Alkane/Alkene reaction with bromine water (1 Viewer)

[mushroom_head]

i just have a slight problem.
--> was it the bromine water that decolourised cyclohexene or the cyclohexene that decolourised the bromine water?

 

[:: ck ::]

bromine water is an aqueous, brownish-yellow solution, cyclohexene is a clear, colourless solution

so the cyclohexene decolourised the bromine water to form a clear, aqueuous layer at the bottom of a test tube when bromine water is added to cyclohexene

 

[skidder]

What happens in the reaction with alkanes and alkenes with Bromine water?

 

[Dragie]

What happens when this experiment is exposed to light? I remember reading somewhere about this...but now I can't find it anywhere...

 

[Riviet]

What happens in the reaction with alkanes and alkenes with Bromine water?

The cyclohexane contains only single bonded carbon atoms and does not react with the bromine water because the alkane is saturated. Therefore no reaction takes place, hence the solution remains red in colour.
However, the cyclohexene contains a reactve double bond which is high in electron density. Therefore it reacts with the bromine water and causes the decolourising of the solution.

 

[Riviet]

What happens when this experiment is exposed to light? I remember reading somewhere about this...but now I can't find it anywhere...

Alkanes will react with chlorine, bromine and iodine when the mixtures are exposed to ultraviolet light. The important thing to note is that alkanes do not react with or do not decolourise Cl, Br, or I solutions in the abscence of u.v. light but they do react quite slowly in the prescence of u.v. light.

 

[Dragie]

Ahhhh...ok that makes sense - thank you

 

[satwik]

could someone plz tell me what a relevant control for this experiment would be to test the reactivity of alkanes and alkenes

 

[tristambrown]

i dont know that we need a control for the BOS for this exp....

i mean maybe if you wanted to make it a hell of alot more complicated you could pour out samples of cyclohexane, cyclohexene and bromine water and keep them all seperate then at the end of the experiment you could compare their colours to the reaction vessles. To be more thourough you could compare the bp's of all liquids using fractional distillation.

The control's should match the data sheet values for their boiling points as they should all still be the same chemical they started as. I dont know if or how well Br vapour will condense so i will advise not to distill the Br water OR the unreacted alkane as Bromine is hazardous to health .. i also dont know which temp it is evolved from br water either.

i would expect the reacted cyclohexene (which is now 1,2dibromocyclohexane) to have a very different BP to the original (and the control) cyclohexene. (use the bp to positively ID the fractions)

If you dont do the UV part of the experiment (which i wouldnt as it doesnt prove the aim) then the bp of the cyclohexane will be the same as the control, supporting the thesis that alkanes are less reactive than alkenes (because in the same situation they dont react). Water and Bromine vapour will come across at some stage depending on what temp they boil off at


If you do the UV part of the experiment then you will have to find out if 1bromocyclohexane has a diff bp to cyclohexane. You may even end up with a stack of multiple substitutions on some molecules and none on others (eg a mix of 1,bromocyclohexane, 1,1dibromocyclohexane, 1,2dibromocyclohexane, etc etc etc) which makes id by bp more difficult.

*gets up on soap box*
The uv part of this experiment to me is bad science - applying uv to one sample to break bonds and allow substitution while not doing it to the other sample is just poor. Even if you apply UV to both, do you re-add bromine (to the now 1,2dibromocyclohexane) to give both the same chance to react under these new conditions ? Also by this point they both are now alkanes and as such the experiment is no longer comparing reactivity of alkenes:alkanes it is simply proving "alkanes can react if you force them to with bond crushing uv energy"

 

[bboyelement]

wat does cyclohexene look like in terms of structure?

and i cant remember what we used, was it cyclohexene with hexane of both cyclo-

 

[tristambrown]

(ignore the dots, they are just there to hold the diagram together coz it usually dies when i push post)

They usually use clyclohexane/ene cos they are cheap .

They would (or should moreso) use either both straight chain or both cyclic. Using a cyclic hexane and a straight chain hexene would mess up the accuracy of your results because you are not comparing the Ane and Ene version of the same chemical and as such your results might be compromised (though practically they would work for this experiment)


Cyclohexene
.... H-C=C-H
......./......\
H2-C.......C-H2
.......\....../
..H2-C--C-H2

----->

1,2 dibromocyclohexane (double bond has opened turning it into a ***hexane and the Bromine has attached to the two newly opened bonding positions)

.......Br..Br
........|...|
....H-C--C-H
......./......\
H2-C......C-H2
.......\....../
..H2-C--C-H2

VS

Cyclohexane
..H2-C--C-H2
......./......\
H2-C......C-H2
.......\....../
..H2-C--C-H2

UV Light Breaks bonds & Br "pushes" H out of the way---->
Bromohhexane
......Br
.......|
...H-C--C-H2
......./......\
H2-C......C-H2
.......\....../
..H2-C--C-H2 + H.

Can someone confirm or deny whether or not the Hydrogen substitued for is infact a free radical once displaced (is displaced th wright word?) from it's Hexane parent ? Or does it split off simply as H+ ..

(note that Bromine addition to the alkene at least has been shown to be through a free radical system
http://pubs.acs.org/cgi-bin/abstract.cgi/joceah/1994/59/i16/f-pdf/f_jo00095a029.pdf?sessid=4320)

 

[laya]

i really doubt u need to know the structures. make that i highly doubt..but its nice u had time to look em up.....

 

[Frantelle_08]

sri for bringing up an old post..

but, can anyone tell me the chemical equation of the two substances used mixed with bromine water??

including states?


dw: another post just answered my question.. =]