Alkyrisk - 15-11-2009 at 04:10
Hello chemistry fans. I`m uninitiated in mysteries of chemistry but i want to start a pretty hard experiment and i don`t have all substances that i
need. The first one is 4-Methylbenzaldehyde. I know only one method to obtain this substance by Friedel-Crafts formylation of toluene under
Gattermann-Koch conditions. If someone else has tried and succeeded in another way he will be very helful for me. Pyrrole and propionic acid are other
substances that i need but i think it may be possible to buy them from somewhere. Here is the process that i know but i can`t realize:
Nicodem - 15-11-2009 at 05:10
It is not at all easy to obtain 4-methylbenzaldehyde by toluene formylation, because all the amateur friendly methods fail on such poorly nucleophilic
substrates. So you are left by either oxidation of p-xylene or varius multistep routes from toluene.
The oxidation of p-xylene can be performed with CrO3 in acetic anhydride. There might be other reagents as well but none particularly readily
available. Oxone would be the most OTC, but the oxidation is a free radical chain mechanism based process and thus non-upscalable and prone to go out
of control. With some care and thought it might be useful nevertheless. See also the methods discussed here and here (most are applicable to xylenes as well). You could use the cheap xylene mixture of regiosomers and then somehow separate out the
4-methylbenzaldehyde (fractionation and/or recrystallization of the bisulfite adduct, etc.).
Two step methods starting from toluene would be either chloro- or bromomethylation followed by either Kornblum oxidation or the Sommelet reaction. The
problem is in that chloromethylation gives essentially a 1:1 mixture of para- and ortho-methylbenzyl chloride and a fume hood is needed because during
the reaction small amounts of the carcinogenic bis(chloromethyl) ether form. This is no problem if you are careful, but beginners are usually not
careful. Bromomethylation would be more selective for the para-methylbenzyl bromide, so it would be a better option but you need HBr. Partial
separation might be possible via fractionation during the vacuum distillation if you have an efficient distillation column. Further separation of
regioisomers could be achieved during the distillation of the benzaldehyde products. The para-methylbenzyl halides can also be prepared via radical
halogenation of para-xylene (or xylenes mixture followed separation). This can be done by heating of NBS in a large excess of xylene in the presence
of benzoyl peroxide or on sunlight. Alternatively trichloroisocyanuric acid can be used instead NBS to give the chloride. Elemental chlorine with a
strong source of light or sunlight can also be used (see here).
The other possibility is to electrophilicaly iodinate toluene (which can be done highly selectively toward the para-iodotoluene), then make a Grignard
reagent and react it with dimethylformamide (this is done in one-pot). An ultrasound bath would come handy here to start the magnesium insertion.
A two step protocol for formylation with chloral hydrate might be aplicable to toluene. I'm not sure if toluene is still within the reaction scope (if
it is it must be on the edge), but essentially you alkylate an aromatic substrate with chloral in the presence of an acid to give Ar-CH(OH)CCl3 which
when treated with a base releases the benzaldehyde. Check if it works on toluene here, here and the references there.
Alkyrisk - 15-11-2009 at 05:50
Firstly, thanks a lot for quickly answer (14.10 - 15.10 exactly one hour) it was just what I needed because tomorrow at school i must start working at
this experiment, oh i forgot to say. I want to obtain finally, meso-tetraphenylporphyrin. Now i will read again your post (including "here`s") and
tomorrow i hope i can succeed in one of them and then to thank you glorious.
Ozonelabs - 15-11-2009 at 06:00
meso-tetratolylporphyrin no?
Otherwise for the meso-tetraphenylporphyrin you'd only need benzaldehyde as opposed to the 4-methyl derivative.
Alkyrisk - 15-11-2009 at 06:46
meso-tetratolylporphyrin??? YES that`s it. My mistake I copied and pasted the wrong name from wikipedia. Thanks for your vigilance. 