What Compound(S) Do You Expect to Be in the Distillate When the Dehydration Reaction Is Complete?

Experiment 6

Emptying Reactions: Acid-Catalyzed Dehydration of two-Pentanol

Objectives

Emptying reactions produce unsaturated compounds. Alcohols undergo elimination to produce an alkene. Alkyl halides undergo eliminations besides. Using a secondary alcohol we will perform an acrid-catalyzed emptying which is second guild (E1), significant that a carbocation is the intermediate and the rate of reaction is dependent on how speedily the carbocation can be formed. As soon as the carbocation is formed, the balance of the elimination reaction occurs very quickly. Starting with 2-pentanol the major product should be the germination of ii-pentene, although a description of how other alkenes could be formed is included.  Since neither GC or IR allow usa to distinguish these other alkenes, we presume that the major (possibly only) product we isolate volition be the two-pentene, although it is possible that some of the other alkenes may be nowadays.  An IR volition exist performed showing that the booze group has been removed.

Groundwork

Dehydration of an alcohol can follow either the E2 or the E1 reaction mechanism. However, in each example, acid is required as a catalyst. Because the OH^- (hydroxide) ion is a poor leaving group (it is a strong base), nosotros perform the reaction in acid to produce h2o (HOH) every bit a leaving group, since information technology is a much weaker base.  The reaction is initiated by adding a strong acid, such as H₂Then_iv (non HCl or HBr because the resultant anion (Cl^- or Br^-) would react and produce the alkyl halide) to the mixture.  Acid volition protonate the hydroxyl group of the alcohol to produce an alkyloxonium ion; R-OH₂ ⁺) to allow for a water molecule to be the leaving group.  Once the alkyloxonium ion is produced, the reaction mechanism that is followed depends on the nature of R-group.  For a primary alcohol, such equally 1-pentanol, the dissociation of water, if it occurred, would produce the very unstable and difficult to form 1° (primary) carbocation.  As a consequence, we would projection that elimination via an E1 mechanism for a primary alcohol will exist difficult to occur.  Equally a consequence, reaction would be expected to keep via the E2 elimination mechanism for any main alcohol, and terminate up producing an ether instead of an elimination product. However, for a secondary alcohol, such as 2-pentanol, dissociation of h2o produces the more stable ii° (secondary) carbocation. Because h2o is non a strong base, the competing E2 mechanism would be slower, which allows for the E1 mechanism to continue faster for 2-pentanol.

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

Adding a stiff acid, such as H2SO4, to the mixture allows protonation of the -OH grouping to give water every bit a leaving group. Once this protonation occurs, the mechanism that is followed depends on the nature of the R group.  As mentioned higher up, 1-pentanol (a 1° booze), dissociation of water would produce the very unstable 1° carbocation, so nosotros would project that elimination via an the E1 mechanism (with carbocation intermediate) will not occur.  Equally a event, reaction would exist expected to proceed via the E2 elimination mechanism.  However, for 2-pentanol, dissociation of h2o produces the more stable two° carbocation. Because h2o is non a very strong base of operations, the competing E2 mechanism will be slow, which will allow the E1 mechanism to continue faster for two-pentanol.   The mechanism beneath depicts reaction by E2 mechanism to product, in a unmarried, concerted footstep, an elimination, producing an alkene.  The but product, via an E2 reaction machinery, would be 1-pentene.

In ii-pentanol, dissociation of h2o will give a more stable two° carbocation. Because h2o is not a strong base (information technology is not readily attracted to one of the H atoms on the b-carbon), the E2 elimination mechanism will exist slow, which will let the E1 mechanism to exist faster for the ii-pentanol.

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

At that place is 1 more result 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 ii° carbocation produced in the E1 reaction of 2-pentanol may rearrange to give the more than stable iii° 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 requite the aforementioned secondary carbocation initially produced with 2-pentanol equally the starting textile.

The only fashion to know for certain whether or non a rearrangement has taken identify is to determine the identities of the products, and the relative amounts of each production, in each reaction mixture must be determined. We will use gas chromatography to do this, very much like we did in Experiment IV.

Process

Safety: ii-pentanol is a volatile and flammable liquid besides every bit being an irritant.  No flames will exist allowed in the lab.  Wear gloves while treatment these chemicals. Concentrated sulfuric acid is strongly corrosive and toxic -- wear gloves while handling it, and exist sure to wash your gloves and your hands immediately later on handling it. Sodium sulfate is an irritant -- gloves are recommended. The alkene products are all highly flammable, and accept irritating vapors -- avert animate their vapors.

Solar day 1

Ready the reaction in the following order.

• Add x mL of water to a 100-mL round-bottomed flask.
• Place the flask in an water ice water bath.
• Slowly add 10 mL of concentrated sulfuric acrid to the flask. Add drop wise and swirl the flask after each addition to thoroughly mix the contents and to absurd the exothermic reaction.

Let the solution to cool to room temperature before proceeding further.

• Slowly add together 10 mL of 2-pentanol to the acid/water mixture already in the flask.
• Add a few boiling stones to maintain smooth the humid. (Discard the boiling stones subsequently the reaction in the solid waste product container.)

Prepare a simple distillation (no thermometer is needed and so you must seal the still caput using a polyethylene seal and cap). The 100-mL flask containing the reaction mixture is used as the distillation pot.

Attach a pre-massed 50-mL round bottom flask as the receiver.  The receiver flask is placed in an water ice water bath to keep it cold during the reaction and distillation because the resultant alkene has a low humid point. Information technology is skilful practice to weigh the receiving flask with a glass stopper in it, since yous volition need to have the stopper in information technology when you determine the mass of your collected product. Begin heating the mixture (you can utilise a high setting for the heating mantel at first to get the reaction started). Once the mixture begins to boil (drops start to collect in receiver vessel), adjust the oestrus setting to give a slow, only steady, boil in the distillation flask. Go along the distillation for about an hour, or until no more distillate is collected, whichever comes offset.

After the distillation, dispose of the strongly acidic waste material contents of the distillation pot in the appropriate liquid waste container, being very conscientious not to spill whatever of it, merely disposing of the boiling stones in the solid waste product container.

Determine the mass of the nerveless crude production (call back, that your collected textile contains both water and organic material).

Wash the isolated liquid in a separatory funnel using some of the v% NaOH(aq). Collect just the organic layer from the separatory funnel.  (Which layer in the separatory funnel is aqueous, and which is organic? How can you lot tell?)

Add together the organic phase from your Separatory Funnel extraction to a make clean and dry l- or 100-mL round-bottom flask. Usually y'all will apply a xix/22 basis-glass flask, since the normal 14/20 flasks are kept in the kits and used by other labs. In society to dry the organic layer you volition use anhydrous sodium sulfate (you should employ virtually two.0 g of anhydrous Na_twoSO₄ to dry out your chemical; that amount of anhydrous common salt will absorb about 2.5 g of water).  Unless you can clearly see a water layer in your flask, the ii.0 chiliad above should be sufficient to dehydrate your organic liquid.

Store your properly labeled flask (with organic phase and dehydrating salt) until the side by side lab period. Attach a ground-drinking glass stopper to a round-lesser flask for storage. You cannot only use parafilm equally your distillate will dissolve the parafilm.

24-hour interval 2

Obtain your flask containing the distillate mixture from the previous lab menstruation (this liquid cloth should incorporate your desired alkene product as well as some ii-pentanol which also distilled over).  When you perform a distillation to re-purify your desired chemical, be certain that no salt crystals from the bottom of the drying flask is added to the distillation pot (containing a couple boiling stones).  Since the table salt crystals incorporate captivated h2o, heating them would release the water, and your drying process would be for nada.

When you re-dribble your stale (anhydrous; absence of h2o) organic cloth, be certain you use a thermometer so that you lot tin monitor the boiling point of the distillate.  Collect the distillate equally before (for this distillation, use the 4-mL collection vial from your organic chemistry kits). Place the collection vial in an ice bath equally earlier and collect your anticipated product, monitoring the temperature, until the 4-mL vial is full (or until the temperature rises above 41^oC) only slightly beneath the footing-drinking glass opening.

Do the following analyses of your product to help characterize it.  Recollect, that whatever ane concrete property (e.thousand., boiling indicate) cannot be used to identify a chemical.  However, when several concrete properties or analyses are performed, you lot can be more sure that the chemic existence analyzed is the chemical you are anticipating when more of the values hold with that chemic.

• GC (optional): If your instructor requests a GC assay, run your sample on the GC. However, since niggling if any differences are observed for the different alkene possibilities, a GC usually will not be performed.
• IR: You will need to perform an IR of your product.  This is a very good procedure to perform since you kickoff with an alcohol, which has a very characteristic IR spectrum.  When you run your IR, remember that the boiling bespeak of your alkene is very depression, and it will evaporate chop-chop.  Let your instructor assist you lot in running the IR.  Obtain an IR spectrum, and compare to the IR spectrum of your starting alcohol.  While you may see a slight dip in the region of an alcohol in the IR spectrum, it is non at the same position, and is often characteristic of these types of compounds. Compare your IR of the re-distilled product to the 2-pentanol you started with. Include both IR spectra in your notebook and report (if written).
• Reaction with Br₂: Alkenes undergo add-on reactions.  Ane of the easiest improver reactions to monitor is that of an alkene reacting with molecular bromine (Br₂). Bromine is a liquid at room temperature.  It is too a colored substance; it is orange in colour. I of the criteria used to classify chemic reactions is a color modify (others include product of a gas, germination of h2o, and germination of a solid). Most alkenes are colorless, and that is truthful of the conceptualize alkene production you produced in this experiment. Add nigh 1 mL of your re-distilled chemic to a small test tube. Once you have your alkene in the test tube, add a few drops of Br₂ (in CH_twoCl_ii) to the test tube. Look at the equation below, and and y'all volition see that the reactant side (if no reaction occurred) would be orangish (the Br_ii color would just be a fleck diluted).  However, if a reaction took place, that orangish colour would disappear, because the resulting production (dibromoalkane) is colorless.  Therefore, if the few drops of Br_ii lead to a colorless solution, you know that a reaction took place and the bromine is no longer a halogen merely is function of the new colorless compound.

Alkene (colorless) + Br₂ (orange) --> dibromoalkane (colorless)

For any reactions, show the complete reaction of reactants and products in your notebook and formal study (if written).

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Optional: Elimination Reaction two: Aridity of Pinacol

Groundwork

As we have seen in lecture, and in Function A of Experiment VI, dehydration of an alcohol can follow either the E2 or E1 reaction mechanism, with protonation of the alcohol equally the outset step of each reaction. (Which reaction gain faster?) With E1 (similar South_None reactions), however, the carbocation intermediate can undergo rearrangement to give a more stable carbocation, which then undergoes the terminal deprotonation pace to give a double bail:

The carbocation rearrangement may involve either an alkyl shift (a methyl group is the alkyl group which shifts in the example depicted above, only 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 attain a conformation which allows overlap between an empty p orbital on the cation carbon, and the s orbital (on a hydrogen cantlet) 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.one thousand, 3^o>ii^o>one^o) be more stable than the original.

In the aridity of pinacol, in that location are two possible mechanisms, one which involves a potential carbocation rearrangement, and one without:

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

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

Process

Safety: Pinacol is a flammable solid and an irritant -- no flames will exist immune.  Wear gloves while handling it. Concentrated sulfuric acid is strongly corrosive and toxic -- wear gloves while handling it.  Be sure to launder your gloves and your hands immediately after handling whatever of the in a higher place chemicals. Anhydrous magnesium sulfate is an irritant -- gloves are recommended. The Pinacolone is a combustible liquid, and have irritating vapors -- avoid animate their vapors.

Put six.0 g of pinacol and 30 mL of 3 Chiliad H₂Then₄ in a 100-mL round bottom flask. Set up a uncomplicated distillation, with the 100-mL flask containing the reaction mixture serving as the distillation pot. Employ a labeled 50-mL round bottom flask as the receiver. One time the mixture begins to dribble, adjust the rut to keep a moderate boil. The distillate will split into two layers, and aqueous layer and your product (you need to decide which layer is your product, based on density). Cease the distillation when the water-insoluble layer is no longer increasing in volume (this should take nearly 20 minutes, or and then, from the time the start drop of distillate is collected).

Dispose of the strongly acidic waste material fabric remaining in the distillation pot into the appropriate waste matter container, being very careful not to spill any of information technology. As in Part A, you should consider neutralizing the distillate using NaHCO₃.  After neutralization, wash the crude product (from your receiver flask) with fifteen mL of deionized water using a separatory funnel.  You need to make up one's mind which of the two phases in the separatory funnel is your desired product, and how you can all-time recover this textile.  Dry out your organic product using anhydrous magnesium sulfate (adding the advisable amount to dry the organic cloth, as described in Office A).

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

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Chemicals and Reagents

Compound	MW	Amount	mmol	mp	bp	Density	η D	msds
2-pentanol	88.fifteen	ten.0 mL	---	-75	119.iii	0.8092		msds
2-pentene (and other alkenes)	70.13	---	---	-140	37	0.65	---	msds
Br2 (in CH2Clii)	159.808	---		-7.25	58.78	3.xi		msds
Compound	g/mol	grams or mL	10-3 G	oC	oC	1000/mL	ηD	msds

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Questions

1. What is an emptying reaction? What type of compound is produced for every elimination reaction?
2. Why do dehydration reactions use acid as catalyst? Why can you non utilize base for dehydration reactions?
3. What is the proper name for the alkyloxonium ion used in the experiment today?
4. Dehydrohalogenation reactions use base of operations as catalyst.  In fact, the base must be a stiff base of operations. Show the machinery for a dehydrohalogenation reaction (E2 elimination).
5. Why can you use a simple distillation setup for the reaction performed in this experiment?  Why is information technology non necessary to do a long reflux to produce your production, and then collect the product by distillation?
6. Show the reaction machinery and the elimination product for the dehydration of both t-butyl alcohol and of isobutyl booze (the same product is produced).

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Footnotes

none

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Source: https://home.miracosta.edu/dlr/210exp6.htm

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