Alkane/Alkene Reaction

[green_appletini]

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Hey guys!

I've always been confused about this so it'd be great ify ou someone can help I'm just really confused about what i need to know about this 1st hand investigation we did. See, all my friends in other schools tell me all i needs to know is:
- alkane + bromine water, no change
- alkene + bromine water, spontaneous/ bromine discoloured

but my teacher at school told us we need to talk about which layers ended up on top and which ones ended up on bottom (cos when we did it, we added the substances and shook the test tube). Then our other conclusion was also that alkane reactions required UV (a catalyst), otherwise a reaction won't occur.

So if they do ask us about this experiment in the exam, is it sufficient to just say alkane/bromine (no change) and alkene/bromine (discolour)?? Obviously i'll answer it in more detail..but how about the stuff my teacher said? I just don't want to write more than necessary, what do you guys think??

thanks heaps!
-marilia- xoxo

[tina_goes_doo]

It is always a good idea to state what substances you used in a first hand investigation.

A question they usually ask is to describe how you would distinguish between "a named alkane and a named alkene". When this is the case give an example, eg. cyclohexane and cyclohexene. Obviously from here on in you describe the reactions with bromine water and the experiment is conducted without the presence of UV light as this would allow the alkane to react as well.

For a question on this you could also draw a structural equation for this reaction (they sure do love it when you include equations).

I'm pretty sure you don't have to describe the layers. It seems a bit specific and i don't think they would ask it so you probably don't need to include that.

Or of course, when in doubt see what the question is out of first, how much space you have and then work out how much you need to write.

[mervvyn]

I don't think we need to know the colours and orders of the layers, i agree that it seems pretty specific.
Being able to name your stuff is good, and my teacher believes that equations are a must for pretty much anything.
Also worth knowing that alkane/bromine is substitution --> alkane + Br2 --> bromoalkane + HBr
and alkene/bromine is addition --> alkene + HOBr --> bromohydroalk[B}A{/B]ne
except you'd do them as condensed structural like tina said.
Alkene is spontaneous, alkane requires UV, both use shaking to mix the layers.

If you wrote all of that, in slightly more organised fashion, i think that would cover it.

[d_elmo]

anywhere you can, stick equations in, it might get you that extra mark...

[Tommy_Lamp]

its good to learn the reaction equations:
alkene + bromine - H2C=CH2 + Br2 ---> CH2BrCH2Br
and if you can learn the alkane one under UV light:
CH3Ch3 + Br2 ---> CH2BrCH3 + HBr

not necessary but will be good to write down, also learning the structures aswell, i.e. if you use cyclohexane and cyclohexene, learn the simplified drawing (as in no 'C's) and add the Br(s) were appropriate

its outlined well in the excel book

[xiao1985]

uhm not sure abt this tommy lamp... maybe tis just me, but i think in addition alkene react with HBr rather than Br2 (l)

[tina_goes_doo]

Bromine water. Do people represent that as Br-Br or Br-OH?

[kacca29]

Bromine water is (using correct subscripts of course) Br2 (aq) ie molecular bromine dissolved in water similar to ionics like NaCl (aq) though without dissociation. So structurally Br-Br.

Cheers

[Tommy_Lamp]

Originally Posted by xiao1985

uhm not sure abt this tommy lamp... maybe tis just me, but i think in addition alkene react with HBr rather than Br2 (l)

i always thought it was HBr that was produced

EDIT: i checked out my excel book and it has the same equation written

[xiao1985]

Originally Posted by Tommy_Lamp

i always thought it was HBr that was produced

EDIT: i checked out my excel book and it has the same equation written

my mistake... tommylamp... yes, tis br2 (l) that is reactin...

[Slidey]

Conquering chemistry has this to say:
Br₂ + H₂O <-> HOBr + Br^- + H⁺
CH₃-CH₂-CH=CH-CH₂-CH₃ + HOBr --> CH₃-CH₂-CH(-OH)-CH(-Br)-CH₂-CH₃
That is: 3-hexene + bromine water gives 4-bromo-3-hexanol.

Now, I've made a thread about bromine water + hydrocarbons before, and once I find it, I'll look over it again, but I'm really confused, because I've been told about 5 different reactions for alkene + bromine. One by my teacher, one by my text book, and another 3 by people on this forum.

Is the one above acceptable enough? I need to know because I'm polishing my study notes and my prac write up, so...

[Slidey]

Also, in my prac we used cyclohexene... does that mean the product is some form of cyclobromohexanol?

[TimeAndTide]

Check my Practical (attachment)

[lum]

thx for the prac man, i didn't get to write the stuff down caz i was away when we did it.
one question though, what are the safetly issues dealing with the cyclo hexane/hexene? since it's poisonous (if u inhale heaps), so we put it under a vent hood, but what happens to the chemicals afterwards? where does the school send it to?

[undecided17]

Well according to what I learnt from tutor not school

the top layer is called the organic layer
the bottom is the aqueous layer

and with cyclohexane+Br2 Water
both layers are amber in colour
Organic layer: Br2 dissolved(not reacted) in cyclohexane
Aqueous layer: Br2 dissolved (not reacted) in H20


while in a reaction between cyclohexene and Br2water
both layers are colourless
and what each layer consists of, i can't give you a definite answer

hopefully this will help anyway