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AKA the Good, the Bad, and the Ugly.

Organometallics aside, the greatest advance since the days of Fieser and Fieser isn’t the development of new chemical transformations [1].  Rather, it’s the gradual replacement of hazardous or toxic reagents with more benign variants.  We do much the same work as before, but with greater substrate scope and less risk.

That said, it’s no surprise that water reactive workhorses like SOCl2/PCl3/POCl3/PCl5 have extensive write-ups in Fieser and Fieser.  We may no longer regularly run reactions in refluxing benzene (never mind the CCl4 recrystallizations), but with proper technique these are perfectly safe and highly effective chlorinating reagents.

PCl5 Synthesis - From Inorg. Syn. 1: 109 (1939).  The tank contains chlorine gas.

Inorg. Syn., 1, 99 (1939). A contains chlorine gas.

That figure up above is from a reference article, and it shows the lengths chemists used to go to for a little PCl5.  Chlorine gas from a lecture bottle is bubbled through sulfuric acid to remove any residual water, then reacted with PCl3 dripping down from an addition funnel.  Cl2 is no treat to play with, but it’s a common trend in F2 for even relatively benign reagents to require hazardous preparations [2].

Pertrifluoroacetic acid is a good example.  A powerful oxidizing agent with dozens of uses, CF3CO3H readily converts amines and oximes to nitro groups, installs phenols in mildy activated aromatic rings, and is an excellent Baeyer-Villiger reagent.  Unfortunately pertrifluoroacetic acid decomposes on standing, and preparation in situ requires 90% HOOH (ie. propellant grade).  Reactions using 30% peroxide and higher generally require the use of an explosion proof screen, and I’m not certain how the 90% solution could be safely stored in a lab filled with organic vapours.

Rocket fuel makes for stressful afternoon, but chemists occasionally get a little desperate.  There are limits though, and this last would probably force me to hang up my lab coat and take a week of unplanned vacation.

Oxygen difluoride, better behaved cousin of FOOF, is prepared by passing fluorine gas through a 2% solution of sodium hydroxide.  The resulting vapor must first be purified to remove the residual F2 and roughly full equivalent of byproduct O2 produced during the reaction.  This is done by cooling the mixture in a bath of liquid oxygen, then fractionally distilling out the desired gas [3].  Once pure, OF2 can be stored at room temperature in large glass bulbs, though it will react violently with silica and molecular sieves.  It is described as having a peculiar, foul odor.

OF<sub>2</sub> Generation.  From Inorg. Syn., 1, 109 (1939).

Inorg. Syn., 1, 109 (1939).

First water, then dry ice/acetone, then liquid oxygen.

First water, then dry ice/acetone, then liquid oxygen.  Oxygen difluoride collects in the third trap.

Once formed the gas is bubbled into trichlorofluoromethane (Freon 11, now banned by the Montreal protocol) at -78 degC, where it will oxidize alkenes and acetylenes to the corresponding mono- and di- alphafluoroketones.  Primary (aliphatic) amines are smoothly converted into the corresponding nitrosos (two equivalents of the amine are lost as the fluoride salt).  The whole setup from start to finish is an ugly beast, and I’m happy to say to those looking for a nitroso prep that there are better ways.

[1] Yes, that’s a pretty big aside. It’s been forty-six years.

[2] Given the plethora of hazardous preparations I was surprised at how cautious Fieser and Fieser are with diazomethane.  Five separate preparations are given, listed in order of preference.  Bis-(N-methyl-N-nitroso)terephthalamide is their preferred precursor, but this is 25 years before the first report of TMS-CN2.

[3] To quote the original authors,

Further purification can be accomplished by pumping
on the trap containing the liquid while it is still immersed
in liquid air.  A water pump is best, since the gas coming
off (principally oxygen at first) will eventually cause loud,
but not damaging, explosions if it comes in contact with the
oil of an oil-filled pump.
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