# Tag Info

28

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 $\sigma_{\ce{C-C}}$ instead of $\pi_{\ce{C=C}}$. Cyclopropane can be considered as a complex of a carbene and an alkene, where the carbene $\mathrm{p}$ orbital interacts with the ...

28

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 carbocation is among the best known. Labeling experiments have shown that the positive charge resides on more than one carbon in the 2-norbornyl ion. Early on, the ...

26

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 carbocations (and secondary carbocations) due to having more inductively donating alkyl groups. The hyperconjugative effect can also be invoked to explain the ...

26

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 of stable carbon monoxide molecule). The t-butyl carbocation isn't as stable as the acylium cation (which is stabilised by resonance), but the difference is ...

24

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 calculations are included below. On this level of theory the depicted reaction has an energy change of $\Delta G = \pu{- 37.1 kJ mol-1}.$ Therefore one could assume ...

23

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, electrons will be more stable (lower in energy) in an $\ce{sp}$ orbital than an $\ce{sp^2}$ orbital, and electrons in an $\ce{sp^2}$ orbital will be lower in ...

23

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 symmetric and methyl migration is the only reaction pathway available. Methyl migration can 1) help stabilize the developing carbocation center and 2) once ...

22

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 data to see which carbocation is more stable in solution, and then we can look at thermochemical data to see how the carbocation stabilities compare in the gas ...

19

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 is ~27.5 kcal.mol (reference). This isn't much of a difference, certainly not enough to drive a reaction one way or the other. The reason that cyclobutyl ...

19

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 affect the affinity of that ligand for a metal was first introduced by Brown and Bartholomay (1944) who suggested that co-ordination of a tertiary amine with ...

18

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 ...

17

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 stable (by about 1–3 kcal/mol)4,5. This prediction has received support from recent experimental work as well6. Taking such relatively small energy differences ...

15

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 hyper conjugation, and therefore have the tendency to sustain the positive charge on the carbon atom and stay like this for long. That is why we call it stable: it ...

14

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 carbocation is see this answer. The hallmark of a non-classical ion is 3 atoms with 2 electrons spread over them. This is called a 3-center 2-electron bond (...

14

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 double bond. Which of the following is a more stable carbocation? This route to vinyl cations has been well studied. As shown in the following diagram, ...

14

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 would be nice to add it to your question. Background The cyclopropyl group is similar to an olefinic double bond in that it is very effective at stabilizing ...

14

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 certainly fail. A bonding picture in the Lewis formalism is almost impossible; hybridisation descriptions in these compounds is extremely complex. As a general tip: ...

13

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 experimental science. A lot of chemistry is concerned with rationalising experimental data, and developing models with predictive power that we can apply to other ...

12

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 ...

12

Very interesting question… I would give two answers, at different levels: At undergrad level, I think the carbenium ion formed at the bridge head is less stabilized that “regular” ternary carbenium ions, because it cannot adopt the planar trigonal geometry which would be ideal for it. At higher level, this is much more complicated: the norboryl cation is an ...

12

$\ce{CF3+}$ 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 stabilises a carbocation to a large extent because of a very good overlap of $\ce{2p -~2p}$ orbitals.

12

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 compounds which are more stable than anti aromatic compounds assuming that all other things are equal. In this case, all other things are not equal because one of the ...

11

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 James was not known to me but its truly an amazing example. James: It’s possible for multiple hydride/alkyl shifts to occur. One amazing example is in ...

11

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 mine once said: The only secondary carbenium ion is the prop-2-yl ion; all others will rearrange to form tertiary carbenium ions. (He did explicitly exclude ...

11

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 persistent, in other words is least reactive towards nucleophiles. With the exception of the tert-butyl cation, one could also ask the question, which of the ...

10

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 as you thought, the cyclobutyl carbinyl carbocation does ring open to the cyclopentyl carbocation (ref_1, ref_2, ref_3). This rearrangement is driven by ...

10

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 of cyclopropylmethyl, cyclobutyl, and homoallyl derivatives. However, the presence of substitution can alter the electronic structure quite significantly. There ...

10

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): The lowering of $\ce{C_\beta–H}$ stretching frequencies in carbocations 1a–d and 2a–c induced by hyperconjugation was tested as a possible probe for ...

9

If this compound can undergo nucleophilic substitution, it can only be via SN1 fashion, as the reverse side attack in the anti-bonding $\ce{C-Br}$ orbital is blocked by the other bridgehead. The reason that is often given why bridgehead carbon cations are not stable is strain. Strain itself is a concept that is difficult to grasp and it more often depends ...

9

As already pointed out by previous contributors, 1-bromobicyclo[2.2.1]heptane will not undergo substitution via $S_{N}1$. A fully solvated and relaxed bridgehead cation is unlike to be formed. Whether a short-lived ion pair in a common solvent cage is possible... who knows. But fortunately, the title compound isn't as dead as it seems: I'm sure I've seen ...

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