Question

Give two tests to difference between alkanes and alkenes

Answer 1

most reactions of organic compounds take place at or adjacent to a functional group. In order to establish a baseline of behavior against which these reactions may be ranked, we need to investigate the reactivity of compounds lacking any functional groups. Such compounds are necessarily hydrocarbons, made up of chains and rings of carbon atoms bonded to a full complement of hydrogen atoms (all carbons are sp3 hybridized). From the previous discussion of formula analysis, the formulas for such hydrocarbons will be CnH(2n+2–2r) , where n is the number of carbon atoms and r is the number of rings. Hydrocarbons of this kind are classified as alkanes or cycloalkanes, depending on whether the carbon atoms of the molecule are arranged only in chains or also in rings. Although these hydrocarbons have no functional groups, they constitute the framework on which functional groups are located in other classes of compounds, and provide an ideal starting point for studying and naming organic compounds. Alkanes and cycloalkanes are termed saturated, because they incorporate the maximum number of hydrogens possible without breaking any carbon-carbon bonds. They are also members of a larger class of compounds referred to as aliphatic. Simply put, aliphatic compounds are compounds that do not incorporate any unsaturated aromatic rings in their molecular structure.

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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