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kitty123
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I thought that unsaturated compounds decolourise bromine, but looking in my revision guide it says that arene compunds are unsaturated but then it says as the bottom none of these compounds contain c=c bonds so do not react with bromine water. Confused!
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spook2575
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unsaturated compounds decolourise bromine. Benzene has no double bonds, however benzene has a ring of delocalised electrons for that reason benzene is less reactive.
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EierVonSatan
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Benzene isn't saturated, but it is less reactive
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Metal_Gear
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Benzene follows it's own set of rules. It is technically an unsaturated hydrocarbon, but it is very unreactive due to it's structure.

What exam board are you doing? For OCR AS, you just need to know that it doesn't react very easily.
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kitty123
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I'm doing OCR salters. Thats why I get confused because it says it is unsaturated yet it doesn't contain c=c bonds? can someone explain that?
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EierVonSatan
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Saturation means only sigma bonds - no pi bonds, benzene doesnt have and double bonds but it does have a pi system - so its unsaturated i.e. you can hydrogenate benzene (unsaturated) to cyclohexane (saturated)
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didgeridoo12uk
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(Original post by kitty123)
I'm doing OCR salters. Thats why I get confused because it says it is unsaturated yet it doesn't contain c=c bonds? can someone explain that?
have you not learnt all about the structure of benzene, with the ring of deloaclised electrons etc etc

or is that not on your course? i'm doing normal OCR chem, and its got like a few pages on it in the book
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kitty123
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No I get thew whole thing about an electron from each carbon makes the ring of electrons just not the unsaturated bit but I think I do now. Can anyone tell me why benzene is colourless? Is it because it only absorbs UV light? If a molecule absorbed all visible light is it white? Or is that if it reflects all visible light?
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IChem
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(Original post by kitty123)
No I get thew whole thing about an electron from each carbon makes the ring of electrons just not the unsaturated bit but I think I do now. Can anyone tell me why benzene is colourless? Is it because it only absorbs UV light? If a molecule absorbed all visible light is it white? Or is that if it reflects all visible light?
If a molecule absorbs all visible light then the molecule will appear black in colour.
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kitty123
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The ring is now only partially delocalised; it has two less electrons and so has a positive charge. The hydrogen gives up its electrons to the benzene ring, reforming the delocalisation

Just found the above on a website about electrophillic substitution, if hydrogen has 1 electron how can it reform the delocalisation? If it has 2 less electrons then surely after the H has given up it's electron it will still be one short? Help!
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EierVonSatan
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there are two electrons in the C-H bond, one of them is hydrogens the other was carbons to start with so C keeps both and H+ (proton) is lost
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zzzzzoe
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i thought benzene was unsaturated? is it JUST alkenes that decolourise bromine water?
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EierVonSatan
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(Original post by zzzzzoe)
i thought benzene was unsaturated? is it JUST alkenes that decolourise bromine water?
Yes and yes
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Icy_Mikki
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(Original post by didgeridoo12uk)
or is that not on your course? i'm doing normal OCR chem, and its got like a few pages on it in the book
For A2? 'A few pages'?!? Bah, Benzene filled about half the chemistry we did this year for AQA A2. :mad:
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didgeridoo12uk
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(Original post by Icy_Mikki)
For A2? 'A few pages'?!? Bah, Benzene filled about half the chemistry we did this year for AQA A2. :mad:
yeh for a2. ahah unlucky

we did have quite a bit on phenol, and other benzene related things. but only 1 chapter out of 16ish that we've done has been based on arenes. and only like 3 other chapters have had arenes in
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gorilla_baby
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i thought only phenol reacted with bromine water, decolourising it and a white ppt form?
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EierVonSatan
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its both phenol and aliphatic alkenes
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122025278
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This is very simple I don't understand the fuss:

Bromine water contains molecular bromine Br-Br

Molecular bromine can be instantaneously polarised, and this polarisation is increased and stablilised by the electron density in the Benzene ring.

The bond in the bromine molecule breaks heterolytically, the bromine cation is a fairly powerful electrophile and bonds with benzene ring using the delocalised electrons. This creates a delocalised positive charge, inducing instablility.

The C-H bond, on the carbon which has bonded to the Bromine ion, forming C-Br, loses a proton, by the breaking of that C-H bond. The electron pair from this bond are delocalised, back into the pi orbital above and below the plane, thus regaining the delocalisation energy.

As for decolouration, you wouldn't see orange - colourless, I'm guessing, because HBr would be formed, this would probably dissociate into its ions in solution, the Br ion if its hydrated and the Bromobenzene may absorb photons from the visible part of the spectrum, any ideas on this?
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EierVonSatan
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(Original post by 122025278)
This is very simple I don't understand the fuss:

Bromine water contains molecular bromine Br-Br

Molecular bromine can be instantaneously polarised, and this polarisation is increased and stablilised by the electron density in the Benzene ring.

The bond in the bromine molecule breaks heterolytically, the bromine cation is a fairly powerful electrophile and bonds with benzene ring using the delocalised electrons. This creates a delocalised positive charge, inducing instablility.

The C-H bond, on the carbon which has bonded to the Bromine ion, forming C-Br, loses a proton, by the breaking of that C-H bond. The electron pair from this bond are delocalised, back into the pi orbital above and below the plane, thus regaining the delocalisation energy.

As for decolouration, you wouldn't see orange - colourless, I'm guessing, because HBr would be formed, this would probably dissociate into its ions in solution, the Br ion if its hydrated and the Bromobenzene may absorb photons from the visible part of the spectrum, any ideas on this?
bromine is a fairly lousy electrophile, and bromine water doesn't react with benzene - you need a FeBr3 do get benzene to react with Br2
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kitty123
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(Original post by EierVonSatan)
there are two electrons in the C-H bond, one of them is hydrogens the other was carbons to start with so C keeps both and H+ (proton) is lost
Ok thanks so is this right
H has 2 electrons; 1 from the carbon and one from H itself.
The electrophile is attracted to the ring and the ring donates a pair of electrons to it leaving the ring partly delocalised an with a + charge. The +charge attracts something with a negative charge eg HSO4-. This donates a pair of electrons to the H atom. The H atom then gives up the pair of electrons so the ring is back to normal?
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