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Topic: Medium for addition reaction to form halogenoalkanes  (Read 3599 times)

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Offline 123456789

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Medium for addition reaction to form halogenoalkanes
« on: August 12, 2012, 11:19:19 AM »
alkenes react with hydrogen halides to form halogenoalkanes (eg ethene with HBr). What is the medium for this reaction? (or is it just pure?) at rtp, presence of what other chemicals will hinder/change this reaction? (Water will not right?)

Thank you. Sorry if my question sounds stupid. Very blur about this whole concept...

Offline discodermolide

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Re: Medium for addition reaction to form halogenoalkanes
« Reply #1 on: August 12, 2012, 11:24:41 AM »
alkenes react with hydrogen halides to form halogenoalkanes (eg ethene with HBr). What is the medium for this reaction? (or is it just pure?) at rtp, presence of what other chemicals will hinder/change this reaction? (Water will not right?)

Thank you. Sorry if my question sounds stupid. Very blur about this whole concept...

If the alkene and HBr are miscible then no solvent should be necessary.
As you go to higher alkenes you will need a solvent.
Water will not get the correct product as you will form hydrobromic acid which may add HOBr as well as HBr across the double bond.
Other chemical, such as bases will hinder this reaction, as will other substances which react preferentially with HBr.
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Offline 123456789

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Re: Medium for addition reaction to form halogenoalkanes
« Reply #2 on: August 12, 2012, 01:32:15 PM »
alkenes react with hydrogen halides to form halogenoalkanes (eg ethene with HBr). What is the medium for this reaction? (or is it just pure?) at rtp, presence of what other chemicals will hinder/change this reaction? (Water will not right?)

Thank you. Sorry if my question sounds stupid. Very blur about this whole concept...

If the alkene and HBr are miscible then no solvent should be necessary.
As you go to higher alkenes you will need a solvent.
Water will not get the correct product as you will form hydrobromic acid which may add HOBr as well as HBr across the double bond.
Other chemical, such as bases will hinder this reaction, as will other substances which react preferentially with HBr.


Thank you very much! Regarding the reaction of HX in water with alkene, From what I understand now, HOX will be formed (which is H+ and OX- ?), and the product formed is ol with halogen group (maybe sth like 2bromoethanol). I would like to ask why is it that it is not XO-CH2-CH2-H that is formed? (not alcohol OH and X, rather OX and H) Since that the ions are H+ and OX-? 

Offline discodermolide

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Re: Medium for addition reaction to form halogenoalkanes
« Reply #3 on: August 12, 2012, 01:49:16 PM »
alkenes react with hydrogen halides to form halogenoalkanes (eg ethene with HBr). What is the medium for this reaction? (or is it just pure?) at rtp, presence of what other chemicals will hinder/change this reaction? (Water will not right?)

Thank you. Sorry if my question sounds stupid. Very blur about this whole concept...

If the alkene and HBr are miscible then no solvent should be necessary.
As you go to higher alkenes you will need a solvent.
Water will not get the correct product as you will form hydrobromic acid which may add HOBr as well as HBr across the double bond.
Other chemical, such as bases will hinder this reaction, as will other substances which react preferentially with HBr.


Thank you very much! Regarding the reaction of HX in water with alkene, From what I understand now, HOX will be formed (which is H+ and OX- ?), and the product formed is ol with halogen group (maybe sth like 2bromoethanol). I would like to ask why is it that it is not XO-CH2-CH2-H that is formed? (not alcohol OH and X, rather OX and H) Since that the ions are H+ and OX-? 

I did not say the formation of HOBr. I said that in water Br+ and "HO-" can add to the double bond, overall an addition of HOBr. to give a bromohydrin.

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Offline 123456789

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Re: Medium for addition reaction to form halogenoalkanes
« Reply #4 on: August 13, 2012, 06:32:26 AM »

I did not say the formation of HOBr. I said that in water Br+ and "HO-" can add to the double bond, overall an addition of HOBr. to give a bromohydrin.


Ok, so how does the reaction take place? (from what I read bromohydrin is usually formed with alkene+H2O+Br2. So for this reaction, does that mean that H2 will be produced? (the most obvious formula I can think of is C2H4 + HBr +H2O --> CH2BrCH2OH + H2 , which I think is obviously wrong, because H2 is very unlikely to be liberated. )

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