Question

(a) Write the initiation, propagation, and termination steps involved in the monofluorination of 2,3-dimethylbutane to give 1-fluoro-2,3-dimethylbutane. (b) Explain why in the monofluorination of \(2,3-\) dimethylbutane the major product is \(1-\) fluoro- \(2,3-\) dimethylbutane, not 2-fluoro-2,3-dimethylbutane.

Short answer

The initiation, propagation and termination steps of the monofluorination of 2,3-dimethylbutane all revolve around the formation and reaction of fluorine radicals. 1-fluoro-2,3-dimethylbutane is the major product due to the stability of the secondary radical compared to the primary radical.

Step-by-step solution

Identify the Initiation Step

The initiation step for the monofluorination of 2,3-dimethylbutane involves the homolytic fission of a fluorine molecule into fluorine radicals: \[F_2 \rightarrow 2F\cdot\]

Propagation Step and Termination Step Identification

The propagation step is a two-step process. First, a hydrogen atom is abstracted from 2,3-dimethylbutane by a fluorine radical, producing H-F and a radical at the 1-position of the butane: \[2,3-\)dimethylbutane + F\cdot \rightarrow 1\cdot-\)fluoro-2,3-dimethylbutane + HF\]In the second step, formation of the monofluoro product happens: \[1\cdot-\)Fluoro-2,3-dimethylbutane + F_2 \rightarrow 1-\)fluoro-2,3-dimethylbutane + F\cdot\]The termination step is when two radicals react to form a covalent bond. In this case, two fluorine radicals can combine to form a fluorine molecule: \[F\cdot + F\cdot \rightarrow F_2\]

Explain why 1-fluoro-2,3-dimethylbutane is the Major Product

1-fluoro-2,3-dimethylbutane is the major product because it results from the abstraction of a hydrogen from the more substituted carbon atom (a secondary carbon). The radical formed is more stable on a secondary carbon compared to a primary carbon due to the greater degree of hyperconjugation, which gives it more structures to help stabilize it. Although both products are possible, 1-fluoro-2,3-dimethylbutane will be formed in greater quantities due to the greater stability of its corresponding radical.

Key concepts

Free Radical Substitution

Free radical substitution is a reaction type where a hydrogen atom in a hydrocarbon, like an alkane, is replaced by a halogen atom through a series of radical reactions. This process occurs in three main steps: initiation, propagation, and termination.

During the initiation step, a halogen molecule is exposed to heat or light to produce two reactive halogen atoms, known as free radicals. These free radicals are highly reactive due to an unpaired electron.

In the propagation steps, these free radicals abstract hydrogen atoms from the hydrocarbon, creating a new free radical in the process. The newly formed radical then reacts with another halogen molecule to produce more halogen radicals and the halogenated hydrocarbon. This step repeats many times, propagating the chain reaction.

The reaction eventually comes to an end through the termination steps, where radicals combine with one another to form stable molecules, ceasing the reaction progression. These steps together constitute the free radical substitution mechanism that leads to the halogenation of alkanes like 2,3-dimethylbutane.

Chemical Kinetics

Chemical kinetics is the study of the rates at which chemical reactions proceed and the factors that affect these rates. It's crucial when considering reactions like halogenation because rate factors determine which halogenation pathway predominates.

The rate of a chemical reaction depends on several factors, including concentration, temperature, and the presence of catalysts or inhibitors. In the context of free radical substitution, the rate at which halogen radicals collide with hydrocarbon molecules affects the overall speed of the reaction. Products form faster if these collisions occur more frequently, which is influenced by the aforementioned variables.

Furthermore, the kinetic stability of the reaction intermediates, such as carbocations or free radicals, plays a significant role. More stable intermediates generally lead to faster reaction rates, so a more stable radical intermediate would favor the formation of certain products over others in monofluorination reactions.

Reaction Mechanisms

A reaction mechanism provides a step-by-step sequence of elementary reactions that show the path taken by molecules to move from reactants to products. Mechanisms are crucial in predicting the outcome of a reaction, such as which products will be favored.

In free radical substitution, the reaction mechanism involves steps where radicals are generated, react with the substrate, and then are quenched. By following each step detailed in the mechanism, one can rationalize the outcome of the monofluorination of hydrocarbons.

Each step in the mechanism contributes differently to the final product composition. For instance, the formation of more stable intermediates during the propagation step leads to preferred product formation. Scientists utilize mechanisms to predict not only the products but also to understand how changes in conditions can affect the outcome of the reaction. This understanding is critical to optimizing conditions for desired chemical transformations, like the selective monofluorination of hydrocarbons.

Stability of Carbocation Intermediates

Stability of carbocation intermediates significantly influences the course and outcome of many organic reactions, including those involving free radicals. Although carbocations are not formed during free radical substitutions, the stability of similar intermediates like free radicals can be compared.

In free radical reactions, the stability of the radical intermediate helps to determine which hydrogen atom is more likely to be abstracted. Radicals located on secondary or tertiary carbons are more stable than those on primary carbons due to the greater degree of hyperconjugation and inductive effects from neighboring carbon atoms.

The increased stability of these radicals influences the chemical kinetics of the reaction, leading to favored formation of certain products based on which radical forms will be more stable. For instance, in the monofluorination of 2,3-dimethylbutane, the resulting secondary radical, which is more stable than a primary radical, leads to 1-fluoro-2,3-dimethylbutane as the major product. An understanding of radical stabilisation helps explain product distribution in halogenation reactions.