16
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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 ...
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9
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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 ...
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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 ...
- 1,261
7
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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 ...
- 68.7k
7
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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 $\ce{O-H}$...
- 1,859
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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 ...
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6
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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 ...
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5
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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_{\...
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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 ...
- 42.3k
4
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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 ...
- 17.2k
4
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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 ...
- 310
3
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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 ...
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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, ...
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3
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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 ...
- 71.9k
3
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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:
- 1,149
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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),...
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3
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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 ...
- 68.7k
3
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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 ...
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3
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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-...
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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 ...
- 12.3k
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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 ...
- 17.6k
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How does hyperconjugation lead to stability?
What you are ignoring is the magnitude of the effect.
While hyperconjugation does decrease the electron density of a localized bond and slightly increases the energy, the overlap with a high-energy, ...
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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 ...
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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 $\ce {C-H}$ $\...
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Why hyperconjugation can't stabilize carbanion?
Hyperconjugation involves the electrons from an adjacent bond to "donate" electrons to an empty p orbital. Now if we consider a filled p orbital, or a carbo-anion, then there will be a repulsive force ...
2
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Why is 1,2,3,4,5,8-hexahydronaphthalene more stable than 1,4,4a,5,8,8a-hexahydronaphthalene?
@ Martin has addressed the shortcomings of assessing “stability” by counting hyperconjugating C-H bonds. If by “more stable” one means which one of the two isomers, 1,2,3,4,5,8-hexahydronaphthalene 1 ...
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Does hyperconjugation only arise from C–H bonds?
No.
Hyperconjugation is the donation of electrons from a one orbital to another, where at least one of those orbitals is a bonding orbital, and there is a net overlap between the orbitals.
A typical ...
- 36
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Carbocation formation and stability from bromo compounds
Yes, both your answers and reasoning are correct. For the second compound a slight ambiguity might arise, because the $\mathrm{sp^2}$-hybridized carbons show $\text{-I}$ effect which would destabilize ...
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Reactivity of Alkenes with HBr
What you should be looking at is the stability of the carbocation intermediate formed when the strong acid $\ce{HBr}$ protonates the alkene. This stability is more sensitive to alkyl substitution than ...
- 62.6k
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