Dehydration of 1- or 2-Butanol

Preparation: Mayo, et al., p 153-154; McM Sect 5.12 and pp. 648-652

Dehydration is an acid catalyzed elimination reaction involving the loss of H₂O from an alcohol to produce an alkene. The mechanism of the dehydration reaction is given for 1-methylcyclohexanol in McM Figure 17.6. The first step in the reaction is the protonation of the alcohol, an acid-base step:

From this point on the reaction is an E-1 elimination involving carbocation intermediate. In the second step, H₂O is separated from the substrate to form a carbocation:

Characteristic of any carbocation, this intermediate may rearrange (see McM p.211) if there is a possibility of forming a more stable carbocation)

The final step in the elimination process is the loss of a proton from the carbon adjacent to the C⁺. This is another acid-base reaction. The proton is returned to restore the catalyst (or passed to another alcohol molecule to begin a new dehydration cycle). Note in Fig 17.6 that the electron pair that bonded this proton (blue) to carbon becomes the p bond of the alkene:

In the laboratory, you will be dehydrating either 1-butanol or 2-butanol and measuring the relative amounts of 1-butene, cis-2-butene and trans-2-butene formed. In addition, you will interpret the data to determine the ratio of rates (and the difference in the activation energies) of two alternative reactions of the 1-butyl carbocation

Procedure:. Start warming the sand bath to about 80 ^oC (sandbaths can be shared by two students). To a 3 mL vial with a magnetic spin vane add 0.15 mL of the alcohol (Either 1-butanol or 2-butanol as assigned), and 2 drops (approx. 50 microliters) of concentrated H₂SO₄. Place the gas delivery tube, with an O ring and cap on the vial as in Fig 3.27.

To a 15x125 mm test tube add 2 mL of water and make a mark; then fill it and a gas collection tube (a simple glass tube with a rubber septum cap - obtain from stockroom) to their tops with water. Fill a 150 mL beaker with about 100 mL of water and use the thumb briefly to stopper both tubes while inverting them in the beaker. Both tubes should be upside down and filled, or almost filled with water. Set the beaker on the cold surface next to the hot part of the sand bath.

The vial is next placed in the sand bath (note the temperature) and the gas transfer tube is immersed in the beaker. Place the test tube on the transfer tube (clamps are not necessary), and collect gas to the 2 mL mark. Now exchange the test tube for the gas collection tube. Most of the gas will evolve when the temperature of the reaction vial is above 110 ^oC if the reactant is 2-butanol or 120 ^oC if the reactant is 1-butanol. Do not allow the temperature of the reaction vial to drop during this process.

Keep the open end of the collection tube under water. When the collection tube is full of gas, remove the vial and delivery tube and allow to cool; disassemble it and rinse carefully in the sink (watch that the spin vane isn't rinsed down the drain). While you await the GC analysis of the gas produced, it is a good idea to clean and dry the vial and re-fill it with the alcohol, H₂SO₄ and the dry spin vane in case you need to repeat the experiment.

Analysis of the Data: When the gas chromatograph becomes available, bring the collection tube in its beaker of water to the GC. Use your tuberculin syringe to withdraw 0.1 mL of the trapped gas into a syringe (do not allow water to enter the syringe) and inject it into the the DC-200 column. The temperature of the column should be about 10 ^oC. As usual, record all GC data on the chart. The 3rd peak (cis-2-butene) may not be well separated from the 2nd so it may be necessary to photocopy the chromatogram and then cut out and weigh the copy of each peak on a milligram balance to determine the percent distribution of the alkene products. If this is done, the peaks are stapled to a page in the notebook along with the original chromatogram. On the DC-200 column the order of elution follows the increasing boiling points of the alkenes: 1-butene, -6.3 ^oC; trans-2-butene, 0.9 ^oC; and cis-2-butene, 3.7 ^oC.

Calculate, using the ideal gas law, the number of mL of gas theoretically possible from this reaction; show your work.

Interpretation of the Data: Data from both reactions are needed for the calculations, so information should be shared between designated student pairs.

Using the diagram below as a model, set up a scheme showing the carbocation intermediates formed from each alcohol that produces all three products. Enter the remaining structures and the percent distributions of alkenes you obtained from your alcohol (indicate which alcohol) and the percent distribution obtained by the person you are sharing data with (note who and the alcohol used by this person).

A. Calculate the ratio of cis- to trans-2-butene formed from each alcohol. This ratio should not depend on which butanol was reacted; it reflects the preference of the 2-butyl carbocation to form the cis- over the trans-2-butene (or is it the other way?). Compare your cis/trans ratios to the ratio obtained by others, particularly those reacting the "other" alcohol. Are the ratios substantially different?

B. The 1-butyl carbocation has two fates: rearrangement to form the 2-butyl carbocation or direct elimination to form 1-butene. We can calculate the relative rates of these rates, Rate_rearr/Rate_elim in the following way:

1. Let us assume that the 2-butyl carbocation formed during the reaction of 1-butanol undergoes the same fate as if it were formed from 2-butanol. In particular, assume that the ratio of combined butene products to 1-butene is consistent no matter which alcohol was used. Thus,

2. From the above equation, determine a', the % 1-butene from 2-butyl carbocation from 1-butanol, using the gas chromatography percents for all the other values. Once determined, this percent needs to be subtracted from the percent of 1-butene actually formed in the 1-butanol experiment. The difference is the percent of the 1-butene formed by direct elimination from the 1-butyl carbocation, "elim":

3. The 1-butyl carbocation either directly forms 1-butene by "elim" or it rearranges, "rearr", to form the 2-butyl carbocation. The percent of the 1-butyl carbocation that rearranges to form the 2-butyl carbocation can be calculated by subtraction:

The relative rates of rearrangement to elimination is the same as the relative percentages calculated above:

C. The activation energy, deltaG^± (McM p. 166),is related to the rate and the temperature, T, of a reaction by

where P and Z are factors relating to probability factors such as steric effects and concentrations, etc. For our calculations below, these factors will be cancelled because we are working with relative rates of optional reactions of a single intermediate, the 1-butyl carbocation.

We will calculate the difference in deltaG^± for rearrangement and deltaG^± for elimination. For each rate,

And the ratio of rates:

Taking the natural logarithm of both sides

If the reaction is carried out at about 120 ^oC

(8.31 joules/mole ^oK)·(393 ^oK)·Ln(Rate_rearr/Rate_elim ) = deltaG^± for elim. - deltaG^± for rearr

Thus the difference in the activation energies for the two reaction paths from the 1-butyl carbocation can be calculated. Fill in the rate ratio you determined in part B above and the actual temperature the reaction was carried out

Conclusions:

1.Report your cis/trans ratio.

Are cis/trans ratios fairly consistent between groups?

Are you justified in assuming that the fate of the 2-butyl carbocation is independent of how it was formed?

2. Calculate your difference in activation energy for the two competing reactions of 1-butyl carbocation.

3. What are the major sources of error in your measurements?

References:

"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991

"McM": McMurry, J. Organic Chemistry, 4th ed., Brooks/Cole Publishing Company, Pacific Grove, CA. 1996

Rev. June, 1999