Problem 23 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Write the structural formula of 4-chloro-2-pentene.

Step-by-Step Solution

Answer

The structure is: \( \text{CH}_3-\text{CH}=\text{CH}-\text{CH}(-\text{Cl})-\text{CH}_3 \).

Step 1: Identify the Base Hydrocarbon

First, identify the base hydrocarbon – pentene, which is an alkene with five carbon atoms and a double bond. Since 'pent' refers to five carbon atoms and 'ene' refers to a double bond, the base structure is a five-carbon chain.

Step 2: Locate the Double Bond

Determine the position of the double bond from the name '2-pentene'. Here, '2' indicates that the double bond starts at the second carbon atom. So, the double bond is between the second and third carbon atoms in the chain.

Step 3: Add the Substituent

Add the chlorine atom to the structure at the position indicated – '4-chloro'. Thus, attach the chlorine atom to the fourth carbon in the pentene chain.

Step 4: Draw the Structural Formula

Draw the full structural formula by placing the carbon chain with the double bond positioned from the second carbon, and attach the chlorine to the fourth carbon. The resulting structure will show the positions of hydrogen atoms to satisfy the carbon valences: \[ \text{CH}_3-\text{CH}=\text{CH}-\text{CH}(-\text{Cl})-\text{CH}_3 \].

Key Concepts

Hydrocarbon Chains

Hydrocarbon chains are the backbone of organic chemistry. They consist of carbon atoms linked together in various configurations, with hydrogen atoms attached to balance the carbon valences. At their core, hydrocarbons can be classified into several groups based on the types of bonds and the shapes of the chains. Here are some critical points to understand about hydrocarbon chains:

• Alkanes: These are saturated hydrocarbons with only single bonds. They follow a general formula of \(C_nH_{2n+2}\).
• Alkenes: Unsaturated hydrocarbons that contain at least one double bond, like pentene. Their general formula is \(C_nH_{2n}\).
• Alkynes: These are unsaturated hydrocarbons with one or more triple bonds, with the formula \(C_nH_{2n-2}\).
• Cyclic Hydrocarbons: Carbon chains that form ring structures are called cyclic hydrocarbons and can include any of the above bond types.

Understanding the type of hydrocarbon is essential because it dictates the molecule's reactivity and physical properties. For instance, the presence of double or triple bonds introduces sites of reactivity. In '4-chloro-2-pentene', the chain forms an alkene due to the presence of a double bond.

Structural Formulas

Structural formulas are visual representations of molecules, showing how atoms are arranged and bonded. This map helps chemists understand and predict how molecules will react.
To build the structural formula, follow these steps:

• Select the Carbon Skeleton: Identify the main carbon chain (e.g., the pentene in our exercise).
• Identify Key Bonds: Mark any significant bonds, like double bonds, that need highlighting on the skeletal structure.
• Add Substituents: Place additional atoms or groups that branch off from the main carbon chain, like the chlorine in '4-chloro'.
• Complete with Hydrogen Atoms: Fill in any remaining valences with hydrogen atoms to complete the stable configuration of each carbon atom.

In '4-chloro-2-pentene', you translate these steps into a linear carbon chain with a double bond and a chlorine atom. This is accurately depicted with the formula: \[\text{CH}_3-\text{CH}=\text{CH}-\text{CH}(-\text{Cl})-\text{CH}_3\]

Double Bonds

Double bonds are a defining feature of alkenes. In hydrocarbon chains, they consist of two pairs of shared electrons between two carbon atoms. The presence of a double bond affects both the physical and chemical properties of a molecule.

• Structure and Geometry: Double bonds create a planar configuration, restricting the rotation around the bond axis. This gives each carbon atom in the bond a sp2 hybridization, leading to a 120-degree bond angle.
• Reactivity: Double bonds are points of higher reactivity compared to single bonds. They can easily participate in reactions such as addition reactions, where other atoms or groups add across the double bond.
• Stability: While double bonds signal unsaturation, making the molecule less stable than single-bonded variants, they also provide essential reactive sites for chemical reactions.

In '4-chloro-2-pentene', the double bond is between the second and third carbon atoms, establishing the compound as an alkene. Understanding the nature of double bonds helps predict how this molecule might react in different scenarios.