Experiment 6

Objectives

1. To perform a reaction which produces an elimination product from 2-pentanol.
2. Isolate your low-boiling-point alkene from the reaction mixture via simple distillation.
3. Extract your chemical in a basic wash to neutralize any carred-over acid.
4. Dry your sample using an anhydrous salt.
5. Re-distill your dry organic liquid and determine yield, percent yield, density, refractive index, and an IR.

Procedure

After performing your elimination reaction, you will analyze the mixture by gas chromatography in order to determine the identities and relative amounts of the products. Dehydration of an alcohol can follow either the E2 or the E1 mechanism. However, in each case, acid is required as a catalyst, because OH^- is a poor leaving group, but HOH is a weaker base, and a better leaving group.  Adding a strong acid, such as H₂SO4 (in contrast HBr, which would add irreversibly) to the mixture allows protonation of the alcoholic -OH group (to produce the alkyloxonium ion; R-OH₂⁺) to give water (a weak base) as the leaving group.  Once this protonation occurs, the mechanism that is followed depends on the nature of R-group.  For a primary alcohol, such as 1-pentanol, the dissociation of water, if it occurred, would produce the very unstable 1° (primary) carbocation, so we would project that elimination via an E1 mechanism (with carbocation intermediate) will not occur.  As a result, reaction would be expected to proceed via the E2 elimination mechanism for any primary alcohol.  However, for 2-pentanol, dissociation of water produces a more stable 2° (secondary) carbocation. Because water is not a very strong base, the competing E2 mechanism will be slow, which will allow the E1 mechanism to proceed faster for 2-pentanol.

Background

Dehydration of an alcohol can follow either the E2 or the E1 mechanism. However, in each case, acid is required as a catalyst, because OH- is a strong base, it is a poor leaving group:

Adding a strong acid, such as H2SO4, to the mixture allows protonation of the -OH group to give water as a leaving group. Once this protonation occurs, the mechanism that is followed depends on the nature of the R group.  As mentioned above, 1-pentanol (a 1° alcohol), dissociation of water would produce the very unstable 1° carbocation, so we would project that elimination via an the E1 mechanism (with carbocation intermediate) will not occur.  As a result, reaction would be expected to proceed via the E2 elimination mechanism.  However, for 2-pentanol, dissociation of water produces the more stable 2° carbocation. Because water is not a very strong base, the competing E2 mechanism will be slow, which will allow the E1 mechanism to proceed faster for 2-pentanol.   The mechanism below depicts reaction by E2 mechanism to product, in a single, concerted step, an elimination, producing an alkene.  The only product, via an E2 reaction mechanism, would be 1-pentene.

In 2-pentanol, dissociation of water will give a more stable 2° carbocation. Because water is not a strong base (it is not readily attracted to one of the H atoms on the b-carbon), the E2 elimination mechanism will be slow, which will allow the E1 mechanism to be faster for the 2-pentanol.

Note that all three possible products are shown.  The actual product mixture will be determined by gas chromatography to determine which of the three possible products is produced.  You will determine the percent composition of this mixture.

There is one more event that is possible in a reaction that involves carbocation intermediates, and is even a possibility with the E2 reaction shown for 1-pentanol, and that is rearrangement. The 2° carbocation produced in the E1 reaction of 2-pentanol may rearrange to give the more stable 3° carbocation as well.  If this occurs, two more possible products can be envisioned:

It is also possible that the 1° alkyloxonium ion formed by the protonation of 1-pentanol has the potential to rearrange via a 1,2-hydride shift (which kicks off the water molecule as the leaving group).  This reaction will give the same secondary carbocation initially produced with 2-pentanol as the starting material.

The only way to know for sure whether or not a rearrangement has taken place is to determine the identities of the products, and the relative amounts of each product, in each reaction mixture must be determined. We will use gas chromatography to do this, very much like we did in Experiment IV.

Procedure

Safety: 2-pentanol is a volatile and flammable liquid and is an irritant.  No flames will be allowed in the lab.  Wear gloves while handling these chemicals. Concentrated sulfuric acid is strongly corrosive and toxic -- wear gloves while handling it, and be sure to wash your gloves and your hands immediately after handling it. Sodium sulfate is an irritant -- gloves are recommended. The alkene products are all highly flammable, and have irritating vapors -- avoid breathing their vapors.

Day 1:

Use a 100-mL round bottom flask for your reaction. Eventually this round-bottom flask will be attached to your distillation setup so that as dehydration occurs, the alkene, which has a low boiling point will immediately be converted into a gas and go into the vapor phase.  The higher boiling point reaction mixture will remain in the distillation flask and continue to produce product.  Alcohols, because they form hydrogen bonds, they boil at a very high temperature.  The alkene, however, since it is not polar nor is it charged has only weak London Dispersion Forces keeping it in the liquid form.  As a result alkenes have a low boiling point and are easily converted into the vapor phase and can be collected when it condenses.  Since the boiling point of 2-pentene is only 36C, you must collect it in a flask kept in an ice bath.

The reaction flask is initially prepared by adding 10 mL DI water and then placing this 100-mL flask in an ice bath.  Slowly add the concentrated sulfuric acid to the flask, swirling it to mix the acid and water together.  Use a dropper to add the acid and remember to swirl the flask after each addition to prevent the chemical mixture from becoming too hot.

Reaction Setup

To the 100-mL round bottom reaction flask, add:

• 10.0 mL DI water (then place the flask on ice)
• 10.0 mL 18 M H₂SO₄ (carefully add the acid dropwise until all of it is added)

Let the cooled reaction flask warm to room temperature and then slowly add the 2-pentanol.

• 10.0 mL 2-pentanol (add slowly using a dropper)
• add a few boiling stones to allow smooth boiling

After you have added all of the 2-pentanol, attach the 100-mL reaction flask to your distillation setup.  Set up a simple distillation (no thermometer is needed -- seal the stillhead using a polyethylene seal and cap it).  The 100-mL flask with the reaction mixture is the distillation pot. Be sure that you added few boiling stones to the reaction flask prior to attaching it to the distillation setup.  The reaction flask which now contains equal amounts of water, H₂SO₄, and 2-pentanol can be heated and the reaction will begin. Bring the liquid to a slow but steady boiling state.

Use a labeled, pre-massed, 50-mL round bottom flask as the receiver, and put the receiver flask in an ice water bath (it is good practice to weigh the receiving flask with a glass stopper in it, since you will need to have the stopper in it when you determine the mass of your product after the distillation/reaction is complete.)

Once the mixture is boiling the reaction will begin.  It might take a few minutes for the first distillate to appear.  Try to maintain a slow steady boil and observe the liquid being collected.  A simple distillation works well for this reaction because the alcohol has a high boiling point but the alkene, which has a very low boiling point, is easily converted into a gas and collected as a liquid after it condenses in the condenser column. (What is the boiling point of 2-pentanol? What is the boiling point of the alkene?).

Continue the reaction/distillation for about an hour, or until no more distillate is collected, whichever comes first. It is quite apparent when no more distillate is visible. The distillate may appears cloudy. It may have a slight color.  Do not be concerned about these things.  The cloudiness is due to both alkene and water being collected.

Remove the receiver flask from the distillation setup and seal it with a ground glass stopper.  You should also disassemble the distillation setup. 

Keep the collected distillate on ice until ready to do the Separatory Funnel extraction.   

Discard the boiling stones in the solid waste container.  Dispose of the strongly acidic reaction mixture in the distillation pot in the appropriate waste container but be very careful because it is extremely concentrated acid.  Do not spill it.

Determine the mass of the collected crude product.  Remember that your collected material contains water (that is often highly acidic) and your organic product.  Add the entire contents of the receiver flask into a Separatory Funnel.  Add about 10 mL of the 5% NaOH(aq) solution to the Separatory Funnel.  Mix vigorously to neutralize the acid that might be present in the organic layer. Collect the upper organic layer (you need to first remove the lower aqueous layer, and you can discard it.  To the remaining liquid (that is, the material that was in the upper layer of the Separatory Funnel) you will transfer it to a clean and dry round bottom flask.  You will then dry the liquid.

To dry your organic material, add anhydrous sodium sulfate which is granular in nature and will absorb water.  You can tell when you have added enough Na₂SO₄ when a little additional sodium sulfate does not stick together when added to the flask.  However, it is not much of a problem since you will need to store your sample until the next lab period.  Usually, about 3 grams of anhydrous sodium sulfate is sufficient.  

In order to store your sample until the next lab period, make certain there are no crystals in the ground glass opening of your flask.  Place the ground glass stopper into the opening.  Gently wrap a little bit of parafilm around the stopper and flask opening.  Store your flask until the next lab period.

Day 2

Distillation of 2-pentene

Your dry alkene liquid will be further purified by distillation.  Be certain not to let any solid material (salt granules) get into your distillation flask as it would quickly release water and  not let you get pure chemical.

When you redistill your organic liquid (you must use a thermometer inserted into the stillhead during this purification), record the temperate (which is a quick way to determine the boiling point of your liquid.  Collect all the liquid that comes over the condenser column. Your chemical should have a boiling point range of about 35-41 °C.  As before, collect your sample in a round bottom flask keep in an ice bucket to prevent evaporation. Continue to collect sample until the liquid is almost completely removed.  Turn off the heating mantle, drop it below the round bottom flask so that it is not still in contact to let it cool.  Some vapors will condense and run back into the flask, leaving some material there.  That is ok.

After collecting your sample do each of the following analyses to help characterize your product.

Determine a density by putting your recovered chemical in a 10-mL graduated cylinder, note the volume and mass of added liquid and calculate its density (g/mL).

Determine a refractive index of your sample.  Be quick, since the boiling point is about 35oC and it will evaporate in less than 10 seconds.  Pour enough to cover the prism, close the top prism, and check the refractive index.  If the "horizon" is not sharp, your sample evaporated.

Ask your instructor to help run an IR, and plot the results of your IR.

Optional: Elimination Reaction 2: Dehydration of Pinacol

Outline: Carry out the dehydration of pinacol to give pinacolone.

Background

As we have seen in lecture, and in Part A of Experiment VI, dehydration of an alcohol can follow either the E2 or E1 reaction mechanism, with protonation of the alcohol as the first step of each reaction. (Which reaction proceeds faster?) With E1 (like SN1 reactions), however, the carbocation intermediate can undergo rearrangement to give a more stable carbocation, which then undergoes the final deprotonation step to give a double bond:

The carbocation rearrangement may involve either an alkyl shift (a methyl group is the alkyl group which shifts in the example depicted above, but more complex alkyl groups may also shift) or a hydride (H:–) shift. The requirements for a shift are: (1) that the carbocation be able the achieve a conformation which allows overlap between an empty p orbital on the cation carbon, and the s orbital (on a hydrogen atom) or the small lobe of the sp3 orbital (on a carbon atom of an alkyl group) of the shifting group; and (2) that the new carbocation (e.g, 3o>2o>1o) be more stable than the original.

In the dehydration of pinacol, there are two possible mechanisms, one which involves a potential carbocation rearrangement, and one without:

Which one is more likely? Remember that a rearrangement will occur only if it gives a more stable carbocation. Look carefully at the carbocation formed after a potential rearrangement.  Moreover, if a resonance structure(s) can be drawn, potential rearrangements will be much more likely.

Resonance stabilizes a cation better than inductive effects alone can, so the new cation, as a result of resonance contributors, is more stable than the initial carbocation, and pinacolone is the major product of this reaction.

Procedure

Safety: Pinacol is a flammable solid and an irritant -- no flames will be allowed.  Wear gloves while handling it. Concentrated sulfuric acid is strongly corrosive and toxic -- wear gloves while handling it.  Be sure to wash your gloves and your hands immediately after handling any of the above chemicals. Anhydrous magnesium sulfate is an irritant -- gloves are recommended. The Pinacolone is a flammable liquid, and have irritating vapors -- avoid breathing their vapors.

Put 6.0 g of pinacol and 30 mL of 3 M H₂SO₄ in a 100-mL round bottom flask. Set up a simple distillation, with the 100-mL flask containing the reaction mixture serving as the distillation pot. Use a labeled 50-mL round bottom flask as the receiver. Once the mixture begins to distill, adjust the heat to keep a moderate boil. The distillate will separate into two layers, and aqueous layer and your product (you need to decide which layer is your product, based on density). Stop the distillation when the water-insoluble layer is no longer increasing in volume (this should take about 20 minutes, or so, from the time the first drop of distillate is collected).

Dispose of the strongly acidic waste material remaining in the distillation pot into the appropriate waste container, being very careful not to spill any of it. As in Part A, you should consider neutralizing the distillate using NaHCO₃.  After neutralization, wash the crude product (from your receiver flask) with 15 mL of deionized water using a separatory funnel.  You need to determine which of the two phases in the separatory funnel is your desired product, and how you can best recover this material.  Dry your organic product using anhydrous magnesium sulfate (adding the appropriate amount to dry the organic material, as described in Part A).

Redistill the dry, crude product, using a pre-massed receiver flask (e.g., a conical vial) and monitor for boiling point. Determine the mass, volume and refractive index of the re-distilled product.  Compare the refractive index to the literature value.  Show your sample to the instructor.  Discard of the material in the appropriate waste container.

Chemicals and Supplies

Compound	MW (g/mol)	Amount Needed	mmol	mp	bp	Density	ηD
2-Pentanol	88.15	10.0 mL (use pipettor)	91.7	-75oC	119.3oC	0.8092	---
conc. H2SO4 (18 M)	96.00	10.0 mL	---	---	---	---	---
5% NaOH (1.25 M)	40.00	10.0 mL
2-pentene (and isomers)	70.13	---		-14oC	36oC	0.66
Na2SO4 (anhydrous)	142.04	3.0 g

Questions

Footnotes