Grignard reagent is a polar molecule: R-MGX, and R can undergo nucleophilic substitution reaction. The structure of Grignard reagent is generally expressed by RMgX. NMR spectrum shows that Grignard reagent is a bimolecular compound, and its real structure may be:
The production difficulty of Grignard reagent is related to the structure of halogenated hydrocarbons and the types of halogens. In terms of reactivity and yield, primary halogenated hydrocarbons > secondary halogenated hydrocarbons > tertiary halogenated hydrocarbons; RI>RBr>RCl .
For example:
Because iodoalkanes are the most expensive and chloroalkanes have the worst reactivity, bromoalkanes with moderate reactivity are often used to synthesize Grignard reagents in the laboratory. Simple iodine can catalyze the reaction, so a small amount of iodine is often added to promote the reaction. Allyl-type and benzyl-type halogenated hydrocarbons are very active and easy to generate Grignard reagents. The generated Grignard reagent is coupled with unreacted halogenated hydrocarbon. Therefore, when synthesizing this kind of Grignard reagent, the reaction should be strictly controlled at a lower temperature. Ethylene halogenated hydrocarbons and aromatic halogenated hydrocarbons with halogen atoms directly attached to aromatic rings cannot form Grignard reagents with magnesium in anhydrous ether. By changing the solvent such as tetrahydrofuran, the reaction can proceed smoothly. For example:
It is believed that this is because the oxygen in cyclic ether is exposed more than that in linear ether, and it is easy to cooperate with magnesium to accelerate the reaction.
Grignard reagent is very active and can react with oxygen, water and carbon dioxide in the air. Therefore, in preparation, in addition to keeping the reagent dry, it should also be isolated from the air.
Grignard reagent is one of the most important reagents in organic synthesis. The prepared Grignard reagent can be directly used in organic synthesis without sharing.
These reactions that use Grignard reagents to synthesize a series of organic compounds such as hydrocarbons, alcohols, aldehydes, ketones and carboxylic acids are called Grignard reactions, which are called Grignard reactions for short.
Reaction of (1) with active hydrogen. Grignard reagent meets compounds with "active" hydrogen, such as water, alcohol, carboxylic acid, ammonia and amine, and generates alkanes. For example:
This reaction can be used for the quantitative reaction of active hydrogen. By measuring the volume of alkanes, the number of active hydrogen contained in each organic molecule can be calculated, which is called active hydrogen determination method.
(2) reacting with active alkyl halide. Grignard reagent is coupled with allyl and benzyl halogenated hydrocarbons to generate hydrocarbons.
The coupling reaction between Grignard reagent and allyl halide is a method to synthesize terminal olefins. Tertiary halogenated hydrocarbons can also react, but the yield is low.
③ React with metal halide. Grignard reagent reacts with metal halide whose reduction potential is lower than magnesium, and alkyl in Grignard reagent replaces halogen in metal halide to generate new organometallic compounds, which is an important method to synthesize organometallic compounds.
Reduction potentials of some metals;
Organocadmium compounds are important reagents for the synthesis of ketones. For example:
Grignard reagent reacts with aluminum trichloride to produce organoaluminum compounds.
RMgX+AlClRAlClRAlClRAl
Organoaluminum compounds are important catalysts for olefin polymerization.
④ Addition with polar double bond or triple bond compound.