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13 votes
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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 ...
Martin - マーチン's user avatar
11 votes
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Gauche effect and bond length

Actually, the more the polarization of a bond, the stronger it is, since both the atoms are drawn close together due to electrostatic attraction, decreasing the bond length and increasing the bond ...
FreezingFire'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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9 votes
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What is the directive influence of the tert-butyl group in electrophilic aromatic substitution?

What you've drawn in your bottom figure is an example of carbon-carbon hyperconjugation. The t-butylbenzene and the charge separated structure drawn to its right are resonance structures (so you ...
ron's user avatar
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8 votes

Why are tertiary carbocations so stable?

What is immediately obvious is that going from a primary to a secondary cation only produces a slight increase in rate, but the change from secondary to tertiary produces an enormous change. Uh-Oh! ...
ron's user avatar
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8 votes
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How to compare stability between 5‐methylcyclohexa‐1,3‐diene and 3‐methylenecyclohexene?

As far as I have read, in general, exocyclic double bonds (which are outside the ring) are stable for 3 or 4 numbered rings while endocyclic double bonds (which are inside the ring) are stable in 5 or ...
V.G's user avatar
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7 votes

Negative hyperconjugation

$\ce{CF3OH}$ is more acidic because of the inductive effect of the $\ce{-CF3}$ group: Fluorine is very electronegative and therefore it pulls the electron-density toward itself weakening the $\ce{O-H}$...
C.X.F.'s user avatar
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7 votes
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Hyperconjugation in bridged carbon systems

Hyperconjugation is indeed very difficult to practically in cases involving bridgehead protons. The underlying reason is essentially the same that also explains why these protons cannot be enolised if ...
Jan's user avatar
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6 votes
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Hyperconjugation in cresol

Above are the resonance forms of m-cresol after deprotonation. As you can see, the negative charge resonates around the aryl ring, but never resides on the carbon directly bonded to the methyl group ...
Ava's user avatar
  • 141
6 votes

Why is 1,2,3,4,5,8-hexahydronaphthalene more stable than 1,4,4a,5,8,8a-hexahydronaphthalene?

Disclaimer: This post might be hard to read for some people. Some may even consider this a rant. You have been warned. This answer will question the validity of the used models, as well as the basis ...
Martin - マーチン's user avatar
5 votes

Why does delocalization lead to stability?

Electrons "want" two thing: be close to a nucleus and have the freedom to move. Also, being fermions, they are "forbidden" by the Pauli exclusion principle to have exactly the same state (e.g. same ...
Karsten's user avatar
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5 votes

Hyperconjugation in ethane conformers

I am not entirely sure what purpose the lower sigma bonding orbital in the MO-diagram serves, because an ethane molecular orbital would, as you have surmised correctly, have six degenerate $\sigma_{\...
Sagnik's user avatar
  • 211
4 votes

Gauche effect and bond length

Many of these cases hinge on some weird effect. Evidently the gauche form of 1,2-Difluoroethane is more stable than the anti form which is sort of weird. This wikipedia article on the Gauche Effect ...
MaxW's user avatar
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4 votes

Stabilisation of carbocations through hyperconjugation

Hyperconjugation Hyperconjugation, as you point out, is able to stabilise carbocation intermediates, IUPAC define this as: the interaction is ... between σ-bonds and an unfilled or partially ...
NotEvans.'s user avatar
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3 votes
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Hyperconjugation and the stability of alkenes

Ordinarily without the contribution of the π* orbital, we would expect the interaction of two filled orbitals to be destabilising, in the sense that $E_2 > E_1$ (as drawn in the diagram). This is ...
orthocresol's user avatar
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3 votes

The delocalised nature of hyperconjugation

I'll try to answer your question through the exemples you give us. Where are the electrons ? If you look at the hybridized orbitals making the C-H and the C-C bonds, you can see that the more the bond ...
Gwendal Grelier's user avatar
3 votes
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Is there a simple way to predict the molecular pi orbitals in conjugated pi systems?

This should work for any even number of conjugated carbons (and with slight modification, for cations/radicals/anions of uneven numbers of carbons). You can follow a simple procedure to generate all ...
Tyberius's user avatar
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3 votes

Why does delocalization lead to stability?

If you think of an electron as a de Broglie wave, and confine it to a box, you can show that its energy $E$ is inversely proportional to the squared inverse of the length $l$ of one side of the box, ...
Buck Thorn's user avatar
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3 votes

Which of the following show hyperconjugation

I guess you've only considered one resonance form of compound C. If you consider the other major resonances, there are alpha C-H bonds that can participate in hyperconjugative effects:
PCK's user avatar
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3 votes

Which of the two structures is more stable?

The explanation can[1] be given by analysing their respective resonance[2] structures which is as follows: (Credits: ChemWriter) Now, inductive effects would work. In case of first (c's resonance),...
Rahul Verma's user avatar
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3 votes
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Hyperconjugation involving other bonds

Your description of hyperconjugation is a very minimalistic one. More generally, hyperconjugation should be seen as the interaction of occupied bonding MOs with unoccupied ones — these unoccupied ones ...
Jan's user avatar
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3 votes

Hyperconjugation, stability of molecules, and heat of formation

Hyperconjugation is an electronic effect which is similar to resonance (one of the most fundamental concepts of organic chemistry). Here's how it works: $$\ce{H3C-CH2+ <-> H+H2C=CH2}$$ You can ...
Nikhil Anand's user avatar
3 votes
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Between compounds having same functional groups, why does the one with greater -I effect have higher acidic strength

Less (absolute, i.e. less negative) local charge on the oxygen atoms of the conjugate base means the energetic penalty for removing the $\ce{H+}$ is lessened. Or in other words: The -I effect of the ...
Karl's user avatar
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2 votes

Why is the eclipsed conformer more stable than the bisected conformer in 1-butene?

Both E1 and E2 are made more stable by hyperconjugation. Hyperconjugation can involve electrons in a sigma bond (either C-H or C-C, no restriction) with an adjacent empty (or partially filled) non-...
getafix's user avatar
  • 8,475
2 votes

How does hyperconjugation affect bond lengths in alkenes?

Because of hyperconjugation, C2-C3 single bond in propene acquires some double bond character ans hence is little shorter (1.49 Å) than the normal C-C single bond length (1.54 Å). However in 3,3-...
Taseen Ahmed's user avatar
2 votes

Why the H+ ion remains with the molecule even though there is no bond between it and hyperconjugative structure?

Resonance structures do not exist on their own! Resonance is not a rapid equilibrium between different structures. It is a description of a single delocalised structure using contributing forms which ...
bon's user avatar
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2 votes

Why does hyperconjugation help for ring cleavage?

This is a bit late, but my instinct tells me you form the 1,3 diradical. The fusion to a cyclopropane seems pretty reasonable. The alkene is formed from a 1,2-hydrogen atom shift (I admit, this is a ...
Zhe's user avatar
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2 votes

Hyperconjugation in cresol

It is known that cresol containing methyl group destabilises the conjugate base ion by releasing electrons which results in decreasing of acidic strength. But in meta-cresol, this destabilisation is ...
Afsal's user avatar
  • 131
2 votes

Does hyperconjugation into sigma-star explain methylamine's increased basicity relative to ammonia?

Yes, I once thought about this interesting phenomenon also. There is no good reason why hyperconjugative interactions cannot take place between the $\ce {N}$ lone pair orbital and the $\ce {C-H}$ $\...
Tan Yong Boon's user avatar

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