Why are subsituted alkenes more stable?

[ipodtouch]

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It makes, little sense to me.
Alkane groups are electron donating, not electron withdrawing. It's not like they can share the negative charge of the double bond.

The double bond is an electron dense nucleophile. I am wondering why shoving more electrons into it would make it more stable.

 

[chiddler]

The answer is hyperconjugation. I don't know what that means, but this question has been brought up frequently enough that I know the term lol

If you want to know enough for the mcat, just know that it happens. If you are still curious, try searching.

 

[Jarudy]

1) Alkyl groups contribute electron density to the pi bond
2) Alkyl groups are best farthest apart from each other. Sp3 = 109.5° apart while sp2 = 120° apart (unstrained)
(From my organic texbook)

 

[folktale]

Double bonds are stronger than single bonds, hence, more stable.

 

[Jdin]

Substituted carbocations are less reactive than non-substituted ones. This is because the increasing substitution decreases the nucleophilicity of the resulting carbocation, making it less reactive and thus more stable.


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[folktale]

Oh okay, I think my previous explanation was kind of shallow and poorly stated. But back to topic, more electrons does not necessarily mean it's going to be weaker. It's adding to the stability of the bond by creating an extra (pi) bond that will help support/connect the molecule together.

 

[chiddler]

Substituted carbocations are less reactive than non-substituted ones. This is because the increasing substitution decreases the nucleophilicity of the resulting carbocation, making it less reactive and thus more stable.


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lol

circular logic, my friend. you just restated the definition of reactivity.

 

[ipodtouch]

Substituted carbocations are less reactive than non-substituted ones. This is because the increasing substitution decreases the nucleophilicity of the resulting carbocation, making it less reactive and thus more stable.


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Quite confused by this.

Carbocations are electrophiles, not nucleophiles.


In anyways, it is a completely different situation. There is a positive charge on the carbon in a carbocation. The alkyl groups are electron donating, and hence diffuse the positive charge. (- stabilizes +)

In a substituted alkene, there is a center high density of electrons. Alkyl groups donating more electrons will not diffuse the positive charge. (- does not stabilize -)

 

[ipodtouch]

Double bonds are stronger than single bonds, hence, more stable.

Wait, haha. I think you may be confused what the topic is about.
I'm not questioning the strength of Single bonds vs double bonds.

This is a question of double bond vs double bond.

I'm wondering why double bonds that are substituted with alkane groups are more stable than isolated double bonds. Alkane groups add more negative charge to the already electron dense double bond, I would think that bringing more (-) to (-) would cause it to destabilize and become more reactive.



But chiddler put it well, hyperconjugation. I'll just remember that.

 

[folktale]

Wait, haha. I think you may be confused what the topic is about.
I'm not questioning the strength of Single bonds vs double bonds.

This is a question of double bond vs double bond.

I'm wondering why double bonds that are substituted with alkane groups are more stable than isolated double bonds. Alkane groups add more negative charge to the already electron dense double bond, I would think that bringing more (-) to (-) would cause it to destabilize and become more reactive.



But chiddler put it well, hyperconjugation. I'll just remember that.

Yea i restated on my second post.

 

[Thang2801]

Hi ipodtouch,

You are correct on the idea that the double bond is electron dense. You don't understand this because I don't view the double bond as a center that withdraw electron from the 2 carbons which make them to bare a very small partial positive charge. And you know the rest of the story.