Joseph K. Myers
4-20-04
(Dehydration of 4-methyl-2-pentanol)
E1 reactions are useful in this systhesis. One molecule is involved in the carbocation transition state.
(Attached are the general reaction and mechanism for an E1 reaction.)
The nonbonding lone pair of electrons rapidly and reversibly undergoes protonation to produce an oxonium ion. The positively charged oxygen atom further polarizes the carbon-oxygen single bond.
In contrast to hydroxide ion, the water is a weak base and an excellent leaving group. For this reason, the oxonium ion may undergo elimination and substitution reactions in presence of a base or nucleophile.
In the case of primary alcohols, an E2 elimination mechanism applies, in addition to an E1 mechanism which produces a highly unstable one-degree carbocation. With 2- or 3-degree alcohols, the ease of the E1 mechanism increases, because they stabilize the carbocation involved in the rate-determining step.
As for the competing substitution products, they can be avoided when they are made reversible. Drawing out the E2 or E1 product as they are formed brings equilibrium to favor the desired product while the reaction continues. Sulfuric acid is often used, because the substitution product readily reionizes to the intermediate carbocation, to be available again for an elimination mechanism.
If isomers of the product are capable of being formed, their distribution is sometimes predictable, depending on the relative free energies of the transition states. There is also the effect of rearrangement of the intermediate carbocation from migration of a hydride or an alkyl group to the cationic center.
Such a rearrangement is more likely if the new carbocation is more stable than the original ion. Subsequent loss of a proton from the rearranged carbocation then leads to additional alkene products.
(Table 01)
| compound | formula | mw | mp | bp | d |
|---|---|---|---|---|---|
| 2-methyl-2-pentene | C6H12 | 84.159 | -135.7 | 67.3 | .6863 |
| 2-methyl-1-pentene | " | " | -135.7 | 62.1 | .6799 |
| trans-4-methyl-2-pentene | " | " | -140.8 | 58.6 | .6686 |
| cis-4-methyl-2-pentene | " | " | -134.8 | 56.3 | .6690 |
| 4-methyl-1-pentene | " | " | -153.6 | 53.9 | .6642 |
| potassium carbonate | K2CO3 | 138.206 | 898 | dec | 2.29 |
| sulfuric acid | H2SO4 | 98.0734 | 3 | 280 | 1.84 |
| 4-methyl-2-pentanol | C6H14O | 102.174 | -90 | 131.6 | .8075 |
(Table 02)
| compound | 4-methyl-2-pentanol |
|---|---|
| equivalence | 1 |
| mmol | 31.6 |
| grams | 3.23 |
| mL | 4.0 |
Note that the elimination reforms the sulfuric acid molecule, so the net sulfuric acid content remains the same. (Examine the mechanism to see this.)
Attached on separate paper.
1) 4.0 mL 4-methyl-2-pentanol + 2.5 mL 9 M H2SO4. Mix well and perform fractional distillation. Collect fraction < 90-deg-C. Use ice water bath to cool the receiving flask.
2) Add 2-3 spatula tips of anhydrous K2CO3. Dry 10-15 min.
3) Decant organic solution to RB flask.
4) Simple distillation. Collect fraction < 70-deg-C.
The solution turned red or darker during the fractional distillation.
Because of the small amount of solution, it was impossible to produce a head temperature of 90 degrees C, so we estimated the time to quit.
Other than these, the same observations were made that are typical of fractional distillation and simple distillation.
Note that after similar experiments are performed many times, it is important to record new observations rather than what is common to all of the procedures.
Table of results
(Table 03)
| compound | theoretical yield | experimental yield | %y | mp exp. (deg-C) | bp lit. |
|---|---|---|---|---|---|
| 2-methyl-2-pentene* | 2.66 g | 0.03 g | 1.13 | 40+ | 67.3 |
(*This is the major product, but the mass includes all products, including 2-methyl-2-pentene, 2-methyl-1-pentene, trans-4-methyl-2-pentene, cis-4-methyl-2-pentene, and 4-methyl-1-pentene. Note that the major product is still the major product even though it underwent a hydride shift. This because shifts of hydrides or alkyl groups are quite common in carbocations, where the shift would increase stability.)
Show calculations:
%y = exp/lit * 100 = 0.03/2.66 * 100 = 1.13.
Almost none of the product could be recovered, because there was such a small amount that it stayed in a condensed state on the inner surfaces of the simple distillation apparatus.
In a similar way to the fact that the head temperature of 90 degrees could not be achieved in fractional distillation, the desired head temperature of 70 degrees could not be obtained in the final simple distillation. All the product had simply evaporated by this time. This hints that vapor pressure was involved in the evaporation, because the true boiling point was not nearly reached.
To produce higher yields, the fractional distillation probably should have been allowed to continue further; likely not all of the product had distilled.
However, the experiment was still successful, because it showed how to produce products by dehydration. It also displayed the interesting technique of using a reagent which is not itself used up during the reaction which it causes.
The experiment was good and fun because it was a low-pressure, relieving procedure, that was easy to do for a last laboratory of the semester.
Cheers! Now we're all this much closer to dying. Where are you going to go? Scientists expect that the true hypothesis for anything has reason and logic, and enough scientific substance in it that it can be tested. Doesn't this mean that science relies on beauty and order? Doesn't this mean that science cannot accept an explanation for life that is non-repeatable, random, and which allows all hypotheses to be equally true?
Either random chance produced one possible way of many ways that life could exist, or somehow all potential ways were considered, and only the best way for life to exist was chosen.
The evidence that everything comes to a conclusion when it is solved, instead of a divergence, seems to show that all ways of existence were considered, and that the way that is chosen is the only possible way.
There could not exist another kind of God besides this kind of God, and if there is one possible truth, then there is one possible way to live and to die and to be with God.
1. Gilbert JC, Martin SF. Experimental Organic Chemistry. 2002. 3rd Edition. p321-325.