroscopic challenges, the two step reaction utilizes simple distillation, vacuum distillation, dropwise addition, extraction, gas chromatography, refractometry and a literature search.
For the elimination of water from 2-methyl-3-butyn-2-ol, the literature suggests a 30% sulfuric acid in water mixture.* Although we have found this mixture to work, the yield and the substantial amounts of residue after the first distillation do leave considerable room for improvement. It might be interesting to vary the percentage of H2SO4 and have students compare yields. As the product boils at 32oC, the heating can and should be provided by a steam bath. Students should determine the infrared and nmr spectra, gas chromatograms and refractive indices on both the starting material and product to verify the course of the reaction.
The primary reason for this reaction sequence was to provide a substrate that on paper could react in two different ways with dichlorocarbene. A literature search provided 2-methyl-1-buten-3-yne as a potential example. 2-Methyl-1-buten-3-yne is commercially available but prohibitively expensive as the initial starting material. The inclusion of the elimination reaction has substantial educational value and is a considerably less expensive source of 2-methyl-1-buten-3-yne. Dichlorocarbene could add to 2-methyl-1-buten-3-yne to give two different products. Students should be asked to predict which product will be formed and should be able to come to the conclusion that the addition will occur at the double bond rather than the triple because of extra strain in the cyclopropene system. Spectroscopy is not only useable for the interpretation of the results but essential as one of the two possible products has not been previously synthesized and therefore physical properties of the second possible product are not available for comparison. Both the ir and nmr should be predicted before running by the students and then compared to the results. The results clearly show that the dichlorocarbene has added to the double bond and not the triple bond (the ir shows the absence of a double bond and sp2 C-H and the continued presence of a triple bond and sp C-H and the nmr is consistent with addition to the double bond). Some additional educational benefits are present in the spectroscopy. The ir shows a cyclopropyl C-H stretch at about 3080 cm-1 and this observation can be coupled with a discussion of hybridization in cyclopropanes. The nmr shows two doublets (with a methyl singlet between them) for the two cyclopropyl H's. Many students do not recognize that the cyclopropyl H's are different when they make their predictions.
The dichlorocarbene
addition is nicely suited for a Chemical Abstracts
search as there are only a few references to the product.
The formula index is first used to find the appropriate name of
the product followed by a name search. The Chemical
Abstracts information will help the students
verify their conclusions.
Preparation of 2-methyl-1-buten-3-yne.
1. Assemble a 500 mL 3 necked
round bottom flask equipped with a dropping funnel, a Claisen
adapter connected to a three way adapter with a water cooled
condenser over a steam bath. A 50 mL round bottom flask
colled by a salt-ice-water bath should be used as a receiver.
2. Cautiously add 17 mL of
concentrated sulfuric acid to 40 mL of ice cold water in
an Erlenmeyer flask. Cool to room temperature and add to
the 500 mL flask.
3. Using the dropping funnel, slowly
add 26 mL of 2-methyl-3-butyn-2-ol to the acid. Slowly
heat the mixture using the steam bath. Collect distillate
until the vapor temperature reaches about 40oC.
Transfer the distillate to a 100 mL Erlenmeyer flask containing
a few grams of anhydrous sulfate. Determine the refractive
index, ir, nmr and store in a tightly sealed container in a
freezer. If you do not get at least 0.06 moles of product,
it may be advisable to combine your product with that of another
student and work together on the next part.
Addition of dichlorocarbene to 2-methyl-1-buten-3-yne.
1. Assemble a 125 mL three necked
flask equipped with a magnetic stirring bar, a thermometer, a
dropping funnel and reflux condenser.
2. Introduce into the flask 7.5 g of
potassium t-butoxide (the potassium t-butoxide
must be dry and preferably from a freshly opened bottle), 40 mL
of pentane and 0.06 moles of 2-methyl-1-buten-3-yne. Cool
the mixture to 0oC in an ice bath and with continuous
stirring, add 4 mL of chloroform dropwise. Maintain the
mixture at 0oC for 1 1/2 hours.
3. Hydrolyze by carefully pouring the
mixture onto about 75 grams of ice. Separate the layers
using a separatory funnel and extract the aqueous layer with an
additional 25 mL of pentane tgwice and add these tothe first
extraction.
4. Dry the pentane extracts over
sodium sulfate and then carefully rotary evaporate or distill
off the pentane and unreacted starting material.
5. Set up a small scale or even a
microscale vacuum (aspirator) distillation and collect the
product which distills at 52oC at 20 mm. The distillation
must be done carefully as commonly only about 1 to 2 mL is
collected. This amount is quite sufficient for the
determination of the desired properties.
Discussion and Questions
1. Analyze the ir and nmr spectra. For the ir
spectra, assign vibrational modes to as many absorptions observed
as possible. Also make assignments for nmr peaks. On
the basis of the spectra, determine the product structure and the
site of carbene attack.
2. Is the double or triple bond more reactive in this case
and why?
3. Name the product.
4. Dichlorocarbene reacts as a singlet state. That is,
the two nonbonding electrons have oposed spins. For
methylene, the triplet state which has electrons with parallel
spins is lower in energy that its singlet state. What
orbitals would the nonbonding electrons be in for each case and
what would the geometry (angle between the hydrogens) of each
species be?
5. In the ir, where does the cyclopropyl C-H stretching
occur and why does it occur at higher frequency than the CH3
stretching?
6. Has the product been reported previously in the
literature?
W. E. Parham and E. E. Schweizer, Organic Reactions, 1963, 13, 55.
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