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31 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 ...
Martin - マーチン's user avatar
25 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 ...
ron's user avatar
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20 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 ...
ron's user avatar
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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 ...
andselisk's user avatar
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18 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 ...
Martin - マーチン's user avatar
17 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 ...
Archer's user avatar
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16 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 ...
orthocresol's user avatar
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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 ...
bon's user avatar
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14 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 ...
Martin - マーチン's user avatar
12 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 ...
user55119's user avatar
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11 votes

Relative stabilities of tropylium and cyclopropenyl carbocations

Both ions are indeed strongly stabilized by aromatic coupling, and salts containing both ions are known. But, of course, aromaticity is not the only thing that determines stability. Tropylium ion, ...
Oscar Lanzi's user avatar
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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 ...
Jan's user avatar
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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 ...
Martin - マーチン's user avatar
11 votes

Is tropylium cyclopentadienide possible?

While I can't give a definitive disproof, my intuition is that an unsubstituted cyclopentadienide anion would be far too nucleophilic and react with the tropylium cation at non-cryogenic temperatures (...
Nicolau Saker Neto's user avatar
10 votes

Why is the acylium ion stable?

The acylium ion is stabilized by the positive mesomeric effect of the lone pair of electrons on the oxygen atom onto the empty p-orbital of the $\ce{C+}$. This helps achieve octet completion of the ...
Gaurang Tandon's user avatar
10 votes
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Are carbocations necessarily sp2 hybridized and trigonal planar?

I actually have a (or many) big issue(s) with the quote: The central C-atom is in an sp2 hybridized state, for which the carbocations have planar geometry. The p$z$-AO remains empty. The authors ...
Martin - マーチン's user avatar
10 votes

Stability of α-chlorocarbocations

As Oscar Lanzi suggested, both $+M$ and $-I$ applies here, but $\ce{Cl}$ stabilizes carbocation, meaning $+M$ is more effective than $-I$. This fact was confirmed by this peer-reviewed paper (Ref.1): ...
Mathew Mahindaratne's user avatar
9 votes
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Which of these carbocations are more stable?

As we know (to some extent), resonance in a molecule occurs due to sideways overlapping of p-π orbitals. Now in the molecule B, there is high steric hinderance near the carbocation due to which the ...
Vaibhav Dixit's user avatar
9 votes

Carbocation rearrangement involving three membered rings

Disclaimer: This is a very simplistic view of the cyclopropyl methyl cation and its reaction given above with $\ce{Br-}$, and is completely wrong. In real conditions, there are a lot of complexities ...
Gaurang Tandon's user avatar
8 votes
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Why is a bridgehead atom bearing a negative or a positive charge unstable?

Since the answer by Aditya Dev has already dealt with the carbocation part, I'll write about the carbanion one. The OP states that: Why is a bridgehead atom bearing a negative or a positive charge ...
Gaurang Tandon's user avatar
8 votes
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Cyclic carbocation rearrangement

1,3- and 1,4- shifts don't really happen. It's just too far away. If you have a double bond at 4 or 5 carbons away, maybe you can do a ring closure or form a non-classical carbocation. The cation you'...
Zhe's user avatar
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8 votes
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Alternative mechanism for carbocation rearrangements

Scheme 1: Orbital model of a Wagner-Meerwein rearrangement. You may think you are proposing something new, but you aren’t. While these Wagner-Meerwein rearrangements are typically referred to as ‘...
Jan's user avatar
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8 votes
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Ring expansion and carbocation formation

To understand which rearrangement pathway is preferred, we must consider the relative stabilities of the intermediates. The rearrangement that puts the carbocation one carbon away from the the oxygen ...
ringo's user avatar
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8 votes

Are carbocations necessarily sp2 hybridized and trigonal planar?

That notion is far from true. There are many examples of carbocations where, through the use of delocalized bonds, carbon can be bonded to five or more atoms. See, for example, https://en.m....
Oscar Lanzi's user avatar
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8 votes
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Can a six member ring expand to achieve octet completion to stabilize a carbocation?

The answer to your question, "can a six member ring expand to achieve octet completion to stabilize a carbocation?" is yes. The most specific and relevant example for that rearrangement is Büchner–...
Mathew Mahindaratne's user avatar
8 votes

8a-methyl-1,2,3,4,4a,8a-hexahydronaphthalen-4a-ylium carbocation rearrangement

While there are examples of 1,2-methide shifts, this question has an uncanny resemblance to the dienone-phenol rearrangement whose mechanism was first elucidated by Woodward and Singh in 1950. Dienone ...
user55119's user avatar
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8 votes
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Comparison of carbocation stability in bicyclo compounds

I think the third option as greatest number of resonance structures can be alluded to it. The following may suffice: In the first structure, the carbocation is isolated except for the presence of a ...
Solid - NMR's user avatar
8 votes
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Why do electron-rich aryl groups have greater migratory aptitudes?

The answer varies slightly depending on whether the migration is one-step or two-step via an intermediate. In either case, the respective transition state or intermediate looks like this: You can see ...
Zhe's user avatar
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7 votes

Carbene mesomeric structure of vinyl carbocation?

The alternative answer to what @levineds said is that you're asking about a carbene. Carbenes are actually reasonably common in chemistry though we generally have them coordinated to metals for ...
Zhe's user avatar
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7 votes

Carbocation formation in electrophilic addition of hydrogen halides to alkenes

The rate determining step is carbocation formation. After which, depending on conditions, the trapping of the cation may or may not be slow enough for [1,2]-shifts. The [1,2]-shifts are probably ...
Zhe's user avatar
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