Mister Junk Pile - 9-12-2010 at 19:43
Could I ask a favor of you guys?
I need mechanisms for the following reactions. I have searched and have been unable to find them. I swear! (and we haven't gone over them in class,
they simply appeared in our final exam review; final is tomorrow):
(1) 2 molecules of alpha-methylstyrene with H+ to yield 1,1,3-trimethyl-3-phenyl-2,3-dihydro-1H-indene. I named that by drawing the
structure in ChemBioDraw.
(2) Benzamide reacted with aqueous Br2 and OH- to give aniline and carbonate. I thought this was "Hoffman Degradation" but I can't find anything by
that name.
(3) Chlorobenzene with an asterisk at the #2 carbon reacting with NH2- to produce aniline with asterisk at #1 carbon and aniline with asterisk at #2
carbon.
(4) 2,5-dichloronitrobenzene reacting with methoxide. Replaces #2 chlorine with methoxide and gives chloride.
Thanks so much in advance. We did not go over any of these mechanisms in class. What the hell? You know?
Magpie - 9-12-2010 at 19:53
Did you check Wiki?
for #2: http://en.wikipedia.org/wiki/Hofmann_rearrangement
UnintentionalChaos - 9-12-2010 at 20:02
For #1, protonate the alkene in a way to make the most stable carbocation. Now, have the carbocation be attacked by a nucleophile, forming the most
stable carbocation you can. You need to close a ring at this point, so you can probably guess the next nucleophile. Finish up by deprotonating the
structure with the counterion to the H+
You spelled #2 wrong, that's why. http://en.wikipedia.org/wiki/Hofmann_rearrangement Google will turn up more detailed mechanisms.
For #4, look up SNar mechanism.
For #3, the asterisk just means they are the same ring carbon in all the pictures. Other times, they may mean isotope labelled carbons.. SNar will
help with one of the two results.
Mister Junk Pile - 9-12-2010 at 21:10
Thank you very much. I have them now.
DJF90 - 9-12-2010 at 22:05
#3 is actually not an SnAr (theres no EWG o/p- to the chloro), but actually one of the classical pieves of evidence for the Benzyne
mechanism.
Mister Junk Pile - 10-12-2010 at 00:27
I noticed that while I was looking it up. I memorized benzyne mechanism for the chlorobenzene to aniline and the SnAr mechanism for the other.
None of these mechanisms are very clear or easy to understand, however. Especially the Hofmann Rearrangement. It's just weird. Like it occurs at
random and does things contrary to what a beginner would expect.
DJF90 - 10-12-2010 at 02:48
The Hofmann rearrangement is nitrene chemistry, and this may explain why it is a little difficult for the beginner to understand. Ionic chemistry is
pretty straightforward, but carbenes/nitrenes and also radicals have their own sets of rules that can make them seem as if they're "misbehaving" most
of the time!
vulture - 10-12-2010 at 11:36
Nitrene pathways are not obvious, but they are made possible by expelling N2 gas, which is energetically very favourable. So checking if you can get
rid of N2 is a good way to spot them. Azides are very good candidates.