37 votes
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What is the reason for the exceptional stability of the cyclopropylmethyl carbocation?

It is commonly said, that a cyclopropane fragment behaves somewhat like a double bond. It can conjugate, and pass mesomeric effect similar to a double bond, but the donor orbital is $\sigma_{\ce{C-C}}$...
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31 votes
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What is a non-classical carbocation?

Here is a picture of a "classical" carbocation, there is an electron deficient carbon bearing a positive charge. There are many examples of "non-classical" carbocations, but the 2-norbornyl ...
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31 votes
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Unusual Friedel–Crafts alkylation with pivaloyl chloride

You were on right path; tert-butyl carbocation is quite stable$\ldots$ so stable that the acylium cation, which normally reacts as an electrophile itself, instead decomposes via decarbonylation (loss ...
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30 votes

Is the t-butyl carbocation more stable than the benzyl carbocation?

I am using a very simplistic quantum chemical approach of the following isodesmic reaction: I have used Gaussian 16 Rev. A.03 and the DF-B97D3/def2-TZVPP level of theory. The summaries of the ...
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28 votes

Why are vinylic and arylic carbocations highly unstable?

Background S-orbitals are lower in energy than p-orbitals, therefore the more s-character in an orbital, the lower its energy and the more stable any electrons occupying the orbital. In other words, ...
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28 votes
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Why are tertiary carbocations the most reactive, if they're already stable?

This is an excellent question. Please correct me if I'm wrong, but I think this is what you are grasping at: First, it is true that tertiary carbocations are generally more stable than primary ...
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26 votes

Migratory aptitude in pinacol-pinacolone rearrangement

In the pinacol rearrangement, a 1,2-diol is treated with acid and rearranges to a carbonyl compound. Here is a reaction scheme showing a mechanism for the rearrangement. In this case the molecule is ...
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23 votes

Is the t-butyl carbocation more stable than the benzyl carbocation?

Is the trimethyl carbocation more stable than the benzylic carbocation? There are a number of approaches we can take to try and answer this question. We'll start by first comparing solvolysis rate ...
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20 votes
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Ring contraction in a carbocation due to ring strain and back bonding

The rearrangement has little to do with ring strain and a lot to do with resonance stabilization. The ring strain in a cyclobutane ring is ~26.3 kcal/mol, while the ring strain in a cyclopropane ring ...
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19 votes
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What exactly is "B-strain"?

B-strain Both B- and F-strains are the terms I found were originally used to describe steric hindrance in complexes [1, pp. 66-67]: The concept that steric hindrance in a monodentate ligand can ...
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18 votes
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Are carbocations that bad?

Are all mechanisms involving carbocations concerted? Many carbocations are stable, observable entities. Many large, delocalized carbocations like trityl perchlorate form very stable salts (reference ...
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17 votes
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Do vinyl cations adopt a classical or non-classical structure?

TL;DR: there have been many theoretical investigations of the relative energies of these two forms for the parent vinyl cation with more recent work indicating that the bridged form is slightly more ...
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17 votes
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Stability Comparison between the Tropylium and Tricyclopropylcarbinyl Carbocation

It seems likely that the tricyclopropylcarbinyl carbocation would be more stable than the tropylium carbocation for the reasons I'll outline below, but if you have a reference proving this point, it ...
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16 votes

Which carbocation is more stable, the ethyl- or 1-propyl-carbocation?

The ethyl carbocation ($\ce{C2H5+}$) has a different structure than what you might have anticipated. It is a non-classical ion with a bridging hydrogen. For a description of just what a non-classical ...
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16 votes

Why are tertiary carbocations the most reactive, if they're already stable?

Very interesting question. This also popped up in my head once. And this is how I convinced myself. The confusing term here is stability. Tertiary carbocations are stable by inductive effect and ...
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15 votes

Which of the following is the more stable carbocation?

Vinyl cations can also be generated by the protonation of acetylenes in strong acid. The vinyl cations are captured by a nucleophile to produce a stable alkene with the nucleophile attached to the ...
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15 votes
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Is the phenyl cation or ethynylium more stable?

Preamble As trivial as this question might seem, it is not. It should certainly never be asked in at an exam level lower than basic quantum chemistry. Any of the tempting explanation schemes will ...
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14 votes

Which of the following is the more stable carbocation?

Addendum 14-5-2017 Please read ron's answer! I went out on a limb writing this, and while there's probably no fault in my logic itself, it's worth remembering that chemistry is fundamentally an ...
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14 votes
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Why is cyclopentadiene more stable than its aromatic counterpart cyclopentadienyl anion?

This is a nice question because it forces you to step back and think rather than fall into the obvious trap. You are entirely correct that aromatic compounds are more stable than non aromatic ...
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13 votes
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Stability of carbocations: CF3+ vs CH3+

$\ce{CF3+}$ is more stable than $\ce{CH3+}$. The comparison between -I and +R effect is made while talking about activation or deactivation of halogens when substituted on benzene. Fluorine always ...
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13 votes
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Carbocation rearrangement involving three membered rings

Unfortunately you have opened the can of worms that is cyclopropylmethyl carbocations. It is known that the parent unsubstituted cyclopropylmethyl carbocation is non-classical, and leads to mixtures ...
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12 votes

Can hydride shift and methyl shift happen one after another in a carbocation?

Multiple rearrangements can happen if the resulting species is stabilized by it. In general hydride shift is more favourable over methyl shift because of its smaller size. The example proposed by ...
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12 votes

Are carbocations that bad?

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-...
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11 votes

What is the reason for the exceptional stability of the cyclopropylmethyl carbocation?

Cyclopropane is a highly strained molecule with a bond angle of $60^\circ$. The normal tetrahedral bond angle is $\pu{109^\circ{28}'}$, so we'd expect a ring strain of $49.28^\circ$! For a ...
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11 votes
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Migratory aptitude in pinacol-pinacolone rearrangement

According to my FIITJEE (a coaching institute in India) textbook, the relative order of migratory aptitude of groups in pinacol-pinacolone rearrangement is: $$\ce{H > Ph > Me_3C > MeCH_2 >...
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11 votes
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Ring expansion from a given cyclic carbocation

I think your friend is thinking of the cyclobutyl carbocation which does ring contract to the cycylopropyl carbinyl carbocation (and also equilibrates with the methallyl carbocation). However, just ...
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11 votes
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Is a sequence of two Wagner-Meerwein rearrangements possible?

There is no reason why a sequence of two Wagner-Meerwein rearrangements wouldn’t happen — assuming low concentrations and thus the absence of directly capturing nucleophiles. In fact, a professor of ...
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11 votes

Is the ethyl cation really more stable than benzylic and allylic carbocations?

I believe this to be a simple typo in the book. In the original image you'll see that in the surrounding schemes $<$ and $\to\text{Stability}$ are given. Only in this line the relations are ...
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11 votes
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What is the reason for the exceptional stability of tricyclopropyl-cyclopropenyl carbocation?

Questions like these always appear simple, when in fact they are not. First of all it has to be clear that stability is no absolute concept. In this case one could ask which of the cations is most ...
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10 votes

Is the t-butyl carbocation more stable than the benzyl carbocation?

Gas phase measurements give: \begin{align} \Delta_\mathrm{f} H^\circ (\ce{PhCH2+}) &= \pu{+219 kcal mol-1} \\ \Delta_\mathrm{f} H^\circ (\ce{t-C4H9+}) &= \pu{+169 kcal mol-1} \end{align} The ...
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