Organic Chem PreLab.. Due early tomorrow.. please help! Will rate!! 2 BASE-PROMO
ID: 907030 • Letter: O
Question
Organic Chem PreLab.. Due early tomorrow.. please help! Will rate!!
2 BASE-PROMOTED ELIMINATION OF AN ALKYL HALIDE (Miniscale)
1. Calculate the molar ratio of base to 2-bromo-2-methylbutane to be used in the elimination experiment(s) you perform and specify which is "limiting reagent.” SHOW your calculations.
2. Why is a Hempel column rather than a regular condenser to be used during the period of reflux?
3. Why is the column to be filled with a packing material during this stage of the procedure?
4. Why is the hold-up of the Hempel column when being used as a fractionating column greater when the column is packed rather than unpacked?
5. Why is the vacuum adapter of the apparatus to be fitted with a drying tube throughout the course of the reaction and the distillation?
6. Why is the receiving flask to be cooled in an ice-water bath throughout the reaction and distillation?
7. Why is it particularly important that the ground-glass joint linking the Hempel column to the reaction flask be properly lubricated for the base-promoted elimination of 2-methyl-2-bromobutane?
8. Write equations for the chemical reactions that you will use to demonstrate the presence of alkenes in your distilled product.
Apparatus
A 10-mL and a 50-mL round-bottom flask, drying tube, apparatus for
fractional distillation, magnetic stirring, and flameless heating.
Setting Up Place 25 mL of a 4 M solution of potassium hydroxide in 1-propanol in
the 50-mL flask containing a stirbar. Attach a drying tube to the flask. Add
2.5 mL of 2-bromo-2-methylbutane and then equip the flask for fractional distillation.
Be sure to lubricate the ground-glass joint connecting the Hempel column to
the flask with a thin layer of stopcock grease. Lubricating this joint is particularly
important in this experiment because the strong base being used may cause the
joint to freeze. To increase the cooling efficiency of the Hempel column, fill it with
Raschig rings, coarsely broken glass tubing, coarse steel wool, or another packing
material as directed by your instructor. Using a short piece of tubing, fit the
vacuum adapter holding a 10-mL receiving flask with a drying tube and immerse
the receiving flask in an ice-water bath. Attach water hoses to the Hempel column
and circulate water through the jacket of the column during the period of reflux for
this reaction.
Elimination and Isolation
Heat the reaction mixture under gentle reflux with stirring for
a period of 1–1.5 h. Allow the mixture to cool below its boiling point, and drain the
water from the jacket of the Hempel column. Connect water hoses to the condenser
so that the apparatus is now set for fractional distillation. Leave in the receiving flask
any low-boiling distillate that has condensed and continue to cool this flask in an icewater
bath. Distill the product mixture, collecting all distillate boiling below 45 °C, and
transfer the product to a tared sample bottle with a tight-fitting stopper or cap.
Analysis
Weigh the distillate and calculate the yield of products. Then put the
container in an ice-water bath until all tests on the distillate have been completed.
Test the distillate for unsaturation using the bromine and Baeyer tests (Secs. 25.8A
and B, respectively). Analyze your distillate by GLC or submit a sample of it for analysis.
After obtaining the results, calculate the relative percentages of the two isomeric
alkenes formed; assume that the response factors are identical for the two
alkenes. A typical GLC analysis of the products from this elimination is shown in
Figure 10.1a. Obtain IR and NMR spectra of your starting material and product.
Compare your spectra with those of authentic samples of 2-bromo-2-methylbutane
(Figs. 10.2 and 10.3) and of the two possible alkenes (Figs. 10.4–10.7). The 1H
NMR spectrum of the product mixture obtained from a representative experiment
is given in Figure 10.8.
Apparatus A 5-mL conical vial, apparatus for simple distillation, magnetic stirring,
and flameless heating.
Setting Up Place a spinvane in the conical vial. Using a calibrated pipet, add 2.5 mL
of a 4 M solution of potassium hydroxide in 1-propanol. Avoid getting any of this
solution on the ground-glass joint of the vial. With a clean syringe or calibrated
pipet, measure 1 mL of 2-bromo-2-methylbutane into the vial. Thoroughly mix the
contents by stirring for a few seconds. Equip the vial for microscale distillation and
circulate cold water through the condenser. Be sure to lubricate the ground-glass
joint connecting the Hickman distillation head to the vial with a thin layer of stopcock
grease. Lubricating this joint is particularly important because otherwise the
strong base being used may cause the joint to freeze.
Elimination and Isolation Heat the reaction vial with a bath that has been preheated
to about 75 °C (bath temperature). The vial should be positioned so that the levels
of the top of the bath and of the liquid in the reaction vial are equal. Stir the mixture
at 75 °C for 1 h and then slowly raise the temperature of the bath to about 90 °C
over a period of about 20 min, collecting all of the distillate until about 2.5 mL of
liquid remains in the vial. It should not be necessary to heat the bath higher than
90–95 °C. Using a Pasteur or a filter-tip pipet, transfer the distillate to a dry sample
vial or a test tube with a tight-fitting cap or stopper.
Analysis Weigh the distillate and calculate the yield of products. Then put the
container in an ice-water bath until all tests on the distillate have been completed.
Test the distillate for unsaturation using the bromine and Baeyer tests (Secs. 25.8A
and B, respectively). Analyze your distillate by GLC or submit a sample of it for analysis.
After obtaining the results, calculate the relative percentages of the two isomeric
alkenes formed; assume that the response factors are identical for the two alkenes. A
typical GLC trace of the products from this elimination is shown in Figure 10.1a.
Obtain IR and NMR spectra of your starting material and product. Compare your
spectra with those of authentic samples of 2-bromo-2-methylbutane (Figs. 10.2 and
10.3) and of the two possible alkenes (Figs. 10.4–10.7). The 1H NMR spectrum of the
product mixture obtained from a representative experiment is given in Figure 10.8.
B ? Elimination with Potassium Tert-Butoxide
Apparatus A dry 50-mL round-bottom flask, drying tube, apparatus for fractional
distillation, magnetic stirring, and flameless heating.
Setting Up Equip the flask with a stirbar. Working quickly, add 25 mL of a 1 M solution
of potassium tert-butoxide in tert-butyl alcohol. This reagent is very moisturesensitive,
so exposure to the atmosphere must be minimized. Then add 2.5 mL of
2-bromo-2-methylbutane to the solution and equip the flask for fractional distillation.
Complete set-up according to the directions in the Miniscale Procedure of Part A.
Elimination and Isolation Follow the procedure given in the Miniscale Procedure of
Part A for completing the reaction.
Analysis Weigh the distillate and calculate the yield of products. Then put the
container in an ice-water bath until all tests on the distillate have been completed.
Test the distillate for unsaturation using the bromine and Baeyer tests (Secs. 25.8A
and B, respectively). Analyze your distillate by GLC or submit a sample of it for
analysis. After obtaining the results, calculate the relative percentages of the two
isomeric alkenes formed; assume that the response factors are identical for the
two alkenes. A typical GLC trace of the products from this elimination is shown in
Figure 10.1b. Obtain IR and NMR spectra of your starting material and product.
Compare your spectra with those of authentic samples of 2-bromo-2-methylbutane
(Figs. 10.2 and 10.3) and of the two possible alkenes (Figs. 10.4–10.7). The
1H NMR spectrum of the product mixture obtained from a representative experiment
is given in Figure 10.9.
WRAPPING UP
Neutralize the residue remaining in the stillpot with 10% aqueous hydrochloric acid;
then flush it down the drain. Place the dichloromethane solution from the bromine
test for unsaturation in a container for halogenated organic liquids; put the
manganese dioxide from the Baeyer test for unsaturation in a container for heavy
metals.
Explanation / Answer
1. Calculate the molar ratio of base to 2-bromo-2-methylbutane to be used in the elimination experiment(s) you perform and specify which is "limiting reagent.” SHOW your calculations.
1:1.1 molar base is required
2. Why is a Hempel column rather than a regular condenser to be used during the period of reflux?
This column is preffered as it maintains the temperature gradullay from hot to cool over a length. To maintain the temperature, because the filling differentiates the temperature and the distillate according to the height the lowest one is hottest and highest one is the coolest.
3. Why is the column to be filled with a packing material during this stage of the procedure?
A normal plain tube does not provide the space for maintain the temperature, saturate and move ahead gradually to achieve the maximum purity in distillation.
4. Why is the hold-up of the Hempel column when being used as a fractionating column greater when the column is packed rather than unpacked?
Even unpacked one is also having the same length, there is not sufficient space for the distillates to pool down and have a efficient fractionation of the compounds.
5. Why is the vacuum adapter of the apparatus to be fitted with a drying tube throughout the course of the reaction and the distillation?
To stop the moisture to enter the distillation flask.
6. Why is the receiving flask to be cooled in an ice-water bath throughout the reaction and distillation?
The distillate is warm and needs effective collection rather than evaporate in to the laboratory. Hence the receiving flask is cooled in ice-water throughout the reaction and distillation.
7. Why is it particularly important that the ground-glass joint linking the Hempel column to the reaction flask be properly lubricated for the base-promoted elimination of 2-methyl-2-bromobutane?
The ground glass joint is efficient for fixing the flask and distillation condensors without difficulties. Even these days the, the lubricant used varies from teflon tape,other than the grease.
8. Write equations for the chemical reactions that you will use to demonstrate the presence of alkenes in your distilled product.
2-bromo-2-methylbutane + 25 mL of a 1 M solution of potassium tert-butoxide in tert-butyl alcohol ----> 2-methyl-1-butene
the elimination with both the bases is evident with the product quantiation by GC.
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