Can you guys explain to me why for the bromine water prac, it's either Br2 or HOBr? Can you show both word and structural equations for the reactions that take place with cyclohexene + bromine water, and cyclohexene + bromine water?
For your first question, it does not matter whether we use Br
2 or HOBr. However, for reference sake, this is what
actually happens.
Generally speaking, rather than using liquid bromine we mix the bromine
into the water. A reaction takes place:
Br
2(l) + H
2O
(l) -> HBr
(aq) + HOBr
(aq)Note that the hydrobromic acid does nothing.
(Not too sure but I think bromine water is preferred over actual bromine because it oxidises more easily.)
____________________
To understand the reactions that take place you need to consider what exactly are substitution and addition reactions. Substitution reactions are exactly that - certain atoms effectively displace other atoms. Addition reactions are when bonds are broken and the atoms form new bonds.
So say you went from ethane to ethanol through reaction with water. Then two of the hydrogens are going to have to be substituted for the H and OH from water.
If you went form ethene to ethanol though, the double bond breaks and the water molecule just adds on. The H goes to one carbon whereas the OH goes to the other.
Now, cyclohexene has formula C
6H
10. We can't really assign it a number as to where the double bond is (unlike say, 1-hexene), but we know that there is a double bond there. It has a high electron density and is very reactive, so when the HOBr is introduced it will readily add on.
(Note that HOBr is what has the brown or whatever colour. The bromine water is what gets decolourised, not the cyclohexene.)
C
6H
10(l) + HOBr
(aq) -> C
6H
10BrOH
(l)On the contrary, cyclohexane only contains single bonds. For a substitution reaction to take place on top of any of those single bonds we need to provide much more energy. In fact, this can only be done so through adding UV radiation.
C
6H
12(l) + HOBr
(aq) -(UV)-> C
6H
10BrOH
(l) + H
2(g)Under laboratory conditions, you're not going to find that much UV at all. So you will see the cyclohexene decolourise it, and not the cyclohexane. That is all.
Structural diagrams can be found at
EasyChem