143 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 ...
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54 votes

Difference between thermodynamic and kinetic stability

To understand the difference between kinetic and thermodynamic stability, you first have to understand potential energy surfaces, and how they are related to the state of a system. A potential energy ...
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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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Relative stability of cis and trans cycloalkenes

Usually trans-olefins are more stable than their cis isomers for steric reasons, like you suggested. However in small and medium size rings this is not the case; here the cis-cycloalkene is more ...
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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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29 votes

Why is 2-methylpropene less in energy than its alkene counterparts?

There are two things we need to understand before we can answer the question. 1) More highly substituted double bonds are generally more stable than less substituted double bonds. This is because ...
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26 votes
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Why is formic acid a stronger acid than acetic acid?

We are discussing the following equilibrium We can make the acid a stronger acid by pushing the equilibrium to the right. To push the equilibrium to the right we can destabilize the starting acid ...
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25 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 ...
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24 votes
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Are geminal disubstituted alkenes more stable than their cis/trans isomers?

This is a very good question and if popular books give conflicting answers, then it must be reasoned out. Unfortunately, Paula Bruice has given the wrong answer while the other two books have given no ...
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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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22 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|} \...
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22 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. ...
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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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Comparing the stabilities of allylic, benzylic, and tertiary radicals

This is a tough question. I think it might even be unfair to ask such a question on a test in non-advanced classes. In advanced classes it could make an interesting topic of discussion, but I'm ...
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18 votes

What is the favoured geometry of chlorine trifluoride?

Short Answer The structure on the left is "preferred" because the structure on the right cannot exist. You cannot put 4 electrons in a p-orbital. Detailed Explanation In $\ce{ClF3}$ the central ...
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17 votes
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For a given element do heavier isotopes form stronger bonds than lighter isotopes?

Isovalent isotopes will have the same force constant. However the different masses of the isotope will affect the position of the vibrational state in its potential well. You can rationalise the ...
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17 votes
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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 ...
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17 votes
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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 ...
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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

Why are noble gases stable

Actually it is not necessary to dig deep into quantum mechanics. There are several reasons why noble gasses are stable (as gasses at room temperature). First of all, there is the obvious full valence ...
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16 votes
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By what mechanisms can molecules with the same empirical formula be so different?

What makes both of those molecules so stable? This is a fundamental question. Let's start by looking at the following diagram; a chemist would call it a "potential energy diagram." The y-axis shows ...
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16 votes

Which is more stable? A phenyl carbanion or a disubstituted vinylic carbanion?

Can the lone pair on benzene carbanion participate in resonance? As Jori has pointed out, the carbanion lone pair is orthogonal to the pi system of the aromatic ring. Therefore it cannot participate ...
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16 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 ...
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16 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 ...
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15 votes
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The effect of hyperconjugation on the stability of alkenes with MO theory

Since you are familiar with how carbocations are stabilized via hyperconjugation I will keep this brief: With alkenes, it really is the same principle at work. Instead of the empty p orbital, you have ...
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15 votes
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Why is picric acid more explosive than TNT?

You're trying to define the stronger explosive by comparing the reagents, but in this case comparing the products is far more important. Trinitrotoluene is a rather oxygen-poor explosive, to the ...
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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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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 ...
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15 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 ...
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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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