Practical session: Wed 2pm/Thurs 10 am Date submitted: 05 May 2015
This experiment aims to conduct a Friedel-Crafts alkylation reaction by synthesizing 1,4 di-t-butyl-2.5,-dimethoxybenzene. This product will be obtained by reacting 1, 4-dimethoxybenzene with t-butyl alcohol and sulfuric acid (Fig 1.0). The final product will then be isolated through filtration and recrystallization. Thin Layer Chromatography (TLC) analysis and melting point measurements will also be performed on the final product to assess its purity (McGeary, 2015).
Fig 1.0 Friedel-Crafts Alkylation reaction (McGeary, 2015) - 1,4-dimethoxybenzene reacts with 2 moles of t-butyl alcohol in the presence of sulfuric acid to form the final product, 1,4 di-t-butyl-2.5,-dimethoxybenzene.
1,4 dimethoxybenzene (1.98 g, 14.48 mmol), t-butyl alcohol (2.75 g, 37.10 mmol) and glacial acetic acid (6.5 ml) were mixed and cooled (0°C - 3°C). Cold sulfuric acid (2.5 M, 10 ml) was then added dropwise to mixture and swirled periodically for 10 minutes. Mixture was then placed at room temperature (20°C-25°C) while being continually swirled. Ice and cooled distilled water (125 ml) were added to mixture to both cool and dilute the sulfuric acid. Mixture was then further cooled (0°C - 3°C) for 10 minutes. Crude product (4.31 g) was collected through suction filtration and washed with cold water. Product was then recrystallized by dissolving in minimum volume of boiling methanol and allowed to crystallise at room temperature and at 0°C - 3°C. Final product was collected through suction filtration and washed with cold methanol. Yield 2.60 g, 71.8 %, M.P: 103.8°C – 104.1°C. TLC analysis was performed on final product and 1,4 dimethoxybenzene to assess identity and purity of isolated sample.
RESULTS AND DISCUSSION:
Melting point (°C) range of final product
Three melting points were recorded: 103.6°C, 103.8°C and 104.1°C.
Melting point range obtained was 103.6°C – 104.1°C which is close to the literature value range of 104°C - 105°C (Williamson et al, 2003). However, it is obvious that the sample melted at temperatures 0.2°C-0.4°C below the expected melting point range. This may be attributed to impurities present in the sample which could weaken the arrangement and interaction of particles in the product thereby lowering the melting point. It is also evident that the product did not melt above the expected melting point range which suggests product dryness.
Thin Layer Chromatography (TLC) plate analysis
Solvent system used: 15% ethyl acetate in petroleum spirit.
Final results of TLC are depicted below (Fig 2.0). The starting material (SM), 1,4- dimethoxybenzene had an Rf value of approximately 0.45 (1.7 cm/3.8 cm) while the final product (P), 1,4 di-t-butyl-2.5,-dimethoxybenzene had an Rf value of approximately 0.66 (2.5 cm /3.8 cm). The third spot containing both SM and product had similar Rf values.
Fig 2.0 TLC results – Starting material (SM) refers to 1,4 dimethoxybenzene while final product (P) refers to 1,4 di-t-butyl-2.5,-dimethoxybenzene. All samples were dissolved in acetone before being spotted onto their assigned positions on the plate. Distance travelled by samples were measured from marked base to center of the spot. Solvent front was 3.8 cm away from marked base.
SM has travelled less than P as reflected by the upper mentioned Rf values. This can be attributed to the polarity of SM and P as well as the polarity of silica on TLC plate. The SM, 1,4-dimethoxybenzene (Fig 1.0), is a polar molecule due to the presence of para-methoxy groups on the benzene ring. This molecule is therefore able to interact with the polar OH bonds on the silica plates (Fig 3.0) through hydrogen