Question

Discuss the direct, concerted addition of \(\mathrm{H}_{2}\) to an alkene from the standpoint of orbital symmetry. (a) In the absence of catalyst, and (b) in the presence of photochemical stimulation.

Short answer

In the absence of a catalyst, the direct, concerted addition of H2 to an alkene is symmetry-forbidden due to the unfavorable overlap of orbitals between the ground state hydrogen molecule and the alkene's pi bond. However, in the presence of photochemical stimulation, the excited hydrogen molecule's asymmetric orbitals allow for a suitable overlap with the pi bond of the alkene, making the reaction symmetry-allowed.

Step-by-step solution

(a) Direct, concerted addition of H2 to an alkene in the absence of a catalyst

In the absence of a catalyst, the hydrogen molecule is in its ground state with both hydrogen atoms having a pair of electrons in their 1s orbitals. An alkene molecule consists of sp2 hybridized carbon atoms, forming a pi bond by overlapping their unhybridized p orbitals. Now, let's consider the interaction between H2 and the alkene. A direct and concerted addition would require a simultaneous interaction between these orbitals. In the ground state, the 1s orbital of H2 is too low in energy and spherically symmetric, making it unfavorable for interaction with the pi bond of the alkene. Therefore, there is no suitable overlap of orbitals, and the reaction is symmetry-forbidden at the ground state of the hydrogen molecule.

(b) Direct, concerted addition of H2 to an alkene in the presence of photochemical stimulation

In the presence of photochemical stimulation, the hydrogen molecule is excited, leading to the promotion of one of the electrons in the 1s orbital to a higher energy level. The resulting singly occupied orbitals become more asymmetric in shape and are suitable for interaction with the pi bond of the alkene. With this asymmetric orbital geometry, the overlap between the pi bond of the alkene and R-H sigma bond of the excited hydrogen molecule becomes possible, thus allowing the direct, concerted addition of H2 to the alkene. The reaction becomes symmetry-allowed due to the alignment of orbitals upon photochemical stimulation. In summary, the direct, concerted addition of H2 to an alkene is orbital symmetry-dependent, with the ground state of the hydrogen molecule making the reaction symmetry-forbidden in the absence of a catalyst. Photochemical stimulation allows for suitable orbital alignment, making the reaction symmetry-allowed.

Key concepts

Alkene

Alkenes are a type of hydrocarbon that contain at least one carbon-carbon double bond. This double bond gives alkenes their unique chemical properties. The carbon atoms involved in the double bond are \( ext{sp}^{2}\)-hybridized, meaning they retain one unhybridized p-orbital. This unhybridized p-orbital is responsible for the formation of a \( ext{pi}\)-bond. This pi-bond is crucial because it contributes to the reactivity and properties of alkenes.

• They are electrons-rich, which makes them susceptible to electrophilic addition reactions.
• With its planarity and electron clouds, the pi-bond allows alkenes to undergo various chemical transformations.

When considering the addition of \( ext{H}_{2}\) to an alkene, it's essential to understand the underlying orbital interactions involving these pi-bonds.

Concerted Addition

Concerted addition refers to a chemical process where all bond-making and bond-breaking steps occur simultaneously. In the context of the addition of \( ext{H}_{2}\) to alkenes, this means that the hydrogen atoms add across the carbon-carbon double bond in one concerted step. This type of reaction is characterized by its synchronized nature, as no intermediates form during the process.

• The reaction coordinates involve a direct transition from reactants to products.
• Concerted pathways tend to preserve more symmetry if allowed by orbital interactions.

It is important in this context to consider orbital symmetry and energy levels because they dictate whether the reaction can proceed concertedly under those conditions.

Photochemical Stimulation

Photochemical stimulation is a process by which a substance, in this case \( ext{H}_{2}\), is excited by absorbing light energy. When the hydrogen molecule is photochemically stimulated, one electron transitions to a higher energy level. This excitation alters its orbital symmetry, making what was once symmetrical and unsuitable more suitable for reaction.
Such stimulation

• breaks electronic symmetry and
• results in an orbital overlap with the alkene's unhybridized p-orbitals, making a concerted addition possible.

This added energy helps in overcoming activation barriers, thus enabling reactions that would otherwise be symmetry-forbidden.

Symmetry-Allowed Reaction

A symmetry-allowed reaction is one where the symmetry properties of the molecular orbitals involved permit the reaction to proceed. In simple terms, the orbitals must align in such a way that they can overlap appropriately. For the addition of \( ext{H}_{2}\) to an alkene, the reaction is symmetry-forbidden in the ground state because the hydrogen molecule's orbitals do not align correctly with those of the alkene.
However, when we excite the hydrogen molecule photochemically,

• the electrons occupy asymmetric higher energy orbitals that can better interact with the alkene's pi-bond,
• enabling a symmetry-allowed reaction.

This principle illustrates how the geometry and symmetry of orbitals are crucial in determining if a reaction can occur smoothly and efficiently.