Reaction of alkanes/alkenes with bromine water (1 Viewer)

[velox]

As you probably know UV light acts as a catalyst in the reaction between bromine water and Cyclohexane. So if you put a drop of bromine water in a hydrocarbon such as cyclohexane, and expose it to UV light itll turn colourless.

However if you put a drop of bromine water in cyclohexene it will also turn colourless (or do u have to shake it?)

So how do u know if its an alkane or alkene cos i cant think of any ways to isolate the uv light. I think the problem may lie with whether you have to shake the cyclohexene solution....(someone remind me lol)

TIA

 

[Dreamerish*~]

hmm well i better not be wrong
there are 2 addition reactions to distinguish between alkanes and alkenes
1. reaction with chlorine or bromine in a non-aqueous solvent such as chloroform
2. reaction of chlorine or bromine in aqueous solution such as bromine water
alkanes react in the presence of uv light so to cut out the uv light you can wrap your test tube in foil, like we did in our prac, or put it somewhere dark.
i think you do have to shake them, but not shaking them doesn't mean they won't react...
well, if i'm wrong i'm sure mitochondria will come and show off his knowledge
oh and (i'm half guessing) by "in presence of uv light" you probably have to put it directly under the sun or something because in classroom conitions and darkish labs there aren't a plentiful supply of uv

 

[Slidey]

"So how do u know if its an alkane or alkene cos i cant think of any ways to isolate the uv light."

The majority the spectrum which reaches us is infrared and visible (not as much UV). Coupled with the fact that it reacts under UV light OVER TIME, the test is simple:

If the hydrocarbon discolours the bromine or bromine water solution immediately, then it is an alkene or alkyne (unsaturated). Otherwise it is an alkane (saturated) and will take a very long time (well, it will take more than a few minutes, put it that way) to decolourise.

See my thorough prac write-up here: http://boredofstudies.org/community/showthread.php?t=55262

 

[funking_you]

UV light!

Do not get UV light confused with normal light.

UV light is (Ultra-violet) which is that 'cool' light you see in nightclubs that makes white shirts 'glow-in-the-dark'

AlkENE will react with bromine water in normal lab conditions, and there is no real need to shake reaction vessel, shaking it simply speeds up the reaction, so you'll see the de-colourisation of bromine water occur faster.

AlkANE will react with bromine-water if u shine a Ultra-violet light on it, ask your school teacher (esp your phyics teacher) i'm sure they have a UV light...

hope that is off help

Cheers
George

 

[Casmira]

you can also test it yourself, buy some UV lights their not that expensive, theres even some that go inside your computer

 

[Dreamerish*~]

you'd probably do this prac at school, or have done it already.
your teacher'll know what to do

 

[Xayma]

you can also test it yourself, buy some UV lights their not that expensive, theres even some that go inside your computer

Or you could just leave it in the sun for a while if its sunny, see it next chem lesson. Just got to have it outside as glass blocks most UV.

 

[Dreamerish*~]

when we did it at school we wrapped some in foil to block out uv and the ones that were meant to be in the presence of uv were also left in the dark fume cupboards...
so the results turned out quite crappy

 

[+:: $i[Q]u3 ::+]

if u do it inside a classroom with the blinds down, (so the only light source is the fluorescent lighting) then only the alkenes will react and decolourise the bromine water.

if u do it outside, the solution is exposed to uv light from the sun and both alkane and alkene will decolourise the bromine water.

it doesnt really have anything to do with shaking it... u should have shaken both test tubes properly (gently tapping the bottom sideways with your finger) in order to allow the reaction to occur (if it is going to occur).

 

[Xayma]

You didn't need to use the blinds. The amount of UV light that gets through glass is negligible. Hence why you can sit with the windows closed all day and not get sunburnt.

If you do Physics you will learn that Hertz detected the photoelectric effect by putting a reciever in glass (which stopped the UV rays getting through) and thin quartz (which didnt stop them) boxes. Although he didn't do further investigation.

 

[Casmira]

i should of done physics instead of IT