Hi SM, I have a quick question and I was hoping maybe I could leave on your doorstep tonight perhaps pick up the answer tomorrow... let the night
transmute my plea into newfound glee, so to speak. Can hydrofluoric acid or any form of fluoride be used to halogenate a basic allylbenzene such as
eugenol or others in the way that bromine and iodine can? What about chlorine? if not, why? THANK YOU!Chordate - 24-2-2011 at 01:48
Hypothetically, maybe. It should be noted that haloacids are usually pretty unreactive in EAS reactions, which is why people use other sources like
the pure halogen with a lewis acid catalyst. Flourine is fairly reactive and people do it without, but any sort of aromatic fluroination is going to
require long elevated reflux and thats something you just don't want to do with HF (well, at least I don't). The classic way I belive is to use
potassium fluoride, but the new and awesome way is to use a quaternary ammonium salt of fluorine, which acts both to give you fluorine and as a PTC to
help keep everything mixed well.Nicodem - 24-2-2011 at 11:12
Can hydrofluoric acid or any form of fluoride be used to halogenate a basic allylbenzene such as eugenol or others in the way that bromine and iodine
can? What about chlorine? if not, why? THANK YOU!
Please open referenceless threads, particularly if they start with questions too vague to be answerable only in the Beginnings section where I'm
moving this.
What is it that you ask? Bromine adds on the double bond of eugenol and then brominates the position ortho to the hydroxy group. You end up with
2-bromo-4-(2,3-dibromopropyl)-6-methoxyphenol. If there is less than 2 equivalents of Br2 available you probably get a mixture of the above,
4-(2,3-dibromopropyl)-2-methoxyphenol and unreacted eugenol. But anyway, I'm sure this is described in the old literature, because bromination with
Br2 was a common method to derivatize such liquid compounds.
Hydrofluoric acid or the usual fluorides are not electrophilic halogenating reagents, so obviously they can not react in any way similar to Cl2, Br2
or I2. Fluorine can not be used as it cleaves the C-C bonds (burns anything organic). Chlorine will give you the corresponding product,
2-chloro-4-(2,3-dichloropropyl)-6-methoxyphenol, if the chlorination is conducted in CH2Cl2 or other inert aprotic solvent, but the reaction might be
less selective than bromination (chlorinations of p-alkylphenols are often accompanied by the ipso chlorination at the para position). Electrophilic
additions of iodine on double bonds are usually ineffective unless the double bond is highly nucleophilic, so I don't know if it I2 would react well
with eugenol.
Edit:
Here are some references for the bromination and chlorination of evgenol with Br2 and Cl2:
J. Med. Chem., 1966, 9 , 618–620. DOI: 10.1021/jm00322a045
J. Chem. Eng. Data, 1982, 27, 209–210. DOI: 10.1021/je00028a033
Apparently, with proper choice of conditions it is possible to brominate eugenol to either 4-(2,3-dibromopropyl)-6-methoxyphenol,
2-bromo-4-(2,3-dibromopropyl)-6-methoxyphenol or 2,3-bromo-4-(2,3-dibromopropyl)-6-methoxyphenol. 4-(2,3-Dichloropropyl)-6-methoxyphenol can be
obtained, but the 2-chloro-4-(2,3-dichloropropyl)-6-methoxyphenol has not yet been described in the literature. None of the possible electrophilic
fluorination or iodination products is known in the literature.
