Problem 165 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr} \stackrel{\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C} \overline{\mathrm{O}} \mathrm{K}^{+}}{\longrightarrow}(\mathrm{A}) \stackrel{\mathrm{NBS}}{\longrightarrow}\) (B) \(\frac{\text { Cold alk. }}{\mathrm{KMnO}_{4}}\) (C). The total number of stereisomers possible for the compound (C) are?

Step-by-Step Solution

Answer

The compound (C) has 0 stereoisomers.

Step 1: Identify Reaction Type

The starting compound \((\mathrm{CH}_{3})_{3} \mathrm{CBr}\) is an alkyl halide, and we are using \((\mathrm{CH}_{3})_{3} \mathrm{C} \overline{\mathrm{O}} \mathrm{K}^{+}\) as a reagent, which suggests a nucleophilic substitution reaction, typically SN1 due to the tertiary alcohol configuration. Therefore, compound A is likely \((\mathrm{CH}_{3})_{3} \mathrm{COC(CH}_{3})_{3}\), an ether.

Step 2: Apply Bromination with NBS

In the presence of N-bromosuccinimide (NBS), allylic or benzylic bromination occurs. However, since compound A is an ether, and there are no allylic or benzylic hydrogen atoms, the bromine is likely to add to the alpha carbon adjacent to the oxygen. Thus, compound B is \((\mathrm{CH}_{3})_{3} \mathrm{CBrC(CH}_{3})_{3}\).

Step 3: React with Cold, Dilute KMnO4

Compound B is treated with cold, dilute potassium permanganate (KMnO4), which will oxidize the brominated ether to form a diol. As the permanganate cleaves C=C double bonds in reactions with alkenes, but here it oxidizes the alpha-brominated ether to potentially a diol or simpler oxidized product on nearby methyl groups. Compound C remains structurally unchanged as suggsted bromine position limits alcohol formation in simple oxidation.

Step 4: Determine Possible Stereoisomers

To find the possible stereoisomers of compound C, we need to examine any chiral centers formed. Given direct oxidation effects was ineffective for largely unchanged tert-butyl groups in C, stereocenters of initial diffractions is unable from KMnO4 perspective. Due to this stability a match for anticipated stereochemistry holds in previous B creation factors but KMnO4 factors limit further isomer counts, uniting to no stereocenters suggested possible of compound, hence: no stereoisomers.

Key Concepts

Stereochemistry

Understanding **stereochemistry** is crucial in organic chemistry. It relates to the 3D arrangement of atoms in molecules. This 3-dimensionality affects how molecules interact with each other and react. Stereochemistry examines isomers, or compounds, that have the same molecular formula but differ in spatial orientation.
Examples of stereochemistry include:

• **Enantiomers**: These are mirror images of each other that cannot be placed on top of one another, similar to left and right hands.
• **Diastereomers**: These are not mirror images and can have different physical properties.

In the exercise, compound (C) was examined for possible stereoisomers, which means checking for chiral centers or differences in spatial arrangement.
However, with tert-butyl groups, this 3D aspect becomes less influential due to limited formation of stereocenters, demonstrating an insight into molecule stability and transformation.

Nucleophilic Substitution

**Nucleophilic substitution reactions** are a fundamental concept in organic chemistry. They involve a nucleophile, an electron-rich species, replacing a leaving group in a molecule. Such reactions are pivotal in the transformation of functional groups.
Two main types of nucleophilic substitution are:

• **SN1 (Unimolecular)**: Goes through a carbocation intermediate stage and is often used in reactions involving tertiary alkyl halides like \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr}\).
• **SN2 (Bimolecular)**: Involves a concerted single step, common in reactions with primary or secondary alkyl halides.

In the exercise, the reaction uses \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C} \overline{O} \mathrm{K}^{+}\) as a nucleophile to transform \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr}\), indicating an SN1 mechanism. This reaction forms a stable carbocation, which is more reactive towards nucleophiles and easier to substitute.

SN1 Reaction

In this context, an **SN1 reaction** proceeds through a two-step mechanism:

• **Step 1**: Departure of the bromide ion leaving group to form a carbocation, which is the slow and rate-determining step.
• **Step 2**: The nucleophile attacks the carbocation, leading to a product, in this case, an ether formation.

SN1 reactions are favored in:

• Good leaving groups like bromide ions.
• Tertiary or secondary carbons that can stabilize the carbocation formed.

The starting compound, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr}\), is typical for an SN1 reaction due to its tertiary carbon structure, which allows the transition to a carbocation before the nucleophile attacks. This reaction pathway leads to compound (A) via its classic mechanism.

N-bromosuccinimide

**N-bromosuccinimide (NBS)** is a popular reagent in organic synthesis for brominating allylic and benzylic positions. However, in this reaction sequence, compound (A) doesn’t contain such specific locations.
The function of NBS here centers upon substituting bromine into adjacent positions leading to potential alpha-bromination near ether formations.
Still, since the tertiary ether lacks classic resonance structures facilitating vivid alliances to either allylic or benzylic facilitators, the apparent and expected bromination in the exercise smoothly customizes around simpler molecular axes. This helps form compound (B), acknowledging limitations and specialized NBS utility.

Potassium Permanganate Oxidation

**Potassium permanganate ( KMnO_4 )** is a strong oxidizing agent commonly used in organic reactions. It usually targets double or triple bonds in alkenes or alkynes to form diols. In the exercise:

• KMnO_4 was used under cold, dilute conditions to limit its vigorous oxidative ability.
• Although primarily effective with alkenes, here it contemplates adjustments via ether to alcohol transformations possibly paired diols.

The reaction concludes with potassium permanganate having a tampered efficiency on brominated ether bonds, affirming an intricate limit on formation of new alcohol or further oxidized byproducts, aligning with tasks to stabilize compound (C) keeping its stereochemistry unchanged.
This outcome eludes stereo structural changes, explaining the absence of chiral centers in final suggestions.