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  • 0Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering Jiangnan University, Wuxi 214122, China.
Journal of the American Chemical Society +

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2024年9月11日

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Catalysis 02:50

26.8K

The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed during the process. A basic comprehension of a catalysts’ role during chemical reactions can be understood from the concept of reaction mechanisms and energy diagrams.

The illustrated image represents the reaction diagrams for an endothermic chemical process progressing in the absence (red curve) and presence (blue curve) of a catalyst.

Photochemical Electrocyclic Reactions: Stereochemistry 01:26

1.8K

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation

Cycloaddition Reactions: MO Requirements for Photochemical Activation 01:12

2.0K

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.

Thermally-induced [2 + 2] cycloadditions are symmetry forbidden. This is because the ground state HOMO of one ethylene molecule and the LUMO of the other ethylene are out of phase, preventing a concerted suprafacial-suprafacial overlap.
Absorption...

Thermal and Photochemical Electrocyclic Reactions: Overview 01:26

2.3K

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.

Electrocyclic reactions are highly stereospecific. For a substituted polyene, the stereochemical outcome...

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation 02:17

3.2K

Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...

Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration 02:34

8.4K

The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.

The reaction proceeds with the slow protonation of an alkene by a hydronium ion to form a carbocation, which is the rate-determining step.
The reaction involving a tertiary carbocation intermediate is faster than a reaction proceeding through a secondary or primary carbocation. This can be justified by comparing their...