SONOMA STATE UNIVERSITY
Chemistry 336-Dale Trowbridge
ELECTROPHILIC AROMATIC SUBSTITUTION
Reference: Experimental Organic Chemistry by Majorie C. Caserio
DISCUSSION:
Any modern Organic Chemistry text book will give a complete discussion of the currently accepted mechanism of electrophilic aromatic substitution reactions including both the directive influence of substituents and the ring activating or deactivating effects of these substituents. In preparation for this experiment you should review these ideas as they are presented in your lecture text book. In this experiment you will use GLPC to study the products of the bromination of benzene and substituted aromatic compounds such as toluene or chlorobenzene.
The major emphasis of your laboratory report should be the analysis of the results of your experiment in terms of what you were and were not able to determine in terms of the nature of this reaction.
Procedures:
A. Bromination of Toluene
Prepare a solution of aqueous nitric acid by carefully adding 16 ml of 70% nitric acid to 100 ml of deionized water. Place 100 ml of this aqueous nitric acid in a 250 ml round bottom flask and carefully add 1 ml (0.02 mol) of bromine(caution) to this nitric acid solution. Due to the toxicity and volatility of bromine this step in the procedure must be done in the hood. Stir the resulting mixture with a magnetic stirring bar until the bromine has dissolved completely (5-10 minutes). Wrap the flask with aluminium foil to exclude light (why?). Add 3.7 ml (0.035 mol) of toluene to the vigorously stirred reaction mixture and allow the reaction to proceed for 15 to 20 minutes. Uncover the flask and if any unreacted bromine remains in the solution destroy it by adding a little solid sodium sulfite. Pour the reaction mixture into a 250 ml separatory funnel, and extract the mixture with several 30 ml portions of diethyl ether. Wash the separated ether extracts with aqueous sodium carbonate to remove
any residual nitric acid and dry the organic phase with anhydrous magnesium sulfate. Decant the dried organic solution from the drying agent, and save this solution for analysis by GCMS.
B. Relative Rates of Bromination of Benzene and Toluene
Repeat the procedure of section A. using a mixture containing 6:1 mole ratio of benzene to toluene instead of the pure toluene used in part A. You can prepared this mixture by the addition of 2.0ml (0.030 mol) of benzene to 0.50ml (0.0050 mol of toluene), Save the reaction product mixture for analysis by GCMS.
C. Bromination of Toluene in the Presence of Silver Ion
If time permits run the bromination of toluene as discribed in part A. of this procure except with the addition of 3.4 g (0.02 mol) of silver nitrate in 10-15 ml of water to the reaction mixture. The addition of silver ion changes electrophile in this reaction from Br2 to
H2OBr+ and an interesting comparison might be made made between the product mixture from the bromination of toluene using these two different procedures.
D. GCMS -Standards
Using the standard samples provided determine the relative retention times of 2-bromotoluene, 3-bromotoluene and 4-bromotoluene. Since these three isomeric bromotoluenes will have very similar retention times this is best done by injecting a mixture containing different amount of the three compounds in the GC. Also determine the retention time for bromobenzene.
E. Unknowns
Inject your three experimental solutionsto determine the relative amounts of the various products formed in each experiment.
SONOMA STATE UNIVERSITY
Chemistry 336-Dale Trowbridge
ELECTROPHILIC AROMATIC SUBSTITUTION
Reference: Experimental Organic Chemistry by Majorie C. Caserio
DISCUSSION:
Any modern Organic Chemistry text book will give a complete discussion of the currently accepted mechanism of electrophilic aromatic substitution reactions including both the directive influence of substituents and the ring activating or deactivating effects of these substituents. In preparation for this experiment you should review these ideas as they are presented in your lecture text book. In this experiment you will use GLPC to study the products of the bromination of benzene and substituted aromatic compounds such as toluene or chlorobenzene.
The major emphasis of your laboratory report should be the analysis of the results of your experiment in terms of what you were and were not able to determine in terms of the nature of this reaction.
Procedures:
A. Bromination of Toluene
Prepare a solution of aqueous nitric acid by carefully adding 16 ml of 70% nitric acid to 100 ml of deionized water. Place 100 ml of this aqueous nitric acid in a 250 ml round bottom flask and carefully add 1 ml (0.02 mol) of bromine(caution) to this nitric acid solution. Due to the toxicity and volatility of bromine this step in the procedure must be done in the hood. Stir the resulting mixture with a magnetic stirring bar until the bromine has dissolved completely (5-10 minutes). Wrap the flask with aluminium foil to exclude light (why?). Add 3.7 ml (0.035 mol) of toluene to the vigorously stirred reaction mixture and allow the reaction to proceed for 15 to 20 minutes. Uncover the flask and if any unreacted bromine remains in the solution destroy it by adding a little solid sodium sulfite. Pour the reaction mixture into a 250 ml separatory funnel, and extract the mixture with several 30 ml portions of diethyl ether. Wash the separated ether extracts with aqueous sodium carbonate to remove
any residual nitric acid and dry the organic phase with anhydrous magnesium sulfate. Decant the dried organic solution from the drying agent, and analyze this solution directly by GCMS.
B. Relative Rates of Bromination of Benzene and Toluene
Repeat the procedure of section A. except in place of toluene substitute a 2:1mole ratio of benzene to toluene (i.e. 0.07 mol of benzene and 0.035 mol of toluene) for the pure toluene in the experiment. Analyze the reaction product mixture by GCMS. Ignoring the hydrocarbon peaks compare the relative amounts of the products from the toluene to the amount of the bromobenzene produced.
C. Bromination of Toluene in the Presence of Silver Ion
If time permits run the bromination of toluene as discribed in part A. of this procure except with the addition of 3.4 g (0.02 mol) of silver nitrate in 10-15 ml of water to the reaction mixture. The addition of silver ion changes electrophile in this reaction from Br2 to
H2OBr+ and an interesting comparison might be made made between the product mixture from the bromination of toluene using these two different procedures.