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Topic: HYperconjugation-stability- inductive effect...  (Read 11796 times)

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Offline 2810713

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HYperconjugation-stability- inductive effect...
« on: February 04, 2005, 08:53:34 PM »
 :) :) Hi, I have some genuine difficulties regarding Hyperconjugation,plaese help.

0] Does the  displacement of bond pair from c-h  to FORM C=C really occurs  or it is a way to explain the  observed properties of the molecules       [ because hyperconj. is   resonance in which of sigma electrons R displaced .]

1] Why is it that more the hyperconjugation effect more is the INDUCTIVE EFFECT of an alkyl group attached to an unsaturated carbon chain?

My logic – In the hyperconjugatives  forms  the  C-H bond pair  is transferred to C-C to form C=C bond , presence of such hyperconjugative forms  leads to the shift of electrons away from the alkyl group as net effect in the   original form . This is nothing but inductive effect.
 

2] What are the hyperconjugative forms of  a carbon anions e.g. tert.-butyl carbonium ion?       When the bond pair is displaced in carbon cations from c-h to c=c the  ion gets discharged , but when  it would happen in the carbon anion the negatively charged C we repel the b.p.  and   this will also provide a negative centre of for  electrophiles and thus , decrease the stability of the molecule ,  so , I have doubt about the formation of the double bond [c=c] , the factor that increases stability of the molecule is  the distribution of the positive charge over the H atoms of the alkyl group , is it so powerful that solely it  makes the molecule more stable as a net effect?

3] Why is the order of  stability of carbon anions ions same as the cabocations  i.e. tert.> sec.>primary>methyl ? More the inductive effect more unstable will be the ion as repulsion between the C- [minus]    & bond pair increases . So the order of stability would be primary>sec.>tert. i.e. revesrse of that of carbon cations. But if the distribution of positive charge is a powerful stabilizer then the order  would be the same as that of carbon.


Please find a file attached with an one more difficulty.


hrushikesh

dexangeles

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Re:HYperconjugation-stability- inductive effect...
« Reply #1 on: February 05, 2005, 01:54:48 AM »
Everything is just a way to explain observations based on experimentation and data. Reactions/mechanisms are not explanations of what actually occurs, but an explanations of what possibly occurs based on the data given.

shouldn't carbon with less H be a cation? as in carbocations

hyperconjugation is just another term for delocalization of sigma electrons, and evetually delocalizing the positive charge as well, thus making the molecule more stable.  

CH3 (methyl) groups have better effect on hyperconjugation than plain Hydrogens; that's why more substituted (more CH3 directly connected) cations or radicals are more stable (delocalization effect); thus: stability with cations or radicals goes  tertiary>secondary>primary>>methyl

and with double bonds its basically the same but named differently: tetrasubstituted>trisubstituted>disubstituted>monosubstituted

Offline movies

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Re:HYperconjugation-stability- inductive effect...
« Reply #2 on: February 06, 2005, 01:13:37 PM »
3] Why is the order of  stability of carbon anions ions same as the cabocations  i.e. tert.> sec.>primary>methyl ? More the inductive effect more unstable will be the ion as repulsion between the C- [minus]    & bond pair increases . So the order of stability would be primary>sec.>tert. i.e. revesrse of that of carbon cations. But if the distribution of positive charge is a powerful stabilizer then the order  would be the same as that of carbon.

You're right that the order should be reversed.  If you look at the pKas of n-butyllithium, sec-butyllithium, and t-butyllithium you will find that the primary carbanion is slightly more stable that the secondary and the secondary is slightly more stable than the tertiary.  

The observed effect is not nearly as great as the effect with carbocations.

The take home message: hyperconjugation is a donating effect so it can't stabilize anions.

Offline 2810713

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Re:HYperconjugation-stability- inductive effect...
« Reply #3 on: February 09, 2005, 12:47:41 AM »
 Thanks movies and thanks dexangels.
Now the order of stability is quite clear.

  If you look at the pKas of n-butyllithium, sec-butyllithium, and t-butyllithium you will find that the primary carbanion is slightly more stable that the secondary and the secondary is slightly more stable than the tertiary.  

How can we decide the stability by  knowing pKas ?
- I think pKas  can tell about the  degree hyperconjugation effect , but
How can we decide the stability by  knowing pKas ?

hrushikesh

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Re:HYperconjugation-stability- inductive effect...
« Reply #4 on: February 09, 2005, 01:05:22 AM »
Hyperconjugation is only used when organic chemist don't have a better answer for a stereo-electronic phenomena.
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Re:HYperconjugation-stability- inductive effect...
« Reply #5 on: February 09, 2005, 02:15:17 PM »
Hyperconjugation is only used when organic chemist don't have a better answer for a stereo-electronic phenomena.

Heh, that's a good point.  It's kind of a catch-all.

With regard to your question about pKa, the higher the pKa, the less stable the anion, right?  A high pKa means that the molecule is unhappy as an anion and reacts readily.  The fact that the carbanion with the most hyperconjugation (t-butyl lithium) is the most reactive indicates that the hyperconjugation either has no stabilizing effect or possibly perturbs the anion to higher energy.

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