last updated Tuesday, May 06, 2008

Problem set  Meeting 12 

Announcement

1. The instructor will offer an in-lab demonstration of the Grignard setup on Wednesday, May 7, 2008 at 4:00 pm in YH 6086. You will receive 5 points when you attend this meeting and sign in.

2. The final version of the paper is due on May 9, 2008 at 5 pm in YH 3077E in printed version (submission via email will not be accepted). Late reports will get 10% of per day late. Make sure that you turn it in online by May 12, 2008 at 10 pm (CourseID: 2266641, Passwd: Chem30CL for Spring 2008).

3. The next quiz (=quiz 5) will be administered on May 8, 2008 at 12 pm and will cover steps 3 of the Lidocaine project, NMR and MS spectrometry. Make sure that you show up on time. Otherwise you will not be allowed to take the quiz.

...and now back to the chemistry....the homework questions

1. Referring to the Grignard reaction, answer the following questions. Show chemical equations when appropriate.

a. It is very critical for the success of the reaction that water is excluded right from the start. Why and how is this accomplished?

b. Prior placing the magnesium in the three-necked flask, it should be crushed a little bit. Why is this necessary and which precautions should be taken here?

c. How does the experimenter know that the reaction initiated? What could he do if the reaction were not to start on its own?

d. It is also important that the bromobenzene solution is added at the proper rate. Explain briefly.

e. After the formation of the Grignard reagent is completed, first the solution is cooled and then added to dry ice. Which purpose does each step serve and which precaution should be taken here?

f. The organic layer is extracted with sodium hydroxide solution. Explain briefly why.

2. Theoretical issues

a. Why are is iodocyclohexane more reactive than chlorobenzene when preparing Grignard reagents?

b. The choice of solvents is very limited when it comes to "normal" Grignard reactions. Most of the time, either diethyl ether or THF is used. Rationalize these choices.

c. Strictly speaking it is wrong to use "RMgBr" to identify a Grignard reagent. Why?