[habbababba]

Greetings!

Treatment of benzene with sodium and methanol or ethanol with liquid ammonia leads to the production of 1,4-cyclohexadiene, a nonconjugated diene.

According to the mechanism of the reaction, the first step gives rise to a benzene anion radical. This intermediate does not however contain conjugated double bonds but rather isolated double bonds. (Please see attached image).

This benzene anion radical can resonate to another form in which the double bonds are conjugated. (Please see attached image). However, the product in this case will be a conjugated cyclic diene, which is not what we observe.

Since the product is a nonconjugated cyclic diene, it follows that the nonconjugated benzene anion radical is the major (if not the only) contributor during resonance as an intermediate in the mechanism.

Is the isolated benzene anion radical more stable than its conjugated resonance form? If yes, why?

Thank you.

[orgopete]

This is a very good question, especially as presented. I can think of one plausible reason. What if that isn't the mechanism? As shown, the conjugated seems extremely likely. What if the first protonation does not occur? If so, a dianion intermediate would result. If protonation of this dianion were concerted, it might favor the non-conjugated diene.

By the way, I had searched for isolation of radical products from a Birch reduction of isopropylbenzene. Despite what seemed like a plausible radical formation, this does not seem to be a problem.

[habbababba]

Thanks for the input.

The diagram I've posted previously shows that resonance takes place after protonation (sorry about that). However, resonance between the conjugated and nonconjugated anion radicals can take place before protonation as shown in this attached mechanism.

If a dianion is to be an intermediate in the Birch reduction mechanism, it can also be formed via the conjugated anion radical too, which would still lead to the formation of a conjugated cyclodiene.

Now if for some reason resonance doesn't take place before protonation (and I don't see why it couldn't) and our dianion forms before protonation, then we would need a third step in which, as you mentioned, protonation should take place in a concerted fashion (otherwise our dianion would be able to resonate to a conjugated form after first protonation). Such third step would require 3 molecules, namely our dianion and 2 molecules of alcohol as sources of protons and accordingly the kinetics of this hypothetical mechanism would differ from experimental evidence.

[orgopete]

If a dianion is to be an intermediate in the Birch reduction mechanism, it can also be formed via the conjugated anion radical too, which would still lead to the formation of a conjugated cyclodiene.

Now if for some reason resonance doesn't take place before protonation (and I don't see why it couldn't) and our dianion forms before protonation, then we would need a third step in which, as you mentioned, protonation should take place in a concerted fashion (otherwise our dianion would be able to resonate to a conjugated form after first protonation). Such third step would require 3 molecules, namely our dianion and 2 molecules of alcohol as sources of protons and accordingly the kinetics of this hypothetical mechanism would differ from experimental evidence.

My thinking was if you draw the conjugated dianion, this would place the negative charges on adjacent atoms. I would argue this would be less favorable. If so, this could also suggest a precursor for the nonconjugated diene. Unless that is an intermediate, I am at a loss to suggest why protonation of a dienyl anion should give the nonconjugated product.

Obviously I made that suggestion simply as a rationalization to give the product found. Since this product is probably a kinetic product, the nonconjugated diene could be the kinetic product. That would be an alternate possibility to my dianion suggestion.

Kinetics of experimental evidence? Tell me more.

[habbababba]

My thinking was if you draw the conjugated dianion, this would place the negative charges on adjacent atoms. I would argue this would be less favorable. 

Good point.

Since this product is probably a kinetic product, the nonconjugated diene could be the kinetic product.

That's what I had in mind too and if it is indeed the kinetic product, does this necessarily mean that the conjugated form could be the thermodynamically favored product? If so, then wouldn't in this case the 2 products compete to form in different proportions rather than only one of the 2 products forming solely? In other words, wouldn't this make the Birch reduction another example of thermodynamic vs kinetic reaction control? If that's the case, then the nonconjugated cyclodiene, the kinetically favored product, can resonate under certain conditions to the more thermodynamically stable conjugated cyclodiene, something that I'm yet to find in the chemistry literature.

Kinetics of experimental evidence? Tell me more.

https://www.erowid.org/archive/rhodium/pdf/birch.reaction.kinetics.pdf

In this paper the author writes (page 6) that the protonation of benzene follows a rate law that is first order with respect to the alcohol (the proton source).
The hypothetical mechanism in which protonation is concerted would suggest a transition state involving 2 molecules of the alcohol. This makes the protonation step of second order with respect to the alcohol, contradictory to what experiments had shown so far.

[orgopete]

I want to take back the dianion notion. It would seem plausible that a kinetic protonation can give the nonconjugated diene. After all, Birch reduction of benzoic acid gives the nonconjugated product. Enones can be deconjugated by preparing the enolate followed by a kinetic protonation. If that is the case, then perhaps it should not be as surprising that protonation of a dienyl anion might also give a nonconjugated product.