Magnesium atoms alkyl halides

Magnesium reacts with alkyl halides to form alkyl magnesium bromide salts known as Grignard reagents. As mentioned throughout this book, these species bear nucleophilic carbon atoms. As illustrated using arrow pushing, the alkyl anion adds to the carbonyl and subsequently eliminates methoxide. This addition-elimination process leads to the formation of cyclohexanone. 

Grignard reagent produces greenish blue luminescence in its reaction with chloropicrin or on its oxidation by air. Only aryl magnesium halides in which Magnesium atom is attached to one unsaturated carbon atom exhibit chemiluminescence s in solution while in solid state, both alkyl and aryl magnesium halides show this phenomenon. 

It will be noted that in the addition of the magnesium alkyl halide, the positive metallic atom remains in combination with the halogen atom and unites with the oxygen of the aldehyde, while the alkyl group joins itself to carbon. The way in which the union takes place should be noted, as the magnesium alkyl halides are much used in synthetic work, and in the formation of addition-products with compounds containing oxygen, the union takes place in a manner similar to that just explained. 

Ketones can be prepared by the replacement of the halogen atom of an acyl halide (e.g. CH3COCI) by an alkyl group. This replacement can be effected as the result of the action of certain organic compounds which contain metals. The zinc alkyls or magnesium alkyl halides are generally used. Methyl ethyl ketone can be prepared in this way from acetyl chloride and magnesium ethyl iodide —... 

An intermediate product is formed as the result of the addition of the acid chloride and magnesium alkyl halide, the addition taking place in a way similar to that which has been explained (180). This synthetic method is an important one, as it serves to replace a halogen atom in an acyl halide by an alkyl group. 

Reactions of Alkyl Halides.—Halogen atoms in organic compounds enter into reactions with many substances. When the alkyl halides are brought into contact with the more active metals, the halogen atom either is removed and a condensation-product formed, or addition takes place. The two classes of reactions are illustrated by those which take place between methyl iodide and sodium and magnesium —... 

Disconnections (a), (b), and (d) in Table 10.1 all require reagents for the carbanion synthon R. Simple carbanions are almost never formed in reactions so we shall need reagents in which carbon is joined to a more electropositive atom such as a metal. The most popular are Li and Mg. Bnty lithium (BuLi) is commercially available and other alkyl lithiums can be made from it by exchange (i). Otignard reagents (2) arc usually made directly from alkyl halides and magnesium metal (iii)—a method also available for RIJ (ii). These methods are available for aryl compounds too. Transformation of RHal into RIa or RNfgBr involves a formal inversion of polarity. 

Grignard reagents are used extensively in organic chemistry to assist in the formation of C-C bonds. For example, magnesium metal reacts with halocar-bons (or alkyl halides) such as bromomethane, in a solvent called ether. The magnesium inserts itself in between the carbon and halogen atoms, and each forms a covalent bond with magnesium. 

The reactions of hexaorganodileads with nitric acid 358,359), dimethyl-acetic acid 358), magnesium/magnesium iodide 296), aluminum chloride 349, 360), and alkyl halides 280,361, 362) also result in Pb—Pb cleavage. Reaction of hexaethyldilead with sulfuryl chloride, thionyl chloride, sulfur dichloride, and sulfur monochloride has been reported (363). The yield of triethyllead chloride was observed to decrease with increasing nucleophilic character of the sulfur atom, in the order listed above. 

Consequently it is proposed that in the black matrix the magnesium atoms are weakly bound to the halogen atom of the alkyl halide thus limiting its movement and selfcondensation to make a film magnesium atoms bound in this manner are readily transferred to other alkyl halides and since methyl halides are more reactive than propyl, the former reacts preferentially. 

Essentially, an atom of magnesium inserts itself in between the C—Cl bond (this reaction works with other halides as well, such as Br or I). This magnesium atom has a significant electronic effect on the carbon atom to which it is attached. To see the effect, consider the alkyl halide (before Mg entered the picture) ... 

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