alkenes and decoloration?

[Unlearner]

which of the following families of compounds will lead to decoloration of bromine in carbon tetra chloride?
a.) alkene
b.) alcohol
c.) ether
d.) ketone

the answer is alkenes. can someone explain how and why? does the double bond lead to coloration or decoloration?

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[loveoforganic]

The first thing you need to know is that bromine solutions are deeply colored and brown. Decoloring the solution implies that you're consuming the solvated bromine, thus turning the solution from brown to clear. This happens with alkenes due to the halogenation reaction, where two bromines are added to the alkene.

 

[linkin06]

http://www.chemguide.co.uk/organicprops/alkenes/halogenation.html

 

[rockaction]

CH2=CH2 + Br2 (red) ---> CH2BrCH2Br (clear)

When you react an alkene with Br2, the bromines add across the double bond. Br2 is red, and the product is clear. When you have an alkene, the Br2 is used up in the reaction and the color becomes clear. If you react Br2 with something else...like an ether...

CH3-O-CH3 + Br2 (red) ---> nothing, so solution stays red

You may have done this in your Organic lab to illustrate this point. It's kind of an old way to test for alkenes. In my Organic I lab we synthesized cyclohexene from cyclohexanol, and used Br2 to see if we successfully got the product.