Dehydration of 2-Methylcyclohexanol Formal Report

A dehydration reaction of an alcohol results in an alkene. This type of reaction requires an alcohol, an acid catalyst and heat. Generally strong concentrated acids, like sulfuric acid and phosphoric acid, are used as the acid catalyst.The acid catalyst protonates the alcohol, to make a much better leaving group. Weakest bases make the best leaving groups, so once the alcohol is protonated the leaving group leaves and produces in a carbocation and water. In order to form the double bond, one of the beta hydrogens, hydrogens on a carbon adjacent to the carbocation, must be removed or eliminated. Therefore, another name for a dehydration reaction is beta elimination. Water acts as a nucleophile and attacks the carbocation, removing a beta hydrogen; the electrons from the C-H bond move to make a C-C pi-bond. The general mechanism is displayed by equation (1.0)Equation (1.0) shows a reaction with a primary alcohol.

Usually primary alcohols form primary carbocation. However, primary carbocations are too unstable to form as intermediates, so they can undergo a rearrangement or E2 mechanism. A rearrangement occurs when an alkyl group or a hydrogen on the neighboring carbon of the carbocation shift to delocalize the positive charge to result in a more stable carbocation. A rearrangement almost always occurs whenever a carbocation can form a more stable molecule. An E2 mechanism means that the reaction is an elimination and the rate-determining step is bimolecular, two species are involved in one step of the mechanism. If the alcohol in equation (1.0) were to undergo an E2 mechanism then the beta hydrogen would be eliminated (the nucleophile would attack the electrophilic carbocation), C-C pi bond would be formed, and the leaving group would leave all in the rate determining step.

Additionally, equation (1.0) only shows one product, which is not true for all alcohols. Certain alcohols can produce more than one product when they undergo dehydration, such as 2-methylcyclohexanol, structure shown on right. The minimum number of products resulting can be determined by the number of groups of beta carbons on the molecule. For example, after the alcohol is protonated and leaves, the carbocation will have two groups of beta hydrogens, so 2-methylcyclohexanol will have at least two products as a result of beta elimination. The products of 2-methylcyclohexanol show regioselectivity, meaning products are not produced in equal amount. Zaitsev’s rule can predict which product will be the major, most favored product. Zaitsev’s rule states that the more substituted alkene will the favored product; the double bond containing the most non-hydrogen substituents will be the major product.

In this lab, 2-methylcyclohexanol will undergo a dehydration with the acid-catalyst, phosphoric acid. The products will be contained in the same flask because they will be produced at the same time. Gas chromatography will be used to separate the mixture of products into its components. Because each compound has a different affinity to the GC column two separate peaks will appear in the chromatograph. The compound that produces the peak with the greatest relative area will be the major product. The gas chromatograph is connected to the mass spectrometer, so the identities of the two compounds can also be determined. A mass spectrometer measures the mass of ions of the sample and matches it with its built in database to determine the compound(s) present.

There is no need for running GC’s of known compounds. Additionally IR spectra will be run on the starting material and final product to determine the functional groups present. Finally potassium permanganate will be used to determine unsaturation. Potassium permanganate (KMnO4), deep purple in color, acts an oxidizing agent in a reaction with alkenes or alkynes to produce a diol and a reduced form of manganese (MnO2), which is brown. Equation (2.0) shows a generic example of this reaction. Alkanes and aromatic compounds do not react with KMnO4; therefore, no color change occurs. Lack of color change indicates that no alkene was present in the reaction.

The purpose of this lab is to determine the products of the dehydration of 2-methylcyclohexanol via distillation and establish if Zaitsev’s rule holds true in this reaction by analyzing results from gas chromatography and the mass spectrometer, performed prior to the experiment. IR spectroscopy will be used to analyze the products obtained from the experiment. The products will also be tested with KMnO4 to confirm that 2-methylcyclohexanol had undergone a dehydration to form unsaturated compounds. Given the structure of 2-methylcyclohexane it will probably form two products, methylcyclohexene and 3-methylcyclohexene, as shown by equation (3.0). If Zaitsev’s rule holds
true, methylcycloxhene will be the major product as it is more substituted than 3-methylcyclohexene. The test for unsaturation will be positive, color change visible.

ExperimentalThe powermite was set to 6 to heat the sand bath, placed on an extended jack. To a 25 mL round bottom flask 5.0 mL (4.21 g or 0.0369 moles) of 2-methylcyclohexanol (from Acros Organics with 99 % purity) was added to the flask. To the round bottom flask, 2.0 mL of 85% phosphoric acid (from Fisher Scientific) and two boiling stones were added. The flask was swirled to mix. The apparatus for distillation, shown in Figure 1 was set up. A 10 mL receiving flask was used, and the 25 mL round bottom flask with 2-methylcyclohexanol, phosphoric acid and boiling stones was used for the boiling flask. Both receiving and boiling flasks were attached. Water tubing was connected and the boiling flask was placed in a sand bath.

The thermometer read 27 °C at the beginning of the distillation. Because the temperature rose very quickly three minutes into the distillation, the powermite was turned down to 3. The first drop occurred at 90 °C. Six minutes later, the temperature began to drop so the powermite was increased to 6. However, the temperature did not rise significantly. At approximately 79.5 °C, fourteen minutes after the distillation began, the majority of the contents in the boiling flask had distilled so the distillation was stopped. Contents of the receiving flask were cloudy. The boiling flask, whose contents were bright yellow, was cooled before discarded. The result of the distillation weighed 3.84 g.

Contents from the receiving flask were transferred to a centrifuge tube, to which 2 mL of 10% sodium carbonate solution (lab sample) was added. Upon transfer to the centrifuge tube, separate layers were visible. The tube was swirled slowly to mix. The centrifuge tube was capped and inverted once. The cap was then removed to expel any gas pressure. Again the cap was replaced and the tube inverted several more times. The tube was placed in a beaker after the cap was removed to allow layers to separate. The top layer was cloudy but the bottom layer was clear. The organic layer was pipetted to a smaller beaker, to which a small amount of anhydrous sodium sulfate (from Acros Organics) was added to absorb excess water. The beaker was then swirled and allowed to sit for 10 min.

A clean dry distillation apparatus was set up with a 10 mL boiling flask and a 5 mL receiving flask. The power mite was turned to 8 to heat the sand bath. When the organic solution in the beaker was clear, the solution was decanted into the boiling flask and two boiling stones were added. The flask was then attached to the distillation set up. Water tubing was connected and heat was applied to the boiling flask. The thermometer read 30 °C at the beginning of the distillation. The contents of the boiling flask distilled in less than a minute, the thermometer read 79 °C.

Because the only distillation product to be collected was between 102 -111°C, the distillation was repeated. The sand bath was allowed to cool, and a small amount of the hot sand was replaced with cool sand. The thermometer read 31 °C at the beginning of the distillation. The contents of boiling flask were the contents of the receiving flask from the previous distillation, massed at 1.1 g. The powermite was set to 4.5. The first drop occurred at 73 °C. The distillation took approximately five minutes, finishing at 90 °C. The receiving final product was massed at 0.88 g or 0.00917 moles. An IR was run on this sample as well as on the dry sample (done previously by another group).

A small sample of the final product was combined with about three drops of 0.8 M KMnO4 in a disposable test tube to test for unsaturation. The tube was agitated, and small brown and red spots appeared. The test tube was allowed to sit as a negative control was run. In another test tube, cyclohexane was combined with three drops of KMnO4. The tube was agitated but no change in color observed. This tube was allowed to sit as well. After five minutes, the tube containing the sample from the distillation had turned brown and separated into two layers with the sample on top. The negative control was still purple, also separated with the sample as the top layer. Finally the GC-MS data was analyzed to determine the validity of Zaitsev’s rule in this reaction.

Table 1.0 Summary of GCRetention TimeRelative Area (100 %)2.7624.02.803.502.8972.5Figure 2.0 shows the GC, done by the instructor. Only the significant peaks were identified in Table 1.0. The area was calculated assuming, total area equaled the sum of the areas of the three peaks (retention time = 2.76, 2.80, and 2.89).

Table 2.0 IR spectral information of 2-methylcyclohexanolPeak Frequency (cm-1)Peak Assignment3356-OH2927CH stretch1449-CH21372-CH3Table 3.0 IR spectral information of products of dehydration of 2-methylcyclohexanolPeak Frequency (cm-1)Peak Assignment3356impurity3002C=C2925-CH32857-CH32836-CH21440CH bendingDiscussionAfter the first distillation, two layers formed in the receiving flask. The top layer was the organic layer, and contained the products of the dehydration of 2-methylcyclohexanol. The bottom layer was the acidic layer and contained the byproduct of the reaction, H3O+. In order to neutralize the layer acid sodium carbonate was added. Since the expected organic products were alkenes, it was important to neutralize the acid to prevent a reverse reaction. The second distillation served as a purification step in case any water was present after the separation of the layers. Presence of water can cause impurities and extra peaks in IRs.

The most significant peak on the IR of 2-methylcylohexanol, Figure 4.0, is the broad alcohol stretch from 3200-3500 cm-1. Peaks in the 3000-2800 cm-1 area usually indicate CH stretch present in alkanes; there is a significant broad peak at 2927 cm-1. A peak at 1372 indicates the presence of a methyl group. The data from this IR is consistent with the structure of 2-methylcyclohexanol.

The IR of the final products of the dehydration of 2-methylcyclohexanol indicates the presence of a double bond; there is a peak at 3002 cm-1. Peaks ate 2925 and 2857 cm-1 indicate the presence of a methyl group. Peaks in the 1500-1200 cm-1 usually indicate CH bending due to -CH2-; there are several small peaks in that range. This data is consistent with the structure of the expected products from the dehydration of 2-methylcyclohexanol.

In addition to these peaks, Figure 5.0 shows a minor stretch at 3356 cm-1. Generally stretches in this area indicate the presence of -OH groups. The purpose of the dehydration was to remove the alcohol group and replace it with a double bond. It is likely that the peak at 3356 cm-1 is due to the presence of water. However, water was not expected to be in the final product becasue the second distillation. This error is probably because the products acquired from the second distillation were from a temperature range below 90 °C. The first time the purification distillation was run, the temperature didn’t even reach 80 °C. Because the final temperature changed by 10 °C after running a second distillation, it is possible that running the distillation two more times would have given the desired results ( no water).

The results of the test for unsaturation confirmed the dehydration of an alkane to an alkane. In the reaction with KMnO4, the alkene (the product) was oxidized to produce an alcohol and MnO2. KMnO4 is a deep purple color and MnO2 is a brown color, making the presence of the alkene more noticeable. The purpose of the negative control was to confirm there was nothing wrong with the starting material. The negative control determined that there were no alkenes already present in the 2-methylcyclohexanol that could cause the reaction with KMnO4 to proceed. The results of the negative control indicated that there were no alkenes present in the starting material as there was no color change.

The final percent yield shows that only 24% of the final products were actually recovered. The major source of error could have occurred during the separation of the organic layer and aqueous layer. It was difficult to tell when all of the organic layer had been removed from the centrifuge tube to beaker so, less of the organic layer may have been removed then perceived. Additionally, because there was still material left in the boiling flask, some of the product may have not been distilled.

Table 1.0 shows a summary of the three main peaks on the GC, all others are irrelevant. The peak with retention time of 2.76 had a relative area of 24.0 %. The peak with a retention time of 2.80 had a relative area of 3.50 %. The final peak, with a retention time of 2.89 had a relative area of 72.5 %. The two peaks with the greatest area represent the two major products, while a more insignificant peak probably represents the minor product. The mass spectrometer was consistent with this data as the machine identified two compounds with retention times of 2.76 and 2.89. The compound with a retention time 2.76 had the structure of 3-methylcyclohexene. The compound with a retention time of 2.89 had the structure of methylcyclohexene. Summarizing data from the GC and the mass spectrometer, two products resulted from the dehydration of 2-methylcyclohexanol: methylcyclohexene and 3-methylcyclohexene. Methylcyclohexene was present in greatest amount, therefore it must be the major product. Methylcyclohexene is more substituted than 3-methylcyclohexene, thus Zaitsev’s rule holds true for this reaction.

Dehydration of 2-methylcyclohexanol followed Mechanism 1.0 to give methylcyclohexene and 3-methylcyclohexene. In the first step, the -OH was protonated to make a better leaving group. In step two the leaving group left, resulting in a carbocation. Beta hydrogen elimination took place in step three. There were two sets of beta hydrogen. If the nucleophile, water, attacked the hydrogen on the carbon attached to the methyl group, methylcyclohexene was produced (A). If water attacked the other beta hydrogen, shown by B, 3-methylcyclohexene was produced. The GC also showed another minor product. This is the product produced if a 1,2 hydride shift occurs. Mechanism 2.0 shows that the hydride shift would take place at step when the formation of the carbocation takes place. The hydride shift would take place to form a tertiary carbocation, a more stable carbocation.

However, it is important to keep in mind that any carbocation is unstable. In step 3 of Mechanism 2.0, the nucleophile could attack either of the two sets of beta hydrogen. Notice, the beta hydrogens are different from the ones seen in Mechanism 1.0. Elimination of a beta hydrogen, seen in A1, results in methylcyclohexene. However elimination of a hydrogen attached to the methyl group results in methylene cyclohexane. Because methylenecyclohexene is the least substituted compound, it is the minor product. Additionally, double bonds inside the ring are more stable than double bonds outside of the ring. Therefore though a tertiary carbocation is more stable than a secondary carbocation, the product is the least substituted and least stable. Zaitsev’s rule favors Mechanism 1.0 over Mechanism 2.0. However some of the molecules undergo a hydride shift, so a small amount of methylene cyclohexane is produced. In the GC, the peak with a retention time of 2.80 is 2-methyl-methylene cyclohexane and makes up 3.50 % of the products. The prediction holds true for the dehydration of 2-methylcyclohexanol as methylcyclohexene was the major product.

ConclusionThe two major products of this dehydration were methylcyclohexene and 3-methylcyclohexene. Methylcyclohexene constituted most of the final product, approximately 73%, and 3-methylcyclohexne made up 24 % of the final product (obtained via analysis of a previously done GC and mass spectrometer). The two major products were formed mostly by following Mechanism 1.0, although one of the products of Mechanism 2.0 was also methylcyclohexane. The other product of Mechanism 2.0 formed a minor product, methylene cyclohexane, making up about 4 % of the final product. As predicted, Zaitsev’s rule held true for this reaction since the most substituted product, methylcyclohexene, was the major product. The test for unsaturation confirmed the reliability of the starting material. The IR’s of the starting material and final products were consistent with structures of the compounds. However, there was contamination by water in the IR of the final product. The percent yield was also very low, which can be accounted for loss in distillation as well as during separation of organic layer from the aqueous layer.

A very important lesson was learned from this experiment: dehydration of 2-methylcyclohexanol is a special case in carbocation rearrangement. Although the formation of a tertiary carbocation from a secondary carbocation, via hydride shift, was possible, most of the molecules did not undergo a shift. The shift resulted in two products where one (methylcylclohexene) was the most substituted of all three products resulting from a dehydration of 2-methylcyclohexanol, and another (methylene cyclohexane) which was the least stable of the three products. This reaction followed Zaitsev’s rule and so the majority of the molecule did not undergo a hydride shift, but formed two more stable products than methylene cyclohexane.

References

• “Introduction to IR Spectrum Interpretation,” Elizabethtown College, Department of Chemistry and Biochemistry, Chem 113, 2009.

•”Dehydration of 2-methylcyclohexanol,” Elizabethtown College, Department of Chemistry and Biochemistry, Chem 113, 2009.