Experiment 6

Elimination Reactions: Acid-Catalyzed Dehydration of 2-Pentanol

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.

Elimination Reaction: Dehydration of 2-pentanol

Outline: Synthesize a potential mixture of alkenes by dehydrating 2-pentanol. Analyze the mixture by gas chromatography in order to determine the identities, and relative amounts, of your product(s).

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.

Put 10 mL of water in a 100-mL round bottom flask.  Place the flask in an ice water bath, and slowly add 10 mL of concentrated sulfuric acid to the flask, carefully and with constant stirring. Allow the solution to cool to room temperature before proceeding.

Slowly add 10 mL of your 2- pentanol to the acid/water mixture in the flask.  (As with any distillation, you need to add a few boiling stones to smooth the boiling.  Discard the boiling stones only in the solid waste container.)  Set up a simple distillation (no thermometer is needed -- seal the still head using a polyethylene seal and cap), with the 100-mL flask containing the reaction mixture as the distillation pot. Use a labeled, pre-massed, 50-mL round bottom flask as the receiver, and put the receiver 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.) Begin heating the mixture (you can use a high setting to get started) to attain a rapid boiling mixture (what is the boiling point of 2-pentanol?). Once the mixture begins to distill (drops start to collect in receiver vessel), adjust the heat to give a slow, but steady boil in the distillation flask. Continue the distillation for about an hour, or until no more distillate is collected, whichever comes first.  

Dispose of the strongly acidic waste from the distillation pot in the appropriate waste container, being very careful not to spill any of it.

Determine the mass of the collected crude product (remember, that your collected material contains both water and organic material), then wash it with10 mL of 5 % aqueous sodium hydroxide using a separatory funnel.  Collect the organic layer (which layer in the separatory funnel is aqueous, and which is organic?).  Dry the organic layer using anhydrous sodium sulfate (you can tell if you have added enough Na₂SO₄ when newly added powder does not cake and stays granular; about 3 grams is usually sufficient).  Cap and parafim your drying distillate mixture until the next lab period.  Redistill your organic material (you must use a thermometer for this purification), collecting the distillate that comes across in the boiling range 35-41 °C, in an iced receiver (make sure your distillation apparatus is completely water-free before starting this distillation).

Perform a GC analysis of your product after you have collected your material. You will need to run two chromatograms -- one on the mixture provided by the instructor (equal volume mixture of the possible products), if not already preformed by the instructor, and one using your product mixture. Use these chromatograms, along with the individual sample chromatograms provided by your instructure (copies are available in instrument room).  You should identify the compounds in your product mixture, by comparing your retention times to the standard compounds. Determine the relative amounts of each compound in your mixture, expressed as a percent of the whole.

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 S_N1 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, 3^o>2^o>1^o) 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.

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Copyright © Donald L. Robertson (Modified: 09/18/2006)