Problem 10 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The hydrocarbon with seven carbon atoms containing a neopentyl and a vinyl group is : (a) 2,2 -dimethyl-4- pentene (b) 4,4 - dimethyl pentene (c) isopropyl-2-butene (d) 2,2 -dimethyl-3-pentene

Step-by-Step Solution

Answer

The hydrocarbon is 2,2-dimethyl-4-pentene.

Step 1: Understanding the Functional Groups

First, identify what a neopentyl and a vinyl group are. The neopentyl group (also known as 2,2-dimethylpropyl) is a branched alkyl group with the formula -C(CH3)3CH2. The vinyl group is an alkene group with the formula -CH=CH2.

Step 2: Counting the Carbon Atoms

Verify that the hydrocarbon contains seven carbon atoms. With a neopentyl group contributing five carbons and a vinyl group contributing two carbons, these groups together make up the seven carbon atoms needed in the hydrocarbon.

Step 3: Matching the Molecular Structure

Examine each option and determine which one matches the description. We need to account for both the neopentyl and vinyl groups as described. None match the neopentyl group's specific requirement until you realize that dissected, these names need matching on total naming, assuming missing description ideas.

Step 4: Checking Satisfying Structural Formula

Although not clear directly within options, the best fit taking functional part ideas would be near theoretical drawn, 2,2-dimethyl-4-pentene, aligning similar abstracting for structural but limited identity specificity against given names.

Key Concepts

Hydrocarbon Nomenclature

Naming hydrocarbons follows the IUPAC system which is designed to give every compound a unique name. This system can be quite detailed and has some basic principles:

• Identify the longest carbon chain which will serve as the base name of the compound. For example, a chain of five carbons is known as 'pentane'.
• Add prefixes for any branches or substituents attached to this main chain. Substituents such as methyl or ethyl are named and positioned by numbering the main chain from the end nearest to a substituent, to give the lowest possible numbers.
• The presence of double or triple bonds requires modification of the suffix: 'ene' for alkenes (double bond) and 'yne' for alkynes (triple bond). The position of the bond is indicated by a number.
  

These rules make it easy to identify the molecular structure from the name of the molecule, thus allowing chemists to deduce all necessary information regarding the hydrocarbon's configuration.

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties. These groups are responsible for the distinctive reactions a compound undergoes. Let's consider the functional groups mentioned in the exercise.

• The neopentyl group is a branched alkyl chain defined as -C(CH3)3CH2. It significantly alters the physical and chemical properties of the hydrocarbons to which it is attached.


• The vinyl group is an example of an olefin, denoting the presence of a double-bonded carbon, specifically -CH=CH2. This double bond gives rise to additional reactivity, such as polymerization with other vinyl groups.

Recognizing these groups within a compound is crucial as they dictate major aspects of molecular behavior and interactions.

Structural Isomerism

In organic chemistry, structural isomerism occurs when molecules with the same molecular formula have different structural arrangements of atoms. This results in different properties and behaviors.

Key types include:

• Chain isomerism, where isomers differ by the branching of their carbon backbone. For instance, butane (a straight-chain isomer) and isobutane (a branched-chain isomer).


• Position isomerism, where functional groups or bonds are positioned differently within the molecule's main structure. An example is the shift of a double bond in butene from position 1 to position 2.

Understanding structural isomers is vital because they can exhibit drastically different chemical and physical properties despite containing the same types and numbers of atoms. This concept is fundamental in designing compounds with desired features and functionalities.