Question

How many structural isomers can be obtained by the replacement of one hydrogen atom of propene with chlorine? (A) 4 (B) 3 (C) 2 (D) 5

Short answer

Two unique structural isomers can be obtained by replacing one hydrogen atom of propene with a chlorine atom: 1) H Cl | | H-C=C-H | H 2) H H | | H-C=C-Cl | H So the correct answer is (C) 2.

Step-by-step solution

Draw the structure of propene.

Propene is an unsaturated hydrocarbon with the molecular formula C3H6, containing a carbon-carbon double bond. The structure of propene can be represented as: H H | | H-C=C-H | H

Replace one hydrogen atom with chlorine.

We will now replace one hydrogen atom with a chlorine atom in the propene structure: H Cl | | H-C=C-H | H H H | | H-C=C-Cl | H H H | | H-C=C-H | Cl

Identify the unique structures.

Now we will compare the structures obtained in step 2 and identify the unique ones. The second and third structures are identical since both have the chlorine atom as a substituent of the double-bonded carbon atom. So we only have two unique structures: H Cl | | H-C=C-H | H H H | | H-C=C-Cl | H

Check the answer options.

Now that we have identified two unique structures, we can check the options given in the problem and find the correct answer. The correct answer is: (C) 2

Key concepts

Understanding Hydrocarbons

Hydrocarbons are the simplest organic compounds, composed exclusively of hydrogen and carbon atoms. These molecules form the foundation of organic chemistry due to their widespread occurrence and importance in various chemical reactions.
Hydrocarbons are categorized into two major types based on the type of bonding between carbon atoms:

• Saturated hydrocarbons (alkanes): These contain only single bonds, with the general formula being \(C_nH_{2n+2}\).
• Unsaturated hydrocarbons: These include alkenes and alkynes, characterized by double or triple bonds. Propene, as highlighted in the exercise, is an example of an alkene.

Understanding these basic categories helps in identifying the nature and reactivity of a given hydrocarbon, aiding in predicting the outcome of reactions such as substitution or addition.

Exploring Propene

Propene, also known as propylene, is a small unsaturated hydrocarbon with the molecular formula \(C_3H_6\). It consists of three carbon atoms forming a chain with one double bond between two of these carbons. The presence of this double bond classifies propene as an alkene.
Its structure can be visualized as:

• One carbon-carbon double bond connected to two hydrogen atoms and one carbon atom (H-C=C-CH3).
• This double bond is the reason for propene's higher reactivity compared to saturated hydrocarbons like propane, as it enables the molecule to participate in reactions such as addition or substitution.

The double bond also causes propene to exhibit isomerism due to the fixed geometric configuration, making it vital to understand its potential transformations, like chlorination.

Chlorination in Organic Chemistry

Chlorination is a substitution reaction where one or more hydrogen atoms of an organic compound is replaced by chlorine. In propene, chlorination typically involves replacing hydrogen atoms bonded to carbon, especially those participating in the double bond.
The chlorination process in hydrocarbons can lead to multiple products because the placement of chlorine can vary:

• In reactions involving alkenes such as propene, the addition of a halogen like chlorine may yield different structures, known as structural isomers.
• The position of chlorine substitution affects not just the structure but also the chemical properties of the resulting isomer, highlighting the significance of determining unique possible structures.

This complexity underlines why understanding possible isomers and their formation mechanisms is crucial in organic chemistry.

Solving Organic Chemistry Problems

Organic chemistry problems often require a methodical approach to explore possible molecular arrangements and reactions. To solve such problems effectively, consider these steps:

• Understand the given structure: Analyze the existing bonds and atom arrangements, like identifying the carbon backbone and any functional groups or double bonds.
• Apply known reactions: Use reactions relevant to the problem, such as substitution in the case of replacing hydrogens with chlorine in propene.
• Compare resulting structures: Identify unique structures by examining the spatial positioning of the new substituents or changes.
• Verify the solution: Cross-check your derived structures against the problem's criteria or provided options to ensure accuracy in problem-solving.

By breaking down problems into these steps, students can systematically approach and solve organic chemistry exercises, gaining a deeper understanding of structural isomers and their complex interactions.