Synthesis and Chemistry of Alkenes

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Table 5: Chemical tests of 4-methyl-2pentanol and alkene product

Compound

Bromine Test

Baeyer Test

Chromic Acid test

4-methyl-2pentanol

(-)

(-)

(+)

Alkene product

(+)

(+)

(-)

Table 6: Blank determination

Blank

Result

A

Orange

B

Purple

C

Light green

Discussion

During the acid catalyzed dehydration reaction of 4-methyl-2-pentanol, in order to achieve the best percent yield, there is a need to continuously distilling the products of the reaction. This allows the reaction, to proceed at the most constant state of equilibrium, shift and drive the forward reaction, hence, increasing the productivity of the reaction. The percent yield of my alkene product was 48.89%.

The product of the dehydration of 4-methyl-2-pentanol was then analyzed using gas chromatography (GC). The large peak at 2.30 minutes is the solvent (diethyl ether) used to prepare the sample and the starting alcohol in my product appeared at about 4.17 minutes. The peak on the GC of my sample corresponded to around the same retention time 2.78 minutes of the GC. This indicates that the favored product is 2-methyl-2-pentene with 1.09% area product. However, the GC provided to me with my own GC does not correspond to the same result. The major product was 4-methyl-2-pentene at 2.55 with the area percent of 54.70%.

In alkenes there is a C=C bond, with the aid of the Bromine test, the Baeyer test and Chromic acid, whether or not this bond was present could be determined. It was found that the reagent Br2 from the Bromine test, changed to the colour brown (+), the reagent KMnO4 from the Baeyer test, changed to the colour purple (+), and the reagent C3H6O from the chromic acid did not change the color to green (-) after the addition of the product whereas, when these tests were done on the 4-methyl-2pentanol the results were opposite, bromine test did not change to colourless (-), Baeyer test also stayed purple (-), and Chromic acid changed to green colour (+). This indicates the presence of alkenes in the product of the dehydration of 4-methyl-2-pentanol.

Questions:

If 2-methylcyclohexanol were dehydrated, either a mixture of both 1-methylcyclohexane, or 3-methycyclohexane would result. Thermodynamically, the formation of 1-methylcyclohexene is favored over the production of 3-methylcyclohexene because it is more substituted and stable alkene that is faster to form. It has a lower activation energy of its intermediate.

Conclusion

From my GC report, I can conclude that my sample is 2-methyl-2-pentene, which was also confirmed by chemical tests.

References

1. P. Marrs, Chem 232 lab Manual, pp.25-29 and Appendix E, pp.A35-A37. (University of Victoria: Victoria, BC). 2016.