Electrophilic addition -halogens

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CurrentlyAtLevel
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#1
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#1
Why does the rate of rxn of Alkene +Halogens decrease as you go down g7 ?

And is this the correct product or the rxn between Alkenes and bromine water ?
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blackvoid17
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#2
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#2
the rate increases since they have more electrons and the molecules form stronger dipoles.
There should be 2 Br atoms attached to that molecule
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Arj9700
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#3
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The rate of reaction should decrease as you go down group 7 as the relative ability of the halogen atoms to accept an electron or pair of electrons (whether "ionically" or in this case to form a dative covalent bond) decreases as you go down the group.

*A- Level Knowledge in brackets (_)* -- ignore if you do GCSE
(These electrons or pair of electrons im talking about are provided by one of the C=C double bonds breaking (Pi bond specifically)) -- these are then used to form a dative covalent bond to one of the Br+ ions produced by heterolytic fission of the Br-Br bond --- look back at the mechanism for this reaction-- its formed after the double bond induces the formation of permanent dipoles across the bromine molecule.)

This is because the atomic radius of the halogen atoms increase as you go down the group i.e the atoms become larger with an increased number of protons, neutrons and electrons. - Have a look at the mass numbers on the periodic table
As a result the outer shell electrons will become placed on on an electron shell which is increasingly further away from the central nucleus of the atom.
( They will also experience an increased shielding effect due to the additional electrons present between the central nucleus of the atom and themselves.)
... and so experience a weaker nuclear force of attraction.
As a result this will make it harder for these atoms to gain more electrons as the additional electrons will need to be held in place by this attraction -- if the attraction gets weaker the further down the group you go the harder its going to be to gain more electrons.

As for the diagram, there should be two halogen atoms as they will be both added across the C=C unless you are carrying out electrophilic addition with a hydrogen halide e.g HBr

- Hope this helps + isn't too confusing -- if you do GCSE dont worry about the brackets.


Edit: Just read you Username, Lmao, you should be good hopefully
Last edited by Arj9700; 3 years ago
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Kian Stevens
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#4
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#4
(Original post by blackvoid17)
the rate increases since they have more electrons and the molecules form stronger dipoles.
There should be 2 Br atoms attached to that molecule
The rate will actually decrease
It doesn't have anything to do with dipoles in this case either, if it did then the rate would increase for the reasons you stated, although you are correct in what you said regardless

Also, there would be two bromine atoms on the molecule, but only if pure bromine was used... However, bromine water is used, and so water will most likely be in a large excess compared to any bromide ions
Because of this, water will be used as a nucleophile more than bromide ions, and so the major product would be a bromohydrin
So, the OP is correct in their drawings

(Original post by Arj9700)
The rate of reaction should decrease as you go down group 7 as the relative ability of the halogen atoms to accept an electron or pair of electrons (whether "ionically" or in this case to form a dative covalent bond) decreases as you go down the group.

*A- Level Knowledge in brackets (_)* -- ignore if you do GCSE
(These electrons or pair of electrons im talking about are provided by one of the C=C double bonds breaking (Pi bond specifically)) -- these are then used to form a dative covalent bond to one of the Br+ ions produced by heterolytic fission of the Br-Br bond --- look back at the mechanism for this reaction-- its formed after the double bond induces the formation of permanent dipoles across the bromine molecule.)

This is because the atomic radius of the halogen atoms increase as you go down the group i.e the atoms become larger with an increased number of protons, neutrons and electrons. - Have a look at the mass numbers on the periodic table
As a result the outer shell electrons will become placed on on an electron shell which is increasingly further away from the central nucleus of the atom.
( They will also experience an increased shielding effect due to the additional electrons present between the central nucleus of the atom and themselves.)
... and so experience a weaker nuclear force of attraction.
As a result this will make it harder for these atoms to gain more electrons as the additional electrons will need to be held in place by this attraction -- if the attraction gets weaker the further down the group you go the harder its going to be to gain more electrons.

As for the diagram, there should be two halogen atoms as they will be both added across the C=C unless you are carrying out electrophilic addition with a hydrogen halide e.g HBr

- Hope this helps + isn't too confusing -- if you do GCSE dont worry about the brackets.


Edit: Just read you Username, Lmao, you should be good hopefully
Yes, the rate should decrease for the reasons you stated
I believe the term you're looking for though is 'electronegativity'...

But as before, the drawing which the OP has suggested is correct
See my above comment as to why
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Arj9700
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#5
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#5
Hey, glad to know that I'm not to far off.
I was pretty reluctant to write "electronegativity" as in this case we are dealing with diatomic molecules of halogens e.g : Br-Br . These are atoms of the same element and hence they have no difference in electronegativity at all. As a result we can't really mention this.

However, I have never seen a reaction with bromine water and am surprised to see this - possibly not to do with my spec but good to know.
(Original post by Kian Stevens)
The rate will actually decrease
It doesn't have anything to do with dipoles in this case either, if it did then the rate would increase for the reasons you stated, although you are correct in what you said regardless

Also, there would be two bromine atoms on the molecule, but only if pure bromine was used... However, bromine water is used, and so water will most likely be in a large excess compared to any bromide ions
Because of this, water will be used as a nucleophile more than bromide ions, and so the major product would be a bromohydrin
So, the OP is correct in their drawings


Yes, the rate should decrease for the reasons you stated
I believe the term you're looking for though is 'electronegativity'...

But as before, the drawing which the OP has suggested is correct
See my above comment as to why
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