The equation for the di-tert-butylation of 1,4-dimethyoxybenzene is as follows: [VISUAL] Procedure A — Organic Chemistry Chemistry Question
Preparation of 1,4-di-tert-butyl-2,5-dimethoxybenzene; an example of a Friedel-Crafts alkylation
The equation for the di-tert-butylation of 1,4-dimethyoxybenzene is as follows:
[VISUAL]
Procedure
A stirred mixture of 1,4-dimethoxybenzene (360 mg) in glacial acetic acid (15 cm 3 ) contained in an Erlenmeyer flask of 50 cm 3 is heated gently (water bath) until dissolved. tert-Butyl alcohol (0.6 cm 3 ) is then added using the pipette and the mixture is cooled in a crushed ice bath, while stirring is continued, and treated dropwise with concentrated sulphuric acid (45 drops). The addition is achieved using a Pasteur pipette and it must be ensured that each drop is mixed thoroughly with the reaction mixture before the next drop is added. The mixture is then removed from the ice bath, allowed to attain room temperature, and then stirred for a further 25 minutes. The mixture is again cooled in an ice-bath (0 °C) whereon water (3 drops) is added VERY CAREFULLY with slow stirring. Crystallization of the product begins and is accelerated by the slow, careful addition of ice water (7.5 cm 3 ). After ca. 10 minutes the crystalline material is collected by filtration using a Hirsch funnel. The crystals are washed twice with water and allowed to dry. The product is recrystallized by dissolving in hot methanol (10 cm 3 ) and cooling in an ice bath and again collected using a Hirsch funnel. The material is allowed to dry on the air. Determine the weight, calculate the yield and determine the melting point. The purity of the product is determined by thin-layer chromatography (silica gel 60 F254) using hexane as the eluent. The starting material is run on the same TLC plate as a reference. Determine the Rf -value of the product, of the starting material and of any contaminant in the product if present.
Record the following data:
1. The mass of the product.
2. The yield of product in percentage of the theoretical yield (show your calculation).
3. The appearance of the crystals and their colour.
4. The melting point
5. Copy your TLC plate on the data sheet + the respective Rf –values.
List of chemicals needed:
1,4-dimethoxybenzene (400 mg)
Acetic acid (2.0 cm 3 )
tert-butyl alcohol (1.0 cm 3 )
conc. sulfuric acid (0.5 cm 3 )
methanol (10 cm 3 )
water (20 cm 3 )
TLC plates (silica gel 60 F254)
Safety: Concentrated H2SO4 must be handled with great care, read the safety rules for working with strong acids. In the event that you spill this acid on your hands and clothes, immediately rinse with running tap water.
Note: Melting points can be determined in various ways, for instance, by using silicon oil in a round bottomed flask equipped with a magnetic stirrer and a thermometer or using a Kofler block (metal block with holes to fit a thermometer and a capillary tube) or employing a Kofler hot bench (a metal alloy strip with gradient heating).
Which species is the electrophile in this alkylation?
Model Answer
tert-butyl cation: (CH3)3C+
Give the structure of the product when 1,3-dimethoxybenzene had been taken as a starting material.
Model Answer
Methoxy group is strongly activating in electrophilic aromatic substitution reactions and will direct the tert-butyl to ortho-para positions.
[VISUAL]
(Note: The visual shows structural representations of 1,3-dimethoxy-4,6-di-tert-butylbenzene and an alternative sterically hindered isomer labeled 'much less likely due to steric hindrance'.)