BRANDEIS UNIVERSITY



BRANDEIS UNIVERSITY

CHEMISTRY 29A

PRELAB WRITEUP AND EXPERIMENTAL REPORT SHEET

NAME Richard Fan______________ T.A.'S NAME David Gay __ ___ ___________

DATE 7/18/06 _____EXPERIMENT# __3___TITLE _Friedel-Crafts Alkylation_____________________

PURPOSE _Synthesizing 1,4-Di-t-butyl-2,5-dimethoxybenzene from alkylation of 1,4-Dimethoxybenzene with 2-methyl-2-propanol

|PRELIMINARY WRITEUP: | |

|Materials: reaction tubes |% yield = (experimental/theoretical) × 100% |

|120mg 1,4-dimethoxybenzene | |

|0.4mL acetic acid | |

|0.2mL t-butyl alcohol | |

|0.4mL H2SO4 | |

|2.5mL water | |

|Ice, stirring rod, pipettes | |

|Methanol | |

|Methylene chloride (TLC eluent) | |

| | |

| | |[pic] | |

|[pic] |1,4-Dimethoxybenzene | |1,4-Di-t-butyl-2,5-dimethoxybenzene |

| |MW 138.16 |[pic] |MW 250.37 |

|[pic] |MP 57ºC | |MP 104-105ºC |

| | | | |

| |2-methyl-2-propanol | | |

| |(t-butyl alcohol) | | |

| |MW 74.12 | |H2SO4 (sulfuric acid) |

| |MP 25.5ºC | |MW 98.18 |

| | | |MP 10ºC |

| | | |BP337ºC |

|1,4-Dimethoxybenzene + 2-methyl-2-propanol ( 1,4-Di-t-butyl-2,5-dimethoxybenzene |

|H2SO4 |

|EXPERIMENTAL PROCEDURES* |MODIFICATIONS AND OBSERVATIONS* |

|(Include in preliminary writeup.) |(Obtained from the lab experience) |

|-Add 0.12g 1,4-Dimethoxybenzene to a reaction tube. |During preparation of the reaction mixture, 1 – 2 drops of sulfuric acid was lost |

|-Add 0.4mL acetic acid to tube, use heat to dissolve. |while it was being added to the reaction tube from a buret. The rest of the |

|-Add 0.2mL t-butyl alcohol to tube. |experiment was carried out, but extra time was allotted for the reaction to complete.|

|-Place tube on ice, add 0.4mL concentrated sulfuric acid while on ice, and |Preparation of the reactants produced a reddish brown colored murky viscous mixture. |

|mix. (mixture should be dark and viscous) |This mixture was stirred to ensure even distribution of the reactants for the |

|-Let mixture stand on ice for 10 minutes. |reaction of 1,4-Dimethoxybenzene with t-butyl alcohol. After remaining in ice for 15 |

|-Add 2.5mL water and mix well on ice. |minutes, the mixture appeared to have a thick light colored precipitate, which was |

|-Use pipette to remove water, and wash with a few more drops of water. |washed with about 3mL of water. The crude product was still wet after the water had |

|-Get weight and melting point of crude product. |been removed, so it was placed on filter paper and pressed dry. The weight of the |

|-Recrystallize in minimal methanol. |crude product was 0.5206g, and the melting point was observed to be 74º-88ºC. The |

|-Use Hirsch funnel to dry product. |crude product was dissolved in approximately 1mL of methanol with light heating and |

|-Get weight and melting point of final product. |recrystallized over ice. The weight of the final product was 0.0395g with a melting |

|-Perform thin layer chromatography on final product and starting material |point of 102º-103ºC. Thin layer chromatography was performed on the final product |

|(standard). |(solution of 20mg final product and 1mL methylene chloride) and a prepared solution |

| |of the starting material (1,4-Dimethoxybenzene). |

| | |

| | |

RESULTS, DISCUSSION AND CONCLUSIONS*

In this experiment, 1,4-Dimethoxybenzene is reacted with t-butyl alcohol in sulfuric acid to create the dialkylated 1,4-Di-t-butyl-2,5-dimethoxybenzene. While this substitution is similar to that of nitration in experiment 2, the difference is that the nitration reaction was a monosubstitution while the alkylation in this experiment was a disubstitution. This means that while only one nitro group was added to the benzene ring in the previous experiment, two t-butyl groups were added to the benzene in this experiment. The reason for this is that the NO2 group is a strong deactivating group, which decreases the reactivity of the benzene upon attachment. After the monosubstitution, the nitrated benzene stopped further reactions. Alkylation is different however, because both the methoxy and t-butyl groups are activating groups to varying degrees. Once one t-butyl group was attached to the benzene, the molecule was still reactive enough to accept another t-butyl group. Their 1,4 positioning was determined by the steric resistance resulting from the locations of the methoxy groups.

An initial concern during the preparation of the reagent mixture in this experiment was the loss of a small amount of H2SO4. To compensate, an effort was made to carry out the reaction for a longer period of time. This may or may not have been effective, as the final product was about 50% of the expected weight. The expected final weight of 1,4-Di-t-butyl-2,5-dimethoxybenzene was around 80mg – 100mg, but the actual experimental final product was only about 39.5mg. Much of this loss probably resulted from product that stayed in solution that was then separated during filtration. The crude weight on the other hand was over 500% of the expected value, clearly indicating the presence of impure compounds. Since the crude product was not dried using aspiration or filtration, much of the weight may have been from incomplete drying and hence from water that remained in the product.

Thin layer chromatography analysis yielded a solvent front at 5.5cm, the final product at 4.1cm, and the starting material at 3.1cm. The Rf values for the final product and starting material were 0.75 and 0.56 respectively. The results show that the relatively larger molecule (1,4-Di-t-butyl-2,5-dimethoxybenzene) actually traveled further than the smaller methyl-3-benzoate. While the larger molecule is normally expected to travel a smaller distance, this may be explained by the interactions with the methylene chloride eluent and the silica plate. While 1,4-Di-t-butyl-2,5-dimethoxybenzene was the larger molecule, perhaps methyl-3-benzoate was less soluble in the eluent, resulting in slower travel than the dialkylated product on the silica plate.

ANSWERS TO END-OF-CHAPTER QUESTIONS*

4.

[pic]

5. Neither 1,3-di-t-butyl-2,5-dimethoxybenzene nor 1,4-dimethoxy-2,3-di-t-butylbezene can be formed in this reaction because both of them would result in much greater steric resistance with the methoxy groups and with each other. For example, when both t-butyl groups and a methoxy group are all next to each other, the steric hindrance would be very high. Likewise, even when the t-butyl groups are not directly next to the same methoxy group and are on 2,3 positions (with methoxy groups at 1,4), the steric resistance between the t-butyl groups themselves would be very high as well.

7. In the IR spectrum of methyl-3-benzoate, the expected peaks appear around 690-710 cm-1 and 750-810 cm-1. The experiment IR spectrum shown does indeed include these peaks at the wavelengths indicated, suggesting that this is the expected disubstituted molecule.

- In the dialkylated molecule, there are 3 different hydrogen groups – those attached to the t-butyl group, those attached to the methoxy group, and those attached to the benzene ring. In the monoalkylated molecule, there are 4 different groups – those attached to the methoxy groups, those attached to the one t-butyl group, two attached to the benzene ring, and a different group attached to the benzene ring.

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