Question

For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (a) \(\mathrm{CH}_{3} \mathrm{O}-\mathrm{OCH}_{3} \rightarrow \mathrm{CH}_{3} \dot{\mathrm{O}}+\dot{\mathrm{O}} \mathrm{CH}_{3}\)

Short answer

The cleavage is homolysis, forming free radicals.

Step-by-step solution

Identifying the Type of Cleavage

In the given reaction, the bond between the two oxygen atoms in \(\mathrm{CH}_{3} \mathrm{O}-\mathrm{OCH}_{3}\) is breaking evenly. Each oxygen atom takes back one electron from the bond, leading to the formation of two radicals. This process is characterized as homolysis because the electrons are equally divided between the two atoms.

Drawing Curved Arrow for Homolysis

To illustrate the electron flow during homolysis, we use a single-headed (fishhook) arrow. This arrow indicates that one electron from the oxygen-oxygen bond in \(\mathrm{CH}_{3} \mathrm{O}-\mathrm{OCH}_{3}\) is relocated to each oxygen atom. This results in two separate \(\mathrm{CH}_{3}\dot{\mathrm{O}}\) radicals.

Classifying Reactive Intermediates

After the bond cleavage, the reactive intermediates formed are \(\mathrm{CH}_{3}\dot{\mathrm{O}}\) and \(\dot{\mathrm{O}}\mathrm{CH}_{3}\). These species contain unpaired electrons, which classifies them as free radicals. Both \(\mathrm{CH}_{3}\dot{\mathrm{O}}\) groups are free radicals because they possess one unpaired electron.

Key concepts

Bond Cleavage

Bond cleavage refers to the breaking of chemical bonds in molecules, resulting in the separation of atoms. It is a significant process in organic chemistry as it helps in the understanding and prediction of reaction mechanisms. There are mainly two types of bond cleavage: homolytic and heterolytic.

• Homolytic bond cleavage involves each atom in the bond taking one electron, forming radicals.
• Heterolytic bond cleavage results in one atom taking both bonding electrons, creating ions.

In our example, the bond between two oxygen atoms in dimethyl peroxide is cleaved homolytically. Understanding the type of bond cleavage is foundational in determining the further course of a reaction.

Homolysis

Homolysis is a process where a bond breaks evenly, distributing one electron to each of the bonded atoms. This even distribution leads to the formation of free radicals. Homolysis is often initiated by energy input, such as heat or light.
For example, when the bond between the two oxygen atoms in \(\mathrm{CH}_{3} \mathrm{O}-\mathrm{OCH}_{3}\) is broken, each oxygen atom retains one electron, forming two separate radical species. In notation, we express this with single-headed curved arrows, showing the movement of an electron to each oxygen atom. Homolysis is critical in initiating radical chain reactions, common in polymerization and combustion reactions.

Free Radicals

Free radicals are highly reactive species characterized by having unpaired electrons. In organic chemistry, free radicals play a crucial role as intermediates in many reactions. These radicals are denoted by a dot next to the atom symbol to represent the unpaired electron.
In the given example, \(\mathrm{CH}_{3}\dot{\mathrm{O}}\) radicals are formed after homolysis. The presence of the unpaired electron makes these radicals very reactive, seeking to pair up by either donating the unpaired electron or capturing another electron. This reactivity allows free radicals to drive chain reactions, impacting how chemical processes evolve or are controlled.

Electron Flow

Electron flow, especially in the context of bond cleavage, is depicted using curved arrows in chemical notation. These arrows indicate the movement of electrons, enabling chemists to visualize and predict chemical reactions.
For homolytic cleavage, single-headed arrows are used to show that one electron moves to each atom involved in the bond. In contrast, double-headed arrows are used in heterolytic cleavage, denoting the movement of electron pairs.
Understanding electron flow is integral to mastering organic reaction mechanisms, as it helps determine how and where bonds will form or break during reactions.

Reactive Intermediates

Reactive intermediates are transient species formed during a chemical reaction, serving as a midpoint between reactants and products. These intermediates are often very unstable and highly reactive.
Some common reactive intermediates include:

• Free radicals, which have unpaired electrons, as seen in the breakdown of dimethyl peroxide.
• Carbocations, which are positively charged and lack electrons.
• Carbanions, which carry a negative charge due to excess electrons.

Identifying reactive intermediates is essential for understanding reaction mechanisms and kinetics, as they determine the reaction pathway and influence the speed and outcome of the chemical reaction.