Filemon - 19-7-2006 at 08:15
An enolate preparation needs low temperatures. Why? if it was made to bigger temperatures that it would happen?
Nicodem - 19-7-2006 at 09:32
Enolate anions add to the carbonyl groups so a self addition or condensation of the ketone itself is the only thing that happens when adding a strong
base to the ketone at room temperature.
At low enough temperatures most such reactions are extremelly slow except for the proton transfers which can still go on. Once all the ketone is
enolized and deprotonated it won't add to the carbonyl groups simply because there are no more carbonyl groups around. Thus you must use a base in at
least equimolar ratio and several pKa units more basic than the enol (LDA, NaH etc.) or else there will be some ketone present due to the low
equilibrium constant.
Nick F - 19-7-2006 at 11:21
Well...
Low temperatures and strong bases will favour the kinetic enolate, ie, deprotonation from the least hindered alpha proton. The kinetic product is the
one which for whatever reason forms fastest, and since a strong base is used here there can be no equilibrium which could allow the formation of the
more stable thermodynamically-favoured product. For example, treating butanone with LDA at -78*C will deprotonate the terminal alpha carbon, since
this has three enolisable protons compared to just two at the other alpha position, and so deprotonation at this site is faster.
Higher temperatures and weaker bases, which allow an equilibrium to be established, will favour the thermodynamic enolate equivalent, ie,
deprotonation at the most hindered possible site. For example, 2-methylcyclohexanone with trimethylsilyl chloride and DABCO in DMF at 100*C will
produce ~78% of the thermodynamically prefered trimethylsilyl enol ether from deprotonation at the tertiary carbon, which can be seperated from the
kinetic product by fractional distilation. Weaker bases, to produce the naked enolate, will cause self-condensation as mentioned above.
Enolate chemistry is actually quite rich in detail and very useful, have a read through it if you have a good organic chemistry textbook.
[Edited on 19-7-2006 by Nick F]
Filemon - 23-7-2006 at 15:51
t-BuONa is sufficiently strong?
sparkgap - 23-7-2006 at 18:15
Compare the pKa's and you can figure it out from there. 
sparky (~_~)
Filemon - 24-7-2006 at 02:45
Where I can find it? I have not found it in internet.
Nicodem - 24-7-2006 at 03:07
Try harder! It's there.