So in the previous post (Hydrocarbons(10)) I explained how electrophilic addition of bromine to an alkene like propene or ethene could take place.
And we were supported in our thinking by my organic chemistry hero ROC Norman!!
So what happens when we do that simple alkene test using bromine water?
How does the water interfere?
If you've done any simple halogen chemistry at this point you should be thinking yea I remember what happens between water and a halogen.
So what does happen say between water and bromine?
The reaction produces two acids:
Br2 + H2O = HBr + HOBr
The two acids are hydrobromic acid (HBr), an analogue of hydrochloric acid, and bromic (I) acid (HOBr)
Both these acids are ionic molecules and that means they can be involved in an ionic mechanism like electrophilic addition.
Let's look at what happens then with ethene and water:
Here's the first stage:
But the bromide ion (Br-) isn't the only species in the reaction that could add to the carbocation.
Water is just as able to add to the carbocation.
When it does a bromohydrin forms.
See the equation below:
CH2=CH2 + Br—Br(aq) = +H2O—CH2—CH2Br = HOCH2—CH2Br + H+
To quote ROC Norman: The intermediate carbocation may be attacked by any nucleophile which is present. For example, the addition of bromine to ethene is aqueous solution gives both the dibromide and the bromohydrin since water is nucleophilic.
And the bromohydrin is also colourless.
The action of hydrobromic acid is similar in ethene the bromoetheae is formed.
CH2=CH2 + H—Br(aq) = +CH2—CH3 + Br- = BrCH2—CH3
But it gets interesting when propene is used instead of ethene as I will discuss in the next post (Hydrocarbons (12)).
Pages on the "Mole" and "Using the Mole" in chemical calculations are here
And we were supported in our thinking by my organic chemistry hero ROC Norman!!
So what happens when we do that simple alkene test using bromine water?
How does the water interfere?
If you've done any simple halogen chemistry at this point you should be thinking yea I remember what happens between water and a halogen.
So what does happen say between water and bromine?
The reaction produces two acids:
Br2 + H2O = HBr + HOBr
The two acids are hydrobromic acid (HBr), an analogue of hydrochloric acid, and bromic (I) acid (HOBr)
Both these acids are ionic molecules and that means they can be involved in an ionic mechanism like electrophilic addition.
Let's look at what happens then with ethene and water:
Here's the first stage:
But the bromide ion (Br-) isn't the only species in the reaction that could add to the carbocation.
Water is just as able to add to the carbocation.
When it does a bromohydrin forms.
See the equation below:
CH2=CH2 + Br—Br(aq) = +H2O—CH2—CH2Br = HOCH2—CH2Br + H+
To quote ROC Norman: The intermediate carbocation may be attacked by any nucleophile which is present. For example, the addition of bromine to ethene is aqueous solution gives both the dibromide and the bromohydrin since water is nucleophilic.
And the bromohydrin is also colourless.
The action of hydrobromic acid is similar in ethene the bromoetheae is formed.
CH2=CH2 + H—Br(aq) = +CH2—CH3 + Br- = BrCH2—CH3
But it gets interesting when propene is used instead of ethene as I will discuss in the next post (Hydrocarbons (12)).
Pages on the "Mole" and "Using the Mole" in chemical calculations are here
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