Question

Michael addition (ORGANIC CHEMISTRY)
We have 2-methylcyclohexanone. Which alpha carbon (or carbon right next to the carbonyl) is going to be deprotonated by a strong base? The one carrying the methyl or the one carying H2.
According to my textbook, addition occurs on the carbon carrying the methyl but it doesn't make sense since methyl is an electron donor and will destabilize a negative charge from deprotonation. Help please?

Answer 1

i think the carbon carrying the hydrogen because they are more stable since there are 2 carbons releasing electrons to atoms attached to it

Answer 2

well the right answer is the carbon carrying the methyl

Answer 3

it depends on the temperature the reaction mixture is at and what base you use. There are 2 results in the formation of enolates, the kinetic product and the thermodynamic product.

The kinetic product is achieved through using an hindered base at a low temperature - for your example it's at the α-carbon with 2 hydrogens that would be deprotonated. Eg. using LDA or nBuLi at -78 C. The thermodynamic product is achieved through a high temp (even room temp) and an unhindered base - the methylated α-carbon in your example.

The difference arises from the reversibility of the reaction since at a high temp there is enough kinetic energy, the most substituted carbon will end up being part of the enolate. Likewise, if you lower the temperature substantially, you wont see this effect.

Answer 4

i was thinking about carbocations and SN1
so it's not about this? @aaronq

Answer 5

is it like free radical substitution?

Answer 6

hm no, there arent any radicals nor carbocations. When the base deprotonates the carbon it only takes the proton and leaves the 2 electrons forming the bond - a carbanion.

More importantly, when it happens beside a carbonyl, you get resonance up on to the oxygen.