Organic Chemistry II Experiment No. 2 - Preparation of Cyclohexene

DEPARTMENT OF PURE AND APPLIED CHEMISTRYVisayas State University, Baybay, Leyte

CHEM126 Organic Chemistry IILaboratory Report

Name : Mark Ryan R. Tripole Date Performed : 11/27/2014Course/Yr : BS Chemistry II Date Submitted :

12/09/2014Group No : 2 Score

Experiment No. 2Preparation of Cyclohexene

I. Introduction

Looking at it from the perspective of study, alkene chemistry is onewhich is quite broad because of the many reactions that the particularclass of organic compound undergoes. The reactions are always reversiblein the sense that if one were to try to convert a particular alkene into,let’s say, an alcohol through the process of hydrolysis, then the reversecould be achieved by the dehydration of the alcohol back into the parentalkene.

The main objective of this experiment was to observe the laboratoryprocedure for the synthesis of cyclohexene from cyclohexanol, which, asalready mentioned before, is though the dehydration of the parent alcoholto introduce the unsaturation. This dehydration, or removal of water, ischaracteristic of a class of reactions known as Elimination reactions. Theclassification and idea is somewhat the same as that of Substitutionreactions, but the key difference lies in the fact that in eliminationreactions, connections are being broken and those molecules that havebroken away are completely removed or “eliminated”. This introduces theother objective, which was to gain a functional understanding of theseElimination reactions and the mechanisms that makes them happen. Thoughthere are two main classifications of Elimination reactions, namely theE1 and the E2, there will be emphasis only on the E1 because that is themain mechanism through which the dehydration of the cyclohexanol actuallytakes place. Further elaboration on the respective mechanisms will beshown in the succeeding sections.

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II. Results

The experimental procedure was all about the synthesis ofcyclohexene from the dehydration of cyclohexanol. Much like the previousexperiment, there was emphasis on the accuracy of the overall procedureso as to obtain the maximum amount of end product possible. Tabulated andshown below are the results obtained from the procedure, including anyrelevant calculations that were made with respect to the data. Furtherelaboration on the results will be in the Discussion section of thelaboratory report.

Calculations relative to the experiment (in reference to tabulated values)

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Taking into consideration that cyclohexanol is the only reactant, thus itis the limiting reagent. It is already known that 20.0g of cyclohexanolwas used in the procedure. Using this information and the molecularweights of the cyclohexanol and the cyclohexene, the theoretical numberof moles of the cyclohexene that can be obtained can be calculated:

Now that it is known that 0.20 moles of C6H10 can be obtained, it’s all amatter of converting it to grams by multiplying the number of moles tothe molecular weight of cyclohexene to obtain the theoretical yield:

Basing on the data from the experiment, we had only obtained 8 grams offinal product, and in comparison with the theoretical yield, thepercentage yield can be calculated as follows:

Tabulated below are the results obtained from the Bromine test and theBaeyer’s test on the final product. The field for the Bromine test isleft blank because the procedure was not conducted due to the danger inhandling such chemicals at the present time.

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III. Discussion

This experiment was all about observing the laboratory procedure inthe preparation of cyclohexene from cyclohexanol. As has already beenestablished in the introductory portion of this laboratory report, thisconversion comes about from the catalytic dehydration of the alcohol tointroduce the unsaturation into the structure. As a preliminary, thisdehydration falls under the mechanisms of Elimination reactions,specifically one of the E1 (Unimolecular Elimination) nature. Basicallyspeaking, the E1 mechanism is similar in pattern with the SN1 mechanismand in the sense that it is also a two step process. Though they arefundamentally different in terms of their fast second step, the twomechanisms share the same first rate determining step, which is the lossof a leaving group and the formation of a carbocation. To illustrate theE1 pathway further, below is a simple schematic diagram for a simple E1reaction:

Similar to the SN1 mechanism, reactivity for the E1 pathway followsthe stability of the carbocation intermediate. The more stable thecarbocation intermediate, then the more favored they are in the E1mechanism (in short, 3° > 2° > 1°). And in retrospect, we also see thedependence of the efficiency of the E1 pathway (similar to the SN1mechanism) on the leaving group. The better the leaving group, then theeasier it is for the E1 mechanism to occur. For the purposes of thislaboratory report, let us take an alcohol as an example. An alcohol is an

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organic compound that possesses the hydroxyl or –OH functional group. Bydefinition, the –OH group is a terrible leaving group, so first it has tobe protonated in order to convert it into a better leaving group. Inrelation to the E1 mechanism, if the substrate is an alcohol, then astrong acid will be required to protonate the OH portion.

In order to look deeper into the reaction mechanism, the simpleconversion of cyclohexanol to cyclohexene was observed in the experiment.As already mentioned, since the hydroxyl group on the cyclohexanol is areally bad leaving group, then it has to be protonated in order toconvert it into a good leaving group. The acidic condition to favor this

was provided through the usage of 85% phosphoric acid. To move on withthe discussion, below is a simple sketch for the experimental procedureconducted:

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Moving on to the mechanisms, when an alcohol like cyclohexanol isplaced in acidic conditions such as that provided by the 85% phosphoricacid, the –OH functional group on the cyclohexanol is protonated:

Now that the OH group has been protonated, it has been convertedfrom a bad leaving group into a better one. The positive charge on theoxygen is something that it doesn’t want, so it will break away from thecarbon it is attached to (hereafter referred to as the α-carbon) toneutralize itself. This loss of the water leaving group facilitates theformation of a carbocation:

At this point, it is important to take note that the SN1 and E1pathways are competing with each other. If the SN1 mechanism were allowedto occur, then the final yield for the desired cyclohexene will be lessthan desirable. One of the purposes of the phosphoric acid is to suppressthe SN1 pathway, because it provides conditions favorable for the E1mechanism and it has an anion that is a relatively poor nucleophile. Nowthat the SN1 pathway is out of the way, the water present in the solutionacts as a base and abstracts a hydrogen from the neighboring β-carbons:

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It is imperative to realize that the abstraction of the hydrogen canoccur at any two of the β-positions, but ultimately in the end theproduct is still the same. There is usually selectivity in terms of fromwhich β-carbon the hydrogen will be abstracted, but since the moleculedescribed is symmetrical in nature, it doesn’t really matter as of now.The resulting hydronium at the end is significant of the regeneration ofthe catalyst as used in the initial protonation step. This whole reactionis basically achieved up until the first distillation portion, but theby-products from the side reactions and eventual work up have to beconsidered as well.

Considering the by-products that might arise from the reaction, onehas to take note that most if not all of the steps are pretty muchreversible. So upon formation of the carbocation, there are a fewpossibilities as to what could happen. First, the carbocation could reactwith the water (by virtue of its nucleophilic tendencies) again to reformthe initial charged substrate and then initial cyclohexanol:

Then of course, from there we have the reaction yielding the desiredcyclohexene which has already been shown and discussed in the previouspage. Another possible byproduct comes from the consideration that sincethe cyclohexanol molecule possesses the hydroxyl functional group, it haspartial charges. The partial negative charge on the oxygen is of note inthis scenario because it allows the molecule to act as a weaknucleophile. Thus a cyclohexanol molecule could react with the formedcarbocation to form dicyclohexyl ether as shown below:

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As a work up, standard techniques have been employed in order toobtain the purest possible product and the maximum amount of yield. Afterthe initial distillation, the solution was first saturated with solidsodium chloride. This was a pre-drying step that aided in removing mostof the water present in the distillate. The principle behind themechanism of the solid sodium chloride is that it pulls water from theorganic layer (which contains the cyclohexene) into the water layer. Thisis connected to the principle that the resulting concentrated saltsolution formed by the salt with the aqueous layer wants to become moredilute and also because salts have a stronger attraction to water than toorganic solvents. Another way to look at this could also be the “saltingout” of the organic material from the aqueous layer. So saturating withthe sodium chloride serves the dual purpose of pulling out water from theorganic layer and expelling any organic material from the aqueous layer,all in the context of “like dissolves like”. The dissolving of sodiumchloride in the water and the presence of ions increases the ionicstrength of the aqueous layer. Since cyclohexene in itself is arelatively non-polar compound, it does not dissolve the sodium chloride.A simple diagram illustrating this concept is shown below:

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After this step, an amount of sodium carbonate was added to thesolution, just enough to make it basic to red litmus paper. This stepensured the removal of any left over phosphoric acid through thefollowing reaction:

2H3PO4 + 3Na2CO3 2Na3PO4 + 3CO2 + 3H2O

The resulting sodium phosphate compound is soluble in the aqueouslayer, and this was easily removed along with the aqueous layer using aseparatory funnel. The final work up step before the final distillationwas the final drying step where anhydrous calcium chloride was added tothe crude cyclohexene. The hygroscopic nature of the calcium chlorideaided in the final removal of any water that could have possibly beenleft over. After separating, the second distillation was then performed,and it is at this stage where any dicyclohexyl ether formed is separatedfrom the cyclohexene. The boiling point of cyclohexene is around the 80to 85°C mark, while the dicyclohexyl ether clocks in at around the242.5°C mark. Any liquid left over in the distilling flask that didn’treally boil over as a distillate was the diclohexyl ether.

DISCUSSION ON YIELD

With regard to the yield, the group had obtained only 48.8% ofproduct, which is a far shot from the expected 79-80% that was to beexpected from the proportions of the chemical species. One of the mainreasons for this low output may have been due to human error. Error inaccurately measuring the amount of initial reactants, error at theseparation stage where some of the organic layer might have actuallyescaped as well as not having set up the distillation apparatus

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correctly, wherein some of the gaseous cyclohexene might have escaped outof the apparatus. The group had encountered this a few times through theexperiment before the connections were adequately sealed and the “fuel-like” odor of cyclohexene was no longer observed.UNSATURATION TESTS

For the purposes of this experiment, tests had to be carried out inorder to determine whether the final distillate was actually thecyclohexene. Thus tests for unsaturation were to be carried out on thefinal product to test for the double bond functionality. Two tests ofnote will be mentioned here, the first being the Bromine test (using Br2

in dichloromethane) and the Baeyer test (using KMnO4). Because of thedanger in the handling of bromine as of the time, the bromine test wasnot carried out, but the mechanism and the expected result will bediscussed here.

Bromine in Dichloromethane

The reaction between bromine and an alkene is characteristic of anaddition reaction, where the bromine “adds” across the double bond toform a dihalide. In its original form, the bromine solution is reddish orquite red in color. On reaction with an alkene, the alkene losesunsaturation and takes in the Bromine, and what happens is that there isa gradual disappearance of the red color of the solution. This is thepositive indication that the unknown compound indeed has unsaturation andis an alkene or an alkyne. The addition of Bromine across a double bondis characteristic of a mechanism known as Electrophilic Addition. A step by stepillustration of this process is shown below with respect to cyclohexene:

STEP 1The loosely held πelectrons of cyclohexeneinduce a dipole inbromine. One of the

bromine leaves as an anion and the other gets connected to the ring.

STEP 2The nucleophilic bromineattacks the carbocationportion of the compound.The resulting ionicspecies is known as abromonium ion.

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STEP 3A bromide ion then goeson to attack thebromonium ion, and thisresults in a dihalide.The reddish color of thebromine solutiondisappears as it is usedup.

The 1,2-dibromocyclohexane formed is not of prime importance in thistest, because it is the physical disappearance of the color that is themajor result. The dichloromethane solvent only serves the purpose ofdissolving the bromine and does not play a part in the reactionwhatsoever. In the succeeding portion will be a discussion on the otherunsaturation test used in the experiment, the Baeyer’s test which employsthe usage of dilute Potassium Permanganate solution (KMnO4)

Baeyer’s Test (Dilute Potassium Permanganate KMnO4)

The use of Potassium Permanganate solution is also one of the morefunctional tests for unsaturation. Compared to the bromine test, this oneis much safer and the results are just as immediate. The original colorof potassium permanganate in its solution form is a dark purple. Onreaction with an unknown that is suspected to have unsaturation, thepurple coloration of the solution is gradually replaced by a brownishsolution that contains a precipitate of the same color. In terms of whathappens to the original compound, the alkene is converted into a diol inthis process. An illustration of the mechanism behind this reaction withKMnO4 is shown below:

First the permanganate ion approaches the alkene and basicallycomplexes it forming the product as shown. Then through a reaction withwater (whose mechanism would be too long and too exhaustive for thesimple purposes of this laboratory report), produces the 1,2-

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cyclohexanediol and the compound Manganese Oxide (MnO2) which is basicallya brown precipitate:

Relating it to the experimental results, when the final cyclohexenewas tested with the potassium permanganate, there was gradual butimmediate change from the purple to the brown solution. Though the browncoloration didn’t really cover the whole solution per se, the change initself is already a significant enough result that the final productcontains unsaturation.

IV. ConclusionAlkene chemistry is one that is broad and demands the full faculties

of attention in order for it to be understood. It provides a simpleplatform for learning where one can understand the machinations behindthe mechanisms and how they could possibly apply to the more complexreactions. Through the study of the conversion of cyclohexanol tocyclohexene, a deeper understanding of elimination reactions has beenobtained. The factors that play an important part in the reactionmechanism and how certain conditions favor them (like the acidicconditions for E1 reactions for example), the dependence of the reactionon the leaving group, the number of connections on the α-carbon and awhole lot more. Though the final yield at 46.8% may not have been allthat desirable to say the least, the group is slowly getting accustomedto the distillation procedures and the precautions that have to be takenin order to obtain the optimum amount of end product possible. Moreover,study of the mechanisms behind the unsaturation tests has provided aninsight into the reactivity of the carbon to carbon unsaturation, be ittriple or double bonds. In conclusion, this experiment has allowed forthe closer scrutiny of catalytic dehydrations of alcohols and theunimolecular elimination pathway that follows it.V. Answers to Questions and Exercises

1. What alcohol would be the most logical choice as starting material for the preparation of1-methylcyclohexene? Write the chemical equation for the reaction. Explain why youthink your choice is the most logical.

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The most logical choice would be to start with using 1-methylcyclohexanol. The chemical reaction for the dehydration of 1-methylcyclohexanol with 85% phosphoric acid is shown below:

Using it as a starting material is the most logical choice because itis the configuration that will yield the highest proportion andpercentage of the desired product. This is in terms of the abstractionof the hydrogen from the β-carbons, and the potential products thatarise from them. Abstraction of hydrogen from the β-positions insidethe ring gives the same product as shown (the desired product). Butabstraction from the methyl group gives another product, and thedetermination of which of the two is the major and the minor productcomes from observing the degree of substitution. The trisubstituted 1-methylcyclohexene (or known as the Zaitsev product) predominates over thedisubstituted compound that forms from the abstraction of the protonfrom the methyl group (also known as the Hofmann product).

2. How much of the alcohol you choose in answering Question 1 would you require asstarting material if you needed to prepare 25g of 1-methylcyclohexene and if your actualpercentage yield were going to be 75%?

Theoretical yield = 25 gramsPercentage yield = 75%

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Given that the theoretical yield is obtained from multiplying thenumber of moles of the limiting reagent by the molecular weight of theproduct:

Then calculating for the amount in grams of the 1-methylcyclohexanolto be used:

3. What is the theoretical yield of 1,2-dibromocyclohexane for the reaction of 20g ofcyclohexene with 20g of bromine? (Hint: Which is the limiting reactant?)

First calculating to check and see which reactant is the limitingreactant:

grams of cyclohexene moles of cyclohexene moles of 1,2-dibromocyclohexane

grams of Br2 moles of Br2 moles of 1,2-dibromocyclohexane

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Since Bromine produces the least moles of product, it is the limitingreagent.

4. A reaction is carried out for which the stoichiometry is A + 2B + 3C D + 2E + F. In thisreaction 0.5 mole of A, 0.75 mole of B, and 1.0 mole of C are allowed to react. The yield ofE is 0.2 mole. What is the percentage yield?

A + 2B + 3C D + 2E + F

(0.5 mol) (0.75 mol) (1.0 mol) (0.2 mol)

Checking for which reactant is the limiting reactant:

Since compound C provides the least number of moles for E, then it isthe limiting reagent. Taking then into consideration that actual yieldof E is only 0.2 moles, the percentage yield then can be calculatedaccordingly. Since we are only given moles, we can use the molar ratioto calculate for an estimate for the percentage yield:

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B B

B

C

CC

VI. References

“Alkenes: Structure and Preparation via Elimination Reactions”David Klein, Organic Chemistry: Page 353 “The Elimination Mechanism”

“Alcohols and Ethers”http://butane.chem.uiuc.edu/cyerkes/chem204sp06/lecture_notes/

lect18c.html “Reaction Mechanism”

http://web.pdx.edu/~wamserc/CH331F97/E3ans.htm “Synthesis of Cyclohexene”

http://www.organicchem.org/oc2web/lab/exp/cyclo/cyclodes2.html “Potassium Permanganate”

http://www.mpcfaculty.net/ron_rinehart/12A/chap6pag.htm “Percentage Yield”

Raymond Chang, General Chemistry: Chapter 8.10 “Stoichiometric Calculations”

Edwin D. Hughes, General Chemistry for Dummies, Excerpt

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http://www.mpcfaculty.net/ron_rinehart/12A/chap6pag.htm

http://www.organicchem.org/oc2web/lab/exp/cyclo/cyclodes2.html

http://web.pdx.edu/~wamserc/CH331F97/E3ans.htm

http://butane.chem.uiuc.edu/cyerkes/chem204sp06/lecture_notes/lect18c.html

http://butane.chem.uiuc.edu/cyerkes/chem204sp06/lecture_notes/lect18c.html

Organic Chemistry II Experiment No. 2 - Preparation of Cyclohexene

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