Question

Give two tests to distinguish between alkanes and alkenes

Answer 1

1. Alkenes can decolourise pink colour of kmn o4 and brown colour of bromine water while alkanes cannot .
2.Alkenes can give ozonisation reaction to form aldehydes and ketones while alkanes cannot .

Answer 2

Number 1 :

By adding bromine solution

Number 2 :

By adding Baeyer's reagnant .

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By adding  cold dilute Bromine solution

If this mixture is added to an unsaturated compounds such as alkenes or alkanes then :

• Bromine water is brown  

• The supernatant liquid gets decolourised.

However saturated compounds do not respond to the above test.

In order to test unsaturation first add bromine solution

If the reagnant decolorises from brown to colourless then the compound is unsaturated

Otherwise if the reaction does not occur , then it is a saturated compound.

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By adding  cold dilute Baeyer's reagnant

More about Baeyer's Reagnant.

• Baeyer's reagnant is cold dilute

• It is an oxidising agent.

If this mixture is added to an unsaturated compounds such as alkenes or alkanes then :

• Purple colour of  changes .

• It is first converted to a green compound   which is unstable in alkaline medium.

• This again changes to brown  

• The supernatant liquid gets decolourised.

However saturated compounds do not respond to the above test.

In simple words........... your answer is :

In order to test unsaturation first add cold dilute Baeyer's reagnant.

If the reagnant decolorises from purple to colourless then the compound is unsaturated

Otherwise if the reaction does not occur , then it is a saturated compound.

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