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Expt 10.1; Acid-catalyzed dehydration of a 2-methyl-2-buranol and a gas chromatographic analysis of the 2-methylbutene product mix

Rebecca Gotthelf CHE210L Section A 11/15/17 ; 11/29/17

I.

II.

Abstract: The entirety of the experiment lasted over the course of two weeks, with the first portion of the lab, a mixture of 2-methyl-2-butanol was dissolved in a strong acid diluted with a strong acid, and distilling it, until pure butane could be collected and placed in a fridge until the second part of the experiment. The amount of butane recovered was .218, which gave a 27% recovery from the initial mixture. The second part of the experiment involved gas chromatography. This allowed the butane to be inserted into an apparatus, and then discover the percent composition of 2-methyl-1-butene, and 2-methyl-2-butene based off the peaks from the graph. Introduction 1. What is gas chromatography and how does it work? Make it clear in the discussion what the stationary and mobile phases are and how they work together to effect separation. Gas chromatography is when a sample is injected into a carrier gas utilizing a syringe and instantly turns to a gas (the temperature of the apparatus being above the boiling point of the sample). The gas that make up the sample then separate as they move along the column which contains a stationary phase. The mobile phase of gas chromatography is the carrier gas, which is typically an unreactive gas, and the stationary phase is what coats the walls of the column, and is typically a high boiling polymer. The different substances making up the initial butene will travel through the column at different speeds, thus becoming separate. 2. In 2-methyl-2-butanol any one of six hydrogens adjacent to the alcohol could be lost in the formation of 2-methyl-1-butene and any one of two hydrogens adjacent to the alcohol could be lost in the formation of 2-methyl-2-butene. If the product distribution of alkenes from the dehydration of 2-methyl-2-butanol were based purely on the statistical availability of hydrogens, what would be the expected ratio of 2-methyl-1-butene to 2-methyl-2-butene? 3:1 3. Explain Saytzeff’s rule and how it relates to this dehydration reaction. Sayzteff’s rule means that during a dehydration reaction, more substituted alkene is formed as a major product, since the greater the substitution of a double bond, the greater stability of the alkene. 4. Make sure your introduction includes a balanced equation AND a detailed mechanism for the dehydration of 2 methyl-2-butanol.

Detailed Mechanism:

5. Each step of the catalyzed dehydration of 2-methyl-2-butanol to give 2-methyl-1butene and 2-methyl-2-butene is reversible. How are the reaction conditions in the lab set up to take advantage of LeChatelier’s Principle to ensure maximum conversion to products. LeChatelier’s Principle states that when a system experiences a disturbance, it will respond to restore a new equilibrium state. In this experiment, the conditions are changed by distilling the butene products out of the system. III.

Experimental Section: Procedure and Changes/Modifications made to the Published Procedure: There were no changes or modifications made to the procedure. Reaction and Theoretical Yield Calculation: Experiment 10.1 H H 3C

moL MW g mL d IV.

CH

C

C

H

OH

3

CH

H 2S O 4 (c a t) 3

H 3C

H

CH

C

C

3

C H

3

+

C H3

C

C

CH

2

+

H 2O

H

2-methyl-2-butanol

2-methyl-2-butene

.00877 [LR] 88.15 0.773 1.01 0.805

0.00877 70.14 0.187 [TY] 1.04 0.662

Results and Discussion: Data: grams methyl-butanol grams methyl-butene Percent recovery Theoretical grams butene Peak Retention

H 3C

H

.78g .218g 27% .620g Peak Area

2-methyl-1-butene 0.00877 70.14 0.0228 [TY] 1.04 0.662

Column

Column

Time (s) 1.09 1.20 1.18 1.31

A B 1 2

% compositionpeakA =

Temperature 314.00 2432.00 5824.00 41120.00

areaunder the curve peak A Σ area under the curve A ∧ B

x 100

areaunder the curve peak A Σ area under the curve A ∧ B

x 100

B

68°C

= (314/2746) x 100 =11.4% % compositionpeakB = = (2432/2746) x 100 = 88.6% % reaction yield = .2178 g .620 g = 35.1%

=

mass isolated product theoretical product yield

x100

x100

2-methyl-2-butene % reaction yield = massisolated product x %composition theoretical product yield .2178 g(.886) x100 = .620 g = 31.1% 2-methyl-1-butene % reaction yield= massisolated product x %composition ¿ theoretical product yield .2178 g(.114 ) ¿ x100 .620 g = 4%

x100

x100

Analysis: The objective of Experiment 10.1 largely consisted of an acid-dehydration reaction of 2methyl-butanol into 2-methyl-2-butene and 2-methyl-1-butene. After collecting this sample, it was analyzed using gas chromatography which allowed the reaction yield of each separate butene to be found. .78g of 2-methyl-2-butanol allows for .620g of the 2-methylbutenes to be produced. That is the theoretical yield of 2-methybutene from 2-methyl-2-butanol, however in this experiment .2178g was yielded. This product then underwent gas chromatography and were

separated and shown through peaks on the graph. The retention time for the first peak was 1.09 minutes and the second one was 1.20 minutes. The reasoning for why the first peak had a lower retention time than that of the second is because the molecule was more volatile, and less stable than the molecule that made up the second peak. Due to these conclusions made, it can be deducted that the molecule that represents the first peak is 2-methyl-1-butene, and the molecule that represents the second peak is 2-methyl-2-butene. After taking the results from the graph, and calculating the percent composition, 2-methyl-1-butene had a percent comp of 11.4 and 2methyl-2-butene had a percent comp of 88.6. Applying these calculated values to the total recovered grams of the 2-methyl-butenes yielded .1929 grams of 2-methyl-2-butene, and .0248 grams of 2-methyl-1-butene. Then applying these values to finding reaction yield, 2-methyl-2butene had a reaction percent yield of 31.1% and 2-methyl-1-butene had a reaction percent yield of 4%. V.

Works Cited 1. Williamson, Kenneth.; Minard, Robert D.; Masters, Katherine M. Macroscale and Microscale Organic Experiments, 6th ed., chapter 10; Houghton Mifflin Co.: New York, 2013....