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Synthesis of 4‐methylcyclohexene Essay


4-methylcyclohexanol was synthesized to 4-methylcyclohexene using dehydration. 4-methylcyclohexanol was heated to reflux and the subsequent distillate (4-methylcyclohexe) was collected. It was then purified using sodium chloride to separate products and an anhydrous solid was then added and filtered. The resulting product had a mass of 0.399 g and a percent yield of 41%. The product was positively identified and characterized as 4-methylcyclohexene via IR and Br2 test.


To synthesize 4-methylcyclohexene from 4-methylcyclohexanol the starting material can be dehydrated resulting in the desired compound. For dehydration to be possible the OH group on the 4-methylcyclohexene must first be converted into H2O by means of an acid-base reaction using a strong acid catalyst such as phosphoric acid and sulfuric acid. The H2O will then become the favored leaving group and leave on its own resulting in the formation of a carbocation in excess water. The water will remove the acidic hydrogen on the carbocation producing the desired alkene as well as regenerating the acid catalyst (fig. 2). A time effective way to collect the 4-methylcyclohexene is to heat the reaction to reflux as it is taking place.

This allows the product to be separated from the starting materials by means of the boiling point discrepancy between the isolated alkene (101-102 C) and the starting alcohol (171-173 C). After the distillate is collected any impurities of water and phosphoric acid can be extracted by adding sodium chloride, drying the resulting organic layer with an anhydrous solid, and filtration. IR of both the starting alcohol and the resulting alkene can be compared as a means of identification and characterization as well as a Br2 test to ensure the correct product was formed.

Procedure and Observations

A mixture of 4-methylcyclohexanol (1.5mL), 85% phosphoric acid (0.40mL), and six drops of concentrated sulfuric acid were heated to reflux. The mixture turned a dark brown upon reaction and darkened with exposure to heat. Initially the heating temperature reached 190C but was then lowered and maintained within the range of 160-180C until the reaction mixture stopped boiling. The resulting distillate was collected and removed. The stillhead the distillate was collected in was then washed with sodium chloride (1.0ml) and the subsequent mixture was added to the product. This mixture was shaken and aqueous and organic layers were allowed to form. The aqueous layer was removed from the organic and the organic dried with anhydrous solid and filtered through a cotton-plugged pipet. The mass of the product as well as a percent yield was determined. An IR was taken and compared to the starting material as well as a Br2 test.

Results and Calculations

4-methylcyclohexanol starting mass: 1.164 g
actual 4-methylcyclohexene mass: 0.399 g
4-methylcyclohexanol molecular weight = 114 g/mol
4-methylcyclohexene molecular weight = 96 g/mol
Percent yield calculations:
% yield = (actual yield/ theoretical yield) * 100%
1.164 g 4-methylcyclohexanol * 1mol 4-methylcyclohexanol/114 g * 1 mol 4-methylcyclohexanol/ 1 mol 4-methylcyclohexene * 96 g/ 1 mol 4-methylcyclohexene = 0.980 g 4-methylcyclohexene (theoretical yield) 0.399 g/ 0.980 g = 0.407 * 100% = 40.7% = 41%

Br2 test:
4-methylcyclohexene: 140 drops to reach excess
4-methylcyclohexanol: 1 drop to reach excess

Discussion and Conclusion

In conclusion, 4-methylcyclohexene was successfully synthesized via dehydration from 4-methylcyclohexanol. This result was verified by comparing the IR spectra of both compounds as well as by performing a Br2 test. The IR spectra for the synthesized material lacked the broad O-H peak found in the spectra of the starting material and which is characteristic of a compound containing an alcohol as well as containing peaks for both sp2 and sp3 hybridized C-H groups, both of which are consistent with the stereochemistry of 4-methylcyclohexene.

Moreover, the results of the Br2 test supported the positive characterization of the product as 4-methylcyclohexene. 140 drops Bromine were required until the compound turned red consistent with the chemistry of Bromine and the alkene forming 1,2‐dibromo‐4‐methylcyclohexane and explaining why the mixture remained colorless for so long. The product was synthesized with a decent to poor percent yield as evidenced by the 41%. This poor yield very likely was caused by the excess heat initially added to the reflux reaction, the result of which was charring and possibly by too much acid catalyst being added to the starting material.

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