haloalkanes reactivity and boiling points

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SupernovaNebula
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Hi,

Can someone please explain to me how/why the boiling points of haloalkanes increase from fluorine to chrlone and yet at the same time there reactivitiy decreases?

I understand that the boiling points increase as the forces of the van der waals increase. For anything to react with the halogenoalkanes, the carbon-halogen bond has got to be broken because that gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order. Why then does the boiling point also increase shouldn't they decrease if its easier to break the bonds like for there reactivity.

Pleas help
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infernalcradle
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to make is look obvious...

iodine has waaaaaaaaaaaaaaaay more electrons that flourine, but is also stupidly less reactive....the large amount of electrons that flourine have mean there are more london forces...lots more london forces...therefore it takes more energy as instead of breaking...say 10 london forces...you have to now break 1000
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SupernovaNebula
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forget fluorine, when you go down the group why do the reactivity of haloalkanes increase and the boiling points if both involve breaking bonds?

shouldnt the reactivity decrease if its harder to break the bonds (thats why the boiling points increase)
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SK-mar
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(Original post by SupernovaNebula)
Hi,

Can someone please explain to me how/why the boiling points of haloalkanes increase from fluorine to chrlone and yet at the same time there reactivitiy decreases?

I understand that the boiling points increase as the forces of the van der waals increase. For anything to react with the halogenoalkanes, the carbon-halogen bond has got to be broken because that gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order. Why then does the boiling point also increase shouldn't they decrease if its easier to break the bonds like for there reactivity.

Pleas help
the bond length between C-Cl is shorter than between lets say C-I or C-Br. Therefore the energy required to break the C-Cl bond is higher than th other two. Therefore it will have a higher boiling point as it requires more energy to break the bond, but also be less reactive as it has the strongest bonding and again requires the most energy to break. does that make sense? hope so
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illusionz
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(Original post by SK-mar)
the bond length between C-Cl is shorter than between lets say C-I or C-Br. Therefore the energy required to break the C-Cl bond is higher than th other two. Therefore it will have a higher boiling point as it requires more energy to break the bond, but also be less reactive as it has the strongest bonding and again requires the most energy to break. does that make sense? hope so
Oh dear god no. Melting and boiling don't involve breaking of covalent bonds.

(Original post by infernalcradle)
to make is look obvious...

iodine has waaaaaaaaaaaaaaaay more electrons that flourine, but is also stupidly less reactive....the large amount of electrons that flourine have mean there are more london forces...lots more london forces...therefore it takes more energy as instead of breaking...say 10 london forces...you have to now break 1000
No. Iodine has more electrons, but core electrons do not cause van der waal's interactions.

The carbon-fluorine bond is polarised. The classic reaction of haloalkanes is nucleophilic substitution by a nucleophile attacking the C-X antibonding orbital. I assume this is A level, so the simple answer is that because the C-F bond is polarised more than the C-Cl one, it's more attractive to a nucleophile, so the compound is more reactive.
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illusionz
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(Original post by SupernovaNebula)
forget fluorine, when you go down the group why do the reactivity of haloalkanes increase and the boiling points if both involve breaking bonds?

shouldnt the reactivity decrease if its harder to break the bonds (thats why the boiling points increase)
There's a huge difference between covalent bonds and intermolecular 'bonds'. Intermolecular attractions are broken when you melt/boil something, but the covalent ones are NOT broken.
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illusionz
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(Original post by SupernovaNebula)
THANKYOU SOOO MUCH!!

apart from obviously boiling dosent involve breaking covalent bonds i get what you meant.
Urm... he got it the wrong way round. He concluded that R-I is more reactive than R-F, which is not the case.
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SK-mar
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(Original post by SupernovaNebula)
THANKYOU SOOO MUCH!!

apart from obviously boiling dosent involve breaking covalent bonds i get what you meant.

*edit actually i dont understand because when you down the group the boiling point increases, you said its because its shorter but then as you go down it gets longer so shouldnt it decreae? omg wth??
sorrrrrry i didnt mean breaking of the covalent bond lol but im glad i could help anywayyy....
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SupernovaNebula
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(Original post by illusionz)
Urm... he got it the wrong way round. He concluded that R-I is more reactive than R-F, which is not the case.

I don't care about Fluorine, I care about the trend between the other elements. Instead of being a smartass explain then.
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Pedus
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This is to do with Bond Enthalpies..

The strength of the C-X bond is the following order (strongest first):

C-F > C-Cl > C-Br > C-I

The reactivity of the halogenoalkanes is in the reverse order:

most reactive ---> least reactive

C-I > C-Br > C-Cl > C-F

The reactivity of the halogenoalkanes depends on the breaking of the C-X bond. AS the C-F bond is the strongest, it is the hardest to break, and therefore fluroalkanes are least reactive of the halogenoalkanes.
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SupernovaNebula
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(Original post by illusionz)
Ok, you don't care about fluorine. I don't give a crap. Go get your exam wrong, see if it effects me.

Next time people offer advice don't be a prick.
okay.
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SupernovaNebula
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(Original post by Pedus)
This is to do with Bond Enthalpies..

The strength of the C-X bond is the following order (strongest first):

C-F > C-Cl > C-Br > C-I

The reactivity of the halogenoalkanes is in the reverse order:

most reactive ---> least reactive

C-I > C-Br > C-Cl > C-F

The reactivity of the halogenoalkanes depends on the breaking of the C-X bond. AS the C-F bond is the strongest, it is the hardest to break, and therefore fluroalkanes are least reactive of the halogenoalkanes.
Thanks
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Asha5692
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F is more electronegative so the C-F bond is more polar, therefore flouroalkanes are more susceptible to nucleophilic attack, the being because the methyl group is elctron-pushing and the F group is electron-withdrawing.
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Asha5692
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(Original post by illusionz)
Oh dear god no. Melting and boiling don't involve breaking of covalent bonds.


No. Iodine has more electrons, but core electrons do not cause van der waal's interactions.

The carbon-fluorine bond is polarised. The classic reaction of haloalkanes is nucleophilic substitution by a nucleophile attacking the C-X antibonding orbital. I assume this is A level, so the simple answer is that because the C-F bond is polarised more than the C-Cl one, it's more attractive to a nucleophile, so the compound is more reactive.
OP this is correct.
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SupernovaNebula
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(Original post by Asha5692)
OP this is correct.
but its not what i'm trying to ask, i'm trying to understand why the boiling point increases as you go down the group but at the same time the reactivity increases, i understand now..
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RobG93
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(Original post by Asha5692)
F is more electronegative so the C-F bond is more polar, therefore flouroalkanes are more susceptible to nucleophilic attack, the being because the methyl group is elctron-pushing and the F group is electron-withdrawing.
No actually, bond polarity has nothing to do with the reactivity of Halogenoalkanes.

It's all do do with bond length. The shorter the bond, the higher the electrostatic attractions between the two Nuclei participating in the bond, hence more Kinetic energy is needed to break the bond and so the shorter the bond, the less reactive it is.

In terms of Halogenoalkanes, the C-F bond is least reactive because the Fluorine atom is tiny, and so is closer to the carbon atom, so the bond is short. In a C-I bond, the Iodine atom is massive and so is quite far away from the carbon atom (in relative terms anyway), so the bond is long, hence the C-I bond being the most reactive.

For boiling points, it's to do with the number of electrons the Compound has. They all have Perminant Dipole - Perminant Dipole forces of attraction, so the number of Instaneous-Induced Dipoles that they form will be the overriding factor is which has the highest boiling point.

Fluoromethane will have the lowest boiling point because it has the least electrons, meaning it doesn't form as many IDID bonds between other molecules. So not alot of Kinetic energy will be needed to separate the molecules apart to become gaseous. Iodomethane will have the highest boiling point because it has the most electrons, and so will form many more IDID bonds between the moleciles, therefore alot of Kinectic energy will be needed to separate the molecules to become gaseous.
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hey_its_nay
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(Original post by SupernovaNebula)
Hi,

Can someone please explain to me how/why the boiling points of haloalkanes increase from fluorine to chrlone and yet at the same time there reactivitiy decreases?

I understand that the boiling points increase as the forces of the van der waals increase. For anything to react with the halogenoalkanes, the carbon-halogen bond has got to be broken because that gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order. Why then does the boiling point also increase shouldn't they decrease if its easier to break the bonds like for there reactivity.

Pleas help
reactivity down the group increases not decreases, this is because the bond enthalpy decreases
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crrrrrash
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I can't really be bothered to read all the replies so this may have been said already but you're confusing the bonds WITHIN the molecules with the bonds BETWEEN the molecules - strong bonds WITHIN the molecule make it unreactive; strong bonds BETWEEN molecules give it a higher boiling point
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Georgeeeee
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it confusing as the intra molecular bonds get weaker which raise the reactivety.

whereas the intermolecular (between the molecules) will effect the boiling point.
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