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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jsg9
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The bonds in a hydrocarbon are non-polar as you said because of the difference in electronegativity between the atoms. C-C have the same electronegativity (around 2.5), and the difference in electronegativity between C-H is very small (2.5 and 2.1), so the bonds are strong. Because of the similar electronegativity values there are no areas of partial positive or partial negative charge that are attacked by other species - apart from in the case of free radicals which have an unpaired electron.

Bond enthalpy just refers to how much energy is needed to break the bonds, in this case, the value is high because of the non-polar C-C and C-H bonds.

However, alkanes have low M.P because the forces that hold the molecules of alkanes together are not covalent, but weak intermolecular forces. It is these weak intermolecular forces that are overcome when alkanes are heated.
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Laraib Chaudhry
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(Original post by jsg9)
The bonds in a hydrocarbon are non-polar as you said because of the difference in electronegativity between the atoms. C-C have the same electronegativity (around 2.5), and the difference in electronegativity between C-H is very small (2.5 and 2.1), so the bonds are strong. Because of the similar electronegativity values there are no areas of partial positive or partial negative charge that are attacked by other species - apart from in the case of free radicals which have an unpaired electron.

Bond enthalpy just refers to how much energy is needed to break the bonds, in this case, the value is high because of the non-polar C-C and C-H bonds.

However, alkanes have low M.P because the forces that hold the molecules of alkanes together are not covalent, but weak intermolecular forces. It is these weak intermolecular forces that are overcome when alkanes are heated.
Thank you for your reply.
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?
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jsg9
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(Original post by Laraib Chaudhry)
Thank you for your reply.
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?
Simply yes. Often bonds with lower bond enthalpies will be more reactive as there is less energy required to break the bonds. Similarly polarity does effect reactivity, such as where salts with ionic bonding e.g. NaCl will disassociate in a solvent like water because of the Na+ charge and Cl- charges involved. However, water is a polar molecule and is a stable molecule. Bond enthalpy and polarity aren't the sole measures of reactivity.

The reactivity down group 7 decreases because the atomic radii increase as you go down the group, therefore, valence electrons and the nucleus are further apart. As halogens all have 7 valence electrons, they gain 1 more to form a full valence shell with 8 electrons. But the nucleus having less electrostatic attraction to electrons going down the group means an electron is less easily gained, and so reactivity down the halogens decreases.
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