Are carbocations that bad?

Question

My professor has an anti-fetish for carbocations. In any mechanism that we draw with a carbocation, the maximum earnable credit is automatically halved. In any case, he draws all mechanisms involving carbocations as concerted reactions; he never wants us to draw an carbocation by itself. If a carbocation could be drawn (as in the case of the formation of an acetal; specifically the step in which water leaves) then we must have electrons flowing from somewhere else to give the carbon an octet.

Now, is this valid? Are all mechanisms involving carbocations concerted? Excluding the obvious case of SN1; I'm pretty sure he'd accept that carbocation (and he hasn't touched on SN1 ever).

Answer 1

Are all mechanisms involving carbocations concerted?

Many carbocations are stable, observable entities. Many large, delocalized carbocations like trityl perchlorate form very stable salts (reference see p. 196), other carbocations are easily observed by nmr.

If free carbocations didn't exist as discrete intermediates how could you explain the stereochemical results from an SN1 reaction where a chiral starting material generates a planar, long-lived carbocation that can be attacked from both sides to produce an achiral product? Any type of concerted process would be expected to proceed with inversion of configuration (SN2).

(image source)

Answer 2

The mention of acetal chemistry leads me to have some sympathy for your instructor, although this person is taking it to the extreme with the grading policy. Below are two mechanisms for a trans-acetalization reaction.

I am guessing that mechanism A is one that your instructor would give half credit (since it goes through a carbocation) and mechanism B is one that your instructor would give full credit. I prefer mechanism B, and I think it's fine for your instructor to agree. However, these are identical mechanisms. How is that? Because the third structure in each mechanism are the same thing, just different resonance structures.

So if they are identical mechanisms, why do I prefer mechanism B? Because that resonance structure explains something about the reaction, particularly in relation to a reaction that you have learned before, the SN1 reaction. In the typical SN1 reaction, formation of a carbocation is slow. However in the trans-acetalization reaction here (really just a variant of the SN1), that carbocation formation is much faster. Why? Because the carbocation formed is stabilized by the ability of the adjacent oxygen to donate electrons, giving an intermediate that has a resonance structure where all atoms obey the octet rule. I think that's an exceptionally important consideration, and one that helped me understand and recognize reactions of carbonyls.

I do think that it would be unfair to give different levels of credit for mechanisms A and B. However, I do appreciate what your instructor is trying to get across, although s/he may not be doing a good job at it.

Answer 3

Your professor does have a case in that many carbocations can be thought of as resonance stabilised, as was depicted by jerepierre in his answer. I once had a professor who would prefer any resonance structure that has complete octets on all p-group elements over any resonance structure that doesn't have that (although he didn't go as far as downgrading for sextet cations).

However, there are also certainly reactions that cannot be thought of as concerted, and also cannot be resonance-stabilised to give an all octet structure: See for example the electrophilic aromatic substitution as depicted by this mechanism (sheepishly stealing the image from Wikipedia):

The mechanism as shown here is for unsubstituted benzene. I did it in an organic practical course with nitrobenzene. There is no way in which nitrobenzene will stabilise a carbocation (indeed, the nitro group is meta directing due to that). And the mechanism can — to the best of my knowledge — cannot be thought of as concerted.