Why do alkenes have lower MPs and BPs? (1 Viewer)

[~ ReNcH ~]

Why do alkenes have lower MPs and BPs than their repsective alkanes? I was under the impression that the double bond in alkenes would be more difficult to break than the single bond in alkanes, hence effectively raising the MP/BP of alkenes. Can anyone explain why this isn't the case, and why the double bond in fact lowers the MP/BP?

 

[Dreamerish*~]

uh.. i thought alkenes had higher MPs and BPs than alkanes?

 

[Pace_T]

because of the strength of the double bond .....i think

 

[stargaze]

isnt it because alkanes have higher molecular weights than their respective alkenes, hence there are more dispersion forces in alkanes. More dispersion forces requires more force to break, which explains the greater MP and BP

 

[illin]

Ok the double bond present in the alkene is much weaker than the C-H, ie saturated and unsatured hydrocarbons

It is for this reason that Alkanes have a higher mp and bp than alkenes

 

[~ ReNcH ~]

Ok the double bond present in the alkene is much weaker than the C-H, ie saturated and unsatured hydrocarbons

It is for this reason that Alkanes have a higher mp and bp than alkenes

So in other words, the covalent C-H bond in alkanes and the fact that they contain more carbons, means that the overall forces in alkanes are stronger than the double bond in their respective alkenes?

 

[~ ReNcH ~]

uh.. i thought alkenes had higher MPs and BPs than alkanes?

Nope...other way around.

 

[illin]

So in other words, the covalent C-H bond in alkanes and the fact that they contain more carbons, means that the overall forces in alkanes are stronger than the double bond in their respective alkenes?

They do not contain more carbons (ie C2H6 - alkane and C2H4 - alkene)
unsaturated hydrocarbons have double or triple bonds (double bonds in this case)
There is a greater difference in electronegativity between C-H than C-C, therefore more dispersion forces

 

[~ ReNcH ~]

They do not contain more carbons (ie C2H6 - alkane and C2H4 - alkene)
unsaturated hydrocarbons have double or triple bonds (double bonds in this case)
There is a greater difference in electronegativity between C-H than C-C, therefore more dispersion forces

My bad...I meant more hydrogens
But now I see what you mean...

 

[(^o^)]

can't be bother to read all of the answers above.
But i believe that it is becos the double bonds of alkenes are easily breakable than the single bonds of alkanes. Hence less energy would be require to break the bonds --> lower bp/mp.
Good enough?
=)

 

[xiao1985]

nice pic smilie face =)

it is not so clear in lower hydrocarbons, but in higher hydrocarbons, it becomes much mroe apparently the reason which accounts for this...

in longer hydrocarbon chains, saturation makes the molecule zigzag... hence it is much easier to stack on on an other, hence dispersion forces can impact on a greater degree...

whereas, if u have doubt bond in there, it greatly reduces the flexibility and ruins the geometry (think 3D, double bond is planar, unrotatable, whereas single bond is tetrahedral, rotatable), so the hydrocarbons cannot stack so easily, hence dispersion forces are weaker => lower mp/bp

this is one of the primary reason why doctors suggest u eat unsaturated fat, as it is less likely to solidify in ur arteries n capillaries...
on one of the fact sheet i recieved last yr, i recall some saturated fat in beef has mp as high as 80C!!! that's why grilling and get rid of those fat is important for a good diet... or imagine what happens if u drink it and it solidifies in ur artery

 

[mitochondria]

isnt it because alkanes have higher molecular weights than their respective alkenes, hence there are more dispersion forces in alkanes. More dispersion forces requires more force to break, which explains the greater MP and BP

hmm.. stargaze was right from the beginning I think.. generally the higher the molecular weight the higher the dispersion force between molecules because as the molecular weight increases, the number of electrons and size also increase. That will mean that the electron density is more dispersed and is easier to distort by an other interacting molecules. This distortion/interaction is what dispersion force is about and therefore the alkanes have higher boiling point than their corresponding alkenes because of the extra hydrogens..

I imagine that the double bond also makes a difference by restricting the region of which the electrons can move, but that probably is insignificant compared to the effect of the extra hydrogen atoms I think..

 

[shafqat]

please people, don't talk about breaking bonds!
wat's happening to the molecules when we melt or boil something?
remember its a physical process, not a chemical one
breaking actual bonds requires HEAPS of energy

 

[mitochondria]

very picky.. *phew* I seem to be safe this time didn't say anything of that sort xD hmm.. just out of curiosity how would you say "breaking intramolecular bonds?"

 

[shafqat]

i would stick to "overcoming the dispersion forces"

 

[mitochondria]

Sounds nice to me doesn't "breaking intramolecular bonds" sound like.. breaking intramolecular bonds to you? but then again, not everyone is a hardcore chemist

I was bored and did a google search for "breaking intramolecular bonds" and "overcoming intramolecular force"/"overcoming intramolecular forces"/"overcoming intramolecular bond"/"overcoming intramolecular bonds"

apparently nobody on the internet says any of the latter 4


Edited: hello shafqat *waves*.. hmm.. talk to you later.. i've got a lecture to go to xD

 

[Slidey]

I quote myself from when I actually knew some chemistry:

Boiling point indicates strength of molecular bonding. As chain length increases so does molecular mass and hence the strength of dispersion forces. Because of this stronger dispersion force, boiling point increases as chain length increases.

Is this correct?

 

[mitochondria]

that looks just fine (I tend to say put inter- or intra- before molecular though, molecular is kinda obscure to me, I actually had to read it three times but I guess it does imply intramolecular) ^_________^ hey slidy, what did you mean by when you actually knew some chemistry? I remember you used to have some amazing Chem things to say last year.. so you should know more Chemisty now? not less?

 

[aim54x]

generally the heavier the molecule the higher the MP/BP

in the case of -anes and -enes;
the -anes have a higher MP/BP due to the heavier mass
the general rule is the longer the chain the higher the MP/BP

 

[mitochondria]

Hmm.. People obviously don't read before posting.. I wonder why O_____o..