Question

Which one of the following gives sooty flame on combustion?
(A) \[{{C}_{2}}{{H}_{4}}\]
(B) \[C{{H}_{4}}\]
(C) \[{{C}_{2}}{{H}_{6}}\]
(D) \[{{C}_{6}}{{H}_{6}}\]

Answer 1

Hint: Saturated hydrocarbons undergo complete combustion with oxygen and produce a clean blue flame whereas unsaturated hydrocarbons burn with sooty flame which is yellow in colour. Use this hint to extract the correct option.

Complete answer:
> Hydrocarbons having double or triple bonds do not burn completely in air and have unburnt carbon which results in blackish smoky flames. This is due to the fact that unsaturated hydrocarbons like \[{{C}_{2}}{{H}_{2}},{{C}_{6}}{{H}_{6}}\], etc have relatively more carbon content than that of saturated hydrocarbons like \[C{{H}_{4}},{{C}_{2}}{{H}_{6}}\], etc.
Let us try to understand it with an example:
> Ethane (\[C{{H}_{3}}C{{H}_{3}}\]) has four hydrogen atoms more than that of ethyne (\[CH\equiv CH\]). In other words, the carbon to hydrogen ratio in acetylene is more than that of ethane. More percentage of carbon requires more oxygen for combustion.
We can rule out methane (\[C{{H}_{4}}\]) and ethane (\[{{C}_{2}}{{H}_{6}}\]) from the above given options.
> Both ethene (${{C}_{2}}{{H}_{4}}$) and benzene (\[{{C}_{6}}{{H}_{6}}\]) have unsaturation in them. But benzene is an aromatic compound having relatively more carbon content (carbon to hydrogen ratio). So it does not get completely oxidized during combustion and gives out sooty flames.

Hence, the correct option would be option (D)- benzene.

Additional Information In qualitative analysis, to find out whether a compound is aliphatic or aromatic, it is usually heated on a flame. If the compound burns with a yellow sooty flame then it is aromatic.

Note: It is to be noted that \[{{C}_{2}}{{H}_{4}}\] is the simplest unsaturated compound and \[{{C}_{6}}{{H}_{6}}\] is an aromatic compound having comparatively higher carbon to hydrogen ratio. Therefore, \[{{C}_{6}}{{H}_{6}}\] is more likely to burn with sooty flame.