Grignard reaction limitations

The conversion of 4,5-dicyanothiazoles to diketones has been attempted (91). A difference in reactivity between the two cyano groups has been observed the least labile is the group in the 5-position. These Grignard reactions are limited and lead to 4-acetyl-5-cyanothiazole (Scheme 25). 

Biguanides are accessible by a Grignard reaction of guanidinomagnesium halide with a substituted cyanamide, and hydrolysis of the resulting complex [59—61). The method is of considerable theoretical interest but so far only of limited practical importance. 

Problem 13.9 Give four limitations of the Grignard reaction. 

Pb was obtained as a solution of PbCl2. Tetra-ethyl lead was synthesised by a Grignard reaction and added to petrol together with ethylene dibromide. The petrol was burnt in a 50-cc four-stroke engine, and the exhaust passed into a wind tunnel (Fig. 7.8). The airflow in the tunnel gave sufficient dilution to limit the concentration of CO to 1000... 

The reaction of ethyl orthoformate and Grignard reagents gives acetals which are hydrolyzed readily by dilute acid to aldehydes. This method has been employed extensively for the preparation of aliphatic and aromatic aldehydes. A study of the optimum conditions has been made, using the conversion of bromobenzene to benzaldehyde as a model synthesis (90%), Comparative studies of various aldehyde syntheses that employ Grignard reagents (methods 154, 166, and 167) show that this one is the most practical however, the possibility of a sudden exothermic reaction limits the size of the run. Longer reaction times at room or reflux tempjerature help overcome this difficulty. Examples of the better preparative procedures are found in those for -hexaldehyde (50%), p-tolualdehyde (TS ), " and phenanthrene-S>-aldehyde (42%)." ... 

Glycols, summary of chemistry, 294 Grignard reaction, 258, 269, 335 with C=0 groups, 335, 353 limitations, 260 with water and D2O, 57 Grignard reagent, 56 Ground state, 230... 

A limitation of the Grignard reaction lies in the sensitivity of the organomagnesium compounds to moisture and oxygen. Therefore, solvents should be carefully dried, and inert gas techniques are required. Small amounts of water in ethereal solvents sometimes are tolerable, however. 

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