Bond energy between CH3-CH3 > CH3-CH2CH3

[kfsa1]

Hi guys,
I was wondering why the bond energy between CH3-CH3 is greater than CH3-CH2Ch3. Or, why is the BE between CH3-CH(CH3)2 > CH3-C(CH3)3

I associate BE with 1)shorter bonds = stronger, 2)the more stable the products after breaking the bonds = weaker. I.E acidic bonds with H-Cl > H-I because iodine can stabilize the negative charge better due to size.

Thanks

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[harryking]

Alkyl groups are somewhat electron-donating which make the bond longer.
Maybe the second rule you talked about only works for acids?
I would just use the first one, since H-Cl bond is still shorter than H-I.

 

[Liquidice07]

With bond energy, the higher it is, the more stable the compound currently is.
Therefore, CH3-CH3 is more stable than CH3-CH2CH3. By more stable, think lower in energy....it is a smaller molecule with less going on.

 

[salim271]

With bond energy, the higher it is, the more stable the compound currently is.
Therefore, CH3-CH3 is more stable than CH3-CH2CH3. By more stable, think lower in energy....it is a smaller molecule with less going on.

? I think you're thinking about bond dissociation energy. The lower the energy in a bond, the more stable it is. The higher its bond dissociation energy (the more energy needed to break it) the more stable it is. If you want to know how stable a organic compound is, look at its carbocation. What carbocation (containing only carbons and hydrogens) is more stable, a methyl, a primary, secondary, tertiary? Tertiary, and like the first poster said, its due to the inductive effect of electron donating and withdrawing groups.

 

[Liquidice07]

Ya, I think I might have. I did some extra hw on the topic and this is what I came up with. Let me know if I have this right....

In my chemistry book, it says, "Bond Energy is the energy required to break 1 mole of the bond in the gas phase. Bond energy can also be called Bond Dissociation Energy". So BOND ENERGY = BOND DISSOCIATION ENERGY

However, there is also bond energy level. The lower the bond energy level is, the more stable it is, as you said above.

Why would I want to look at a carbocation as opposed to a carbanion for stability?

 

[salim271]

Why would I want to look at a carbocation as opposed to a carbanion for stability?

Carbanions would follow the opposite trend if you look at methyl, primary, etc. Carbons without electron withdrawing groups on them are electron donating groups. So if you look at a tertiary carbocation, the carbon carrying the positive charge is stabilized by electron donating carbon groups that are attached that are donating negative charge and reducing the positive charge. If the carbocation is more stable (less energetic), then the bond formed would also be more stable and less energetic.