Mg-X bonds

The broken bonds (boldface = recommended data reference in parentheses) Methods (reference in  

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The halide radical anion (6) dissociates to give a carbon radical (R, where a radical is defined as a reactive intermediate with one electron in a p-orbital see Chapter 7, Section 7.4.3) and a halide anion, X. If X and 5 react with each other, a Mg-X bond is formed and the magnesium complex becomes a radical (6) rather than a radical cation. Note that the radical cation can accept electron density from the electron-rich X. If 6 subsequently reacts with the carbon radical (R ), Grignard reagent (7) is generated. This sequence is taken to be the mechanism of Grignard reagent formation from alkyl halides 1. The ether... 

Reductive methods form B—B bonds from B—X bonds. For B2X4 (X = Cl, Br, I) from BXj, an electric discharge is supplemented by the presence of a metal, or metal atoms, as halide scavenger. The passage of BX3 at low pressure through a rf discharge in the presence of Hg produces the diboron tetrahalides B2X4 at 300 mg h ... 

The mechanism for this insertion of magnesium is beyond the scope of this course, and therefore, we will not go into it. For now, we should just know that we can insert Mg into a C—X bond (where X is Cl, Br, or 1). Here are some examples ... 

Once a magnesium atom is inserted between a C—X bond, the newly formed C— Mg bond is somewhat ionic in character (because carbon is so much more electronegative than magnesium). Therefore, there are two acceptable ways to draw a Grignard reagent ... 

Scheme 3. Competition between Mg-X and Mg-R bond metathesis in the reactions of MITp1 ] with Grignard reagents. 

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