Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. A compound with the molecular formula, \(\mathrm{C}_{4} \mathrm{H}_{6}\), does not exist in cis or trans form. It does not produce a precipitate with ammoniacal \(\mathrm{AgNO}_{3}\) solution. The compound as such is optically inactive but a monohalogen derivative can be optically active. The compound is (a) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{C}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\) (d)

Short answer

Question: Identify the correct structure of the compound with the molecular formula C4H6 that fulfills the following properties: 1) Does not exist in cis or trans form. 2) Does not produce a precipitate with ammoniacal AgNO3 solution. 3) Optically inactive but a monohalogen derivative can be optically active. Answer: Option (b): CH3-CH=C=CH2

Step-by-step solution

Option (a): CH3C≡C-CH3

Alkynes do not exist in cis or trans forms. Since it's a symmetric molecule, it's optically inactive. Adding a halogen to the molecule results in the same symmetry, so the monohalogen derivative remains inactive. This option fulfills property 1 but not properties 2 and 3.

Option (b): CH3-CH=C=CH2

This is an allene molecule. Allenes cannot display cis or trans isomers. They don't react with ammoniacal AgNO3, fulfilling property 2. Although the molecule is optically inactive, adding a halogen to it results in an optically active derivative fulfilling property 3. Therefore, option (b) satisfies all the given properties.

Option (c): CH3-CH2-C≡C-H

Alkynes do not exist in cis or trans forms, fulfilling property 1. However, this molecule is optically inactive and its monohalogen derivatives are also optically inactive, not fulfilling property 3. It also doesn't fulfill property 2 about the precipitate formation.

Option (d): Not given

Since option (d) is not given in the exercise, we cannot analyze it. Putting all this information together, we can conclude that option (b) is the correct answer as it fulfills all the given properties of the compound C4H6.

Key concepts

Optical Isomerism

Optical isomerism is a fascinating concept in organic chemistry, mainly associated with compounds that can rotate plane-polarized light. This rotation causes some compounds to exist as non-superimposable mirror images, known as enantiomers. These mirror-image molecules have identical chemical and physical properties but differ in their interaction with polarized light.

• Enantiomers are specific cases of optical isomers.
• A molecule's chirality is what allows for optical isomerism. A chiral center, typically a carbon atom bonded to four different groups, will lead to chirality.

A compound is considered optically inactive if its molecules are symmetrical or if it exists in a form known as a racemic mixture, where two enantiomers are present in equal proportions. This balance leads to no overall rotation of polarized light. In the context of the exercise, even if the base compound is optically inactive, a monohalogen derivative can lead to chirality, thus becoming optically active. It highlights how structural changes can impact optical isomerism.

Allene Chemistry

Allenes are an intriguing part of organic chemistry, characterized by a central carbon atom double-bonded to two other carbon atoms on each side. This unique structure,

• gives them a linear geometry at the central carbon, which is unusual for most organic compounds, leading to distinctive chemical behavior.
• Unlike alkenes, allenes do not show cis-trans isomerism due to their linear structure.

Allenes are particularly interesting as they can sometimes exhibit optical activity, even though they lack the usual chiral center. This happens when the substituents on the two terminal carbons differ. If proper conditions are met, allene compounds can lead to optically active monohalogen derivatives. In the exercise, the identified compound was an allene that, while initially optically inactive, could form a chiral derivative upon halogenation.

Organic Reactions

Organic reactions are the backbone of organic chemistry, involving transformations that convert one organic compound into another. Several types of reactions exist,

• including additions, eliminations, substitutions, and redox reactions.
• The specific reagents and conditions lead to varying outcomes in these transformations.

For example, in the context of the given exercise, an addition reaction involving a halogen to an initially inactive compound can result in optical activity, demonstrating a significant structural change. Halogenation is a common reaction in organic chemistry where a halogen atom is introduced into the compound.
This reaction is an example of how relatively minor changes in a molecule's composition can lead to significant changes in its chemical behavior, demonstrating the dynamic nature of organic reactions. The exercise underscores the intricacies of organic transformations and how they can impact molecular properties.