A hydrocarbon X which decolorizes bromine water... | WAEC SSCE - Chemistry - 1990 (Essay)

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A hydrocarbon X which decolorizes bromine water but has no action on ammoniacal silver trioxonitrate (V) solution was found to have a molar mass of 58 g mol\(^{-1}\)

(a) Deduce the molecular formula of X. (H = 1, C = 12)

(b) Write the structures of two isomers of X.

Answer

Interpreting the clues

X decolorises bromine water, so X is unsaturated (it contains a carbon-carbon double or triple bond and undergoes addition).
X has no action on ammoniacal silver trioxonitrate(V) (Tollens' reagent gives no precipitate), so X is not a terminal alkyne (it has no \(\equiv C-H\) at the end of the chain). It is therefore an alkene, a diene, or an internal alkyne.

(a) Molecular formula

Test the unsaturated general formulae against the molar mass:

Alkene \(C_nH_{2n}\): \(14n = 58 \Rightarrow n = 4.1\) (not a whole number).
Alkyne / diene \(C_nH_{2n-2}\): \(14n - 2 = 58 \Rightarrow n = 4.3\) (not a whole number, and \(14n-2 = 54\) for \(n=4\)).

The only whole-number hydrocarbon fitting the unsaturated, non-terminal-alkyne description is \(C_4H_6\) (molar mass \(4\times12 + 6 = 54\ \text{g mol}^{-1}\)); the quoted value 58 appears to be a misprint for 54. Taking the consistent value:

\[\textbf{Molecular formula of X} = C_4H_6\]

(b) Two isomers of X (both unsaturated, neither a terminal alkyne)

But-2-yne: \(CH_3-C\equiv C-CH_3\) (internal triple bond, so no reaction with ammoniacal \(AgNO_3\)).
Buta-1,3-diene: \(CH_2=CH-CH=CH_2\) (two double bonds, decolorises bromine, no reaction with ammoniacal \(AgNO_3\)).

Read lesson note on Chemistry Of Carbon Compounds (Nigeria Only) (WAEC)

Answer Details

Interpreting the clues

X decolorises bromine water, so X is unsaturated (it contains a carbon-carbon double or triple bond and undergoes addition).
X has no action on ammoniacal silver trioxonitrate(V) (Tollens' reagent gives no precipitate), so X is not a terminal alkyne (it has no \(\equiv C-H\) at the end of the chain). It is therefore an alkene, a diene, or an internal alkyne.

(a) Molecular formula

Test the unsaturated general formulae against the molar mass:

Alkene \(C_nH_{2n}\): \(14n = 58 \Rightarrow n = 4.1\) (not a whole number).
Alkyne / diene \(C_nH_{2n-2}\): \(14n - 2 = 58 \Rightarrow n = 4.3\) (not a whole number, and \(14n-2 = 54\) for \(n=4\)).

\[\textbf{Molecular formula of X} = C_4H_6\]

(b) Two isomers of X (both unsaturated, neither a terminal alkyne)

But-2-yne: \(CH_3-C\equiv C-CH_3\) (internal triple bond, so no reaction with ammoniacal \(AgNO_3\)).
Buta-1,3-diene: \(CH_2=CH-CH=CH_2\) (two double bonds, decolorises bromine, no reaction with ammoniacal \(AgNO_3\)).

Read lesson note on Chemistry Of Carbon Compounds (Nigeria Only) (WAEC)

Chemistry Of Carbon Compounds (Nigeria Only)

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A hydrocarbon X which decolorizes bromine water but has no action on ammoniacal silver trioxonitrate (V) solution was found to have a molar mass of 58 g mol...

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A hydrocarbon X which decolorizes bromine water but has no action on ammoniacal silver trioxonitrate (V) solution was found to have a molar mass of 58 g mol...

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