Question

What is major product expected from the monobromination of 2,2,3 -trimethylpentane?

Short answer

The major product expected from the monobromination of 2,2,3-trimethylpentane is 2-bromo-2,2,3-trimethylpentane.

Step-by-step solution

Understanding the Mechanism

The monobromination of 2,2,3-trimethylpentane involves a free radical halogenation. This process is characterized by a mechanism that includes three stages: initiation, propagation, and termination. In the initiation stage, the bromine molecule undergoes homolytic fission under the influence of ultraviolet light or heat to form two bromine radicals. In the propagation stage, the bromine radicals react with 2,2,3-trimethylpentane to form a bromo-2,2,3-trimethylpentane and another radical, which in turn can react with another bromine molecule. The termination stage occurs when two radicals react with each other to form a stable molecule.

Structural Analysis of 2,2,3-trimethylpentane

Looking at the structure of 2,2,3-trimethylpentane, there are three kinds of hydrogens: those on the tertiary carbon (3°), those on the secondary carbon (2°), and those on the primary carbon (1°). In free radical halogenation, bromine will tend to substitute the hydrogen on the tertiary carbon more frequently. This is due to the stability of the free radical formed at this stage in propagation step.

Determine the Major Product

Given the structure of 2,2,3-trimethylpentane and knowing that bromine will predominantly substitute the hydrogen of the tertiary carbon, it can be concluded that the major product will be 2-bromo-2,2,3-trimethylpentane. In this product, the bromine atom is bound to the tertiary carbon atom, which was originally bonded to one of the hydrogen atoms.

Key concepts

Free Radical Halogenation

Imagine a dance where molecules pair and swap partners; this is akin to free radical halogenation, a fundamental type of chemical reaction. In this process, a halogen molecule, such as bromine (Br₂), is persuaded to split into two highly reactive atoms, known as free radicals, often with ultraviolet light or heat as the matchmaker.

Once formed, these radicals don't stay lonely for long – they seek out and replace a hydrogen atom in an organic molecule such as 2,2,3-trimethylpentane. This substitution dance happens in three main steps: initiation (the halogen breakup), propagation (the radical bonding and breakup cycle), and termination (the cooling down phase where radicals pair off).

The nature of free radicals is such that they prefer certain types of carbon partners. A radical will generally choose a tertiary carbon atom to bond with, due to the resulting molecule's greater stability. This preference affects the outcome of the reaction, steering us towards predicting the major product of the substitution.

Why heat or UV light?

These forms of energy are proficient at breaking the strong bond holding the bromine molecule together, setting the stage for the reaction without adding unnecessary products to the mix.

Chemical Reaction Mechanisms

To truly understand a chemical reaction, it's like peering into the inner workings of a clock – we dissect the chemical reaction mechanism. This is the detailed step-by-step account of the transformations occurring from reactants to products.

Each reaction has its own unique sequence, much like a choreographed performance. The mechanism for free radical halogenation follows a specific pathway, involving the radical formation and the subsequent substitution reaction. Through this lens, we appreciate the nuances of reactivity, stability, and the influence of molecular structure on the behavior of those involved.

Importance of Stability

In the free radical halogenation process, the stability of the free radical intermediates is crucial. It guides the reaction towards the most stable (and often, the most abundant) product. In the case of 2,2,3-trimethylpentane, the stability of a tertiary radical can be thought of as a predictable plot twist that eventually leads us to the main outcome.

Tertiary Carbon Atom Substitution

When the free radical halogenation dance begins, not every partner is equally appealing to the radical. The tertiary carbon atom substitution is the chemical equivalent of winning the perfect match on a dating show. Tertiary carbon atoms are bonded to three other carbon atoms, making them more enticing due to the stability they offer to their free radical suitors.

This stability is due to a concept in chemistry called hyperconjugation, which is the delocalization of electrons that provides extra stabilization to the electron-deficient radical.

Hyperconjugation: The Chemistry Charm

Think of hyperconjugation as the tertiary carbon's charisma, attracting more radicals than its less charismatic secondary and primary counterparts. Consequently, in the monobromination of 2,2,3-trimethylpentane, it's the tertiary carbon's allure that dictates the most probable outcome - the formation of 2-bromo-2,2,3-trimethylpentane, where the bromine atom has replaced a hydrogen atom previously attached to a tertiary carbon.