Heating of 2-Methyl-2-Butanol to Produce 2-Methyl-1-Butene & 2-Methyl-2-Butene

Substitution and Elimination

Heating of 2-methyl-2-butanol to produce 2-Methyl-1-butene & 2-methyl-2-butene

Gaia Carradori

Student number 9

Lab Teacher: Dagmara Kaczynska

24.10.2018

Summary

This experiment involved dehydration/elimination of the alcohol 2-methyl-2-butanol to produce two alkene products; 2-methyl-1-butene and 2-methyl2-butene, by fractional distillation. 3 ml of 2-methyl-2-butanol was heated with 6 ml of distilled water and 3 ml of sulfuric acid (H2SO4) to 40°C. This resulted in 0.19g of 2-methyl-1-butene and 0.12g 2-methyl-2-butene being collected in receiving flasks at 31°C and 38°C respectively.

Introduction[pic 1]

2-methyl-2-butanol or tert-Amyl alcohol is a branched pentanol which exists as a tertiary alcohol. It is a colorless liquid with a strong odor and is commonly used as a recreational drug as it acts like a depressant on the central nervous system when ingested. Even if ingested in small doses it can be deadly and induce alcohol poisoning that can lead to death.  (1)

Though, as a typical alcohol, it undergoes elimination to produce alkenes when heated with an acid such as H2SO4 or H3PO4. Elimination is a regioselective reaction that follows the Zaitsevs rule through the E1 mechanism. (2) E1 stands for “elimination unimolecular reaction”, which involves a two step mechanism; the rate determining step(slow step) and the fast step. In the rate determining step, only the alcohol is involved and the bond of the leaving group, -OH breaks. The second step is when then π bond is formed producing the alkene. In general, this reaction a fast because it is a tertiary alcohol. The carbocation intermediate formed is very stable which allows for a faster reaction to occur. (3)

Since tert-Amyl alcohol follows Zaitsevs rule, when it is heated in the presence of sulfuric acid and water, it produces 2 products, 2-methyl-1-butene and 2-methyl-2-butene. 2-methyl-1-butene is the minor product because it has the least alkyl substituents and 2-methyl-2-butene is the major product as it has the most alkyl substituents. Tert-Amyl Alcohol is heated under fractional distillation, as both products formed have different boiling points of 31°C and 38°C. This is a favored experimental technique as it allows the two alkene products to be easily separated and collected. The mechanism of this reaction is shown in the figure below.

[pic 2]

Method

First a distiller apparatus was set up then 6 ml of water was added into a 25-ml round bottom flask with a magnetic stir bar. Next 3 ml of concentrated sulfuric acid (H2SO4) was carefully added to the flask dropwise. The flask was cooled on an ice bath. Following the addition of sulfuric acid, 2-methyl-2-butanol was also added of 3 ml in drops. The flask was then attached to the distiller. Before reflux was started, three receiving flasks were weighed on a balance and labelled; # 1, #2 and #3. The round bottom distillation flask was heated in a PEG bath to start the reaction. It was heated until the first distillate, 2-methyl-1-butene, was collected at 31°C in receiving flask #3. Once the first compound was collected, the receiving flask was turned to allow the second distillate, 2-methyl-2-butene to be collected at 38°C in receiving flask #2. The receiving flasks were places in an ice bath throughout the heating to prevent evaporation of the compounds collected. Finally, once the temperature had reached 38°C, the heat was turned off and the receiving flasks #2 and #3 were removed and weighed accurately to obtain weight of the products collected.

             Table 1: Reagents table with volume

Results

When 3 ml of 2-methyl-2-butanol was heated with 3ml of sulfuric acid and 6 ml of water, 0.12g of 2-methyl-2-butene and 0.19 of 2-methyl-1-butene was obtained.

                                   Table 2: Weight of flasks before and after distillation

                         Table 3: Weight of products

Theoretical and Percentage yield:

Limiting reagent: 2-methyl-2-butanol

Molecular Weight (2-methyl-2-butanol) : 88.15g/mol

Molecular Weight(2-methyl-butene): 70.1 g/mol

Density (2-methyl-2-butanol) : 0.815g/mol

Mass of 3 ml, 2-methyl-2-butanol = density(2-methyl-2-butanol) x volume (2-methyl-2-butanol)

                                                             =  0.815 g/ml x 3.0ml

                                                             = 2.445 g

Theoretical yield = (88.15/70.1) x 2.445 g

                                = 3.0745g

Percentage (%) yield = [(0.20 + 0.12)g / 3.0745 g ] x 100                                  

                                     = 10.4 %

     Table 4: Theoretical and Percentage yield                                                                                 

Pre-lab Questions