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157 votes
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Why doesn't H₄O²⁺ exist?

I myself was always confused why $\ce{H3O^+}$ is so well-known and yet almost nobody talks of $\ce{H4O^2+}$. I mean, $\ce{H3O^+}$ still has a lone pair, right? Why can't another proton just latch onto ...
Nicolau Saker Neto's user avatar
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
26 votes

Which molecule is more acidic: CHF3 or CHBr3?

The following table contains some relevant data, the $\mathrm{p}K_\mathrm{a}$'s of the various haloforms along with the Pauling electronegativity of the corresponding halogen. \begin{array}{|c|c|c|} \...
ron's user avatar
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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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23 votes
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Why is cyclooctateraene shaped like a "tub" rather than a "chair"?

It's all about the 3D structure of double bonds. If we look at the tub form we see that all dihedral C-C=C-C angles are 0°. the C=C-C angles are 125°, also pretty close to the optimal value. ...
DSVA'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
  • 85k
20 votes

Why doesn't H₄O²⁺ exist?

In chemistry ask "why" only after you ask "if". Given a sufficiently strong superacidic medium, $\ce{H3O^+}$ can be protonated to $\ce{H4O^{2+}}$. Evidence for this reaction, by ...
Oscar Lanzi'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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19 votes
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Thermodynamic stability of meta-xylene over ortho- and para-isomers

The thermodynamic stability of m-xylene over o-xylene or p-xylene can be deduced by considering the hyperconjugative effect. More specifically, resonance forms with a positive charge on methyl-...
Soumik Das's user avatar
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18 votes
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Why are 7-membered rings less likely to form than 5- and 6- membered rings?

There are two factors that need to be considered to answer your question - entropy and enthalpy. Entropy favors the formation of smaller rings. If we consider a hydrocarbon chain, the number of ...
ron's user avatar
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18 votes
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Why are bromine oxyanions uncommon?

I can only really speak for bromine(VII). Bromine(V) is pretty common and I'm not entirely sure what's the deal with Br(I) readily disproportionating to Br(V) + Br(-1). (The tendency of Br(I) to ...
orthocresol'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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17 votes
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Why can't NH5 form?

I have pursued such questions regarding the hypervalency of particular molecules before and thus, I would like to provide some insights to the matter. A common superficial answer that one can ...
Tan Yong Boon's user avatar
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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15 votes
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Why is cis-1,2-dichloroethene more stable than trans-1,2-dichloroethene?

Here's a pre-MOT rationalisation of the cis form being more stable than the trans form for 1,2-dihaloethylenes: The lone pair of chlorine atoms is involved in resonance with the double bond, as it ...
Archer's user avatar
  • 5,491
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
13 votes

Why doesn't H₄O²⁺ exist?

$\ce{H4O^{2+}}$ would be much smaller than $\ce{SO4^2-}$. The charge density is too high much higher. The oxygen in water already becomes electron-deficient when it forms $\ce{H3O+}$. In your (not ...
Karl's user avatar
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13 votes

Does germanium(IV) chloride have to be handled under inert atmosphere?

At times like this, you always refer to the material safety data sheet (MSDS). Have a look: Upon contact with water or moist air this material reacts to release corrosive and toxic hydrogen chloride ...
Nilay Ghosh's user avatar
  • 25.9k
12 votes

Why is cyclopropane planar?

Why is cyclopropane planar? This is like asking why $\ce{HCl}$ is linear–just as two points define a line, three points define a plane. There isn't any way to orient the three methylene groups in ...
ringo's user avatar
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12 votes
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If the ferric ion is more stable than the ferrous ion, then why is it readily reduced to the latter?

As already pointed out, the notion of stability needs to be defined much more accurately. The fact that the standard reduction potential is positive has no direct bearing on the gas-phase stabilities ...
orthocresol's user avatar
  • 71.3k
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
  • 16.8k
12 votes
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Long-term storage of Ascorbic Acid mixed with Sodium Bicarbonate

The reason that your solid mixture is clumping and hardening inside your airtight container is that the reaction between the two compounds is producing water as follows: $$\ce{C6H8O6 + NaHCO3 <=&...
airhuff's user avatar
  • 17.5k
12 votes

Can a molecule contain a linear series of conjugated pi bonds twisted through 180 degrees?

The material you imagine has been a target for Natta, best known together with Ziegler for they work on isotactic polymers. Natta was interested in what you describe - polyacetylene - as the ...
Alchimista's user avatar
  • 3,636
11 votes

Comparing the stabilities of allylic, benzylic, and tertiary radicals

Imagine your favorite activities are playing Video games (Most Favourite), and other ones are reading chemistry, spending time in a chemical lab. (these two are activities are those activities in ...
Aditya Shrivastava's user avatar
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
  • 68.2k
11 votes

Why is the cis isomer of 1,3-dimethylcyclobutane more stable than the trans isomer?

The four carbon atoms in cyclobutane are not coplanar, as that would lead to a large degree of unfavourable eclipsing interactions. So, it is not a flat square; it adopts a puckered conformation. One ...
getafix's user avatar
  • 8,495
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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Why is a C–D bond stronger than a C–H bond?

Physics is better able to answer "how" questions than "why" questions, but here goes. The quantum mechanical description of the C–D system versus the C–H system gives the former a lower zero-point ...
alphonse's user avatar
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