Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: satha13 on November 07, 2006, 12:43:59 AM
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Can anyone please explain me why the boiling point of Alkane, Alkene and Alkyne is the following order
Alkyne > Alkane > Alkene (assume that same number of carbon atom) such as C2H2 > C2H6 > C2H4
In this situation, we assume that molecular weight of them are not much differrent. So London dispersion (intermolecular force) is the same. But why alkyne is the most boiling point when compare with alkane and alkyne.
Is it decided by Molecular shape??
Satha
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nice question
I'm just guessing but i'd say you have to consider structure and molecular weight
for alkanes and alkenes the effect of the decrease in mass is probably has more of an effect than the structural change.
for alkynes due to its linear structure it can be packed together more easily and with greater stability meaning so it has a higher boiling/melting point. As this effect is greater than the decrease in molecular weight.
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nice question
I'm just guessing but i'd say you have to consider structure and molecular weight
for alkanes and alkenes the effect of the decrease in mass is probably has more of an effect than the structural change.
for alkynes due to its linear structure it can be packed together more easily and with greater stability meaning so it has a higher boiling/melting point. As this effect is greater than the decrease in molecular weight.
I disagree. I think the decrease in mass is negligible to the other structural changes. As jubba noted, alkynes are linear and therefore can pack more closely. Since van der Waals forces scale with r-6, having a very small distance between two molecules can greatly increase the strength of the intermolecular force. Similarly, alkenes have planar regions which can pack more efficiently than alkanes. Both alkenes and alkynes also benefit from pi-pi stacking interactions, and I would also argue that their pi bonds are more polarizable than the sigma bonds in alkenes, so the dispersion forces are stronger.
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nice question
I'm just guessing but i'd say you have to consider structure and molecular weight
for alkanes and alkenes the effect of the decrease in mass is probably has more of an effect than the structural change.
for alkynes due to its linear structure it can be packed together more easily and with greater stability meaning so it has a higher boiling/melting point. As this effect is greater than the decrease in molecular weight.
I disagree. I think the decrease in mass is negligible to the other structural changes. As jubba noted, alkynes are linear and therefore can pack more closely. Since van der Waals forces scale with r-6, having a very small distance between two molecules can greatly increase the strength of the intermolecular force. Similarly, alkenes have planar regions which can pack more efficiently than alkanes. Both alkenes and alkynes also benefit from pi-pi stacking interactions, and I would also argue that their pi bonds are more polarizable than the sigma bonds in alkenes, so the dispersion forces are stronger.
that makes sense but
Can anyone please explain me why the boiling point of Alkane, Alkene and Alkyne is the following order
Alkyne > Alkane > Alkene (assume that same number of carbon atom) such as C2H2 > C2H6 > C2H4
In this situation, we assume that molecular weight of them are not much differrent. So London dispersion (intermolecular force) is the same. But why alkyne is the most boiling point when compare with alkane and alkyne.
Is it decided by Molecular shape??
Satha
thats what was puzzling me
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Thank you for answer by Jubba and yggdrasil
Anyway, the order Alkyne>Alkane>Alkene make me ??? I read it from text book(but the author didn't mention about a clear reason). At first I agree with yggdrasil and new arrangement of order should be Alkyne>Alkene>Alkane.
I try to understand about saturation, if we order the boiling point following with saturation, So
Alkane(most saturation) >Alkyne>Alkene (most unsaturation)
Beacuse the compact structure of alkyne, so it is more influence than saturation and make more dispersion force and order will be
Alkyne>Alkane>Alkene
Is it make sense???? Please let me know
Oh Organic chemistry can make anyone mad..
Satha