Answer

Introduction

Dehydration of alkanols is an elimination chemical reaction whose ultimate result is removal of water from the alcohol giving an alkene product. According to Hunt (2016), alcohols undergo a 1,2-elimination reaction to generate water and alkene when heated with strong and catalytic acids such as H₂SO₄ and H₃PO₄ (P. 1). The water eliminated is formed from the hydroxyl group, -OH that is the functional group of the alkanol and a hydrogen atom. The dehydration of a secondary alcohol or tertiary alkanol can produce a mixture of alkene isomers. Methylbutan-2-ol is a complex alcohol and therefore, has a possibility of giving more than one alkene after dehydration.

The dehydration reaction of 2- Methylbutan-2-ol using phosphoric acid results in the production of two alkenes; 2-methylbut-2-ene and 2-methylbut-1-ene. Consequently, the product of the reaction will be a mixture of two alkenes. The gaseous contents of the mixture can be analyzed by gas chromatography involving vaporization of the contents without decomposition. The gas chromatography will separate the alkenes based on the difference in their boiling points. According to Marriott, Chin, Maikhunthod, Schmarr, and Bieri (2012), the analytical multidimensional gas chromatography with excellent separation efficiency achieves advanced characterization of complex semi-volatile and volatile compounds (p. 1). 2-methylbut-2-ene has a boiling point of 32°C and therefore, will be expected to evaporate first.

The safety precaution to be taken in this experiment is performance in a fume cupboard because Methylbutan-2-ol is toxic and causes upper respiratory tract irritation when inhaled. The experiment on separation of the alkene isomers is applicable in many scientific fields. Luong, Gras, Hawryluk, Shellie, and Cortes (2013) posit that volatile compounds like dienes and light hydrocarbons are encountered in manufacturing, chemical, and petrochemical environments and should be monitored by gas chromatography (p. 105). This experiment involved dehydrating Methylbutan-2-ol and identifying the components of the product.

Materials and Reagents

Measuring cylinder

Round bottomed flask

Ice bath

Separating funnel

Gas chromatograph

Granules

85% phosphoric acid

2-methylbutan-2-ol

Heat source

Anhydrous sodium sulphate

Method

25mL of 85% phosphoric acid was measured using a measuring cylinder and poured into a 250mL round-bottomed flask before cooling in an ice-bath. The measurements were then recorded.

After that, 36mL of 2-methylbutan-2-ol was measured and added slowly to the cooled ice while swirling the flask gently in the cold ice-bath. The mixture was shaken thoroughly and measurements recorded.

A few anti-bumping granules were then added to the resultant mixture in the round-bottomed flask and the apparatus for distillation was set with the arrangement for cooling the distillate with ice.

The flask was heat very gently until the distillation of the alkenes was complete with the distillation temperature recording 40°C.

The distillate was then transferred to a separating funnel and shaken with 10mL dilute sodium hydroxide to remove acid traces. The aqueous solution was allowed to sink to the bottom before running off and discarding.

After that, the hydrocarbon layer was dried over an anhydrous sodium sulphate in a stoppered flask before filtration under gravity into a previously weighed dry flask. The stoppered sample of the dry hydrocarbons was then taken to the chromatography laboratory and o.5µL of the product injected into the gas chromatograph.

Finally, the retention times and relative concentration of the two major alkenes were recorded.

Results

Attach a copy/photograph of your GC data below.

Tabulate your results in your lab book in the following manner:

QUESTIONS

1. Which alkene would be expected to emerge first from the gas chromatograph? Why?