Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: jnouriel on August 12, 2011, 04:10:20 PM
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Hi,
I was reading about Enamine Formation from secondary amines and ketones in Francis Carey's _Organic Chemistry_. One question about the mechanism is whether a regioselectivity exists for where the double forms? Does the double bond for between the carbonyl carbon and the adjacent carbon that is most substituted? I thought it might be that because the transition state my have carbocation character, but I also thought that if the adjacent carbon is more substituted then there is possibly a steric hindrace blocking the base from deprotonating the carbon. So does is it the other option: does the double bond form between the carbonyl carbon and the adjacent carbon that is less substituted? I thought it might be this because the less substitution means that carbon has more hydrogens and therefore there is a higher probablity of the base interacting with a carbon that has 3 hydrogens then if it has 1 or 2. I also thought it might be the first option because the hydrogen's on a more substituted carbon are more acidic meaning the bond is weak and so lost easier, but I don't remember if that assumption about acidity is correct. Would appreciate information on this matter.
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In order for an enamine to react, the electron of the nitrogen must align with the electrons of the alkene. That means the pi-electrons will be perpendicular to the plane of the alkene. If you could make a model, compare the two possibilities. That will give you the correct answer.
Acetone is more acidic than 3-pentanone which is more acidic than diisopropyl ketone. Even though a methyl ketone is more acidic (kinetic), more substitution gives a more stable double bond (thermodynamic).