Exploring Reaction Mechanisms
CHY 116 Exploring Reaction Mechanisms
Learning Goals
By evaluating a mixture of reaction products by chromatography and 1H-NMR spectroscopy, the mechanisms of two reactions will be determined.
Overview
You will carry out two different chemical reactions in which competing reactants produce mixtures of products. You will use gas chromatography (GC) and proton nuclear magnetic resonance (1H-NMR) spectroscopy to measure the relative amounts the products. The results will allow you to verify or refute possible mechanisms that are based on observed rate laws for the reactions.
Lecture Connections
Additional Resources
The two most basic substitution reactions in organic chemistry are the SN1 and SN2 reaction mechanisms. The "S" stands for substitution, the "N" for nucleophilic and the "1" for unimolecular or "2" for bimolecular. The slow step in the SN1 reaction mechanism has one species in the rate law while the SN2 mechanism's rate law has two species. The two videos below demonstrate a molecular view of each reaction.
(1) SN1 Reaction (University of Surrey, 2.5 minutes). In this video, 2-methyl-2-bromopropane undergoes an SN1 reaction forming 2-methylpropan-2-ol.
(2) SN2 reaction (University of Surrey, 2 minutes). In this video, bromoethane undergoes an SN2 reaction forming ethanol.
Experiment Background
Reactions of Alcohols with Halide Ions
An alcohol is a covalent compound in which an -OH group is attached to a tetrahedral (sp3-hybridized) carbon. Examples are methyl alcohol (CH3-OH), also called methanol or wood alcohol, a component of gasoline drying solutions like DryGas®, and ethyl alcohol (CH3-CH2-OH), also called ethanol or grain alcohol, the alcohol in beer, wine, and spirits. An alkyl halide is a covalent compound in which a halogen atom is attached to a tetrahedral carbon. Examples are methyl chloride (CH3-Cl) and ethyl bromide (CH3-CH2-Br). In any type of compound, the name "methyl" refers to a CH3- group, and "ethyl" refers to a CH3-CH2- group. Generic formulas for alcohols and alkyl halides are R-OH and R-X, in which R represents some group of carbons and hydrogens (like methyl or ethyl), and X represents any halogen atom.
Alcohols react with halide ions in acidic solution to produce alkyl halides, as in this reaction between ethyl alcohol and chloride ions to produce ethyl chloride:
CH3-CH2-OH (aq) + Cl- (aq) + H+ (aq) --> CH3-CH2-Cl (l) + H2O (l) (Eqn. 1)
This reaction is known as a substitution reaction, because the chloride ion substitutes for, or displaces, the -OH group of the alcohol. The substituting agent, in this case chloride ion, is called a nucleophile ("nucleus lover") because it readily adds itself to nuclei such as carbon, displacing other atoms, especially if carbon is joined to a very electronegative atom like oxygen. So this kind of reaction is known specifically as nucleophilic substitution.
Hydrogen ions are essential to this kind of reaction because they assist the loss of the -OH group by protonating it. Protonation turns the -OH group into an -OH2+ group, which separates easily from its attached carbon atom as a stable, neutral water molecule. When groups like -OH2+ are displaced in nucleophilic reactions, they are called leaving groups.
When organic chemists study the rates of nucleophilic substitution reactions by changing concentrations of reactants, they find that for some alcohols,
rate = k [ R-OH ] (Eqn. 2)
implying that the rate-determining step is unimolecular (only involves one molecule, first order). For other alcohols, however,
rate = k [ R-OH ] [ X - ] (Eqn. 3)
For alcohols in the latter group, the rate-determining step in nucleophilic substitution is bimolecular, implying that both an alcohol molecule and a halide ion are involved in the rate-determining step (second order). For reactions obeying rate laws (Eqn. 2) and (Eqn. 3), the following mechanisms have been proposed:
Mechanism 1, for reactions obeying rate law (Eqn. 2):
1) R-OH + H+ <--> ROH2+ (fast, acid-base equilibrium reaction)
2) ROH2+ --> R+ + H20 (slow, rate-determining, unimolecular)
3) R+ + X- --> R-X (fast)
Mechanism 2, for reactions obeying rate law (Eqn. 3):
1) R-OH + H+ <--> ROH2+ (fast, acid-base equilibrium reaction)
2) ROH2+ + X- --> R-X + H2O (slow, rate-determining, bimolecular)
Reactions that follow Mechanism 1 are called SN1 reactions (substitution, nucleophilic, unimolecular rate-determining step), while those that follow Mechanism 2 are called SN2 (substitution, nucleophilic, bimolecular rate-determining step).
How could we test examples of first- and second-order reactions to see if these mechanisms are actually at work? One test is suggested by the observation that not all nucleophiles are alike. Some nucleophiles react -- that is, displace leaving groups -- faster than others; for example, among halides, iodide reacts faster than bromide, which reacts faster than chloride. Note that in SN1 reactions (Mechanism 1), the nucleophile is not involved in the rate-determining step, but acts later, after the slow step has occurred. Such reactions should go at the same rate no matter whether the nucleophile is fast or slow, because the rate is determined by a prior process (separation of water from R-) that does not involve the nucleophile. So in SN1, there is no direct displacement of the leaving group, and thus the differing reactivities of the nucleophiles have no effect on the rate. On the other hand, fast reacting nucleophiles should make SN2 reactions (Mechanism 2) faster, because they are involved in the rate-determining step.
This suggests a simple strategy to see which mechanism is at work with a particular alcohol. Allow the alcohol to react with an equimolar mixture of two nucleophiles, say, bromide (fast) and chloride (slow). If the reaction is SN1, you should get a mixture of products containing equal amounts of alkyl bromide and alkyl chloride, because the R+ ion formed in the slow step reacts rapidly with any nucleophile it encounters in Mechanism 1, step 3. If the reaction is SN2, you should get a mixture of products containing more of the alkyl bromide than of the alkyl chloride, because bromide ions, being faster, will capture more of the ROH2+ intermediates in Mechanism 2, step 2.
In this experiment, you will carry out two reactions, each allowing a different alcohol to react with a mixture of chloride and bromide ions. One of the reactions is known to be first order [obeys rate law (Eqn. 2)], and the other is known to be second order [obeys (Eqn. 3)]. You will use gas chromatography or nuclear magnetic resonance spectroscopy to determine the relative amounts of alkyl bromide and alkyl chloride formed in each reaction, and finally, you will determine whether the results of each reaction are consistent with the mechanisms proposed.
The two alcohols you will use are CH3-CH2-CH2-CH2-OH (called n-butyl alcohol or n-BuOH) and (CH3)3C-OH (t-butyl alcohol or t -BuOH). The questions you are asking in this experiment are
1) Does the reaction of halide ions with t -BuOH proceed by a unimolecular or a bimolecular rate-determining step?
2) Does the reaction of halide ions with n-BuOH proceed by a unimolecular or a bimolecular rate-determining step?
Analyzing Product Mixtures
t -Butyl Halides
A substitution reaction with competing nucleophiles gives a mixture of products, one product for each nucleophile. You will analyze the product mixture from your t -BuOH reaction by 1H-NMR spectroscopy. 1H-NMR spectra (see Figure 1) exhibit peaks for the hydrogen atoms (protons) in a compound. Positions of peaks in a spectrum depend upon the environment of each proton. A t -butyl halide gives a very simple 1H-NMR spectrum, because all nine of its protons are in identical chemical environments. So each of your products (t -butyl bromide and t -butyl chloride) gives only one absorption peak, whose area is proportional to the number of moles of that product. Your mixture of two products exhibits two peaks and their relative areas gives the molar ratio of your two products.
Here is a very brief introduction to 1H-NMR spectroscopy (pdf).
Figure 1 shows the 1H-NMR spectrum of a mixture of two alkyl halides, t-BuBr and t-BuCl (Lewis diagrams shown above spectrum). The signal at 1.80 ppm comes from the 9 identical protons in the t-BuBr, while that at 1.62 ppm arises from the 9 identical protons in t-BuCl. The signal at 0.00 ppm is due to a compound that has been added as a standard for calibration of the ppm scale. The number beside the peaks are proportional to the areas of the peaks. They indicate that the two products are present in a 9.36/9.00 ratio. In other words, the mixture contains [(9.36)/(9.36 + 9.00)]*100 = 51.0% t -BuBr and 49.0% t -BuCl.
n- Butyl Halides
You saw in CHY 114 that chromatography is useful for analyzing and separating mixtures, such as the pigments in dyes. You will use gas chromatography (GC) to analyze the product mixture from reaction of n-butyl alcohol. GC is a form of column chromatography in which the stationary phase is a polar solid in a long, metal tube (the column), and the mobile phase is an inert gas like nitrogen or helium, which is pumped through the column. Special plumbing allows you to inject a sample into the column with a syringe.
(1) For animations that review thin-layer chromatography and introduce gas chromatography, go to https://www.wooster.edu/academics/areas/chemistry/facilities/instrumentation/chromatography/ then click Play repeatedly to see the full set of animations.
(2) For a brief animation of the principle of chromatography: Chromatography. Animation (IQOG-CSIC), 1 min 11 sec.
(YouTube https://www.youtube.com/watch?v=0m8bWKHmRMM)
(3) For a brief animation of gas chromatography: Gas Chromatography (IQOG-CSIC), 4 min 12 sec.
(https://www.youtube.com/watch?v=iX25exzwKhI)
In thin layer chromatography (TLC), you saw spots on a silica-gel plate representing each component of a mixture. In GC, after you inject a mixture into the instrument, components of the mixture move through the column at different rates, emerge from the column into a detector, generating a signal to a recorder, which in turn prints out peaks representing the different components of the mixture (shown in the last part of the animation). The retention time (tr) of a compound is the length of time (minutes) after the injection until the compound elutes from the column, as measured at the top of its peak. The relationship between the molar amounts injected and the area of the peaks is complicated, so you must calibrate the GC for each component, by injecting known amounts of components and determining the area of the resulting peaks. In this experiment, you will calibrate the GC with a mixture of the two expected products of the n-BuOH reaction. You will obtain a chromatograph that looks somewhat like Figure 2. Each peak represents a pure component from the injected mixture, and the relative areas of the peaks reflect the molar ratios of the components. The areas of the peaks are calculated by the instrument (this is done more or less by the computer treating the peaks as triangles; recall that the area of a triangle of height h and base w is (1/2)(h•w)).
Figure 2. GC Recorder Output
The following problems require skills and calculations similar to those called for in the Report Form of this experiment. Learn how to work these problems, showing all calculations with units. For problems involving calculations, answers are provided. Practice these.
Experimental Procedure
Introduction
You will work in teams to run two reactions in this experiment. To complete both reactions in the allotted time you must a.) come to lab prepared, b.) coordinate the work with the members of your team, and c.) carry out the procedures in the order specified. Your instructor will suggest a division of labor that will allow your team to carry out all tasks.
The products of the two reactions are volatile. The boiling point of n-butyl bromide is 100 oC, while that of n-butyl chloride is 77 oC. The boiling points of t-butyl bromide and t-butyl chloride are 72 oC and 51 oC. The volatility of these four compounds poses two problems. First, unless you prevent it, the samples will evaporate rapidly, and you will not have enough sample to isolate. Second, because the chloro compounds have lower boiling points, they evaporate faster than their bromo analogs. Evaporation will mean that the n-butyl bromide/n-butyl chloride ratio that you measure will not reflect the composition of the mixture that was formed initially. The same applies to the t-butyl bromide/t-butyl chloride ratio. So evaporation will introduce error into your results. You cannot completely prevent the evaporation of your samples, but you can take steps to minimize it.
WARNING: Alkyl halides are toxic, irritating to eyes and skin, and potentially carcinogenic. Avoid contact with liquid or vapor. Keep in sealed tubes. Dispose of all alkyl halides in the specially marked hazardous-waste container in the hood.
Part 1. Preparation of equimolar chloride and bromide in sulfuric acid
Weigh 1.76 ± 0.05 g of ammonium chloride (32.8 mmol) and 3.21 ± 0.05 g (32.8 mmol) of ammonium bromide. Mix them together in a small beaker.
WARNING: Concentrated sulfuric acid is corrosive and causes severe burns. Wear gloves in handling all solutions containing concentrated sulfuric acid, and handle them with care. Dilute and wash up spills with plenty of water.
Using a graduated cylinder, measure 10.0 mL of water into a 30 mL beaker that contains a magnetic spin bar. Start the magnetic stirrer and -- WEARING GLOVES -- using a second 10 mL graduated cylinder, add 7.6 mL of concentrated sulfuric acid to the water. The mixture will become very hot.
Immediately add the ammonium chloride and ammonium bromide to the aqueous sulfuric acid and continue to stir until all the solid has dissolved. If necessary, use a pasteur pipet with a dropper bulb to draw up solution and rinse down any solid clinging to the sides of the flask. All of the solid should dissolve within 5 minutes. Before the solution has a chance to cool, pour 6 mL of this mixture into a graduated centrifuge tube. Pour the remaining liquid along with the spin bar into a 25 mL round bottom flask, being careful not to get ANY of the solution onto the frosted rim of the flask. Place the flask into the well of an electric heater and clamp it to a ring stand.
Part 2. Reaction of n-butyl alcohol
Using a plastic syringe, add 1.0 mL of n-butyl alcohol to the ammonium halide mixture in the round bottom flask. Assemble the reflux apparatus as shown in Figure 1, making sure that there are good seals between the glass joints. Note that the cooling water enters through the bottom of the condenser and leaves from the top.
Caution: To prevent the apparatus from drawing water in from the acid trap, DO NOT ALLOW THE WATER IN THE ACID TRAP TO COVER THE MOUTH OF THE FUNNEL.
Turn the heat controller to high. The solution should begin to boil within 5 minutes. Allow the solution to reflux for 60-75 minutes (reflux is a continuous boil, with vapors being returned to the flask by the condenser).
During the refluxing, carry out the reaction of Part 3, and if time permits, the GC calibration of Part 4.
Figure 1. Reflux apparatus: The condenser captures vapors and returns them to the flask.
At the end of the reflux period, turn off the heat controller and remove the flask from the heating well. Place the flask into a beaker of ice/water and allow the solution to cool for 10 minutes before disassembling the glassware; keep the cooling water turned on. Transfer the solution to a threaded centrifuge tube (CAUTION: remember that the lower layer contains concentrated H2SO4!). Add 1 mL of pentane. Cap the tube and shake vigorously for 1 minute. After the layers have separated, use a pasteur pipet to transfer the bulk of the upper layer (pentane solution of products), to a 13 x 100 mm threaded test tube, taking care not to transfer any of the aqueous layer. Add about 50 mg of anhydrous sodium bicarbonate to the tube. Cap it tightly and shake the mixture gently for 1or 2 minutes. Meanwhile, prepare a filter pipet by using a 9" Pasteur pipet to push a small amount of cotton into a 5" pasteur pipet until the cotton forms a small plug in the neck of the pipet (Figure 2). Make the plug very small to avoid losing product in the cotton during filtration. Avoiding the NaHCO3, remove the solution with a pasteur pipet, and filter the solution through the filter pipet into a second 13 x 100 mm threaded test tube with a rubber gasket. Cap the tube tightly. Label the sample with your names, lab section, and “n-butyl”, and give it to your instructor for storage in a freezer to minimize evaporation. Next week, you will analyze the sample by GC. Discard the aqueous layer (CAUTION: contains concentrated H2SO4!) by pouring down the drain followed by plenty of water.
Figure 2. Filter pipettes: Use a small plug to reduce loss of product in the cotton.
Part 3. Reaction of t-butyl alcohol
Using a plastic syringe, add 1 mL of t-butyl alcohol to the 6 mL of the ammonium halide solution in the threaded centrifuge tube. Cap the tube tightly and shake the mixture vigorously and continuously for 1 minute. Place the sample into a beaker of ice/water and allow it to cool for 5 minutes. Using a pasteur pipet, transfer the bulk of the top layer liquid (products) to a 13 x 100 mm threaded test tube, taking care not to transfer any of the aqueous layer. Add about 50 mg of anhydrous sodium bicarbonate to the tube. Cap it tightly and shake the mixture gently for 1 or 2 minutes. Avoiding the NaHCO3, remove the solution with a pasteur pipet, and filter the solution through a newly prepared filter pipet (Figure 2) into a second 13 x 100 mm threaded test tube. Cap the tube tightly. Label the sample with your names, lab section, and “t-butyl”, and give it to your instructor for storage in a freezer to minimize evaporation. He or she will obtain an NMR spectrum of your mixture and provide it to you at the second lab meeting. Discard the aqueous layer (CAUTION: contains concentrated H2SO4!) by pouring down the drain followed by plenty of water.
Part 4. Analyzing products by gas chromatography
Gas chromatography provides a quick and convenient way to determine the composition of a mixture of volatile materials. During the first week of this experiment you will have time to familiarize yourself with this technique by analyzing mixtures of known composition.
A. Filling a 10-µL syringe
Immerse the syringe needle beneath the surface of your sample and pump the plunger back and forth repeatedly to lubricate the barrel and to expel small air bubbles. Then slowly draw 1 µL of liquid into the syringe. Remove the needle from your sample and continue to pull back the plunger until you can see about 5 µL of air in front of your sample. When the directions call for you to inject 1 µL of your sample, you should always inject about 5 µL of air along with it.
B. Cleaning the syringe
Clean the syringe immediately after each injection, as follows:
Remove the plunger from the barrel of the syringe and gently wipe it with a clean Kimwipe. To clean the barrel of the syringe, use a vacuum to draw a small volume of pentane through it. For this purpose, a suction flask attached to an aspirator is available in the hood. Insert the end of the syringe into the hole in the rubber stopper and turn on the aspirator water all the way. Using a pasteur pipet, add a few drops of pentane to the top of the syringe. The vacuum will draw the liquid into the barrel. It will then draw air through, which will dry the barrel. Insert the clean plunger into the clean barrel so that the syringe is ready for the next person to use.
C. Analyzing a known mixture of products by GC
In GC analyses, everything is relative. It is assumed that the proportions of the components of a mixture are directly reflected in the proportions of the areas of each peak in the GC of that mixture. However, a mixture that contains 1 mL of n-butyl chloride and 1 mL of n-butyl bromide, for example, will not necessarily produce a GC in which the areas of the two peaks are in a 1/1 ratio. This is because a mixture that is 1/1 by volume is not 1/1 by mass if the densities of the two liquids are different. Nor is it 1/1 on a molar basis, since the molecular weights of the compounds are different. Nor is the response of the GC the same for equimolar amounts of two different compounds. The following analysis of a known mixture allows you to calibrate the response of the GC to the two reaction products, and thus be able to calculate the percentage of each product.
WARNING: Alkyl halides are toxic, irritating to eyes and skin, and potentially carcinogenic. Avoid contact with liquid or vapor. Keep containers sealed when not in use. Dispose of all alkyl halides in the specially marked hazardous-waste container in the hood.
Near the GC, you will find a 3:1 (mol/mol) calibration mixture of n-butyl bromide/n-butyl chloride. Using a 10-µL syringe as described in Part 4A, inject a 1 µL sample of the calibration mixture into injection port of the GC. A complete chromatogram requires less than 1 minute. When the chromatography is complete, determine the retention times and areas of each peak.
Note: since your sample is dissolved in pentane, you may also see a fairly large pentane peak – it should not interfere with the n-BuCl or n-BuBr peaks.
Construct the following table in your lab notebook, and enter your data there, not here, for the standard injected into the GC [3:1 (mole/mole)]) calibration mixture of n-butyl bromide/n-butyl chloride). Refer to Practice problem 3.
D. Analyzing your reaction mixture from Part 2
By the procedure of Part 4C, obtain and analyze a chromatograph of your reaction mixture from the reaction of Part 2. Construct a table in your lab notebook similar to the one above and record the results (retention times and areas).
E. Analyzing your reaction mixture from Part 3
The products of Part 3 decompose during GC analysis due to heat. You will determine the product ratio from Part 3 by NMR spectroscopy. In a table in your lab notebook, record the ppm values and
integration values for each peak printed on your NMR spectrum. Use the peak areas to determine the molar ratio of t-BuBr to t-BuCl in your reaction mixture of Part 3.
Pre-lab Assignment
Practice Problems
1) What is an alcohol; an alkyl halide; a nucleophile?
2) Draw the product of a nucleophilic substitution reaction involving iso-propyl alcohol, (CH3)2CH- OH, and iodide ion (I- ).
3) You mix 1.00 mL of n-BuBr with 1.00 mL n-BuCl to make a mixture for calibrating the GC. What is the molar ratio of n-BuBr to n-BuCl? Express the ratio as [(mol n-BuBr)/(mol n- BuCl)]. The molecular weight of n-BuBr is 137.03 g/mol, and the molecular mass of n-BuCl is 92.58 g/mol. The density of n-BuBr is 1.276 g/mL; the density of n-BuCl is 0.886 g/mL. Hint: Calculate the number of moles of each halide in 1.00 mL. Divide the two results to get the ratio.(Answer: [(mol n-BuBr)/(mol n-BuCl)] = 0.97; in other words, equal volumes of n-BuBr and n- BuCl contain roughly equal numbers of moles.)
4) An alcohol R-OH reacts with an equimolar mixture of Br- and Cl-, giving 65% R-Br and 35% R- Cl. Is this reaction likely to be SN1 or SN2?
5) Explain why Mechanism 1 is compatible with rate law (Eqn. 2), but not (Eqn. 3).
6) Explain why Mechanism 2 is compatible with rate law (Eqn. 3) but not (Eqn. 2).
Notebook Preparation
In your lab notebook, prepare the following information:
· A brief (2-3 sentence) objective of the lab.
· A table of glassware, equipment and chemicals to be used. Include relevant properties and safety information for each chemical. Use this helpful link for online SDS https://chemicalsafety.com/sds-search/ find out the hazards associated with substances you use and make in this experiment.
· Several “bullet points” summarizing the tasks involved in the procedure.
Be sure to read and study the BACKGROUND material, along with the procedure, before coming to lab.
Experimental Videos
(1) Preparation of NH4Cl/NH4Br solution & set up of n-butyl alcohol reflux, 19.7 min
(2) t-butyl alcohol reaction and work up, 11.3 min
(3) n-butyl alcohol reflux close up, 1.0 min
(4) Cooling of n-butyl alcohol reaction mixture, 1.3 min
(5) Preparation for work up of n-butyl alcohol reaction mixture, 2.8 min
(6) n-butyl alcohol reaction mixture work up continued, 3.7 min
(7) n-butyl alcohol reaction mixture work up continued, 6.2 min
Experimental Data
GC chromatogram of n-butyl chloride and n-butyl bromide (pdf)
1H NMR spectrum of t-butyl chloride and t-butyl bromide (pdf)
References
Grading Rubric
(pdf)
Post Experimental Analysis
After completing all calculations in your lab notebook, complete the following REPORT FORM on your own. Link to pdf version of Report Form.
Report Form
CHY 116: Exploring Reaction Mechanisms Name
A.) Calculations and Results
Keep complete records of your lab work, including observations and data, in your Laboratory Notebook and carry out all calculations and obtain final results called for in the Procedure. Refer to the Background information concerning rate laws and proposed mechanisms for nucleophilic substitution reactions (Equations 4 and 5, Mechanisms 1 and 2).
1.) What is the molar ratio of the n-BuBr : n-BuCl standard that you determined experimentally from the GC results for the 3:1 (mole/mole) standard of n-BuBr/n-BuCl?
Answer (Lab Notebook, page _ .)
2.) If the 3:1 (mole/mole) standard of n-BuBr : n-BuCl did not experimentally yield a 3:1 molar ratio, provide an explanation for any discrepancies (consider errors involved and limitations of the GC).
3.) According to your GC results, what is the molar ratio of the n-BuBr : n-BuCl product mixture synthesized in the reaction of Part 2?
Answer (Lab Notebook, page _ .)
4.) See Mechanisms 1 (SN1) and 2 (SN2) in the Background section to help you answer the following questions.
a.) From your answer to Question 4 (and 5), which Mechanism, 1 (SN1) or 2 (SN2), appears to apply to the reaction of n-BuOH with halide ions? Explain your reasoning briefly.
b.) Which rate law, Equation 2 or Equation 3, applies to this reaction?
5.) According to your 1H-NMR results, what is the molar ratio of the t-BuBr : t-BuCl product mixture synthesized in the reaction of Part 3?
Answer (Lab Notebook, page _ .)
6.) See Mechanisms 1 (SN1) and 2 (SN2) in the Background section to help you answer the following questions.
a.) From your answer to Question 7, which Mechanism, 1 (SN1) or 2 (SN2), appears to apply to the reaction of t-BuOH with halide ions? Explain your reasoning briefly.
b.) Which rate law, Equation 2 or Equation 3, applies to this reaction?
B.) Questions
1.) For the reaction of Part 2, would evaporation of your reaction products increase (I), decrease (D), or not change (NC) the molar ratio of the n-BuBr : n-BuCl product mixture synthesized? Hint: n-BuCl boils at 78 C; n-BuBr boils at 101 C.
Circle your answer: I D NC
Explain briefly.
2.) For the reaction of Part 3, would loss of some of the products during filtration increase (I), decrease (D), or not change (NC) the molar ratio of the t-BuBr : t-BuCl product mixture synthesized?
Circle your answer: I D NC
Explain briefly.
3.) In Mechanism 1 (SN1), step 2, the product R+ is called a carbocation (a cation of carbon). Much research suggests that carbocations are more stable if the positively charged carbon is connected directly to other carbons. In other words, the more carbons directly connected to the charged carbon, the more stable the carbocation. With these research results in mind, suggest a reason why t-BuOH and n-BuOH each react by the mechanism that you observed.
4.) Considering the observations of Question 3, suggest a reason why t-BuOH reacts much more rapidly (reaction time, one minute without heating) than does n-BuOH (reaction time, one hour with heating).
5.) An alcohol R-OH reacts with an equimolar mixture of Br- and Cl-, giving 65% R-Br and 35% R-Cl. Is this reaction likely to be SN1 or SN2? Explain briefly.
6.) Type a brief summary of the experiment.
Turn in as one pdf document:
1.) all pages from your laboratory notebook for this experiment,
2.) a copy of your NMR spectrum and your GC chromatogram,
3.) all pages of this Report Form, and
4.) a typed Summary.
The following should be included in the summary: topic sentence describing the goal of the experiment; in the body of the summary, focus on the results and outcomes (not procedural details, not intermediate calculations, but final results); a final conclusion sentence. Staple them together as a single package. Be sure that the spectrum and chromatograph meet all report guidelines for graphs