156
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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 ...
- 28.3k
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 ...
- 43.8k
27
votes
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Why is sulfur hexafluoride more stable than selenium or tellurium hexafluoride?
$\ce{SF6}$ is extremely stable for purely steric reasons, because S is completely blocked by fluorine atoms from all directions, so the reactions starting with an attack on S that otherwise would ...
- 31.1k
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|} \...
- 84.2k
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 ...
- 84.2k
22
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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. ...
- 5,712
19
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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 ...
- 84.2k
19
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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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19
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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 ...
- 53.4k
18
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Is iron the most stable element in the periodic table?
Yes, $^{56}\ce{Fe}$ has the most stable nucleus, and $\ce{He}$ is the most chemically inert element. These are different and unrelated qualities, pretty much like physical fitness and intelligence in ...
- 31.1k
18
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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 ...
- 43.8k
17
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Why copper(I) is unstable in aqueous medium?
You are quite correct in that it appears at first sight that $\ce{Cu+}$ should be more stable than $\ce{Cu^2+}$, but in aqueous media it isn’t.
Stability in aqueous conditions depends on the ...
- 639
17
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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 ...
- 84.2k
17
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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 ...
- 71.1k
16
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 ...
- 84.2k
16
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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-...
- 5,593
16
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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 ...
- 71.1k
15
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Why are aromatic compounds so stable?
Let me start by saying that aromatic compounds are not exceedingly stable, they are just much more stable than antiaromatic compounds. However, they can still be hydrogenated exothermically to the ...
- 67.3k
15
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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 ...
- 5,375
15
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 ...
- 10.2k
14
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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 ...
- 71.1k
14
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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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14
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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 ...
- 15.3k
14
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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 ...
- 43.8k
13
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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 ...
- 146
13
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Why is the eclipsed conformer more stable than the bisected conformer in 1-butene?
In alkanes staggered conformers (all anti) are local minima, while eclipsed conformers represent transition states. The first is "stable", while the latter is not. The rotational barrier has been ...
- 43.8k
13
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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 ...
- 12k
13
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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 ...
- 25.2k
12
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Is iron the most stable element in the periodic table?
No, nickel-62 is the most stable on a binding energy per nucleon basis. Fe-58 is second and Fe-56 is third.
See Fewell, M. P., "The Atomic Nuclide with the Highest Mean Binding Energy", Am. J. Phys....
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12
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Which complexes are stable: ammine complexes or carbonyl complexes?
That quotation from Wikipedia is bad and should be reworded (I can’t think of good ways to edit it, though). Ligands in a spectrochemical series are not ordered by bonding strength or anything, they ...
- 67.3k
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