Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following compound yields only one product on monobromination? (a) neopentane (b) aniline (c) phenol (d) toluene

Step-by-Step Solution

Answer

Neopentane yields only one product upon monobromination.

Step 1: Evaluate Neopentane

Neopentane is an alkane with the chemical structure \((CH_3)_4C\). Upon bromination, all the hydrogen atoms attached to the tertiary carbon in neopentane are equivalent. Hence, substitution by bromine will yield only one product: \(C_5H_{11}Br\).

Step 2: Evaluate Aniline

Aniline, \(C_6H_5NH_2\), when subjected to bromination, can lead substitution at the ortho or para positions relative to the amino group on the benzene ring. This can produce multiple mono-brominated products.

Step 3: Evaluate Phenol

Phenol, \(C_6H_5OH\), also directs bromination to the ortho and para positions on the aromatic ring with respect to the hydroxyl group. Hence, more than one mono-brominated product can be formed.

Step 4: Evaluate Toluene

Toluene, \(C_6H_5CH_3\), undergoes bromination mainly at the methyl group due to the stability of benzyl radicals. However, bromination could also occur at the ortho and para positions of the aromatic ring, leading to multiple products.

Step 5: Select the Correct Compound

Among all options, only neopentane yields a single monobromination product due to the symmetric structure and equivalent hydrogen atoms on the tertiary carbon.

Key Concepts

Bromination Reaction

A bromination reaction involves adding bromine atoms to a molecule through a substitution process. It is a crucial transformation in organic chemistry, particularly for alkanes and aromatic compounds.
The reaction often requires conditions such as elevated temperatures or a catalyst because bromine is less reactive than other halogens like chlorine.

• In alkanes, bromination typically occurs at the most reactive site, which is usually secondary or tertiary carbon atoms because these sites form more stable radicals.
• For aromatic compounds, the substituents on the benzene ring direct where bromination takes place, thanks to their activating or deactivating effects.

Understanding the nature of bromination helps predict the structure of products formed, essential in multi-step synthesis, and aids in the identification of specific organic compounds.

Neopentane Structure

Neopentane is a simplest alkane, specifically a form of isopentane, characterized by a high degree of symmetry. Its formula is \[ (CH_3)_4C \]. The central carbon atom in neopentane is bonded to four equivalent methyl groups.

• When bromination occurs, each hydrogen atom attached to these methyl groups is equivalent, thanks to the symmetry and tetrahedral geometry of the central carbon.
• This leads to only one possible product upon bromination, as every hydrogen on the tertiary carbon is indistinguishable from the others.

Neopentane is a great example of how chemical structure can influence chemical reactions and product outcomes.

Aromatic Substitution

Aromatic substitution is a type of reaction involving benzene or its derivatives. This process replaces a hydrogen atom on the aromatic ring with another atom or group.
For instance, bromination of aromatic compounds typically follows an electrophilic aromatic substitution mechanism, driven by the presence of substituents:

• Substituents like amino (in aniline) or hydroxyl (in phenol) direct incoming bromine atoms to the ortho or para positions.
• These groups are activating, making certain positions on the benzene ring more reactive compared to others.

However, this can result in multiple products due to the presence of more than one reactive site on the aromatic ring.

Equivalent Hydrogen Atoms

In organic molecules, equivalent hydrogen atoms are those that cannot be distinguished from each other in terms of their chemical environment.
In practice, this means they will react in the same way to a given chemical reagent, such as bromine in a bromination reaction.

• In neopentane, all the hydrogens attached to the tertiary carbon are equivalent, leading to only one monobrominated product.
• This equivalency arises because of the symmetric distribution of the methyl groups around the central carbon atom.

Understanding the concept of equivalent hydrogen atoms is vital in predicting the number of potential products in many organic reactions.