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I am planning to synthesize a grignard and I have a choice between the iodo or chloro aryl halide. The iodo is a solid and the chloro is borderline. For my previous reaction I melted the chloro and then tranfered it using a flushed sringe. As an alternative I could just weigh out the iodo and then pull vacuum on it a few times before running my reaction. Is there a better option between the two or they are just the same?

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The chloride can be a bit slow to react, particularly if the aryl halide is hindered. You might need a small amount of iodine to get the reaction started. (Maybe you ran into this already?)

Once you form the Grignard, they should be identical for most applications.

Personally, I would just go with iodide because the chloride is probably a bit less reactive, and frankly, weighing out substances that are almost solid is extremely annoying. Strongly prefer the actual solid.

The main reason I would elect the chloride over the iodide in this case is if the cost of the chloride were much lower, and even then only if the reactivity weren't seriously impeded by using the chloride.

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  • $\begingroup$ Also, I should point on that the air-free part doesn't matter in terms of the choice. You'll going to pull vacuum on the substrate, and if you really need to avoid oxygen/water vapor, you'd backfill with an nitrogen/argon and then transfer into your reaction vessel with a cannula. $\endgroup$ – Zhe Oct 13 '16 at 0:25
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While I haven’t done this with halocarbons to create Grignards, I have performed quite a large number of reactions where I either had a very highly viscous liquid or a solid.

If you have a choice, my advice is to go for the solid for these reasons:

  • If a compound is ‘borderline’, it’ll typically be rather viscous as a liquid just above its melting point.

  • Viscous liquids are a pain to deal with. Taking them into a syringe is hard.

  • Getting exact amounts of liquids is harder than getting exact amounts of solids. Typically, the scale on a syringe is too broad to adequately determine how much substance you have added. You would have to weigh the syringe with its contents. Adding or removing a few milligrams is much harder than just picking a few crystals/needles.

  • Solids, having relatively high melting points, are unlikely to evaporate notably when being subjected to HV conditions. (But this applies to viscous liquids, too.) This makes them easy to handle as you can simply add them to a flask and then evacuate.

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