Starting recalcitrant Grignard reactions - Journal of Chemical

Starting Recalcitrant Grignard Reactions Grignard reactions have been used in the sophomore organic chemistry laboratory for many years because they serve well as representatives of synthetically useful organometallic chemistry but do not involve the use of potentially dangerous or toxic metals. They also offer the opportunity of introducing techniques and skills necessary for carrying out reactions under anhydrous conditions. That organometdlie chemistry at the hench is still somewhat of an art is annually demonstrated by the observation that in a laboratory of 24 students all using the same reagents and solvents and identical glassware there are almost always one or more students who cannot seem to get their Grignard reactions started. Boudjouk and co-workers have reported that a number of heterogeneous reactions can be promoted with ultrasonic treatment1. Recently Sprich and Lewandos reported that preparation of Grignard reagents even in the presence of trace amounts of water or alcohol were d r a m a t i d y enhanced by low intensity sonificatian2. We have found in our sophomore organic laboratory that recalcitrant Grignards ean be started typically within 5 min by placing the reaction flask in a sonic bath. In order to investigate this technique in our laboratory in a controlled manner, two identical reaction mixtures were made. One was placed in an ultrasonic bath (Branson Model 220) while the second served as aeontrol. The time required to initiate a self-sustaining3 reaction was noted. The data in the table shows that ultrasonic treatment indeed does substantially reduce the time required to generate a self-sustaining reaction between an alkyl or aryl halide with magnesium. Ratio of InRlatlon Times lsonlc SamoleIControl Samolel Run

1

Chlorocy~lohexan& l%romonaphthalentr lOdine was ad-

0.47 0.21

2 0.47 0.21

3 0.42 0.23

4 0.45 0.17