alkene stability

[Xtian]

---

Members do not see this ad.

if the most stable alkene is the most substituted, how come the most substituted alkene has the lowest heat of combustion and hydration? That doesn't sound very stable?

Xtian

 

[artanis]

The more substituted the alkene is lower in energy state because it is more stable. Think of the carbocation produced in the electrophilic addition reactions. Refer to the energy diagrams.

 

[klutzy1987]

if the most stable alkene is the most substituted, how come the most substituted alkene has the lowest heat of combustion and hydration? That doesn't sound very stable?

Xtian

Well if something is stable, then it has very little energy contained in it. Therefore when broken up, it will not have much energy to give off.

 

[PreDent2009]

To add onto klutzy's response: The reason why the more substituted alkene is lower in energy is due to inductive effects. Since the surrounding alkyl groups are sp3 hybridized there is a dipole from going from the flanking sp3 groups towards the sp2 hybridized carbons located on the double bond. Since sp2 orbitals are lower in energy the neighboring alkyl group will shift as much e-density towards the sp2 hybridized carbon as possible. This helps to lower the energy of the molecule as a whole, so there is less energy contained in the structure, hence releasing less energy when combusted.

Well if something is stable, then it has very little energy contained in it. Therefore when broken up, it will not have much energy to give off.

 

[Contach]

To add onto klutzy's response: The reason why the more substituted alkene is lower in energy is due to inductive effects. Since the surrounding alkyl groups are sp3 hybridized there is a dipole from going from the flanking sp3 groups towards the sp2 hybridized carbons located on the double bond. Since sp2 orbitals are lower in energy the neighboring alkyl group will shift as much e-density towards the sp2 hybridized carbon as possible. This helps to lower the energy of the molecule as a whole, so there is less energy contained in the structure, hence releasing less energy when combusted.

i don't get this..
why do sp2 want electron? it makes sense for radicals whcih dont have a complete octet and carbocations to want electrons. But what apart of an sp2 hybridized carbon makes it want electrons?

are you mixing up your theory by any chance?

to the OP: think of a potential energy diagram. the higher the molecule is, the harder it will fall and the more heat will be released. So something that is very stable is a low ona potential energy diagram and will release the least heat to get to its products.

 

[PreDent2009]

I am pretty sure about this one. The thought process would be the same for a carbocation because the carbon there is also sp2 hybridized. The molecule is always looking to be in the lowest possible energy state possible. Sp2 orbitals are lower in energy than sp3 so there is polarity in a sp3 to sp2 carbon-carbon bond. Alkene stability and carbocation stability are alike in the way that inductive effects are the answer to why the more substituted double bond or carbocation are more stable and preferred for the compound.

i don't get this..
why do sp2 want electron? it makes sense for radicals whcih dont have a complete octet and carbocations to want electrons. But what apart of an sp2 hybridized carbon makes it want electrons?

are you mixing up your theory by any chance?

to the OP: think of a potential energy diagram. the higher the molecule is, the harder it will fall and the more heat will be released. So something that is very stable is a low ona potential energy diagram and will release the least heat to get to its products.