格氏反应（Grignard Reaction）
Grignard Reaction
Halogenated hydrocarbons in anhydrous ether or tetrahydrofuran react with metal magnesium to form alkyl halide magnesium RMgX, an organic magnesium compound known as Grignard reagent (Grignard, Reagent). Grignard reagent can react with aldehydes, ketones and other compounds to produce alcohols after hydrolysis. This reaction is called Reaction reaction (Grignard). Grignard reagent is one of the most widely used reagent in organic synthesis, it is by the French chemist gelinniya invented (V.Grignard).
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In 1871, seeborg Grignard was born in France (Cherbourg Frace). When he was in Lyon (Lyons) University, worked from Babia (P.A.Barbier) professor. At that time, mainly engaged in research of Babia organic zinc compound, he got two methyl zinc to zinc and iodine methane reaction, the organic zinc compounds are used as Methylating reagent. Later, the metal magnesium zinc Babia alternative to try, also have similar metal organic compounds, but relatively harsh reaction conditions. Therefore。 Babia let Green that continues to make further research on the preparation of organic magnesium compound system. The study found that the reaction can be conveniently obtained new compounds with methyl iodi
de and magnesium in ether medium without separation with aldehyde or ketone occurs further reaction, the reaction product can be obtained after hydrolysis of the corresponding alcohols. The reaction process can be expressed as:
Later studies show that alkyl magnesium halides (Grignard
reagents) can be used in many reactions and have wide range of applications, thus becoming one of the most commonly used reagents in organic synthesis.
The invention of Grignard reagent will greatly promote the development of organic synthesis, Grignard and therefore won the 1912 Nobel prize in chemistry.
In general, reactions of various halogenated hydrocarbons and magnesium can produce Grignard reagents. However, the reaction activity of different halogenated hydrocarbons with magnesium is different. Generally speaking, when the alkyl group reacts with the alkyl iodides at the same time, the activity of fluorinated hydrocarbon is the worst (in fact, no one has RI RCI RBr > RF)
When the halogen atoms remain unchanged, benzyl halides and allyl halides have the highest activity, and vinyl halogenated hydrocarbons have the lowest activity: ArCH2X, CH2=CHCH2X > 30RX, > 20RX > 10RX > CH2=CHX
Grignard reagent is very sensitive to water. In fact, all compounds with active hydrogen can and Grignard reaction, such as alcohols, amines and carboxylic terminal alkynes, etc.. Therefore, anhydrous reagents and drying instruments should be used when preparing Grignard reagents.
In addition, Grignard reagents react with oxygen in the air, such as:
2RMgX+O2==2ROMgX
However, in the Grignard reaction using ethyl ether as solvent, the oxygen in the air is not influenced by the oxygen in the ether because the vapor pressure of the ether is larger and the reaction liquid is surrounded by ether atmosphere. Solvent selection is also critical in the preparation of Grignard reagents. Absolute ether is often used as a solvent. This is because the oxygen atom in the ether molecule has lone pair electrons, which can react with Grignard reagent to form a solvent soluble complex:
If other solvents are used, such as alkanes, the reaction product will be deposited on the magnesium metal surface because it is insoluble in solvent, thus terminating the reaction.
In addition to ether, tetrahydrofuran is a good solvent for Grignard reactions. Especially when some hal
ogenated hydrocarbons, such as vinyl chloride and chlorobenzene, are difficult to react with magnesium in ether, the reaction can be carried out smoothly if tetrahydrofuran is used instead of ether as solvent. Because the boiling point of tetrahydrofuran is higher than that of ether, it is safer to do Grignard reaction with tetrahydrofuran as solvent than with ether.
2. Grignard reaction method
(1) preparation of anhydrous ether
First, take a small amount of the ether, adding volume of 2% potassium iodide solution and a few drops of dilute
hydrochloric acid, after shaking the starch solution is purple, namely that the ether containing a peroxide (also available starch potassium iodide peroxidase test). The method of removing peroxides is as follows: ether is transferred into the separating funnel, and the H2SO4 ferrous solution equivalent to 1/5 of the ether volume is added. The rock is shaken vigorously, and the layered water is removed and the water phase is removed. Then, the ethyl ether removed peroxide was fractionated two times [1] and each was collected at a temperature of 33 DEG C to 37 DEG C.
The ether must be kept in a brown glass bottle, pressed into the sodium thread and covered with a bottle cap with a capillary to escape the hydrogen it produces.
When pressed, the surface of the sodium wire is still lustrous, or solvent is no longer bubbling, indicating that the solvent can be used for Grignard reaction.
Note: if you remove the peroxide ether, the queen will still produce peroxide, and the ether will easily absorb moisture in the air. Therefore, the treated ether should be used as soon as possible and should not be used for a long time.
(2) preparation of Grignard reagent
On 250mL three flasks, a mixer, a constant pressure drop funnel and a reflux condenser with a CaCl2 drying pipe [see Figure 1.9 (4)]. 2.9g (0.12mo1) magnesium chips were placed in three flasks and submerged with 20mL anhydrous ether, [2].
Stir,
First drip into 5mL 25% halogenated ether solution (made from 0.12mol halogenated hydrocarbons and anhydrous ether). If the reaction is cloudy and the temperature rises, it indicates that the reaction
has started. If the above phenomenon does not occur, then 1~2 small crystals of iodine will be added and slightly heated [3].
For a moment, the color of iodine began to fade away, and the solution became cloudy. The reaction started and stopped heating. Pour the remaining halogenated hydrocarbon solution into the reaction bottle to accelerate the boiling of the reaction liquid. Plus, use warm water bath heating reflux for about half an hour, so that the reaction is complete, that is, Grignard reagent, ether solution [4].
Notes：
[1] don't steam the ether at any time.
All instruments used in the [2] are dry. Both solvents and reagents must be dried.
[3] can be heated by hot water bath. It must not be heated by open flame.
[4] oxygen in the air will react slowly with Grignard reagent. That is why the ether solution of Grignard reagent emits beautiful light in the darkroom. Therefore, Grignard reagent can not be used for a long time, usually with the system.

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