A gaseous hydrocarbon rapidly decolourizes bromine water in tetrachloromethane, but fails … — JAMB UTME

A gaseous hydrocarbon rapidly decolourizes bromine water in tetrachloromethane, but fails to form a precipitate when bubbled through an ammoniacal solution of silver nitrate. Which of the following hydrocarbons exhibits this chemical behavior?

PropeneAnswer
B
Propyne
C
Ethyne
D
Propane

Answer

Propene

Propene is an alkene (

CH_3CH=CH_2

). Its carbon-carbon double bond allows it to undergo electrophilic addition with bromine water, decolourizing the reddish-brown solution. However, because it is an alkene rather than a terminal alkyne, it lacks an acidic hydrogen atom bonded to a triply-bonded carbon atom, so it does not produce a precipitate when treated with ammoniacal silver nitrate.

Step-by-Step Solution

Analyze the reaction with bromine water.

Rapid decolourization of bromine water confirms the presence of unsaturation (a double or triple carbon-carbon bond). Saturated hydrocarbons like propane are eliminated.

Unsaturated hydrocarbons undergo addition reactions across their double or triple bonds to absorb bromine.

Analyze the outcome with ammoniacal silver nitrate solution.

The absence of a precipitate indicates that the hydrocarbon is NOT a terminal alkyne (

R-\text{C}\equiv\text{C}-\text{H}

Only terminal alkynes possess acidic acetylenic hydrogen atoms capable of being replaced by silver ions to form insoluble metallic acetylides.

Identify the hydrocarbon matching both criteria.

Propene (

CH_3CH=CH_2

) is an alkene. It decolourizes bromine water due to unsaturation but yields a negative test with ammoniacal silver nitrate.

Alkenes contain carbon-carbon double bonds but lack acidic acetylenic hydrogen atoms.

Key Concept

Distinguishing alkenes from terminal alkynes using unsaturation and acetylenic hydrogen reagents

Estimated Time:1m 0s

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