Question

Two jars \(\mathrm{A}\) and \(\mathrm{B}\) are filled with hydrocarbons. \(\mathrm{Br}_{2}\) in \(\mathrm{CCl}_{4}\) is added to these jars. A does not decolourise the \(\mathrm{Br}_{2}\) solution but B decolourises. What are \(\mathrm{A}\) and \(\mathrm{B} ?\) (1) Alkane and alkene (2) Alkene and alkane (3) Alkene and alkyne (4) None of the above

Short answer

Jar A contains an alkane, and Jar B contains an alkene. The correct answer is (1) Alkane and Alkene.

Step-by-step solution

Identify the reaction

When \(\text{Br}_{2}\) in \(\text{CCl}_{4}\) is added to hydrocarbons, it reacts with alkenes and alkynes by decolorizing due to the addition reactions at the double or triple bonds. Alkanes do not react and hence do not decolorize the \(\text{Br}_{2}\) solution.

Analyze Jar A

Jar A does not decolorize \(\text{Br}_{2}\) solution. This indicates that it contains a hydrocarbon that does not have double or triple bonds. Therefore, it is most likely an alkane.

Analyze Jar B

Jar B decolorizes the \(\text{Br}_{2}\) solution. This indicates that it contains a hydrocarbon with double or triple bonds that reacts with \(\text{Br}_{2}\). Therefore, it is most likely an alkene or alkyne.

Determine the specific hydrocarbons in A and B

Since the options provided are distinguishing between alkanes, alkenes, and alkynes, and only one jar does not react (Jar A), the specific hydrocarbons can be identified as follows: Jar A contains an alkane and Jar B contains an alkene. The appropriate answer is (1) Alkane and Alkene.

Key concepts

Hydrocarbons

Hydrocarbons are organic compounds made of only carbon (C) and hydrogen (H) atoms. These compounds are the primary constituents of many fuels like petroleum and natural gas. Hydrocarbons can be broadly classified into several categories based on their chemical properties and structures, but the most common ones are:

• Alkanes
• Alkenes
• Alkynes
• Aromatics

Understanding hydrocarbons is crucial because their reactions form the basis for various chemical processes, such as the Bromine test used to distinguish different types of hydrocarbons.

Alkane

Alkanes are a type of hydrocarbon that contain only single bonds between carbon atoms. This makes them saturated hydrocarbons. Alkanes have a general formula of \(\text{C}_n\text{H}_{2n+2}\), where \(n\) is the number of carbon atoms. Here are some important characteristics of alkanes:

• They are generally unreactive
• They do not easily participate in addition reactions
• Common examples include methane (CH₄), ethane (C₂H₆), and propane (C₃H₈)

Because alkanes lack double or triple bonds, they do not decolorize Bromine solution, making them easily distinguishable from alkenes and alkynes in tests like the Bromine test.

Alkene

Alkenes are hydrocarbons that contain at least one double bond between carbon atoms, making them unsaturated hydrocarbons. The general formula for alkenes is \(C_nH_{2n}\). These double bonds give alkenes unique properties:

• They are more reactive than alkanes
• They readily participate in addition reactions
• Common examples include ethene (C₂H₄), propene (C₃H₆), and butene (C₄H₈)

In the Bromine test, alkenes react with Bromine water, leading to the decolorization of the reddish-brown solution. This reaction occurs because the Bromine atoms add across the double bond of the alkene, forming a dibromo compound.

Addition reaction

An addition reaction is a chemical reaction where atoms or groups of atoms are added to a molecule without taking any atoms away. This type of reaction often occurs in unsaturated hydrocarbons like alkenes and alkynes:

• Involves breaking of a double or triple bond
• Atoms are added to the carbon atoms that were involved in the multiple bonds

A classic example of an addition reaction is the reaction between an alkene and Bromine (\text{Br}_{2}). The general reaction can be written as: \[\text{R}-\text{CH}=\text{CH}-\text{R'} + \text{Br}_{2} \rightarrow \text{R}-\text{CHBr}-\text{CHBr}-\text{R'}\] In this reaction, the double bond in the alkene breaks, and the Bromine atoms add to the former double-bonded carbons, decolorizing the Bromine solution. This is why alkenes like the hydrocarbon in Jar B decolorize the Bromine solution, unlike alkanes.