Friedel-Crafts Alkylation

Friedel-Crafts Alkylation Experiment 10 Introduction: The discovery of the alkylation of aromatic hydrocarbons was found by Charles Friedel and James Craft. They discovered that by reacting benzene with an alkyl halide and aluminum chloride results in the formation of an alkylbenzene. This reaction is known as the Friedel-Crafts alkylation.1 The Friedel-Crafts alkylation reaction is important synthetically. It forms new carbon-carbon bonds, and adds alkyl groups to aromatic rings. This process involves the attack of the benzene ring with a strong electrophile. The first step of this reaction is the Lewis acid-Lewis base reaction. An alkyl halide, which is the Lewis base, and a Lewis acid, react. This reaction produces a carbocation. In the second step, the carbocation, which is a strong electrophile, is attacked by the pi electrons of the benzene ring, which is a weak nucleophile.1 The benzene ring allows a carbocation intermediate due to resonance-stabilization. In the third step, a proton is pulled off the benzene ring to reform the benzene double bond.1 There are some drawbacks to the Friedel-Crafts alkylation reaction. Vinyl and aryl halides do not react because they contain sp2 hybridized carbons. In order for these reactions to take place, a high activation energy is required, thus they do not reaction.1 Another limitation is the possibility of rearrangement of the alkyl group due to the carbocation intermediate. This can occur when a rearrangement would produce a more stable carbocation. The next drawback is that this reaction cannot occur if there is more than one strong electron-withdrawing group on the benzene ring. The final drawback is that it is difficult to stop the reaction after one addition. This is due to the fact that the alkylated product formed is more reactive than the benzene ring.1 During this experiment, a Friedel-Crafts alkylation reaction was done. The first step was the formation of the carbocation. The oxygen of t-butyl alcohol was protonated by sulfuric acid. This caused water to act as a leaving group and form the t-butyl cation. This mechanism is shown in figure 1. Next, the t-butyl cation is attacked by pi electrons on 1,4-dimethoxybenzene. This forms a carbocation intermediate. Water then pulls off a hydrogen to reform the conjugated aromatic system. Next, the t-butyl cation is attacked by another set of pi electrons. This forms another carbocation intermediate. Water, again, pulls off another hydrogen to reform the conjugated aromatic system. This forms the product, 1-tert-butyl-2,5-dimethoxybenzene. This reaction is shown in figure 2. All chemicals used in this experiment are shown in table 1. Figure 1: Figure 1 shows the mechanism for the formation of the t-butyl carbocation from t-butyl alcohol. Figure 2: Figure 2 shows the mechanism for the formation of 1-tert-butyl-2,5dimethoxybenzene from the reagents 1,4-dimethoxybenzene and t-butyl carbocation. Table 1: Table of Reagents2 Compound 1-tert-butyl-2,5dimethoxybenzene acetic acid 1,4-dimethoxybenzene Molecular Weight (g/mol) Boiling Point (°C) Melting Point (°C) Density (g/ cm3) 250.38 — 104.0-105.0 — 60.05 117.9 16.6 1.05 138.17 212.6 58.0-60.0 1.05 methanol 32.04 64.7 -97.8 0.79 sulfuric acid 98.07 337.0 10.3 1.83 t-butyl alcohol 74.12 82.3 25.8 0.79 water 18.02 100.0 0.0 1.00 Experimental: First, 0.325 g of 1,4-dimethoxybenzene were crushed and added to a 25 mL Erlenmeyer flask. Then, 1.6 mL of acetic acid were poured into the flask along with 0.5 mL of t-butyl alcohol. Next 5 mL of sulfuric acid were added dropwise 1 mL at a time. This addition caused the solution to turn from clear to yellow. The resulting mixture was stirred with a glass rod for three minutes before being cooled in an ice bath. Next, 5 mL of cold water was slowly added. The solution turned pink as a result. Upon the addition of another 3 mL of cold water, a precipitate fell out of solution. This white crystal precipitate was suction filtered, and washed with 1 mL of methanol three times. After drying, the product was weighed, and the melting point was taken. Results: During this experiment, a Friedel-Crafts alkylation reaction occurred. 1,4Dimethoxybenzene and t-butyl alcohol were reacted to form the product, 1-tertbutyl-2,5-dimethoxybenzene. The percent yield was calculated for the product. First, the limiting reagent was determined, which was 1,4-dimethoxybenzene. Then the theoretical yield was calculated. This is shown in equation 1. Then, the actual yield was calculated. This is shown as equation 2. Finally, the percent yield was calculated to be 6.25%. This calculation is shown as equation 3. Next, the melting point was taken on the product formed. The melting point was a range from 102.0°C to 106.0°C. The melting point data, along with the rest of the data collected for the 1-tert-butyl-2,5-dimethoxybenzene product, are shown in table 2. Equation 1: Equation 1 shows the two calculations to determine the limiting reagent. 1,4-Dimethoxybenzene is the limiting reagent, and 2.35x10-3 mol is the theoretical yield. Equation 2: Equation 2 shows the calculation for the actual yield, which is 1.36x10-4 mol. Equation 3: Equation 3 shows the formula and calculation for percent yield which is 6.25%. Table 2: Data for 1-Tert-butyl-2,5-dimethoxybenzene Percent Yield (%) 6.25 Melting Point (°C) Appearance 102.0-106.0 crystal white solid Discussion: In this experiment, 1,4-dimethoxybenzene and t-butyl alcohol were reacted to form 1-tert-butyl-2,5-dimethoxybenzene. The reagent, 1,4-dimethoxybenzene is highly reactive. This is because it is substituted with two methoxy electron donating groups, which are activating groups, and ortho-para directors. When, 1,4-dimethoxybenzene reacts with the t-butyl carbocation the first time, it adds ortho to one of the methoxy groups. A tribsubstituted product called 1-tert-butyl-2,5-dimethoxybenzene. This product contains two methoxy activating groups and one activating t-butyl group. This product is even more reactive than the initial, 1,4-dimethoxybenzene, product. Thus, this allows for another addition of the t-butyl cation.3 This addition occurs in the ortho position to the other methoxy group. The final product formed is 1-tert-butyl-2,5-dimethoxybenzene. In this experiment, acetic acid is used as a solvent. Acetic acid makes a good solvent for this reaction because it is able to dissolve polar and non-polar compounds. It also is able to act as a nucleophile to trap carbocations.4 An ice bath was used in this reaction to aid in the process of recrystallization. Slow cooling of the solution allows for the formation of pure crystals. Initially stirring the mixture with a glass rod also aided in the recrystallization process by scratching off small pieces of glass off the beaker. These small pieces of glass act as a nuclei for crystal formation.5 The addition of cold water to the solution also helped with recrystallization. Methanol was used in this reaction to aid in the purification of the solid product. Washing the product with methanol helped to remove some organic impurities and water.6 The percent yield calculated in this experiment was 6.25%. Several errors could have occurred to give this low percent yield. Multiple transfers of the mixture between glassware occurred during the course of this experiment. Anytime a mixture is transferred between glassware, there is a risk for the loss of product. Also, not all of the crystallized product was able to be removed from the flask. Another error that could have occurred, was that not all of the product could have crystallized out of the mixture. If the mixture was cooled for a longer period of time, then there could have been more product that crystalized out.5 The melting point obtained for the final product was a range from 102.0°C to 106.0°C. The melting point of pure 1-tert-butyl-2,5-dimethoxybenzene is a range from 104.0°C to 105.0°C.2 The melting point observed is slightly lower and broadened. This indicates the presence of impurities. Impurities could have been recrystallized from the rapid cooling of the mixture. Water, methanol, or unreacted products could have also still been present in the product mixture, which could have given this melting point range. Conclusion: In this experiment, 1,4-dimethoxybenzene and t-butyl alcohol were reacted to form 1-tert-butyl-2,5-dimethoxybenzene. The final product obtained was fairly pure 1- tert-butyl-2,5-dimethoxybenzene. This was confirmed by analysis of melting point and calculation of percent yield. More analysis could have been done on the final product to determine its purity. IR spectroscopy could have been done to confirm the functional groups on the final product. Proton NMR spectroscopy could have been done to determine the hydrogen structure of the final product.