Sigmadelocalization

Sigmadelocalization refers to the movement of a sigma bond electron density. This phenomenon is distinct from pi delocalization, which involves the movement of pi electrons. While pi delocalization is commonly discussed in conjugated systems, sigmadelocalization is often associated with situations where a carbocation or other species with a partial positive charge is present. In such cases, the electron density of a sigma bond, typically adjacent to the positively charged center, can shift to help stabilize the charge. This shifting of sigma electrons can be viewed as a form of hyperconjugation, where the electrons of a sigma bond interact with an adjacent empty or partially filled p orbital.

The effect of sigmadelocalization is generally considered to be weaker than pi delocalization. However, it can still play a significant role in determining the stability of reaction intermediates and the regioselectivity of certain reactions. For instance, sigmadelocalization is invoked to explain the enhanced stability of secondary and tertiary carbocations compared to primary carbocations. It can also influence the reactivity of molecules, particularly in electrophilic addition reactions. The concept is particularly relevant in organic chemistry for understanding reaction mechanisms and predicting product formation.