Friedel Crafts Lab Report, final draft PDF

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Friedel Crafts Alkylation of 1,4-Dimethoxybenzene

Lead Author: Lois Metuge Reviewer: Amber Sanders Editor: Elliott Burnett

Introduction Friedel Crafts Alkylation of 1,4-dimethoxybenzene is an electrophilic aromatic substitution reaction.1 The purpose of this experiment was to react the t-butyl alcohol in the presence of an acid catalyst, in this case sulfuric acid (H2SO4), to form a carbocation intermediate. The carbocation intermediate would then react with 1,4-dimethoxybenzene to form a tetra-substituted ring. The tetra-substituted ring occurs because this electrophilic aromatic substitution reaction produces a poly-substitution, specifically a double substitution, of the tbutyl alcohol on the 1,4-dimethoxybenzene ring.1 Electrophilic aromatic substitution reactions prefer strong electrophiles that are cations, and this is why the t-butyl cation is a great electrophile for this reaction.1 In the experiment, the mixture of 1,4-dimethoxybenzene, t-butyl alcohol, and sulfuric acid is placed in an ice bath and isolated via vacuum filtration to obtain the solid product, 1,4-di-t-butyl-2,5-dimethoxybenzene.1 One of the proposed theories for the resulting experimental product is that the two t-butyl groups add on opposite sides of the ring in the ortho position relative to the two present substituents on the 1,4-dimethoxybenzene ring.1 The two t-butyl groups add in the ortho position on opposite ends because there is less steric hinderance in those locations. Another theory relates to the product’s electron withdrawing or donating groups; in this case, t-butyl alcohol is an electron withdrawing group in the electrophilic substitution reaction.2 As seen in Figure 1, before the t-butyl alcohol group reacts with 1,4 dimethoxybenzene, it initially reacts with acetic acid to make a good leaving group, removing OH, forming a stronger electrophilic carbocation.2 The general mechanism of this reaction is shown below in Figure 1:

Figure 1: The general mechanism for Friedel Crafts Alkylation of 1,4-dimethoxybenzene.2

Reagents 1,4Dimethoxybenzene T-Butyl Alcohol Sulfuric Acid Glacial Acetic Acid Methanol 1,4-di-t-butyl-2,5dimethoxybenzene Water

Table 1: Table of Reagents1,3 Molecular Melting Point Boiling Point Weight (g/mol) (C) (C) 138.16 57 212.6

Density (g/mL) 1.053

74.1 98.08 60.052

26 10 16.8

83 337 117.9

0.78 1.83 1.05

32.04 250.37

-97.6 104

64.7 336.3

0.79 0.924

18

0

100

1.00

Experimental First, the starting material was obtained by weighing out 300 mg of 1,4 dimethoxybenzene.3 There were visible, though relatively small in size, chunks present in the solid; instead of using a mortar and pestle, these small chunks were crushed using the bottom of glassware. After ensuring there were minimum to no more chunks, the solid was added into a 25 ml Erlenmeyer flask along with 1.5 mL of glacial acetic acid.3 The contents of the Erlenmeyer flask were swirled until the solution appeared visually homogenous. Once mixed, 0.5 mL of tbutyl alcohol was added after running it under warm water as it is solid at room temperature, and the mixture was swirled together one more time.1,3 At this point, the Erlenmeyer flask was carefully placed into an ice bath. Next, using a glass pipette, 1.5 mL of concentrated sulfuric acid was added into the Erlenmeyer flask dropwise due to its high reactivity and corrosive nature.1,3 It was necessary to briefly place the flask in a warm water bath to fully dissolve the compounds; while there, they were mixed together for 4 minutes with a glass stirring rod. Once dissolved, the Erlenmeyer flask was returned to an ice bath and 5 mL of ice cold water was added to the Erlenmeyer flask.3 Once the water was added, a precipitate was observed to have formed. Finally, using a Hirsch funnel, the product was isolated by vacuum filtration. During filtration, the product was washed three times with 1 mL of cold methanol (3 mL total).3 The solid was allowed to dry and the weight of the product was taken, then used to calculate the percent yield. Finally, a capillary tube and a melting point apparatus were employed to determine the melting point of the product in an effort to establish a level of purity. Results and Calculations After letting the solid dry, the final yield of the product was weighed out using a zeroed scale, giving a final weight of 0.15 grams. Before the percent yield of 25% could be found (Eq. 1), the theoretical yield of 0.60 grams had to be calculated from the formula shown in Eq. 2 below.3 0.33 grams of 1,4-dimethoxybenzene was divided by its molar mass, then multiplied by the molar mass of 1,4-di-t-butyl-2,5-dimethyloxybenzene (250.375 grams).1,3 The actual yield was 0.15 grams, which was divided by the theoretical yield of 0.60 grams and multiplied by 100.3

𝑎𝑐𝑡𝑢𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 (𝑔)

Percent Yield = 𝑡ℎ𝑒𝑜𝑟𝑒𝑐𝑡𝑖𝑐𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 (𝑔) ∗ 100 = 25% = ( (

0.33 𝑔 1,4 dimethoxybenzene 1

)(

1 𝑚𝑜𝑙 1,4 𝑑𝑖𝑚𝑒𝑡ℎ𝑜𝑥𝑦𝑏𝑒𝑛𝑧𝑒𝑛𝑒 138.16 𝑔

250.375 𝑔 1 𝑚𝑜𝑙 1,4−𝑑𝑖−𝑡−𝑏𝑢𝑡𝑦𝑙−2,5−𝑑𝑖𝑚𝑒𝑡ℎ𝑦𝑙𝑜𝑥𝑦𝑏𝑒𝑛𝑧𝑒𝑛𝑒

) = 0.60 g

)(

0.15𝑔 0.60𝑔

* 100

Eq. 1

1 𝑚𝑜𝑙 1,4−𝑑𝑖−𝑡−𝑏𝑢𝑡𝑦𝑙−2,5−𝑑𝑖𝑚𝑒𝑡ℎ𝑜𝑥𝑦𝑏𝑒𝑛𝑧𝑒𝑛𝑒 ) 1 𝑚𝑜𝑙 1,4 𝑑𝑖𝑚𝑒𝑡ℎ𝑜𝑥𝑦𝑏𝑒𝑛𝑧𝑒𝑛𝑒

Eq 2.

The melting point of the product was then determined using a melting point apparatus and a capillary tube, resulting in a measurement of 101.7 C.3 Based on the literature values, this was very near the melting point of 1,4-di-t-butyl-2,5-dimethoxybenzene, which is approximately 104 C.1,3 Only one out of three results could be gathered from group members. The two other group members were unable to obtain a final product after suction filtration and drying. Discussion The objectives of this lab were to react t-butyl alcohol in the presence of acid catalyst (H2SO4, sulfuric acid) to form a carbon intermediate that would eventually react with 1,4dimethoxybenzene to form a tetra-substituted ring.1,2 The carbocation intermediate was formed after adding sulfuric acid dropwise and it was indicated by the formation of a precipitate which did occur during the experiment.1 The formation of the tetra-substituted ring was achieved through a series of step wise processes beginning by using the solid form of 1,4dimethoxybenzene.1 Based on the results obtained from the experiment, the percent yield of 25% (Eq. 1) was quite low, which indicated the errors were made during the lab process.3 Some product could have been lost during the suction filtration step, the acid may have not worked well enough as a catalyst to bring the reaction to completion, so some liquid reactants stayed as reactants. Product could have also been lost during the drying process. Next, the results of the melting point of the obtained product was 101.7 C.3 This temperature indicated that 1,4-di-t-butyl-2,5-dimethoxybenzene was formed because the melting point of the synthesized product, 101.7 C, compared to the theoretical melting point of 104 C, were very close in value.1,3 This also denoted that the product had very few impurities. Based on these results, it can be determined that the theories were proven true because the steric and electronics of the final product were as expected. Two of the three group members failed to obtain a final product after suction filtration and drying. This could have been a result of not weighing out enough starting product of 1,4dimethoxybenzene, not using a enough acid catalyst to bring the reaction to completion, not leaving the solution in the ice bath for long enough, or not putting the sold t-butyl in a warm water bath.

Conclusion According to the results of this lab, it can be determined that the carbocation intermediate for this lab was reached and the final tetra-substituted product, 1,4-di-t-butyl-2,5dimethoxybenzene, was formed. This was proven by the melting point obtained using a melting point apparatus and inserting the synthesized product into a capillary tube, resulting in a melting point 101.7 C.3 This value is very similar to the theoretical melting point of 1,4-di-t-butyl-2,5dimethoxybenzene of 104 C so the final product was synthesized to completion and had very few impurities.1 Unfortunately, the percent yield was rather low at 25%, meaning not all of the product was formed.3 This indicates there was probable and significant loss of product during the drying process. That and the mistakes made by the other two group members, ranging from possibly not weighing out enough starting product of 1,4-dimethoxybenzene, not using a enough acid catalyst to bring the reaction to completion, or not leaving the solution in the ice bath for long enough, can be amended with care and consideration. This lab could be improved overall by warming the t-butyl alcohol up as one of the first steps so it can be easily utilized as soon as it is needed in the process of adding it to the Erlenmeyer flask.

References 1. Casselman, B. Friedel Crafts Friedel Crafts Procedure. https://uab.instructure.com/courses/1532137/files/64054838?module_item_id=15762165 (Accessed September 14, 2020) 2. Wang, P. Chapter 22 Notes on Friedel Crafts Alkylation. https://uab.instructure.com/courses/1532144/files?preview=64158664 (Accessed September 15, 2020) 3. Burnett, E.; Metuge, L.; Sanders, A. University of Alabama at Birmingham, Birmingham, AL. Laboratory Notes for CH 238: Organic Chemistry Laboratory I, 2020....