Stable hydrocarbon

[YoonS]

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branched and linear hydrocarbon.

There was a question about the lowest heat of combustion from DAT dest

so the answer was the most branched hydrocarbon since it is the

most stable form, according to DAT dest.


But isn't the linear hydrocarbon is more stable than branched one?

since the boiling and melting point of linear hydrocarbon is higher

than the branched one thus inferring linear hydrocarbon is more stable?

 

[HannibalLecter]

branched and linear hydrocarbon.

There was a question about the lowest heat of combustion from DAT dest

so the answer was the most branched hydrocarbon since it is the

most stable form, according to DAT dest.


But isn't the linear hydrocarbon is more stable than branched one?

since the boiling and melting point of linear hydrocarbon is higher

than the branched one thus inferring linear hydrocarbon is more stable?

Boiling and melting points of hydrocarbons are only dealing with intermolecular forces between molecules.

But the heat of combustion is actually dealing with breaking intramolecular bonds (bonds between atoms) because you form CO2 and H20.

The lowest heat of combustion goes to the molecule that is most stable by itself. Which in this case would be a branched molecule over a linear hydrocarbon.

Be careful not to mix intermolecular forces b/w molecules with the actual bonds of the molecule

 

[yappy]

I agree with what you said. Is the branched more stable because a C-C bond takes more energy to break than C-H bonds?

I say this because the heavier branched molecule has more C-C bonds while the less branched will have less C-C bonds.

Boiling and melting points of hydrocarbons are only dealing with intermolecular forces between molecules.

But the heat of combustion is actually dealing with breaking intramolecular bonds (bonds between atoms) because you form CO2 and H20.

The lowest heat of combustion goes to the molecule that is most stable by itself. Which in this case would be a branched molecule over a linear hydrocarbon.

Be careful not to mix intermolecular forces b/w molecules with the actual bonds of the molecule

 

[HannibalLecter]

I agree with what you said. Is the branched more stable because a C-C bond takes more energy to break than C-H bonds?

I say this because the heavier branched molecule has more C-C bonds while the less branched will have less C-C bonds.

Actually if you look at a structural isomer for a pentane molecule
http://en.wikipedia.org/wiki/Structural_isomer

For n-pentane:

12 C-H bonds
4 C-C bonds

For neopentane:

12 C-H bonds
4 C-C bonds

So you see the C-C bonds and C-H bonds stay the same:

But to better explain it:

"It is generally true for alkanes that a more branched isomer is more stable than a less branched one.
The small differences in stability between branched and unbranched alkanes result from an interplay between attractive and repulsive forces within a molecule (intramolecular forces). These forces are nucleus–nucleus repulsions, electron–electron repulsions, and nucleus–electron attractions, the same set of fundamental forces we met when talking about chemical bonding and van der Waals forces between molecules.

When the energy associated with these interactions is calculated for all of the nuclei and electrons within a molecule, it is found that the attractive forces increase more than the repulsive forces as the structure becomes more compact."

Source: http://reptilespedia.com/konfuciy.asp?tda=dt&t=10492&fs=chemical+properties.+combustion+of+alkanes

And if you want to talk about it on the quantum level:
http://pubs.acs.org/doi/abs/10.1021/jp108577g

Basically branched alkanes are just more stable than linear alkanes (of the same carbon number)

 

[jwnichols21]

Actually if you look at a structural isomer for a pentane molecule
http://en.wikipedia.org/wiki/Structural_isomer

For n-pentane:

12 C-H bonds
4 C-C bonds

For neopentane:

12 C-H bonds
4 C-C bonds

So you see the C-C bonds and C-H bonds stay the same:

But to better explain it:

"It is generally true for alkanes that a more branched isomer is more stable than a less branched one.
The small differences in stability between branched and unbranched alkanes result from an interplay between attractive and repulsive forces within a molecule (intramolecular forces). These forces are nucleus–nucleus repulsions, electron–electron repulsions, and nucleus–electron attractions, the same set of fundamental forces we met when talking about chemical bonding and van der Waals forces between molecules.

When the energy associated with these interactions is calculated for all of the nuclei and electrons within a molecule, it is found that the attractive forces increase more than the repulsive forces as the structure becomes more compact."

Source: http://reptilespedia.com/konfuciy.asp?tda=dt&t=10492&fs=chemical+properties.+combustion+of+alkanes

And if you want to talk about it on the quantum level:
http://pubs.acs.org/doi/abs/10.1021/jp108577g

Basically branched alkanes are just more stable than linear alkanes (of the same carbon number)

This is kind of abstract but I guess I understand if it's the same # of C's. What about if there's a branched hexane vs. a linear heptane? Which is more stable then? & a branched hexane vs. a linear pentane?