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

Question

\(\underset{\mathrm{H}_{3} \mathrm{C}}{\mathrm{OH}} \stackrel{\mathrm{CH}_{3}}{\stackrel{\mathrm{H}^{+}}{-\mathrm{H}_{2} \mathrm{O}}}[\mathrm{F}] \stackrel{\mathrm{Br}_{2}, \mathrm{CCl}_{4}}{\longrightarrow} \underbrace{\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{Br}_{2}}_{\begin{array}{l}5 \text { such products } \\ \text { are possible }\end{array}} .\) How many structures for \(\mathrm{F}\) are possible? (a) 2 (b) 5 (c) 6 (d) 3

Step-by-Step Solution

Answer

(d) 3 possible structures.

Step 1: Understanding the Reaction

In this sequence, methanol ( OH group) undergoes dehydration in the presence of an acid, typically forming an alkene ( F). It's important to identify potential alkenes that could be formed.

Step 2: Identifying Alkene Structures

The dehydration process of the initial alcohol generates different alkene structures. Considering various possible configurations around double-bond formation, each structure corresponds to a positional isomer or stereo isomer. Here, we have to find how many alkene structures ( F) are possible.

Step 3: Considering Alkene Types

The dehydration leads to F with a double bond in different positions along the carbon chain or in different stereocontrolled formations (cis/trans or E/Z isomers). In this case, C4 implies that potential alkenes belong to butene isomers. Possible isomers include 1-butene and 2-butene, along with their cis/trans forms.

Step 4: Counting Possible Alkene Isomers

For a molecule C4H8 obtained through dehydration, the possible alkenes include 1-butene, cis-2-butene, and trans-2-butene. Thus, three different structures of F are possible.

Step 5: Conclusion about Potential Structures

Thus, the total number of possible structures for F, based on different positional or geometric isomers, is computed to be three.

Key Concepts

Alkene Isomers

Alkene isomers are variations of alkenes that differ in the arrangement of atoms or groups within the molecules. When we talk about alkenes, we are considering compounds that have a carbon-carbon double bond, represented as C=C. The location of this double bond and the attachments around it create different structural forms.

• **Positional Isomers**: These are alkenes where the carbon-carbon double bond's position changes in the carbon chain. For example, moving the double bond from one carbon position to another in a four-carbon chain like butene results in different positional isomers such as 1-butene and 2-butene.
• **Stereoisomers**: These occur when the spatial arrangement around the double bond changes, but the overall structure remains the same. In the case of butenes, 2-butene further divides into cis-2-butene and trans-2-butene, based on the relative position of substituents around the double bond.

Understanding these concepts is crucial as each type of isomer can impact the chemical properties and reactivity of the compounds significantly.

Dehydration Reaction

A dehydration reaction is a type of chemical reaction where water (H₂O) is removed from a molecule. In organic chemistry, the dehydration of alcohols is a common pathway for synthesizing alkenes.In this specific reaction, methanol ( \( ext{CH}_3 ext{OH}\) ) loses a water molecule in the presence of an acidic catalyst. The removal of water involves heating and acidic conditions, which favors the formation of a more stable alkene via the elimination of the hydroxyl group (OH) and a hydrogen (H), typically from an adjacent carbon.

• **Mechanism**: The reaction mechanism usually involves protonation of the alcohol, forming an oxonium ion, followed by the loss of a water molecule to generate a carbocation intermediate. This carbocation can then lose a proton to form the alkene.
• **Product Formation**: The position of the double bond in the resulting alkene is crucial and can lead to different positional isomers, as discussed. Temperature and catalyst type can influence which isomer is more prominently formed.

Stereochemistry

Stereochemistry involves the study of the spatial arrangement of atoms within molecules. It has profound implications in chemistry, particularly regarding the reactivity and physical properties of organic compounds, such as alkenes. In the case of alkenes, stereochemistry is primarily concerned with the orientation of groups attached to the double bond:

• **E/Z Notation**: This is used to describe the stereochemistry of double bonds. The notation "E" (from the German "entgegen," meaning opposite) and "Z" (from "zusammen," meaning together) indicate whether the higher priority substituents on either end of the double bond are on opposite sides or the same side, respectively.
• **Cis/Trans Isomers**: These are specific types of stereoisomers. For alkenes like 2-butene, 'cis' means that similar or higher priority groups are on the same side of the double bond, whereas 'trans' means they are on opposite sides. This type of geometric isomerism is pivotal for the three-dimensional shape and interactions of the molecules.

By understanding stereochemistry, chemists can predict and explain how different molecules behave in chemical reactions and what properties they may exhibit.