[Kobsessed]

So I have p-tert-butylcyclohexanone and benzaldehyde. How would I connect these two structures together to make an Aldol condensation? Does it have a name? I'm not too good with these reactions and we never did one with rings involved so I'm really lost. Help would be really appreciated!

[jpg28]

First, for an aldol reaction to take place, you should generate a carbanion. In this case, only the 4-tert-butylcyclohexanone has 4 acidic hydrogen atoms that are located at either of the two alpha (or the hydrogens attached to the carbons next to the carbonyl group) positions. This then adds to your partially positive carbonyl carbon in the benzaldehyde molecule, and then undergoes elimination of water to generate and alpha-beta unsaturated ketone.

Attached herein is the mechanism. Have a nice day!

[Juto]

Hey I'm new to these forums, but I've got a question concerning this reaction. So I was studying for the DAT and I came across a problem similar to the one shown above. My question is why does the carbon anion not occur at the aldehyde? Also wouldn't heat be necessary for the E1 elimination to generate the alkene double bond?

I have a DAT review book and one of the practice problems shows an aldol reaction that is reacting with NaOH and H₃O ions and it states the answer as being the alpah beta unsaturated ketone but would this be correct if they did not show  :delta: in the reagents?

Thanks for help.

[jpg28]

Hi Juto! The carbon anion (carbanion) can also occur in the aldehyde, here's a question for you: why do you think the carbanion did not occur on the aldehyde in this example (benzaldehyde). If you need more hints, maybe consult a pKa table in your textbook. It has something to do with the acidic alpha-hydrogen atom (the hydrogen attached to the carbon right next to your carbonyl [C=O] group) on your molecule.

You were also correct in saying that most elimination reactions occur with the addition of heat, but some solutions are sufficiently basic for the elimination process to occur.

[KritikalMass]

jpg do you use chemdraw? That rxn mechanism you drew is awesome!

[jpg28]

Hi KritikalMass! I used ChemSketch (from ACD Labs). It's a freeware that can be downloaded from the web. Thanks for the compliment!

[KritikalMass]

Hi KritikalMass! I used ChemSketch (from ACD Labs). It's a freeware that can be downloaded from the web. Thanks for the compliment!

Hey no problem at all, thanks for the tip and keep up the great work 

[Jay]

Ok, just a few minor points.

The carb-anion or enolate as it is more commonly referred to can happen to carbonyl compounds, which have alfa-protons. Your aldehyde does not have these protons, so the only enolisable compound is ketone.

Also, technically the drawing is not correct. The elimination is not an E1 reaction, but an E1cb reaction. Basically the proton in the elimination step is taken away and the negative charge is conjugated onto the ketone, so hence you do not need heat and hence OH- (which normally is a really bad leaving group) can be a leaving group in this reaction. http://www.chemtube3d.com/model/elimination/e1cb/3-hydroxypropanal/acrolein.JPG

Hope this helps. xx

[jpg28]

thanks for pointing that out.