notmydayjob - 12-9-2011 at 09:35
I am having difficulty understanding a reaction about half way down on page 76 of the following PDF:
http://mmagniez.free.fr/chimie/parfums/ChemTechFlav.pdf
It states: "Isopropylbenzene (135) undergoes a Friedel–Crafts
reaction with a protected form of methacrolein, the diacetate, to form an enol acetate (136) which is readily hydrolysed to the aldehyde [19]."
My questions are:
1. How does hydrolysis yield an aldehyde? I thought the result would be an alcohol.
2. Why would they use methacrolein instead of 2-methylpropanal which is much cheaper? I don't see how the double bond to carbon would affect the
result.
Thanks in advance for any help!
[Edited on 12-9-2011 by notmydayjob]
[Edited on 12-9-2011 by notmydayjob]
Chordate - 12-9-2011 at 11:48
those double bonds make all the difference in the world. It gives a conjugated pi system that allows electrons from the terminal carbon of the alkene
to migrate to the oxygen atom.
Go read this:
http://chemistry2.csudh.edu/rpendarvis/a-carbaldol.html
especially the portion on alpha-beta unsaturated carbonyls。
As for hydrolysis yielding the aldehyde，The reason it yields the aldehyde is the double bond is there. enols spontaneously tautomerize to
aldehydes and ketones: typically the carbonyl form is favored.
notmydayjob - 13-9-2011 at 20:34
Chordate,
Thank you very much for pointing me in that direction and describing how the enol tautomerizes to the aldehyde! Did quite a bit of reading and I can
see now why that is necessary.
One more question, I hope you don't mind! And I'm not sure if it is easy to tell from the very terse description of the reaction that they provide,
but I can't seem to figure out exactly how they are performing the Friedel Crafts acylation of the ring. I don't think they are starting with a
halogenation of the ring, and I don't think they are using an acid anhydride to form a ketone on the ring (as then they would have a ketone, and I
certainly don't see one there).
I think they are using a lewis acid to simultaneously remove the acetoxy groups from the acylal such that acetic acid is formed when a proton is
removed from the benzene ring to form acetic acid. Then somehow the other acetoxy group migrates? Unless there is some kind of
1,3-diacetoxy-2-methyl-propene isomer of methacrolein diacetate (actually not sure that would be the right name for the isomer).
In any case, any help appreciated as I am totally confused about the Friedel crafts mechanism that is going on in this case too.