Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Grignard
dapper
Hazard to Self
**




Posts: 66
Registered: 8-11-2006
Member Is Offline

Mood: No Mood

[*] posted on 21-7-2008 at 12:41
Grignard


I'll be making a Grignard soon and I haven't done the procedure before. Any tips?

Thanks
Dapper
View user's profile View All Posts By User
Ritter
National Hazard
****




Posts: 370
Registered: 20-6-2008
Location: Earth
Member Is Offline

Mood: Curious

[*] posted on 21-7-2008 at 13:36


Quote:
Originally posted by dapper
I'll be making a Grignard soon and I haven't done the procedure before. Any tips?

Thanks
Dapper


A good place to start is by reviewing the detailed procedures used (and checked) by professionals. These are published in Organic Syntheses at http://www.orgsyn.org/orgsyn/default.asp?formgroup=&data...

Just type Grignard in the box under this legend:

Quote:
and has this phrase anywhere


Grignards can be very difficult to get going. The major factors are the dryness of your glassware & reagents, the exclusion of air & moisture, etc.

[Edited on 21-7-2008 by Ritter]




Ritter
=============================
\"The production of too many useful things results in too many useless people.\"

Karl Marx
View user's profile View All Posts By User
Klute
International Hazard
*****




Posts: 1378
Registered: 18-10-2006
Location: France
Member Is Offline

Mood: No Mood

[*] posted on 21-7-2008 at 13:49


Reda the procedures also in Vigels and other preparative chemistry books, lots of usefull details.

As Ritter said, the dryness of the ether and exclusion of humid atmospher is critical (dessicants like MgSO4 will not suffice, standingover KOH for a few dasy, after testing for peroxides, seems to work enough, but a distilaltion over sodium, or benzophenone ketyl, is the must), aswell as the purity of the magnesium and it's freshness.

here is some pretty good info Lugh posted at the Hive, to get started:


Quote:
From Chapter 12 of Migrdichian's Organic Synthesis

Preparation of Organomagnesium Compounds
The general procedure followed in preparing an organomagnesium halide is as follows:

The required quantity of magnesium; in the form of turnings, 3 mm long and 0.6 mm thick, is placed in a dry flask provided with a sealed stirrer and a reflux condenser; the air is displaced with a current of dry nitrogen, a little ether and a crystal of iodine are added, then a small amount of the halide is introduced to initiate the reaction. If reaction fails to proceed spontaneously, the mixture is heated to 45°, although more frequently it is necessary to apply external cooling in order to moderate the reaction. After the initial vigorous reaction has subsided to some extent, the halide, mixed with 1 to 3 volumes of ether, is added slowly and at a constant rate, while the liquid in the flask is stirred. After the addition of all the halide mixture, the flask is heated until the reaction is complete.
It is important that both the reflux condenser and the dropping funnel used for the in­troduction of the halide-ether mixture, be protected from atmospheric moisture by means of calcium chloride tubes. The magnesium turnings must be clean and, in particular, free from any oily matter. Commercial magnesium in the form of turnings or of coarse powder may be used. The quality of magnesium employed may greatly affect the yields obtained_ The magnesium should be stored dry and, if desirable, it may be washed with pure, dry ether by decantation. The impurities present in the metal remain as a dark powder alter the completion of the reaction. The metal is used in an amount corresponding to the halide or in slight excess. The ether must be anhydrous and free from alcohol. Com­mercial alcohol-free ether distilled over phosphorus pentoxide is satisfactory. The ether may also be dried by storing it over sodium wire. The halide must be in a very pure form and free from acids, water or alcohol.
It is important that the halide be added slowly in order to avoid the presence of any large excess of unreacted halide and thus to prevent the reaction
RMgX + XR -. R.R + MgX2
which proceeds quite readily when iodides are employed. This reaction is favored if an excessive amount of iodine is used for initiation of the reaction.

and

Procedure in Carrying out Grignard Reaction

The Grignard reaction is carried out in the following manner:
The compound dissolved in ether is added to the solution of the organomagnesium halide at such a rate that the solution boils continuously. If the compound is insoluble in ether, it may be dissolved in benzene, petroleum ether, etc. Solid compounds may be added to an ethereal solution of the organomagnesium halide in the form of a powder. If there is a possibility of reduction, any temporary excess of the organomagnesium halide is avoided, by gradually adding the solution of the Grignard reagent to the solution of the compound. The reaction generally proceeds quite rapidly in most cases The magnesium complex resulting from the reaction is then decomposed, carrying out the operation with great care in order to assure the success of the preparation and to avoid danger. Rapid dilution and cooling by the addition of water and ice generally ac­complishes the desired result. Decomposition with water results in the formation of mag­nesium hydroxide. The formation of this compound may be avoided by adding dilute hydrochloric or sulfuric acid to the diluted, cooled reaction mixture. It is preferable to pour the reaction mixture on ice, and then to add the acid. If the reaction product is sensitive to acids and likely to polymerize when decomposi­tion is carried out with an acid, ammonium chloride may be used in place of the acid.
The success of the Grignard reaction often depends upon the rapidity with which the operation is carried out.
The ether, as well as other reagents must be quite dry. The ether is best kept dry by storing it over metallic sodium. The reaction often proceeds only when ether dried over phosphorus pentoxide is employed, though it has been carried out successfully in some cases with moist ether, using an excess of the re­agent.
If it becomes necessary to employ a high-boiling solvent as the reaction medium, the ether used in the preparation of the organomagnesium halide is re­moved as much as possible by distillation under vacuum at 100-105°, and the desired solvent is added. Benzene and ethers of higher alcohols are suitable solvents. Among the latter, methyl benzyl, methyl cyclohexyl, amyl and butyl ethers are often employed. The use of rubber stoppers should be strictly avoided, since rubber inhibits the reaction. Glass, or new, paraffin coated cork stoppers are satisfactory.
The rate of reaction is proportional to the concentration of the organomag­nesium halide in the solution. The optimum concentration of the reagent appears to be 30-40%, although occasionally concentrations as high as 70% are employ­ed.
High molecular products may be formed and may separate out in the course of the reaction; if they are liquids, sufficient benzene is added to bring them into solution. Solids, if they appear, are not readily brought into solution and may cause considerable difficulty.




\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"

-Alice Parr
View user's profile View All Posts By User
vulture
Forum Gatekeeper
*****




Posts: 3330
Registered: 25-5-2002
Location: France
Member Is Offline

Mood: No Mood

[*] posted on 22-7-2008 at 12:28


Instead of using iodine for starting a unwilling grignard (which can cause all kinds of side reactions), try using a very small amount of dibromoethane.
View user's profile View All Posts By User
dapper
Hazard to Self
**




Posts: 66
Registered: 8-11-2006
Member Is Offline

Mood: No Mood

[*] posted on 23-7-2008 at 11:58


Great advice! Thanks! I'll let everyone know how it goes
View user's profile View All Posts By User

  Go To Top