Grignard reaction

Topics: Distillation, Diethyl ether, Magnesium Pages: 8 (1137 words) Published: April 25, 2014
Experiment C.
Aim:
To protect one of two carbonyl groups (C1) in order to allow the other to react twice with a Grignard followed by removal of the protecting group by acid hydrolysis to give final product (C2). Method:

Ethyl acetoacetate (30.03g), ethylene glycol (15.01g) and toluene-p-sulphonic acid (0.13g) were added to a 250 cm3 round bottomed flask, containing a stirrer bar and toluene (100 cm3), fitted with a condenser and dean-stark head. Solution was heated strongly under reflux using a heater stirrer block for approximately one hour. During heating 4.5 cm3 of water was collected. Solution was cooled, placed in a separating funnel and washed with NaOH (2M, 35 cm3) and water (2 x 50 cm3), organic layers were collected and dried over potassium carbonate and Buchner filtered. Excess toluene was removed by rotary evaporation ensuring bath temp was around 40 °C. Vacuum distillation apparatus was set up and residue placed in a round bottomed flask. Residue was distilled under reduced pressure by way of vacuum for approximately one hour. Residual toluene was collected in one vial between 24–28 °C then a slight temperature drop was observed indicating all toluene had been removed. Pig was turned to allow collection of pure product into a separate flask from 78 °C. a clear liquid was collected as the product (C1) weighing 21.94g. Percentage yield was calculated and IR spectrum was run. Next, apparatus for Grignard reaction was placed in an oven for an hour to ensure no moisture was present then set up for reflux with a dropping funnel to equalise pressure and a jack to allow easy removal of apparatus from heat. Magnesium turnings (2.68g), anhydrous diethyl ether (15 cm3), a crystal of iodine and a stirrer bar were added to the round bottomed flask (250 cm3) and anhydrous diethyl ether (25 cm3) and dry bromobenzene (15.7g) were added to the dropping funnel, ensuring tap was closed. Dropping funnel was opened slightly allowing a small amount of bromobenzene solution through until initiation was confirmed. Upon initiation, the remaining bromobenzene solution was slowly added to maintain a steady reflux and heated for approximately 25 minutes. After 25 minutes, solution was cooled on an ice bath whilst still under vacuum pressure. C1 ester (8.7g) in anhydrous diethyl ether was added to the dropping funnel and dropwise added to the flask with stirring. The solution inside the flask was stirred for a further 30 minutes then ice (25g) was added. Once ice was melted, water (25 cm3) was added and a gummy grey/white residue was formed. Diethyl ether (25 cm3) was added and stirred until gummy residue was mostly dissolved. Aqueous layer was extracted using diethyl ether (2 x 25 cm3), organic layers were combined and washed with water (25 cm3), dried over magnesium sulphate, Buchner filtered and rotary evaporated to remove diethyl ether. Once diethyl ether was removed, the product was recrystallised in hot methanol (minimal) to give a white powder. IR was run of the final product (C2) and melting points acquired. Table of materials:

Chemical
Mr
Moles
Amount
Ethyl acetoacetate
130.14
0.23
30.03g
Ethylene glycol
62.07
0.24
15g
Toluene-p-sulphonic acid

0.13g
Toluene

100 cm3
Sodium hydroxide solution (2M)

35 cm3
Potassium carbonate

~5g
Magnesium metal

2.68g
Iodine

~10g
Diethyl ether (DRY)

65 cm3
Diethyl ether

150 cm3
Bromobenzene

15.7g
Magnesium sulphate

~5g
Methanol

~5 cm3
Acetone

50 cm3
Hydrochloric acid

1.5 cm3
C1 ester
174.19
0.05
8.7g

Results:
C1:
Boiling point: 78 °C
IR table:
Wavenumber cm-1
Assignment
Intensity
2984 & 2888
C-H
Weak
1732
C=O
Strong, sharp
1370 & 1320
C-O
Weak-medium

Percentage yield:
1:1 reaction, ethyl acetoacetate limiting reagent, Mr = 130.14, mass = 30.03g. 30.03/130.14 = 0.23 moles
Moles C1 = 0.23, Mr C1 = 174.19
Theoretical mass C1 = 0.23 x 174.19 = 40.06g
Actual mass C1 =...
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