distinguishing between alkanes and alkenes prac (1 Viewer)

[xbimbo]

We did a prac on distinguishing between cyclohexane and cyclohexene by adding bromine water. Has anyone else done that one? I couldn't do some of the questions on that prac. Here are the ones I wasn't sure of:

1. What are the structural equations if pure bromine was added to the cyclohexane and cyclohexene? Explain why.
2. What are the structural equations if bromine water was added? Explain.
3. Why is bromine soluble in both water and hydrocarbons?
Also, can anyone explain the equilibrium of bromine and hydoxide molecules in bromine water?

Any ideas?
Thanks

 

[Riviet]

Well, i just done some work on this section but haven't done the prac yet, but this is what you should observe from the prac. Firstly if you look at the two reactions one will lose colour and the other remains browny red. The one that loses colour is the one with cyclohexene because in cyclohexene, there is a double bond which causes the molecule to be more reactive with other substances. So when the Br2 reacts with cyclohexene, Br2 breaks into 2 Br's and that is why you see the solution lose colour since Br by itself has no colour unless if its diatomic ie Br2. Now in the other reaction with cyclohexane, the solution stays browny red and means the Br2 is still in solution, and that it did not react and therefore hasn't broken up into Br's and hence you see the colour remain in solution. That should help with the explaining required in Q1.

 

[s_t_a_r1234]

its something to do with it being a saturated compound..dunno meself, guess that makes 3 clueless ppl

 

[Dreamerish*~]

This is more than you need to know for this particular prac.

I'll assume you know what cyclohexane and cyclohexene looks like and are able to apply my explanation to them because it's not very easy to draw them here. Hence I'm going to explain this using hexane and hexene.

1. What are the structural equations if pure bromine was added to the cyclohexane and cyclohexene? Explain why.

Pure bromine is Br₂. When reacted with an alkane under U.V. radiation, a subsitution reaction occurs.

C₆H₁₄ + Br₂ → C₆H₁₃Br + HBr

Because alkanes are saturated compounds - that is, they have the maximum number of H atoms to each C atom - they are unable to undergo addition reactions. They only react (slowly) with bromine, chlorine and iodine in presence of U.V. light.

Alkenes, on the other hand, undergo addition reactions.

C₆H₁₂ + Br₂ → C₆H₁₂Br₂

Sorry I can't demonstrate via structural formulae, but basically, the double bond in the alkene opens up to join onto the two Br atoms.

2. What are the structural equations if bromine water was added? Explain.

Pure bromine is Br₂ while bromine water is HOBr, so the same principles apply. It will react with hexene to form HO-CH₂-CH₂-Br, which is 2-bromoethanol. Under U.V. light, it will react in the same way with alkanes - via substitution. I'm not clear on whether the H atom will be replaced with the OH or Br, but I don't see why it can't be either.

3. Why is bromine soluble in both water and hydrocarbons?

Bromine is non-polar. Since like dissolves in like, it dissolves in hydrocarbons because hydrocarbons are non-polar. However (now I'm just making an educated guess), it seems it reacts with water to form HOBr as bromie water is HOBr. So I guess it's more of reacting than dissolving.

Also, can anyone explain the equilibrium of bromine and hydoxide molecules in bromine water?

I doubt you need to know this. I suppose it's like the self-ionisation of water:

HOBr ←→ OH^- + Br⁺

 

[Casmira]

does pure bromine discolour? or do schools use broime water because its cheaper?

 

[Dreamerish*~]

does pure bromine discolour? or do schools use broime water because its cheaper?

Yes, pure bromine does discolour when it's reacted with alkenes or alkanes.

Ethene: CH₂=CH₂ + Br₂ → Br-CH₂-CH₂-Br

Ethane: CH₂-CH₂ + Br₂ + U.V. → CH₂-CHBr + HBr

Hydrogen bromide (HBr) is colourless. Therefore ethane also decolorises Br₂.

 

[s_t_a_r1234]

..................................................

do alkanes ONLY undergo substitution reactions, do alkenes ONLY undergo addition reactions.?

 

[Dreamerish*~]

do alkanes ONLY undergo substitution reactions, do alkenes ONLY undergo addition reactions.?

No, they both undergo combustion.

 

[nit]

well alkenes undergo addition and hardly ever substitutions, alkanes undergo substitutions and never additions. As far as alkane reactions go, there isnt a wide variety of commonly encountered reactions - mostly radical reactions. Alkenes have a much wider set of chemical reactions - oxidation (ie combustion above), hydration, hydrohalogenation etc and this is due to the reactivity of the double bond, promoting addition reactions.

 

[.ben]

How did you get all this when ur in Eastment's class?

 

[Dreamerish*~]

How did you get all this when ur in Eastment's class?

I still can't quite figure it out - who are you?

Oh and about that Eastment comment ... Well, if it wasn't for excessive studying before the start of year 12 and tutoring, I would be failing Chem. She's a good head teacher, but it's just that being a head teacher means she has a lot to do, which results in not being in class a lot of the time. At the end of the year she was very helpful, but it was a little late.

 

[bassqueen16]

lol i have a superduper chem teacher, and a small class which actually loves chem, so im happy, im just a little thick on stuff so it takes me a while to get things... we did this experiment with potassium permanganate, cos we didnt have bromine water-what properties of potassium permanganate make this experiment still work?

 

[bassqueen16]

yea wat exactly makes this work with potassium permanganate?

 

[Riviet]

yea wat exactly makes this work with potassium permanganate?

Potassium permanganate will probably react with the alkene only, not sure what causes this to occur.