Cyclohexene + Bromine Water (1 Viewer)

[js992]

What exactly is the equation of the reaction taking place?

I was thinking C6H10 + Br2 ---> C6H10Br2

But somewhere else i saw

C6H10 + Br2 +H2O ---> C6H10BrOH + BrOH



Also : is the state of bromine water (l) or (aq)?

thanks

 

[Darnie]

Bromine water is actually BrOH, as the reaction occurs in the bottle; Br2 + 2H2O ---> 2BrOH + H2 so the reaction proceeds:

C6H10 + BrOH --> C6H10BrOH

 

[brenton1987]

Bromine water is actually BrOH, as the reaction occurs in the bottle; Br2 + 2H2O ---> 2BrOH + H2 so the reaction proceeds:

Br₂ + H₂O --> BrOH + HBr

 

[Aerath]

For pretty much all of my HSC, I wrote the formula for bromine water as Br2. =\
So C6H10 + Br2 --> C6H10Br2.

Not 100% sure though.

 

[Aquawhite]

The correct way to write it is BrOH as said above.

 

[study-freak]

Bromine water is actually BrOH, as the reaction occurs in the bottle; Br2 + 2H2O ---> 2BrOH + H2 so the reaction proceeds:

C6H10 + BrOH --> C6H10BrOH

Br₂ + H₂O --> BrOH + HBr

+1 to highlighted ones

and BrOH is aqueous.

 

[DrDream_Cardio]

Also...can someone give me some safety precautions for this experiment...
thanks.

 

[mtsmahia]

Also...can someone give me some safety precautions for this experiment...
thanks.

Bromine water is Highly toxic if ingested, and slightly corrosive
Cyclohexene/ane is poisonous if ingested... they also give off fumes as they are highly volatile and flammable.

 

[tarnished]

For pretty much all of my HSC, I wrote the formula for bromine water as Br2. =\
So C6H10 + Br2 --> C6H10Br2.

Not 100% sure though.

Confirmed this with my Chem teacher and a girl who topped Chem, that you can write it as Aerath has (you won't get marked down), although BOS prefer the equation involving HOBr. It's one of those Board things that pisses off Chem teachers who religiously follow the IUPAC. xD

But yeah, the HOBr one is right.

 

[ChemNerd]

I was going through my notes from last year.. and i saw that we did hexene/ane not cyclohexene/ane.

My friend is having trouble with safety precautions...
anyone help?!?

 

[dog on heat]

yeh m8

 

[monzilicious]

I was going through my notes from last year.. and i saw that we did hexene/ane not cyclohexene/ane.

My friend is having trouble with safety precautions...
anyone help?!?

Isnt hexene/ane too dangerous to use in the labs... so
thats why everyone has to do cyclohexene/ane???

 

[study-freak]

Isnt hexene/ane too dangerous to use in the labs... so
thats why everyone has to do cyclohexene/ane???

more dangerous than cyclohexene/ane but still usable
I'd use cyclohexene/ane in exams though.

 

[study-freak]

I was going through my notes from last year.. and i saw that we did hexene/ane not cyclohexene/ane.

My friend is having trouble with safety precautions...
anyone help?!?

Hexane/ene are highly volatile and flammable. Hence the experiment must be conducted in the absence of naked flames/sparks that could ignite fumes. Well ventilated area and fume cupboard must be used to prevent the build-up of flammable fumes. If ignition occurs, immediately wash the affected area with plenty of cold water until the fire ceases.
Bromine water and hexane/ene are toxic and safety googles and protective clothing must be worn at all times during the experiment to prevent ingestion. Wash immediately with flowing water if ingestion occurs and consult with doctors. Disposal of organic wastes should be done in an organic waste bottle and special procedures need to be followed (companies will do this).

 

[maximuscheesius]

Re: Cyclohexene + Bromine Water - Exemplar Response; EasyChem

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This was actually a question in the 2002 HSC Exam. Fortunately, that was a year when a standards package was released, including an 'examplar' (perfect score) answer to each question. Here is the ideal answer to the cyclohexene and bromine water question:

http://arc.boardofstudies.nsw.edu.a...050/files/samples/qexemplar/qexemplar_q16.pdf


I would also recommend EasyChem's entry on this syllabus dot-point, as they have a great stylised version of the structural equation used in the exemplar response:

http://www.easychem.com.au/production-of-materials/fossil-fuel-products/bromine-water-experiment


You can't argue with the HSC markers, so I hope this helps resolve all of the discussion!

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[Pwnage101]

Re: Cyclohexene + Bromine Water - Exemplar Response; EasyChem

I was taught:

C6H10(l) + Br2(aq) --> C6H10Br2(aq)

 

[mitochondria]

Re: Cyclohexene + Bromine Water - Exemplar Response; EasyChem

To answer js992's original question, the most pedantic way to answer the question in an exam is the formation of both the halohydrin and dibromo adducts:

-CH=CH- + Br_{2 (aq)} ----> -CHBr-CHOH- + -CHBr-CHBr- + HBr_(aq)

This should also have answered your question as to what the state of bromine should be. Note that I haven't indicated the state of the alkene above because it's not soluble water and actually appears as a separate phase in the case of cyclohexene.

The two reasons why there are such discrepancies between answers is most likely due to the fact that 1. people don't think enough; and or 2. those who teach chemistry wish to simplify things in case students can't cope with it.

First of all, we need to settle the identity of bromine in bromine water. The options we have are Br₂, HOBr and HBr. The majority of bromine most likely exists as molecular bromine in bromine water, that is, Br_{2 (aq)}. Let's consider the scenario where hypobromous acid (HOBr) is produced:

Br₂ + H₂O ----> HOBr + HBr


Hypobromous acid is actually unstable; particularly when under acidic conditions (which is true in this case as one equivalent of HBr, a strong acid, is produced for every part of HOBr) - the equilibrium tends largely to the L.H.S. where molecular bromine is. Therefore the reactive component in the reaction that is being considered above is unlikely HOBr. Even if the reactive species were to be HOBr, the product produced would not actually be either the halohydrin or the dibromide unless decomposition occurs first - which brings us back to molecular bromine after all.

Moving along. Now we consider the mechanism of the reaction. The reaction proceed firstly by polarisation of the Br-Br bond (this can potentially happen in many ways - particular in an aqueous solution of bromine) and then the formation of the bromonium cation intermediate, that is:

Then the following can happen (clearly in an aqueous solution the latter is more likely than otherwise):

Please note that ethylene was used instead of cyclohexene for simplicity.


Hence you get can get either the dibromo adduct or the halohydrin adduct. It is worth noting that in an aqueous solution both can happen.

Ultimately, for the reaction between cyclohexene and bromine water, I think that giving an answer with only the dibromo adduct is strictly correct. This is because cyclohexene is not soluble in water and therefore diffusion of Br₂ into the cyclohexene layer should happens first, followed by the reaction with cyclohexene; and that way the reaction should produce much less of the halohydrin.

C6H10 + Br2 +H2O ---> C6H10BrOH + BrOH

Please note that this equation is wrong. See above.

Bromine water is actually BrOH, as the reaction occurs in the bottle; Br2 + 2H2O ---> 2BrOH + H2 ...

I don't think the reaction you have there is right - bromine doesn't undergo a redox reaction with water. See above (or below).

Br₂ + H₂O --> BrOH + HBr

Please see the edited argument above - thanks for pointing out the mistake earlier and, once again, apologies for my ignorance.

The correct way to write it is BrOH as said above.

I really don't think so - please see the explanation above.

Confirmed this with my Chem teacher and a girl who topped Chem, that you can write it as Aerath has (you won't get marked down), although BOS prefer the equation involving HOBr. It's one of those Board things that pisses off Chem teachers who religiously follow the IUPAC. xD

But yeah, the HOBr one is right.

Ummm. Are you sure that your chem teacher and the girl who topped chem know what they are talking about? Where did they get the information that BOS prefers HOBr from?

Also, this has absolutely nothing to do with the IUPAC... Teachers often get pissed off and blame BOS when they are unsure (although that's not to say BOS isn't to be blamed for a lot of things - just not this particular one).

I was taught:

C6H10(l) + Br2(aq) --> C6H10Br2(aq)

For pretty much all of my HSC, I wrote the formula for bromine water as Br2. =\
So C6H10 + Br2 --> C6H10Br2.

Not 100% sure though.

Both of you are technically right - regarding the state of bromine and the actual equation. For reasons detailed above.

Also...can someone give me some safety precautions for this experiment...
thanks.

I was going through my notes from last year.. and i saw that we did hexene/ane not cyclohexene/ane.

My friend is having trouble with safety precautions...
anyone help?!?

Isnt hexene/ane too dangerous to use in the labs... so
thats why everyone has to do cyclohexene/ane???

Like most things, you just need to be sensible and deal with it. Concentrated bromine water can cause severe burns and inhaling cyclohexene/cyclohexane may give you highs (they are possibly carcinogenic - again, like everything else and may cause infertility).

I highly doubt that the "sensible" people who came up with the syllabus would actually put people (potentially incompetent students and, occasionally, teachers at risk) in order to cover their bums.

 

[Pwnage101]

Re: Cyclohexene + Bromine Water - Exemplar Response; EasyChem

Thankyou mitochondria, for an excellent explanation. I'm sure this will assist many HSC Chemistry students.

 

[brenton1987]

First of all, we need to settle the identity of bromine in bromine water. Bromine most likely appears as molecular bromine in bromine water, that is, Br_{2 (aq)}. There are a couple of reasons (feel free to make any corrections should I make a mistake):

1. Br⁺ would be energetically quite unstable
2. In order to form Br⁺OH^- in solution the balanced equation would be Br₂ + H₂O ---> BrOH + HBr. Now HBr is a strong acid, if there is an equivalent amount of HBr in solution, wouldn't all the BrOH be neutralised?
3. If I'm not mistaken, quick Googling showed that Br⁺ is colourless. If BrOH were to exist in large quantities, as suggested by several people in the post, bromine water should be colourless or has only a very faint colour - which isn't the case.

HOBr is hypobromous acid not bromine hydroxide. The ionisation products are H⁺ and BrO^-.
The products of Br_2(l) + H₂O_(l) --> HOBr_(aq) + HBr_(aq) are both acids.

 

[mitochondria]

HOBr is hypobromous acid not bromine hydroxide. The ionisation products are H⁺ and BrO^-.
The products of Br_2(l) + H₂O_(l) --> HOBr_(aq) + HBr_(aq) are both acids.

You are absolutely correct. Apologies for my ignorance.

I will edit the relevant sections of my post. Thank you very much.