Laraib Chaudhry
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I am working on the reactivity of alkanes. I wanted to ask how the polarity of the bonds within alkanes affects their reactivity. I am aware that the bonds are non-polar, but how does this make alkanes unreactive . I thought reactivity depends on an atoms ability to gain or lose electrons depending on whether its a non-metal or a metal. Or does reactivity depend on its ability to make or break bonds? How does this link with bond enthalpy? AGHHH, I'm so confused!?
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username3434964
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I should be good at chemistry...

But I'm not.

Sorry.
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Laraib Chaudhry
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(Original post by Bill Nye)
I should be good at chemistry...

But I'm not.

Sorry.
Hhahah thanks anyway!
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Ethan9133
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When a bond is polar the species can become more reactive because it can be attacked by other species more easily i.e. a nucleophile or electrophile.
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Dat1Guy
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The reactivity of a compound depends on many factors which you won't have covered yet. But the basics yes, an alkane is not significantly polar, but neither is an alkene.

The difference between an alkene and an alkane is that an alkene has an area of high electron density, which is in the double bond. Now, an alkane has sigma bonds which are the single bonds between say c-c which are strong. But an alkene has a sigma bond and a pi bond. The pi bond is weaker than the sigma bond and can be attacked by a molecule or ion which has an area of positive charge, this could be another polar molecule or induced-polar molecule. This process is electrophillic addition.

But to really answer your questiom, this is not to do with polarity, it's to do with the unsaturated double bond that has the ability to react with something else. An alkane does not have this ability. Once you saturate the double bond it's reactivity is the same of the alkane equivalent that didn't start with a double bond.
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Dat1Guy
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meeravirdee
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Reactivity is influenced by several things The non-polar C-C bonds are part of it.

Electronegativity is the attraction of a bonded atom for the pair of electrons in the covalent bond. So if two bonded atoms have significantly different electronegativities they are polar, which generally causes the molecule to be soluble and usually reactive.
Yes, reactivity is the ability to lose/gain electrons, but that depends on electronegativity, which also causes polarity.
It's also due to the fact that all of the carbons outer electrons are engaged in a bonded pair with hydrogen, so there aren't any spare electrons ready to bond with another molecule.
They contain strong C-C and strong C-H bonds. The C-H bonds are only slightly polar; so there are no portions of alkanes that carry any significant amount of positive or negative charge that can attract other molecules or ions.

Bond enthalpy is basically about how much energy is needed to be supplied to break bonds of a molecule. A molecule's reactivity will have a slight correlation with bond enthalpy but generally its a separate topic. That will relate to alkanes because they have strong C-C and C-H bonds (as above) which means the molecule will need a lot of energy to break those bonds hence the high boiling points.

That's a lot of info but hope that helps
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Laraib Chaudhry
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(Original post by BDunlop)
The reactivity of a compound depends on many factors which you won't have covered yet. But the basics yes, an alkane is not significantly polar, but neither is an alkene.

The difference between an alkene and an alkane is that an alkene has an area of high electron density, which is in the double bond. Now, an alkane has sigma bonds which are the single bonds between say c-c which are strong. But an alkene has a sigma bond and a pi bond. The pi bond is weaker than the sigma bond and can be attacked by a molecule or ion which has an area of positive charge, this could be another polar molecule or induced-polar molecule. This process is electrophillic addition.

But to really answer your questiom, this is not to do with polarity, it's to do with the unsaturated double bond that has the ability to react with something else. An alkane does not have this ability. Once you saturate the double bond it's reactivity is the same of the alkane equivalent that didn't start with a double bond.
Hi! Thank your reply!
Could you please clear up a few things?
First of all, with elements Im aware that reactivity of non-metals depends on its ease of gaining electrons,however with alkanes, what effects the reactivity? Would it be how easily it can break bonds, therefore its bond enthalpy, which I am aware for alkanes, is high?
Also, I'm doing OCR A Chemistry A level, and in the spec it mentions the following:
(d) the low reactivity of alkanes with many reagents in terms of the high bond enthalpy and very low polarity of the σ-bonds present (see also 2.2.2 j) HSW1 Use of ideas about enthalpy and polarity to explain macroscopic properties of alkanes.
How would you go about explaining this?
Many thanks
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Plantagenet Crown
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pKa of alkane hydrogens is very high so few things can rip them off.
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Laraib Chaudhry
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(Original post by Plantagenet Crown)
pKa of alkane hydrogens is very high so few things can rip them off.
Im confused? How does this answer my question? Sorry, what is pKa?
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spidle
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Polarity is the separation of electric charge in a molecule, so it's how much each electron is attracted to either side of the molecule. Since alkanes are non-polar there is no electric charge on either part of the molecule and so it isn't very reactive. This relates on it's ability to gain or lose electrons because it is almost electrically neutral, it isn't reactive.
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Texxers
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(Original post by Laraib Chaudhry)
Im confused? How does this answer my question? Sorry, what is pKa?
pKa is an A2 chemistry thing lol..

10^-Ka
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Plantagenet Crown
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(Original post by Laraib Chaudhry)
Im confused? How does this answer my question? Sorry, what is pKa?
pKa, in simple terms, is the acidity of a proton and how easily it can be removed. The way you’re going to react alkanes is by removing hydrogens and since they’re not very acidic (high pKa) they’re inert to most substances.
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Laraib Chaudhry
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(Original post by meeravirdee)
Reactivity is influenced by several things The non-polar C-C bonds are part of it.

Electronegativity is the attraction of a bonded atom for the pair of electrons in the covalent bond. So if two bonded atoms have significantly different electronegativities they are polar, which generally causes the molecule to be soluble and usually reactive.
Yes, reactivity is the ability to lose/gain electrons, but that depends on electronegativity, which also causes polarity.
It's also due to the fact that all of the carbons outer electrons are engaged in a bonded pair with hydrogen, so there aren't any spare electrons ready to bond with another molecule.
They contain strong C-C and strong C-H bonds. The C-H bonds are only slightly polar; so there are no portions of alkanes that carry any significant amount of positive or negative charge that can attract other molecules or ions.

Bond enthalpy is basically about how much energy is needed to be supplied to break bonds of a molecule. A molecule's reactivity will have a slight correlation with bond enthalpy but generally its a separate topic. That will relate to alkanes because they have strong C-C and C-H bonds (as above) which means the molecule will need a lot of energy to break those bonds hence the high boiling points.

That's a lot of info but hope that helps
Thank you so much for your reply! So would I be right in saying that molecules which are polar, are generally more reactive? But why is it that when it comes to Haloalkanes, it is the other way round as Iodoalkanes are more reactive?
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Laraib Chaudhry
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(Original post by Ethan9133)
When a bond is polar the species can become more reactive because it can be attacked by other species more easily i.e. a nucleophile or electrophile.
Thank you for replying!
So would I be right in saying that molecules which are polar, are generally more reactive? But why is it that when it comes to Haloalkanes, it is the other way round as Iodoalkanes are more reactive?
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Dat1Guy
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Interesting, I'm also doing OCR but B 😊. What effects the reactivity will be be determined by the groups attached. If it's just a normal alkane CxH2x+2 then it will be very unreactive. Firstly, the difference in electronegativity is small, thus the bonds aren't considered 'polar', and secondly the hydrogen has a strong attraction to the carbon's electron because it's so close to its nucleus. This means the bond enthalpy is high (a large amount of energy is needed to break the bond) which means it's not very feasible for the bond to be broken, something needs to provide this energy to break it, so under normal conditions it would be unlikely.

If you look at the values for electronegativity for hydrogen and carbon you'll see they aren't to dissimilar, you can look up the considered difference in electronegativity for a bond to be polar, which c-h will fall below. Now the sigma bone has hitch is that o looking shape is just a way of describing the single bond so don't let it confuse you.

In terms of macroscopic properties..thie could take a while.. We would look at the 'bigger' picture, thus it's physical sort of properties: melting point, boiling point etc.

If the alkane has lots of branches then the chains aren't able to order themselves up all nicely which would mean that there are lots of gaps, gaps where there are no intermolecular forces of attraction between molecules, meaning the more branched the molecule, the less intermolecular bonds thus the less energy needed to overcome all of these bonds, so the lower the boiling point.

But, you have to consider as the chain gets longer, there'll be more electrons everywhere and so there will be more intermolecular bonds around. This and how branched an alkane is will determine the macroscopic properties. (the enthalpy is just energy) Alkanes are not polar enough for polarity to affect reactivity. .
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Dat1Guy
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You have to consider that polarity is not the only consideration. Bond enthalpy will also need to be considered. If you look at iodine in the periodic table you'll see it has a much bigger nucleus than say chlorine. This means that there are much more electrons within the atom. These electrons will be attracted to the positive nucleus of the iodine atom more weakly than they would be to say a chlorine atom because the distance between the outermost electrons and the iodine nucleus is far greater (there is the shielding effect). Thus iodoalkanes are more reactive because the bonds between the C-I are weaker. They require less energy to break.
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Laraib Chaudhry
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(Original post by BDunlop)
You have to consider that polarity is not the only consideration. Bond enthalpy will also need to be considered. If you look at iodine in the periodic table you'll see it has a much bigger nucleus than say chlorine. This means that there are much more electrons within the atom. These electrons will be attracted to the positive nucleus of the iodine atom more weakly than they would be to say a chlorine atom because the distance between the outermost electrons and the iodine nucleus is far greater (there is the shielding effect). Thus iodoalkanes are more reactive because the bonds between the C-I are weaker. They require less energy to break.
Wow! You must be well smarrttttt! Do you cover this in your spec and are you in Year 13??
Therefore, does both bond enthalpy and bond polarity effect the reactivity. So are most polar molecules reactive? Also the difference in electronegativity between C and Iodine is less than that between and F, so shouldn't flouroalkane be more reactive?
So for asking so many questionss!! I must be driving you insane!
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Laraib Chaudhry
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(Original post by Ethan9133)
When a bond is polar the species can become more reactive because it can be attacked by other species more easily i.e. a nucleophile or electrophile.
Also, a C-I bond is less polar than a C-F, yet Iodoalkane is more reactive?
Thanks
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Dat1Guy
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You're right to be questioning this it's weird that polarity can affect the reactivity of one reaction but not the other.

The thing to try to understand is that there's not one underlying factor. They all culuminate together, bond enthalpy and polarity. In the case of the halogenalkanes the lower bond enthalpy for C-I than say C-Cl means that C-I is more reactive, despite C-Cl being more w polar than C-I.
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