Lab 4 – Friedel-Crafts Alkylation of p-Dimethoxybenzene Full Lab Rep PDF

Preview

Summary

orgo lab report...

Description

Organic Chemistry Lab 654.02 2/28/20 Friedel-Crafts Alkylation of p-Dimethoxybenzene Laboratory Report Introduction The goal of this lab is to determine the structure of the di-alkylated p-dimethoxybenzene product. In this lab we are using Friedel-Crafts Alkylation reactions to get to the final product. To get the product 1,4-dimethoxybenzene will be reacted with 3-methyl-2-butanol and sulfuric acid as shown in figure 1. The electrophile for this reaction is a carbocation that forms when 3-methyl-2-butanol reacts with sulfuric acid. The nucleophile is the 1,4-dimethoxybenzene because it is an aromatic ring.

Figure 1: Friedel-Crafts Dialkylation of 1,4-Dimethoxybenzene with 3-Methyl-2-Butanol. 1 The Friedel-Crafts Alkylation reaction was discovered in 1877 by Friedel and Crafts. They discovered that alkyl groups can be substituted onto aromatic rings by reacting an alkyl halide with a Lewis acid like aluminum chloride.2 The first step of the mechanism shows the electrophile (carbocation) being generated when the alkyl halide reacts with the aluminum chloride present. The electrophile will then react with the aromatic ring through an electrophilic aromatic substitution as seen in figure 2. The last step is when there is a loss of a proton from the structure and then the final structure is present. The reaction run during lab was different because the electrophile in our case is a carbocation that is formed when the 3-methyl-2-butanol reacted with sulfuric acid. In this reaction excess equivalents of 3-

1 Joiner, S. L. Fried

ampshire, Durham,

NH, 2020 2 Joiner, S. L. Fried NH, 2020

ampshire, Durham,

methyl-2-butanol is also used to encourage the distribution and produce a higher yield. The OH in the beginning material 3-methyl-2-butanol is also a bad leaving group so H 2O had to be made because a protic acid was needed instead of the usual Lewis acid. Figure 2: Friedel-Crafts Alkylation with an Alkyl Halide and AlCl 33 Results and Discussion There were multiple steps to obtaining the final product during this lab. The first step was to mix together 1,4-dimethyoxybensene, acetic acid, and 3-methyl-2-butanol. This mixture was then treated with concentrated sulfuric acid in an ice bath over a course of 5-10 minutes as to not cause the solution to gain too much heat. It was then left to mix slowly for 20 minutes out of the ice bath. Two 5 mL portions of icecold water were added to the mixture to keep it cool. The mixture was then transferred to a 125 mL Erlenmeyer flask to vacuum filtrate mixture and obtain the crude product. To ensure as many of the impurities were out of the crude as possible when in the vacuum filtration system, the crude product was rinsed with 25 mL of ice water and 2 portions of 2 mL ice cold water. The crude product was then left to dry out before being removed for a recrystallization. The recrystallization was used to obtain a purer product then the crude product. After the product was purified a melting point was collected. The melting point was 107.2-108.3 degrees Celsius. The official melting point of di-alkylated p-dimethoxybenzene is recorded at 110 degrees Celsius. The melting point taken from the sample obtained in lab is close to that of the melting point of a pure product as well as there not being a large range in the melting point. The small range and nearness to a pure products melting point indicates that the product generated in lab was relatively free of impurities.

3 Joiner, S. L. Friedel-Crafts Alkylation of p-Dimethoxybenzene Procedure University of New Hampshire, Durham, NH, 2020

Based off the Friedel-crafts alkylation mechanism of 1,4-dimethyoxybenzene, 3-methyl-2butanol, and sulfuric acid the proposed product formed in this reaction is 1,4-Dimethoxy-2,5-bis(2methylbutan-2-yl) benzene. The first step of the mechanism uses 3-methyl-2-butanol and sulfuric acid reacting together to generate an electrophile. The next step of the mechanism has the pi-system of 1,4dimethyoxybenzene attack the electrophile generated from the first step. This produces three resonance forms, all of which can be used for the last step in the mechanism. The final step takes one of the resonance forms generated from step 2 in the mechanism and reacts with the H 2O that is present and causes the loss of a hydrogen and the final product (figure 3).

Figure 3: Friedel-crafts alkylation mechanism of 1,4-dimethyoxybenzene, 3-methyl-2-butanol, and sulfuric acid The IR spectra run on the product obtained showed peaks at 2989.72 cm -1, 1505.51 cm-1, 1483.14 cm-1, and 1063.25 cm-1. These points all help to support statement that the product obtained from lab is 1,4-Dimethoxy-2,5-bis(2-methylbutan-2-yl) benzene. The peak at 2989.72 cm -1 indicates that there is a CH bond present in the structure of the product. The IR also showed peaks at 1505.51 cm -1 which indicates C=C bonds present. There were also peaks present at 1483.14 cm -1 where C-C bonds show up. The final

peak present on the IR spectra was that at 1063.25 cm -1 this peak is for C-O bonds. All these peaks and their correlating bonds are present on the structure proposed to be the product generated from lab. The 1H NMR had points at 6.77, 3.80, 1.83, 1.34, and 0.65. Each of these points had different multiplicities which helped identify where the carbons with hydrogens attached to them were on the structure. The 13C NMR helped identify the structure by indicating what type of substituents were attached to different carbon atoms. For example, the carbon labeled E in figure 4 had a point at 33 on the 13

C NMR which indicated that it was a methyl substituent attached to an aromatic ring

Figure 4: Proposed structure of product Experimental 1,4-dimethoxybenzene (0.743g, 5.4 mmol), acetic acid (2.5 mL), and 3-methyl-2-butanol (1.75 mL, 0.0162 mol) were added to a 25 mL round bottom flask with a stir bar and mixed together while submerged in the ice bath. Over the course of 5 minutes sulfuric acid (5 mL) was added to the ice bath system. Once the acid was added the mixture was removed from the ice bath and allowed to stir for 20 minutes and was returned to the ice bath. Ice cold water (5 mL x2) was added to the reaction flask and stirred for 2 minutes. The reaction was transferred to a 125mL Erlenmeyer flask and was placed in the ice bath and stirred. During the transfer ice cold water (5 mL x2) was used to complete the transfer with 2 minutes in between portions. After the transfer ice cold water was added to reach a volume of around 3540 mL and the mixture was mixed for a few minutes. The reaction was vacuum filtered with a Buchner funnel and rinsed with ice cold water (25 mL) and ice-cold ethanol (2 mL x2). The product was dried, and

a recrystallization was performed. After recrystallizing the product a white powder was formed and the following data was obtained: [0.993 g, 66.2% yield, MP: 107.2-108.3 degrees C, IR: 2989.72, 1505.51, 1483.14, 1063.25 cm-1, 1H NMR (440 MHz, CDCl3) δ 6.77 (s, 2H), 3.80 (s, 6H), 1.83 (q, 4 H), 1.34 (s, 12H), 0.65 (t, 6H), 13C NMR (101 MHz, CDCl3) δ 151, 134,113, 56, 39, 33, 28, 10]

References 1.) Joiner, S. L. Friedel-Crafts Alkylation of p-Dimethoxybenzene Procedure University of New Hampshire, Durham, NH, 2020

2.) Joiner, S. L. Friedel-Crafts Alkylation of p-Dimethoxybenzene Procedure University of New Hampshire, Durham, NH, 2020

3.) Joiner, S. L. Friedel-Crafts Alkylation of p-Dimethoxybenzene Procedure University of New Hampshire, Durham, NH, 2020...