Question

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3} \stackrel{\mathrm{Br}_{2} / \mathrm{hv}}{\longrightarrow}(\mathrm{A}) \stackrel{\mathrm{Mg} / \mathrm{Ether}}{\longrightarrow}(\mathrm{B}) \stackrel{\mathrm{D}_{2} \mathrm{O}}{\longrightarrow}(\mathrm{C})\) The major product \(C\) formed is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{D}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CHD} \mathrm{CH}_{3}\) (d)

Short answer

Answer: (c) \(\mathrm{CH}_{3} \mathrm{CHD} \mathrm{CH}_{3}\).

Step-by-step solution

\(Br_{2} / hv\) reaction

The first reaction involves bromination of the initial compound under light (hv). This light-catalyzed reaction is a free-radical halogenation reaction, where the main intermediate is a carbon radical. In this reaction, the halogen (Br) is added to the carbon-hydrogen (C-H) bond. The major product will be formed at the most stable carbon radical, which is the more substituted carbon (the secondary carbon in this case). So, the major product (A) will be 2-bromopropane: \(\mathrm{CH}_{3}\mathrm{CHBr}\mathrm{CH}_{3}\).

\(Mg / Ether\) reaction

The second reaction is the formation of a Grignard reagent using magnesium (Mg) in the presence of ether. The magnesium inserts itself between the carbon-bromine bond, forming a carbon-magnesium bond. The major product (B) after this reaction will be a Grignard reagent: \(\mathrm{CH}_{3}\mathrm{CHMgBr}\mathrm{CH}_{3}\).

\(D_{2}O\) reaction

The third and final reaction involves the addition of deuterium oxide (\(\mathrm{D}_{2}\mathrm{O}\)) to the Grignard reagent. The Grignard reagent will react with the deuterium from \(\mathrm{D}_{2}\mathrm{O}\), and the carbon-magnesium bond will be replaced with a carbon-deuterium bond. The major product (C) after this reaction will be: \(\mathrm{CH}_{3}\mathrm{CHD}\mathrm{CH}_{3}\). Now that we have determined the major product C, we can match it with the given options. The correct answer is: (c) \(\mathrm{CH}_{3} \mathrm{CHD} \mathrm{CH}_{3}\).

Key concepts

Free radical halogenation

Free radical halogenation is a fundamental organic chemistry process that involves the substitution of a hydrogen atom with a halogen (such as chlorine or bromine). This reaction occurs in the presence of ultraviolet light (hv), which provides the energy needed to break the halogen-halogen bond and create reactive halogen radicals.
In the context of the exercise, we initiated a free radical bromination using bromine (\(\mathrm{Br_2}\)) and light. A radical is an atom with an unpaired electron, making it extremely reactive. This reactivity allows the radical to react with the hydrogen atom on a hydrocarbon chain, producing a more stable carbon radical.
The process favors the most substituted carbon radicals because they achieve stability through hyperconjugation and the inductive effect. In particular, for a propane molecule, this means that a bromine atom attaches itself to the secondary carbon (the middle one in propane). This results in the formation of 2-bromopropane, which is the major product (A) described in the exercise.

Grignard Reagents

Grignard reagents are organometallic compounds crucial in organic synthesis, typically characterized by the formula R-Mg-X, where R is an alkyl or aryl group and X is a halogen. They are prepared by reacting an organohalide with magnesium in the presence of an ether solvent, usually diethyl ether or THF (tetrahydrofuran).
In the given exercise, once the bromination yields 2-bromopropane, it undergoes further transformation into a Grignard reagent. Magnesium inserts itself between the carbon-bromine bond, ultimately forming isopropylmagnesium bromide: \(\mathrm{CH_3CHMgBrCH_3}\).
Grignard reagents are valuable due to their ability to act as strong nucleophiles. This makes them highly useful for forming carbon-carbon bonds by attacking electrophilic centers in various chemical reactions.

Deuterium Exchange

Deuterium exchange is a reaction where a hydrogen atom in a molecule is replaced by deuterium (an isotope of hydrogen with a neutron). It is often used to label molecules for tracing reactions or studying molecular pathways.
In our exercise, the Grignard reagent interacts with deuterium oxide (\(\mathrm{D_2O}\)). The nucleophilic carbon of the Grignard reagent attacks the electrophilic deuterium, thereby replacing the magnesium-bromine group with deuterium. This process leads to the product \(\mathrm{CH_3CHDCH_3}\) as the deuterium replaces the hydrogen atom on the carbon in the Grignard reagent.
This selective replacement serves as a powerful tool to track hydrogen movement and study reaction mechanisms in more complex organic synthesis.

Bromination Reaction

The bromination reaction is a specific type of electrophilic halogenation involving bromine that allows for the introduction of bromine atoms into organic molecules. It is a member of the broader family of halogenation reactions, characterized by the substitution of hydrogen atoms in hydrocarbons with halogens.
Differing from chlorination, bromination proceeds via a more selective mechanism influenced by the reaction conditions, such as temperature and the presence of light. The issue of selectivity becomes crucial, since bromine radicals preferentially form at the most stable carbon radicals, typically the tertiary or secondary carbons, due to their higher radical stability.
In the solution, light catalysis (\(hv\)) fosters the initial breakage of a \(\mathrm{Br}_2\) bond. The resulting bromine radical then attacks the propane molecule, leading to the formation of a stable secondary radical at the carbon where bromine is eventually added, giving rise to 2-bromopropane as the major product.